A regional drainage evaluation for the Avon Basin, Western Australia · 2011. 12. 6. · A Regional...

A Regional Drainage Evaluation for the Avon Basin,

Western Australia R Ali, N Viney, G Hodgson, S Aryal and W Dawes

A final report to the Engineering Evaluation Initiative of the Department of Water, Western Australia

October 2010

Water for a Healthy Country Flagship Report series ISSN: 1835-095X

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Citation: Ali R, Viney N, Hodgson G, Aryal S and Dawes W (2010). A Regional Drainage Evaluation for Avon Basin, Western Australia. A final report to the Engineering Evaluation Initiative of the Department of Water, Western Australia from CSIRO Water for a Healthy Country National Research Flagship. 225pp.

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Cover Photograph: Lake Grace in 2004 (photo: Department of Water, Western Australia)

A regional drainage evaluation for the Avon Basin Page iii

CONTENTS

Acknowledgments ................................................................................................... xviii

List of Abbreviations ................................................................................................. xix

Executive Summary .................................................................................................... xx

1. Avon Basin ........................................................................................................... 1 1.1. Introduction ............................................................................................................... 1

1.2. Hydrology .................................................................................................................. 3

1.3. Rivers and lakes ....................................................................................................... 3

1.4. Soils and geology ...................................................................................................... 4

2. Background and Objectives ............................................................................... 6

3. Project Description ............................................................................................. 6

4. Model Development ............................................................................................ 9 4.1. Model selection ......................................................................................................... 9

4.2. Model description ...................................................................................................... 9

4.3. Model modifications .................................................................................................. 9

5. Input Data ........................................................................................................... 10 5.1. GIS and DEM data .................................................................................................. 10

5.2. Rainfall data ............................................................................................................ 11

5.3. Streamflow data ...................................................................................................... 12

5.4. Leaf Area Index (LAI) .............................................................................................. 12

5.5. Soil data .................................................................................................................. 13

5.6. Evaporation data ..................................................................................................... 15

5.7. Lake area and volume ............................................................................................ 16

5.8. Natural channels cross-sectional data and curve fitting ......................................... 20 5.8.1. Channel characterisation ..................................................................................... 20 5.8.2. Velocity-discharge calculations in streams .......................................................... 22

5.9. Landmonitor salinity data ........................................................................................ 23

5.10. Artificial drainage .................................................................................................... 26 5.10.1. Zone of effectiveness (ZOE) of artificial drainage ................................................ 26 5.10.2. Determination of current and future artificial drainage lengths ............................ 27

5.11. Arterial channels ..................................................................................................... 31

6. Model Calibration .............................................................................................. 33 6.1. Calibration procedure.............................................................................................. 33

6.2. Calibration data and processing ............................................................................. 33 6.2.1. Streamflow data ................................................................................................... 33 6.2.2. Salt load data ....................................................................................................... 34

6.3. Calibration assumptions and targets ...................................................................... 35 6.3.1. Calibration assumptions ...................................................................................... 35 6.3.2. Calibration targets ............................................................................................... 35

6.4. Calibration results ................................................................................................... 36 6.4.1. Streamflow........................................................................................................... 36 6.4.2. Salt load .............................................................................................................. 44 6.4.3. Lake storage and discharge ................................................................................ 50

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7. Baseline Conditions .......................................................................................... 52 7.1. Current and future hydrological drivers................................................................... 52

7.2. Current and future predictions of baseline scenario ............................................... 52 7.2.1. Impacts on subcatchment-average groundwater trends ...................................... 52 7.2.2. Impacts on streamflow and water yield ................................................................ 53 7.2.3. Salt outflow and yield ........................................................................................... 59 7.2.4. Lake volumes and loads ...................................................................................... 65 7.2.5. Summary of the baseline scenario ...................................................................... 67

8. Water Management Scenarios ......................................................................... 68 8.1. Artificial drainage .................................................................................................... 68

8.1.1. Background to artificial drainage scenarios ......................................................... 68 8.1.2. Impacts of Drainage SSe ..................................................................................... 69 8.1.3. Impacts of Drainage SSr and Drainage SSf ........................................................ 95 8.1.4. Impacts of Drainage EB ....................................................................................... 98 8.1.5. Impacts of Drainage LB (lake bypass) ................................................................. 99

8.2. Woody perennials ................................................................................................. 102 8.2.1. Background to revegetation scenarios .............................................................. 102 8.2.2. Impacts of revegetation on groundwater ........................................................... 102 8.2.3. Impacts of revegetation on streamflow .............................................................. 105 8.2.4. Impacts of revegetation on salt load .................................................................. 114 8.2.5. Impacts of revegetation on lake storages and discharges ................................. 122

8.3. Climate impacts .................................................................................................... 123 8.3.1. Background to climate scenarios ....................................................................... 123 8.3.2. Climate impacts on groundwater ....................................................................... 123 8.3.3. Climate impacts on streamflow .......................................................................... 126 8.3.4. Climate impacts on salt loads ............................................................................ 137 8.3.5. Climate impacts on lake storages and discharges ............................................. 148

8.4. Summary of catchment scenarios ........................................................................ 149

9. Regional Drainage Discharge Management .................................................. 152 9.1. Introduction ........................................................................................................... 152

9.2. Leveed and open drainage with natural channel discharge management ........... 152 9.2.1. Leveed drainage with natural creek discharge management ............................ 152 9.2.2. Open drainage with natural creek discharge management ............................... 153 9.2.3. Leveed and open drainage with arterial channel discharge management ......... 154 9.2.4. Leveed drainage and subcatchment retention system (evaporation basins) ..... 163 9.2.5. Determining basin costs .................................................................................... 165

9.3. Summary ............................................................................................................... 172

10. Conclusions ..................................................................................................... 173

Appendix 1. Model Development ................................................................. 176

Appendix 2. GIS Data Preparation ............................................................... 183

Appendix 3. Leaf Area Index (LAI) ............................................................... 189

Appendix 4. Lake Bathymetry and Volume ................................................. 191

Appendix 5. Discharge Velocity Curve a and b Values .............................. 193

Appendix 6. Flow and Quality Data .............................................................. 195

Glossary .................................................................................................................... 208

References ................................................................................................................ 209

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LIST OF FIGURES Figure 1.1. Map of the Avon Basin in the wheatbelt of Western Australia .............................. 2

Figure 1.2. Distribution of remnant vegetation in the Avon Basin ........................................... 2

Figure 1.3. Main catchments and drainage system of the Avon Basin ................................... 4

Figure 1.4. Main soil types of the Avon Basin ......................................................................... 5

Figure 5.1. Subcatchment delineation for the Avon Basin .................................................... 11

Figure 5.2. Estimated annual average subcatchment LAI for the Avon Basin in 1975 ......... 13

Figure 5.3. Area-weighted average solum depth for Avon subcatchments .......................... 14

Figure 5.4. NRAG soil attribute Unrestricted Rooting Depth used to determine soil depth for each subcatchment for input into the LASCAM. .................................................................... 15

Figure 5.5. Thirty years of daily pan evaporation values for Beacon (each unique symbol is a separate year, not listed) ....................................................................................................... 16

Figure 5.6. Schematic of outlet height and next overflow height used for determining dead storage and maximum volume of a salt lake ......................................................................... 17

Figure 5.7. Area-volume curve for Lake Walyormouring obtained by constructing a 3D model of the lake from DEM data ..................................................................................................... 18

Figure 5.8. Cross-section of the outlet at Lake Walyomouring extracted from DEM ............ 18

Figure 5.9. Method for the rough estimation of dead storage (DS) and maximum volume (Vmax) for lakes without a 3D model ..................................................................................... 19

Figure 5.10. Comparison between surveyed and DEM based natural channel cross-section at Baird Road ......................................................................................................................... 21

Figure 5.11. Comparison between surveyed and DEM based natural channel cross-section at Doolakine – Baandee cross-section .................................................................................. 21

Figure 5.12. The extent of mapped salinity for the Avon Basin ............................................ 23

Figure 5.13. Landmonitor valley hazard predictions for the Avon Basin at hydrological equilibrium ............................................................................................................................. 24

Figure 5.14. Percent area of the Avon Basin mapped as saline in 1998 and valley hazard area at equilibrium ................................................................................................................. 25

Figure 5.15. Area of subcatchments of the Avon Basin with valley hazard at equilibrium .... 25

Figure 5.16. Drainage effectiveness regions of the Avon Basin ........................................... 27

Figure 5.17. Modelled arterial channel network showing various reaches ........................... 32

Figure 6.1. Observed and predicted daily streamflow for the Avon River at Walyunga, 1986 ............................................................................................................................................... 38

Figure 6.2. Observed and predicted daily streamflow for the Yilgarn River at Gairdner’s Crossing, 1986 ....................................................................................................................... 38

Figure 6.3. Observed and predicted monthly streamflow for the Avon River at Walyunga .. 39

Figure 6.4. Observed and predicted monthly streamflow for Wooroloo Brook at Karl’s Ranch ............................................................................................................................................... 39

Figure 6.5. Observed and predicted monthly streamflow for the Brockman River at Yalliawirra .............................................................................................................................. 40

Figure 6.6. Observed and predicted monthly streamflow for the North Mortlock River ........ 40

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Figure 6.7. Observed and predicted monthly streamflow for the East Mortlock River .......... 41

Figure 6.8. Observed and predicted monthly streamflow for the Avon River at Northam ..... 41

Figure 6.9. Observed and predicted monthly streamflow for the Avon River at Broun’s Farm ............................................................................................................................................... 42

Figure 6.10. Observed and predicted monthly streamflow for the Dale River at Waterhatch 42

Figure 6.11. Observed and predicted monthly streamflow for the Salt River at Qualandary Crossing ................................................................................................................................. 43

Figure 6.12. Observed and predicted monthly streamflow for the Lockhart River ................ 43

Figure 6.13. Observed and predicted monthly streamflow for the Yilgarn River .................. 44

Figure 6.14. Observed and predicted monthly salt load for the Avon River at Walyunga .... 45

Figure 6.15. Observed and predicted monthly salt load for Wooroloo Brook at Karl’s Ranch ............................................................................................................................................... 45

Figure 6.16. Observed and predicted monthly salt load for the Brockman River at Yalliawirra ............................................................................................................................................... 46

Figure 6.17. Observed and predicted monthly salt load for the North Mortlock River .......... 46

Figure 6.18. Observed and predicted monthly salt load for the East Mortlock River ............ 47

Figure 6.19. Observed and predicted monthly salt load for the Avon River at Northam ....... 47

Figure 6.20. Observed and predicted monthly salt load for the Avon River at Broun’s Farm 48

Figure 6.21. Observed and predicted monthly salt load for the Dale River at Jelcobine ...... 48

Figure 6.22. Observed and predicted monthly salt load for the Salt River at Qualandary Crossing ................................................................................................................................. 49

Figure 6.23. Observed and predicted monthly salt load for the Lockhart River .................... 49

Figure 6.24. Observed and predicted monthly salt load for the Yilgarn River ...................... 50

Figure 6.25. Predicted storage volume in Lake Kondinin. The dashed line shows the assumed dead storage level .................................................................................................. 50

Figure 7.1. Predicted groundwater depth in the absence of artificial drainage for representative subcatchments in each major region of the Avon Basin ................................ 53

Figure 7.2. Predicted cumulative flows in the absence of artificial drainage for eight subcatchments ....................................................................................................................... 55

Figure 7.3. The relative flow contributions (percentage of total flow) from the eight key subcatchments for the baseline scenario during the period 2004–2031 ............................... 55

Figure 7.4. Predicted annual flows at major gauging sites for the baseline case ................. 56

Figure 7.5. Predicted annual peak flows at major gauging sites for the baseline case ........ 57

Figure 7.6. Mean annual water yield of subcatchments in the Avon Basin, 1976–2003 ...... 58

Figure 7.7. Mean annual water yield of subcatchments in the Avon Basin, 2004–2031 ...... 58

Figure 7.8. Predicted cumulative areally-weighted salt loads in the absence of artificial drainage for eight subcatchments .......................................................................................... 60

Figure 7.9. The relative salt load contributions (percent of total salt load) from the eight key subcatchments for the baseline scenario during the period 2004–2031 ............................... 61

Figure 7.10. Predicted annual salt loads at major gauging sites for the baseline case ........ 62

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Figure 7.11. Predicted annual flow weighted salinity at major gauging sites for the baseline case. Gaps in the salinity curves correspond to years when no flow is predicted ................. 63

Figure 7.12. Mean annual salt yield of subcatchments in the Avon Basin, 1976–2003 ........ 64



Figure 7.15. Predicted daily lake storages for Lake Kondinin (green line). The thin red line is a smoothed average storage volume and the broken black line is the dead storage level. 66

Figure 8.1. Predicted groundwater depths for subcatchment 54 (Avon South River); drainage density 0.15 km–1 ................................................................................................................... 70

Figure 8.2. Predicted groundwater depths for subcatchment 61 (lower Lockhart River); drainage density 0.47 km–1 .................................................................................................... 70

Figure 8.3. Predicted groundwater depths for subcatchment 87 (lower Yilgarn River); drainage density 1.20 km–1. Note that the red and blue lines overlap ................................... 71

Figure 8.4. Predicted groundwater salinities for subcatchment 54 (Avon South River); drainage density 0.15 km–1 .................................................................................................... 72

Figure 8.5. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River); drainage density 0.47 km–1 .................................................................................................... 72

Figure 8.6. Predicted groundwater salinities for subcatchment 87 (lower Yilgarn River); drainage density 1.20 km–1. Note that the red and blue lines overlap ................................... 73

Figure 8.7. Predicted net water yields for open drains with high zone of effectiveness, 2073–2100 ....................................................................................................................................... 74

Figure 8.8. Predicted net water yields for open drains with low zone of effectiveness, 2073–2100 ....................................................................................................................................... 74

Figure 8.9. Predicted cumulative streamflow for the Avon River at Great Northern Highway. ............................................................................................................................................... 75

Figure 8.10. Predicted cumulative streamflow for the North Mortlock River ......................... 75

Figure 8.11. Predicted cumulative streamflow for the East Mortlock River .......................... 75

Figure 8.12. Predicted cumulative streamflow for the Avon River at Northam ..................... 76

Figure 8.13. Predicted cumulative streamflow for the Salt River at Qualandary Crossing ... 76

Figure 8.14. Predicted cumulative streamflow for the Lockhart River .................................. 76

Figure 8.15. Predicted cumulative streamflow for Wakeman Creek at Narembeen ............. 77

Figure 8.16. Predicted cumulative streamflow for Yilgarn River ........................................... 77

Figure 8.17. Predicted annual streamflow for Avon River at Great Northern Highway ......... 79

Figure 8.18. Predicted annual streamflow for North Mortlock River ..................................... 79

Figure 8.19. Predicted annual streamflow for East Mortlock River ....................................... 80

Figure 8.20. Predicted annual streamflow for the Avon River at Northam ............................ 80

Figure 8.21. Predicted annual streamflow for Salt River at Qualandary Crossing ................ 80

Figure 8.22. Predicted annual streamflow for Lockhart River ............................................... 81

Figure 8.23. Predicted annual streamflow for Wakeman Creek at Narembeen ................... 81

Figure 8.24. Predicted annual streamflow for Yilgarn River ................................................. 81

Figure 8.25. Predicted annual peak streamflow for Avon River at Great Northern Highway 82

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Figure 8.26. Predicted annual peak streamflow for North Mortlock River ............................ 82

Figure 8.27. Predicted annual peak streamflow for East Mortlock River .............................. 82

Figure 8.28. Predicted annual peak streamflow for Avon River at Northam ......................... 83

Figure 8.29. Predicted annual peak streamflow for Salt River at Qualandary Crossing ....... 83

Figure 8.30. Predicted annual peak streamflow for Lockhart River ...................................... 83

Figure 8.31. Predicted annual peak streamflow for Wakeman Creek at Narembeen ........... 84

Figure 8.32. Predicted annual peak streamflow for Yilgarn River ......................................... 84

Figure 8.33. Predicted net annual salt yields for open drains with high zone of effectiveness, 2073–2100 ............................................................................................................................. 85

Figure 8.34. Predicted net annual salt yields for open drains with low zone of effectiveness, 2073–2100 ............................................................................................................................. 86

Figure 8.35. Predicted cumulative salt load for Avon River at Great Northern Highway ...... 86

Figure 8.36. Predicted cumulative salt load for Mortlock North River ................................... 87

Figure 8.37. Predicted cumulative salt load for Mortlock East River .................................... 87

Figure 8.38. Predicted cumulative salt load for Avon River at Northam ............................... 87

Figure 8.39. Predicted cumulative salt load for Salt River at Qualandary Crossing ............. 88

Figure 8.40. Predicted cumulative salt load for Lockhart River ............................................ 88

Figure 8.41. Predicted cumulative salt load for Wakeman Creek at Narembeen ................. 88

Figure 8.42. Predicted cumulative salt load for Yilgarn River ............................................... 89

Figure 8.43. Predicted annual salt load for Avon River at Great Northern Highway ............. 89

Figure 8.44. Predicted annual salt load for Mortlock North River ......................................... 89

Figure 8.45. Predicted annual salt load for Mortlock East River ........................................... 90

Figure 8.46. Predicted annual salt load for Avon River at Northam ...................................... 90

Figure 8.47. Predicted annual salt load for Salt River at Qualandary Crossing .................... 90

Figure 8.48. Predicted annual salt load for Lockhart River ................................................... 91

Figure 8.49. Predicted annual salt load for Wakeman Creek at Narembeen ....................... 91

Figure 8.50. Predicted annual salt load for Yilgarn River ..................................................... 91

Figure 8.51. Predicted annual flow-weighted salinity for Avon River at Great Northern Highway ................................................................................................................................. 92

Figure 8.52. Predicted annual flow-weighted salinity for Mortlock North River ..................... 92

Figure 8.53. Predicted annual flow-weighted salinity for Mortlock East River ...................... 92

Figure 8.54. Predicted annual flow-weighted salinity for Avon River at Northam ................. 93

Figure 8.55. Predicted annual flow-weighted salinity for Salt River at Qualandary Crossing 93

Figure 8.56. Predicted annual flow-weighted salinity for Lockhart River .............................. 93

Figure 8.57. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen ... 94

Figure 8.58. Predicted annual flow-weighted salinity for Yilgarn River ................................. 94

Figure 8.58. Predicted groundwater depths for subcatchment 54 (Avon South River) for different revegetation scenarios ........................................................................................... 103

A regional drainage evaluation for the Avon Basin Page ix

Figure 8.59. Predicted groundwater depths for subcatchment 61 (lower Lockhart River) for different revegetation scenarios ........................................................................................... 103

Figure 8.60. Predicted groundwater depths for subcatchment 87 (lower Yilgarn River) for different revegetation scenarios ........................................................................................... 103

Figure 8.61. Predicted groundwater salinities for subcatchment 54 (Avon South River) for different revegetation scenarios ........................................................................................... 104

Figure 8.62. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River) for different revegetation scenarios ........................................................................................... 104

Figure 8.63. Predicted groundwater salinities for subcatchment 87 (lower Yilgarn River) for different revegetation scenarios ........................................................................................... 105

Figure 8.64. Predicted cumulative streamflow for the Avon River at Great Northern Highway for various revegetation scenarios ....................................................................................... 106

Figure 8.65. Predicted cumulative streamflow for the North Mortlock River for various revegetation scenarios ......................................................................................................... 106

Figure 8.66. Predicted cumulative streamflow for the East Mortlock River for various revegetation scenarios ......................................................................................................... 107

Figure 8.67. Predicted cumulative streamflow for the Avon River at Northam for various revegetation scenarios ......................................................................................................... 107

Figure 8.68. Predicted cumulative streamflow for the Salt River at Qualandary Crossing for various revegetation scenarios ............................................................................................ 107

Figure 8.69. Predicted cumulative streamflow for the Lockhart River for various revegetation scenarios ............................................................................................................................. 108

Figure 8.70. Predicted cumulative streamflow for Wakeman Creek at Narembeen for various revegetation scenarios ......................................................................................................... 108

Figure 8.71. Predicted cumulative streamflow for the Yilgarn River for various revegetation scenarios ............................................................................................................................. 108

Figure 8.72. Predicted annual streamflow for the Avon River at Great Northern Highway for various revegetation scenarios ............................................................................................ 109

Figure 8.73. Predicted annual streamflow for the North Mortlock River for various revegetation scenarios ......................................................................................................... 109

Figure 8.74. Predicted annual streamflow for the East Mortlock River for various revegetation scenarios ......................................................................................................... 109

Figure 8.75. Predicted annual streamflow for the Avon River at Northam for various revegetation scenarios ......................................................................................................... 110

Figure 8.76. Predicted annual streamflow for the Salt River at Qualandary Crossing for various revegetation scenarios ............................................................................................ 110

Figure 8.77. Predicted annual streamflow for the Lockhart River for various revegetation scenarios ............................................................................................................................. 110

Figure 8.78. Predicted annual streamflow for Wakeman Creek at Narembeen for various revegetation scenarios ......................................................................................................... 111

Figure 8.79. Predicted annual streamflow for the Yilgarn River for various revegetation scenarios ............................................................................................................................. 111

Figure 8.80. Predicted annual peak streamflow for the Avon River at Great Northern Highway for various revegetation scenarios ........................................................................ 111

A regional drainage evaluation for the Avon Basin Page x

Figure 8.81. Predicted annual peak streamflow for the North Mortlock River for various revegetation scenarios ......................................................................................................... 112

Figure 8.82. Predicted annual peak streamflow for the East Mortlock River for various revegetation scenarios ......................................................................................................... 112

Figure 8.83. Predicted annual peak streamflow for the Avon River at Northam for various revegetation scenarios ......................................................................................................... 112

Figure 8.84. Predicted annual peak streamflow for the Salt River at Qualandary Crossing for various revegetation scenarios ............................................................................................ 113

Figure 8.85. Predicted annual peak streamflow for the Lockhart River for various revegetation scenarios ......................................................................................................... 113

Figure 8.86. Predicted annual peak streamflow for Wakeman Creek at Narembeen for various revegetation scenarios ............................................................................................ 113

Figure 8.87. Predicted annual peak streamflow for the Yilgarn River for various revegetation scenarios ............................................................................................................................. 114

Figure 8.88. Predicted cumulative salt load for the Avon River at Great Northern Highway for various revegetation scenarios ............................................................................................ 114

Figure 8.89. Predicted cumulative salt load for the North Mortlock River for various revegetation scenarios ......................................................................................................... 115

Figure 8.90. Predicted cumulative salt load for the East Mortlock River for various revegetation scenarios ......................................................................................................... 115

Figure 8.91. Predicted cumulative salt load for the Avon River at Northam for various revegetation scenarios ......................................................................................................... 115

Figure 8.92. Predicted cumulative salt load for the Salt River at Qualandary Crossing for various revegetation scenarios ............................................................................................ 116

Figure 8.93. Predicted cumulative salt load for the Lockhart River for various revegetation scenarios ............................................................................................................................. 116

Figure 8.94. Predicted cumulative salt load for Wakeman Creek at Narembeen for various revegetation scenarios ......................................................................................................... 116

Figure 8.95. Predicted cumulative salt load for the Yilgarn River for various revegetation scenarios ............................................................................................................................. 117

Figure 8.96. Predicted annual salt load for the Avon River at Great Northern Highway for various revegetation scenarios ............................................................................................ 117

Figure 8.97. Predicted annual salt load for the North Mortlock River for various revegetation scenarios ............................................................................................................................. 117

Figure 8.98. Predicted annual salt load for the East Mortlock River for various revegetation scenarios ............................................................................................................................. 118

Figure 8.99. Predicted annual salt load for the Avon River at Northam for various revegetation scenarios ......................................................................................................... 118

Figure 8.100. Predicted annual salt load for the Salt River at Qualandary Crossing for various revegetation scenarios ............................................................................................ 118

Figure 8.101. Predicted annual salt load for the Lockhart River for various revegetation scenarios ............................................................................................................................. 119

Figure 8.102. Predicted annual salt load for Wakeman Creek at Narembeen for various revegetation scenarios ......................................................................................................... 119

A regional drainage evaluation for the Avon Basin Page xi

Figure 8.103. Predicted annual salt load for the Yilgarn River for various revegetation scenarios ............................................................................................................................. 119

Figure 8.104. Predicted annual flow-weighted salinity for the Avon River at Great Northern Highway for various revegetation scenarios ........................................................................ 120

Figure 8.105. Predicted annual flow-weighted salinity for the North Mortlock River for various revegetation scenarios ......................................................................................................... 120

Figure 8.106. Predicted annual flow-weighted salinity for the East Mortlock River for various revegetation scenarios ......................................................................................................... 121

Figure 8.107. Predicted annual flow-weighted salinity for the Avon River at Northam for various revegetation scenarios ............................................................................................ 121

Figure 8.108. Predicted annual flow-weighted salinity for the Salt River at Qualandary Crossing for various revegetation scenarios ........................................................................ 121

Figure 8.109. Predicted annual flow-weighted salinity for the Lockhart River for various revegetation scenarios. Gaps in the salinity curves correspond to years when no flow is predicted .............................................................................................................................. 122

Figure 8.110. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen for various revegetation scenarios. Gaps in the salinity curves correspond to years when no flow is predicted ................................................................................................................... 122

Figure 8.111. Predicted annual flow-weighted salinity for the Yilgarn River for various revegetation scenarios. Gaps in the salinity curves correspond to years when no flow is predicted .............................................................................................................................. 122

Figure 8.112. Predicted groundwater depths for subcatchment 54 (Avon South River) for different climate scenarios ................................................................................................... 124

Figure 8.113. Predicted groundwater depths for subcatchment 61 (lower Lockhart River) for different climate scenarios ................................................................................................... 125

Figure 8.114. Predicted groundwater depths for subcatchment 102 (upper Yilgarn River) for different climate scenarios ................................................................................................... 125

Figure 8.115. Predicted groundwater salinities for subcatchment 54 (Avon South River) for different climate scenarios ................................................................................................... 125

Figure 8.116. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River) for different climate scenarios .............................................................................................. 126

Figure 8.117. Predicted groundwater salinities for subcatchment 102 (upper Yilgarn River) for different climate scenarios .............................................................................................. 126

Figure 8.118. Predicted net annual water yields for a climate that is 10 % wetter than present, 2073–2100 ............................................................................................................. 127

Figure 8.119. Predicted net annual water yields for a climate that is 10 % drier than present, 2073–2100 ........................................................................................................................... 128

Figure 8.120. Predicted net annual water yields for a climate that is 20 % drier than present, 2073–2100 ........................................................................................................................... 128

Figure 8.121 Predicted net annual water yields for a climate that is 30 % drier than present, 2073–2100 ........................................................................................................................... 128

Figure 8.122. Predicted cumulative streamflow for the Avon River at Great Northern Highway ............................................................................................................................... 129

Figure 8.123. Predicted cumulative streamflow for the North Mortlock River ..................... 130

Figure 8.124. Predicted cumulative streamflow for the East Mortlock River ...................... 130

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Figure 8.125. Predicted cumulative streamflow for the Avon River at Northam ................. 131

Figure 8.126. Predicted cumulative streamflow for the Salt River at Qualandary Crossing 131

Figure 8.127. Predicted cumulative streamflow for the Lockhart River .............................. 131

Figure 8.128. Predicted cumulative streamflow for Wakeman Creek at Narembeen ......... 132

Figure 8.129. Predicted cumulative streamflow for the Yilgarn River ................................. 132

Figure 8.130. Predicted annual streamflow for the Avon River at Great Northern Highway 132

Figure 8.131. Predicted annual streamflow for the North Mortlock River ........................... 132

Figure 8.132. Predicted annual streamflow for the East Mortlock River ............................. 133

Figure 8.133. Predicted annual streamflow for the Avon River at Northam ........................ 133

Figure 8.134. Predicted annual streamflow for the Salt River at Qualandary Crossing ...... 133

Figure 8.135. Predicted annual streamflow for the Lockhart River ..................................... 134

Figure 8.136. Predicted annual streamflow for Wakeman Creek at Narembeen ............... 134

Figure 8.137. Predicted annual streamflow for the Yilgarn River ....................................... 134

Figure 8.138. Predicted annual peak streamflow for the Avon River at Great Northern Highway ............................................................................................................................... 135

Figure 8.139. Predicted annual peak streamflow for the North Mortlock River .................. 135

Figure 8.140. Predicted annual peak streamflow for the East Mortlock River .................... 135

Figure 8.141. Predicted annual peak streamflow for the Avon River at Northam ............... 136

Figure 8.142. Predicted annual peak streamflow for the Salt River at Qualandary Crossing ............................................................................................................................................. 136

Figure 8.143. Predicted annual peak streamflow for the Lockhart River ............................ 136

Figure 8.144. Predicted annual peak streamflow for Wakeman Creek at Narembeen ....... 137

Figure 8.145. Predicted annual peak streamflow for the Yilgarn River ............................... 137

Figure 8.146. Predicted net annual salt yields for a climate that is 10 % wetter than present, 2073–2100 ........................................................................................................................... 138

Figure 8.147. Predicted net annual salt yields for a climate that is 10 % drier than present, 2073–2100 ........................................................................................................................... 138



Figure 8.150. Predicted cumulative salt load for the Avon River at Great Northern Highway ............................................................................................................................................. 140

Figure 8.151. Predicted cumulative salt load for the North Mortlock River ......................... 140

Figure 8.152. Predicted cumulative salt load for the East Mortlock River .......................... 141

Figure 8.153. Predicted cumulative salt load for the Avon River at Northam ..................... 141

Figure 8.154. Predicted cumulative salt load for the Salt River at Qualandary Crossing ... 141

Figure 8.155. Predicted cumulative salt load for the Lockhart River .................................. 142

Figure 8.156. Predicted cumulative salt load for Wakeman Creek at Narembeen ............. 142

Figure 8.157. Predicted cumulative salt load for the Yilgarn River ..................................... 142

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Figure 8.158. Predicted annual salt load for the Avon River at Great Northern Highway ... 143

Figure 8.159. Predicted annual salt load for the North Mortlock River ............................... 143

Figure 8.160. Predicted annual salt load for the East Mortlock River ................................. 143

Figure 8.161. Predicted annual salt load for the Avon River at Northam ............................ 144

Figure 8.162. Predicted annual salt load for the Salt River at Qualandary Crossing .......... 144

Figure 8.163. Predicted annual salt load for the Lockhart River ......................................... 144

Figure 8.164. Predicted annual salt load for Wakeman Creek at Narembeen ................... 145

Figure 8.165. Predicted annual salt load for the Yilgarn River ........................................... 145

Figure 8.166. Predicted annual flow-weighted salinity for the Avon River at Great Northern Highway ............................................................................................................................... 145

Figure 8.167. Predicted annual flow-weighted salinity for the North Mortlock River ........... 146

Figure 8.168. Predicted annual flow-weighted salinity for the East Mortlock River ............ 146

Figure 8.169. Predicted annual flow-weighted salinity for the Avon River at Northam ....... 146

Figure 8.170. Predicted annual flow-weighted salinity for the Salt River at Qualandary Crossing ............................................................................................................................... 147

Figure 8.171. Predicted annual flow-weighted salinity for the Lockhart River. Gaps in the salinity curves correspond to years when no flow is predicted ............................................ 147

Figure 8.172. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen. Gaps in the salinity curves correspond to years when no flow is predicted ......................... 147

Figure 8.173. Predicted annual flow-weighted salinity for the Yilgarn River. Gaps in the salinity curves correspond to years when no flow is predicted ............................................ 148

Figure 9.1. Layout of arterial channels in the Avon Basin ................................................... 157

Figure 9.2. Relationship between potential net evaporative loss and basin area ............... 169

Figure 9.3. Design inflow and basin area relationship for the Avon Basin .......................... 169

Figure 9.4. Evaporation basin size required in various subcatchments of the Avon Basin during first quarter of the twenty-first century ...................................................................... 170

Figure 9.5. Evaporation basin size required in various subcatchments of the Avon Basin during last quarter of the twenty-first century ....................................................................... 170

Figure 9.6. Indicative evaporation basin costs in various subcatchments of the Avon Basin during first quarter of the twenty-first century ...................................................................... 171

Figure 9.7. Indicative evaporation basin construction costs in various subcatchments of the Avon Basin during last quarter of the twenty-first century ................................................... 171

Figure A1.1. Schematic of a conceptual hillslope in LASCAM and the arrangement of the conceptual stores ................................................................................................................. 179

Figure A1.2. Flowchart of water fluxes in LASCAM ............................................................ 180

Figure A2.1 Geoscience Australia Geodata 250K Hydrological features for the Avon Basin ............................................................................................................................................. 183

Figure A2.2. Department of Water subcatchments and hydrology for the Avon Basin ....... 184

Figure A2.3. Digital Elevation Model of the Avon Basin ..................................................... 186

Figure A2.4. The DEM derived drainage lines for the Avon Basin ..................................... 187

Figure A2.5. DEM derived drainage and stream ordering for the Avon Basin .................... 187

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Figure A2.6. Final subcatchment delineation for the Avon Basin ....................................... 188

Figure A3.1. Estimated annual average subcatchment LAI for the Avon Basin in 1975 .... 190

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LIST OF TABLES

Table 5.1. Evaporation parameters computed for five representative stations in the Avon Basin ...................................................................................................................................... 16

Table 5.2. Dead storage, outlet height and next overflow height for selected lakes. ............ 17

Table 5.3. The normalisation parameters for salt lakes in the Avon Basin ........................... 20

Table 5.4. List of surveyed natural channel cross sections .................................................. 20

Table 5.5. Parameters for determining roughness coefficient in Manning’s equation .......... 22

Table 5.6. Drainage length required for various proportions of the salt-affected area for a range of ZOEs in the Avon Basin .......................................................................................... 26

Table 5.7. Drainage lengths based on mapped 1998 salinity and Landmonitor valley hazard ............................................................................................................................................... 29

Table 5.7 (Continued). Drainage lengths based on mapped 1998 salinity and Landmonitor valley hazard .......................................................................................................................... 30

Table 6.1. Streamflow data used in this study ...................................................................... 34

Table 6.2. Salt load data used in this study (R stands for River; and Ck stands for creek)) . 34

Table 6.3. Calibration performance of the model for the major gauging stations ................. 37

Table 6.4. Comparison of expert estimates of the frequency of lake discharges with those predicted by the model, 1965–2003. The model predictions are divided into events greater

than and less than 1 GL discharge ........................................................................................ 51

Table 7.1. Mean annual streamflow and mean annual salt load for eight selected subcatchments over three time periods ................................................................................. 59

Table 7.2. Comparison of predicted lake discharge characteristics for two different 28-year time periods ........................................................................................................................... 66

Table 8.1. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the open drain scenario expressed as a proportion of the corresponding quantities for the leveed drain scenario. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness ................................. 69

Table 8.2. Comparison of predicted lake discharge characteristics for two different drainage scenarios for the period 2073–2100 ...................................................................................... 95

Table 8.3. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the regional retention scenario (Drainage SSf with open drains) with no lake discharge, expressed as a proportion of the corresponding quantities for open drains without retention (Drainage SSe). Predictions are for the 2073–2100 period and drainage is assumed to have a high ZOE .............................................................................. 96

Table 8.4. Predicted lake discharge characteristics for the regional retention scenario with raised discharge heights for the period 2073–2100 ............................................................... 97

Table 8.5. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the regional retention scenario (open drains) with lake discharge heights raised by 30 cm, expressed as a proportion of the corresponding quantities for open drains without retention. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness ..................................................................................... 97

Table 8.6. Predicted lake discharge characteristics for the subcatchment retention scenario

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for the period 2073–2100 ....................................................................................................... 99

Table 8.7. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the subcatchment retention scenario, expressed as a proportion of the corresponding quantities for the total discharge from the leveed drainage scenario without retention. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness ...................................................................... 99

Table 8.8. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for leveed drains (total discharges) without lakes, expressed as a proportion of the corresponding quantities for leveed drains with lakes. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness ...... 101

Table 8.9. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for leveed channels without lakes, expressed as a proportion of the corresponding quantities for total leveed drains without lakes. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness ............ 101

Table 8.10. Comparison of predicted lake discharge characteristics for two different revegetation scenarios for the period 2073–2100 ............................................................... 123

Table 8.11. Comparison of predicted lake discharge characteristics for two different climate scenarios for the period 2073–2100 .................................................................................... 149

Table 9.1. Main assumptions for farm and subcatchment scale leveed artificial drains ..... 153

Table 9.2. Excavaion cost of farm and subcatchment scale leveed drainage .................... 153

Table 9.3. Main assumptions for farm and subcatchment scale leveed artificial drains ..... 153

Table 9.4. Farm and subcatchment scale open drainage excavation costs ....................... 154

Table 9.5 Arterial channel lengths, subcatchments involved, upstream subcatchments and contributing drainage lengths ............................................................................................... 157

Table 9.6. Daily peak flows of 1 in 10 year recurrence interval used as design flows (calculated at end points) for various sections of the leveed and open arterial channels .... 158

Table 9.7. Main assumptions for channel design parameters ............................................ 161

Table 9.8. Size and excavation cost of leveed arterial system with salt lake storage option (Drainage SSe) .................................................................................................................... 161

Table 9.9. Size and excavation cost of leveed arterial system without salt lake storage option (Drainage LB) ...................................................................................................................... 161

Table 9.10. Size and excavation cost of open arterial system ............................................ 162

Table 9.11. Main assumptions for the excavation cost of arterial channels ....................... 163

Table 9.12. Assumptions for cost estimation of evaporation basins ................................... 166

Table 9.13. Design inflows and construction cost of subcatchment scale evaporation basins ............................................................................................................................................. 167

Table 9.13 (Continued). Design inflows and construction costs of subcatchment scale evaporation basins ............................................................................................................... 168

Table A1.1. Model maximisation matrix for the evaluation of various models .................... 178

Table A4.1. List of salt lakes surveyed. .............................................................................. 191

Table A5.1. Values of a and b for discharge velocity power curve relation of 88 subcatchments of the Avon Basin ....................................................................................... 193

Table A5.2. The a and b values for the remaining streams ................................................ 194

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Table A6.1. Predicted net water yields (mm y–1) for eight scenarios. Some small non-zero values may have been rounded down to zero. .................................................................... 196

Table A6.1 (continued). Predicted net water yields (mm y–1) for eight scenarios. Some small non-zero values may have been rounded down to zero. ..................................................... 197

Table A6.2. Predicted mean annual streamflows (GL) for eight scenarios. Some small non-zero values may have been rounded down to zero ...................................................... 198

Table A6.2 (continued). Predicted mean annual streamflows (GL) for eight scenarios. Some small non-zero values may have been rounded down to zero ............................................ 199

Table A6.3. Predicted ten-year return period peak streamflows (GL d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero .................................. 200

Table A6.3 (continued). Predicted ten-year return period peak streamflows (GL d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero ................ 201

Table A6.4. Predicted net annual salt yields (t km–2) for eight scenarios. Some small non-zero values may have been rounded down to zero ...................................................... 202

Table A6.4 (continued). Predicted net annual salt yields (t km–2) for eight scenarios. Some small non-zero values may have been rounded down to zero ............................................ 203

Table A6.5. Predicted mean annual salt loads (kt) for eight scenarios. Some small non-zero values may have been rounded down to zero ..................................................................... 204

Table A6.5 (continued). Predicted mean annual salt loads (kt) for eight scenarios. Some small non-zero values may have been rounded down to zero ............................................ 205

Table A6.6. Predicted ten-year return period peak salt loads (kt d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero .................................. 206

Table A6.6 (continued). Predicted ten-year return period peak salt loads (kt d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero ................ 207

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ACKNOWLEDGMENTS This research was a joint initiative to carry out regional drainage evaluation of the Avon Basin. The study was funded by Water for a Healthy Country Flagship, CSIRO Land and Water, and the Engineering Evaluation Initiative of the Department of Water, Western Australia. The Department of Agriculture and Food, Western Australia and the Department of Conservation and Land Management provided in-kind support for this project. Discussions with the WA Channel Management Group helped define various modelling scenarios and management options for the project. The authors thank numerous farmers of the Avon Basin who allowed access to their properties during field survey work of the lakes and creek/river sections. In-kind support from John Ruprecht, Ken McIntosh, Melinda Burton and David Rowlands of the Department of Water, Western Australia was crucial for the successful completion of this project. The authors especially thank Dr. Richard George of the Department of Agriculture and Food, Western Australia for continued in kind support and reviewing the summary and final report.

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LIST OF ABBREVIATIONS AHD Australian Height Datum

ARB Avon Basin

AVHRR Advanced Very High Resolution Radiometer

BOM Bureau of Meteorology

BM Bench mark

DAFWA Department of Agriculture and Food Western Australia

Dd drainage density

DEM Digital Elevation Model

DLI Department of Land Information

DOW Department of Water, WA

EMSS Environmental Management Support System

ET Evapotranspiration

GA Geoscience Australia

GPS Global Positioning System

IQQM Integrated quantity and quality modelling

LAI Leaf Area Index

Landmonitor Landmonitor is a multi-agency initiative to improve the quality of natural resource data through remote sensing – see http://www.landmonitor.wa.gov.au/

LASCAM LArge Scale CAtchment Model

NRAG Natural Resource Assessment Group

PPD Patched Point Data

RTK Real Time Kinetic

SSM Standard Survey Mark

TIN Triangulated Irregular Network

SHPA Soil Hydrological Properties of Australia

ZOE: Zone of effectiveness

GLy-1 Giga litres per annum

gL-1 Grams per litre

tkm-2y-1 tonnes per square kilometre per year

kt kilotonnes

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EXECUTIVE SUMMARY

Introduction This project was aimed to evaluate regional drainage and water management strategies for the Avon Basin, and assess feasibility and cost implications of various engineering options for salinity, waterlogging and flood control in the basin. After evaluating three models (EMSS, IQQM and LASCAM) LASCAM (LArge Scale CAtchement Modelling) was selected for this study. LASCAM was developed to predict the impact of land use changes on fluxes of water, salt, sediment and nutrients in forested and agricultural catchments in Western Australia. It operates on a daily time step and relies on calibration of model parameters against one or more observed records of streamflow and load. LASCAM was modified to enable its use for this project. The required modifications included lake storage, natural creek/stream/river storage and flow, lake and natural channel evaporation and artificial drainage.

Input data To calibrate the modified model for the Avon Basin, data about rainfall, stream flow, leaf area index (LAI), soil and evaporation were required. Existing GIS datasets were sourced from the Department of Environment. A DEM of the Avon Basin was constructed including delineation of basin boundaries, drainage network, subcatchment boundaries, flow direction and drainage density. The delineation of the basin boundary revealed that about 3,500 km2, previously believed to be within the Avon Basin, were in fact outside its boundary. Daily rainfall data from 358 rain gauges from 1965 to 2003 were used for model calibration, providing a good coverage of the Avon Basin. Stream gauging data were available from 26 stream gauging stations.

AVHRR data that estimated LAI over the whole of Australia was available electronically for the period from July 1981 to December 2000, covering most of the proposed modelling period. The relevant data for the Avon Basin was extracted and used to estimate actual monthly LAI for all 108 identified subcatchments. The data about soil properties including topsoil depth (subcatchment mean and minimum), regolith depth, porosity, field capacity, specific yield, Brooks-Corey texture index and bubbling pressure were obtained from various sources. Evaporation data were mainly from the Bureau of Meteorology’s (BOM) evaporation network and from the patched point dataset (PPD) containing daily rainfall, minimum and maximum temperatures, radiation, evaporation and vapour pressure. Data about lake and stream storage capacities were derived from DEM and field surveys. The lengths of artificial drainage required in each subcatchment of the Avon Basin under present conditions and at the reach of new equilibrium, were derived from LandMonitor salinity predictions assuming areal effectiveness of 100 to 600 m.

Model calibration These data are processed and used to calibrate LASCAM. The calibration process involves assessing the quality of model predictions for different sets of parameter values. For LASCAM, this is achieved by a semi-automated procedure that is based on the Shuffled Complex Evolution method. The calibration procedure seeks to find a set of model parameters that optimises the value of a prescribed objective function. An objective function is a linear combination of the model efficiencies (statistical measures of model fit) of the daily streamflows and salt loads at several locations in the catchment. The calibration technique yields a single set of parameters that has wide applicability across the catchment and which has taken advantage of any spatial heterogeneities existing in the observed data. This provides some confidence in the quality of predictions on the ungauged subcatchments, as well as on the gauged subcatchments. No measures of soil moisture (including watertable depths), soil salinity, lake stage or lake volume are employed in the calibration process. The subcatchment attribute data used by the model (e.g., land cover, soil depths, soil physical properties, climate, lake and channel characteristics) are all prescribed a priori, and are not

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subject to local modification to improve model fits. In calibrating LASCAM for the Avon Basin, in excess of 200,000 model runs have been conducted. The calibration period was from 1965 to 2003. The first five years of this time series is used as a spin-up period to allow the initial storages time to reach equilibrium.

A set of calibration performance targets was developed. These targets specify the quality of the model predictions that the calibration process should aim to achieve. For streamflow predictions, the targets for each of the major flow gauging sites are that the total flow over the calibration period should be within 30 % of the observed total, and prediction efficiencies should at least be 0.5 for daily flows, 0.6 for monthly flows and 0.7 for annual flows. For salt load predictions the targets for efficiency are the same, but the total load (or bias) target is relaxed to fall within 40 % of the observed total. The targets are not designed to be absolutely prescriptive. They are intended merely as convenient yardsticks on model performance across the catchment.

All but one of the 16 gauging sites used for calibration meet the water balance target for bias, with Wooroloo Brook missing only narrowly. All four targets are achieved by seven of the streamflow sites. All the others meet at least one target. Some sites have substantially poorer daily than monthly and annual efficiencies. For salt load, there is an overall tendency towards underprediction (negative bias). The calibration statistics for salt load are compromised to some extent by the limited amounts of observed data for some gauging sites. Five of the fifteen sites used for calibrating the salt parameters meet all four prediction targets, while only two meet none. In all, 50 of a possible 64 targets are met for the water parameters and 40 of 60 targets for the salt parameters.

Of the twenty modelled lakes, seven failed to fill to the dead storage level during the calibration period and may therefore be considered as terminal for streamflow during this period. These are the lakes that we expect to be terminal. All other lakes discharge at least once during the calibration period. Overall, the model predictions for lake discharge and storage appear to be quite encouraging given the extreme difficulty in establishing reliable and robust a priori accumulation and discharge characteristics for the modelled lakes. In most cases these characteristics were estimated without access to any reliable bathymetry data and in some cases without any survey data.

Baseline conditions Current and future hydrological drivers

In simulation mode, all LASCAM model runs are for the period 1965 to 2100. For the years after 2003, the 28-year period of observed meteorological data (rainfall and potential evaporation) from 1976 to 2003 was repeated about three and a half times to extend the meteorological forcing data to 2100.

Since 1975s a regime of reduced winter rainfall commenced over the southwest Western Australia. Some climate projections suggest that this reduction will continue to intensify over coming decades. The baseline conditions used in this project do not account for these predictions and there has been no attempt to incorporate any projections of possible future climate change into the future weather data. The 28-year weather cycle is sourced entirely from the dryer period from the mid-1970s onwards. As such, the baseline and drainage scenario predictions in this project are based on an implicit assumption that the climate of the region will remain similar to that of the past 30 years. Whether this climate eventuates or not is not particularly important since we are mainly concerned with assessing differences between scenarios and not so much with establishing absolute predictions. The LASCAM predictions also assume that land use, vegetation cover and lake characteristics will remain static in the catchment at their 2003 levels.

A set of baseline hydrological predictions that might be called the ‘do-nothing’ or ‘no-drainage’ case are reported below. These predictions are made on the assumption that there is no artificial drainage in the catchment (previously constructed artificial drainage is

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ignored in this scenario). The purpose of these predictions is to provide a benchmark against which we can assess the impacts of all other management and climatic scenarios.

Subcatchment-average groundwater trends The groundwater depths predicted by LASCAM are averages over an entire subcatchment; they are not intended to be representative of any specific point in the subcatchment. Most of the wheatbelt subcatchments exhibit continuing rises in average groundwater level, but at a slower rate than is currently occurring. There is considerable variability in groundwater response within and between regions. The wheatbelt subcatchments have average rates of watertable rise of between 20 mm y–1 and 26 mm y–1 in 2006. Over the course of the twenty-first century these rates of rise decline at all sites and by 2100, the rates of rise in the Lockhart and Yilgarn subcatchments reduce to 4 mm y–1 and 6 mm y–1, respectively, while further west the subcatchments are essentially at equilibrium. As well as being slower to reach equilibrium, there is also a general tendency for subcatchments in the east of the catchment to reach a higher equilibrium watertable level than subcatchments in the west.

Streamflow

A consequence of the rising watertables is that streamflows increase over the simulation period. The rate of increase in streamflow varies across the catchment and is greatest for the eastern subcatchments. For the Avon River at Northam, mean annual streamflow increases by about 50 % from 128 GL y–1 to 193 GL y–1 over the course of the twenty-first century and the proportion of days with flow increases from 46 % to 60 %. In contrast, for the Lockhart, mean annual streamflow increases from 6 GL y–1 to 30 GL y–1 over the same period and the proportion of days with flow increases from 29 % to 94 %.

Currently, the largest sources of streamflow are in the wetter subcatchments in the far west, with annual streamflow generation exceeding 40 mm in Wooroloo Brook and parts of Brockman River. The parts of the catchment that exceed 25 mm are all below Dunbarton Bridge. Above Northam, the most productive areas are Spencers Brook and the western branch of the Dale River. There is very little streamflow generated in the more than 50 % of the catchment east of Kellerberrin. In the main, this distribution of water yields reflects the spatial patterns of rainfall. By the end of the twenty-first century there are small increases in streamflow generation in almost all subcatchments. The greatest absolute increases (by more than 5 mm) are in the region between Northam and Beverley, and in the subcatchments closest to the catchment outlet.

Salt loads The increasing discharge of groundwater into shallower aquifers also has a significant impact on stream salinities and salt loads, since the source of that discharge is quite saline. This increased salinity is exacerbated by the rising watertables dissolving some of the substantial amounts of salt stored in the unsaturated zone. The largest increases in salinity are in the eastern wheatbelt and are associated with increases in the discharge of saline groundwater and from increased volumes and salinities of lake discharges.

For the period 1976–2003, the largest predicted salt yields (more than 60 t km–2 y–1) are from parts of the Brockman River and some of the subcatchments near Northam. Areas producing more than 40 t km–2 y–1 include the subcatchments along the Avon River between Toodyay and Yealering, and in the lower Mortlock East. By 2073–2100, there are predicted to be substantial increases in salt yield across most of the catchment. The largest increases (by more than 200 t km–2 y–1) are between Northam and Beverley and make some of these subcatchments the largest contributors of salt per unit area in the catchment. There are now also significant salt sources in the cleared parts of the Lockhart and Yilgarn catchments, and downstream of the main lakes in the two Mortlock branches.

Lake volumes and loads Increased flows have also manifested as increased frequencies and volumes of lake discharge. Discharge frequencies increase for all but one of the non-terminal lakes, although only one of the previously identified terminal lakes (Gulsen) discharges in the final 28 years

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of the simulation period. Several of the lakes that are near-terminal in the twentieth century fill quite frequently by the late twenty-first (e.g., Kurrenkutten, Jilakin). There are also significant increases in flow-weighted discharge salinity, which reflect the increased salinity of lake inflows.

Summary of baseline scenario

The foregoing sections have highlighted that the current hydrological state of the Avon catchment is not static. The catchment is not at equilibrium. We are presently in a phase of steadily increasing watertables, streamflows, lake storages and salt loads. These increases have been occurring in some parts of the catchment since European settlement and are in response to the widespread replacement of native vegetation with shallow-rooted crops and pasture. The results presented in this report suggest that while these increases continue, the rates of increase are now slowing and that most parts of the catchment will (in the absence of further land management changes) reach equilibrium some time during the current century. A consequence of this non-stationary catchment behaviour is that in assessing the impacts of any future management or climatic changes, we should strive to make comparisons against the non-stationary benchmark scenario that has been established here.

Water management scenarios Catchment scenarios

This report investigates the impacts on streamflows, salt loads, salinities, lake discharge rates and salinities and groundwater levels and salinities of the following water management scenarios:

Leveed drainage and open drainage with low and high ZOE and disposal of drainage discharge into the natural channels

Leveed and open subcatchment drainage and arterial channels with low and high ZOE and storage of drainage discharge in salt lakes up to: a) existing storage capacity or outlet heights (Drainage SSe); b) elevated outlet heights, i.e., making the lakes terminal (Drainage SSr); and full capacity (Drainage SSf).

Leveed and open subcatchment drainage and arterial channels with high ZOE and no storage of drainage discharge in salt lakes i.e., lakes are bypassed (Drainage LB). All flows and loads generated within the catchment are routed through the stream network without impediment. When lakes are removed, the arterial leveed channel is fully connected down to the junction of the Mortlock and Avon Rivers.

Leveed subcatchment drainage with high ZOE and storage of drainage discharge in subcatchment scale evaporation basins (Drainage EB). The subcatchment retention option applies only to the leveed drains with high zones of effectiveness.

Two revegetation scenarios are modelled: revegetation to 50 % of the landscape; and revegetation to 100 % of the landscape. Revegetation is assumed to occur at the beginning of 2001, and is implemented in the model as fully-mature forests from the outset. The model also takes no account of any biological growth limitations that may be imposed by the salinity of the landscape.

Changed climate with 10 % more and 10 %, 20 % and 30 % less rainfall. The impacts of five climate scenarios are assessed for the baseline case. The wetter climate (rainfall 10 % greater than the benchmark) is typical of what might be expected if the region’s climate returns to pre-1975 conditions. The three drier climates (10 %, 20 % and 30 % less rainfall than the benchmark) give an indication of what might happen in the catchment if the current trend of climate change continues. In all cases, the changed climate is applied from the beginning of the simulation period, 1965.

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For each scenario, hydrological impacts are assessed in detail for eight key sites within the Avon Basin: Avon River at Great Northern Highway, Mortlock North River, Mortlock East River, Avon River at Northam, Salt River at Qualandary Crossing, Lockhart River, Wakeman Creek at Narembeen and Yilgarn River. The model simulation runs for the various scenarios are done in the same way as those for the benchmark case. The simulations commence in 1965 and end in 2100 and include the same initial values and model parameters.

Artificial drainage is assumed to commence in 1999 and its length in each drained subcatchment is assumed to increase annually throughout the succeeding 101 years of each simulation run. One of the main factors affecting the impact of artificial drainage is the ZOE of drains. We choose to model the upper and lower limits on the ZOE. The low ZOE limit varies between 100 m and 300 m and the high between 200 m and 600 m.

The project separately assesses the impacts of implementing subcatchment scale open and leveed artificial drainage. For each type of subcatchment scale drain, equations governing the discharge of groundwater to the drains, the drain dimensions and lengths for a given subcatchment are the same. As a consequence, the predicted groundwater discharge rates from the two types of drain are identical. Although the groundwater dynamics are affected slightly by the different treatments of surface water, the impacts of these differences on subcatchment and catchment discharges is extremely small. In the light of the predicted differences between the two scenarios being so slight, especially for mean discharges, the ensuing results are presented for either open or leveed drains.

Impacts of subcatchment scale drainage with arterial channels and storage of drainage discharge in salt lakes up to their existing outlet heights (Drainage SSe)

In all subcatchments with artificial drainage, watertables are lower and reach an earlier maximum than in the no-drainage case. The watertables then begin to decline towards a new, lower equilibrium value. The impact of artificial drainage on groundwater levels is greater for the high ZOE than for the low ZOE. Watertable response rates vary from subcatchment to subcatchment, with those subcatchments having greater drain densities showing the fastest responses. Groundwater salinities are slightly reduced by artificial drainage. This is because the increased groundwater discharge through drains is more saline than the relatively unchanged recharge, thus leading to a net freshening of the groundwater.

Artificial drainage leads to substantial increases in subcatchment water yields in the heavily drained areas upstream of Beverley. There are also, however, some substantial decreases in water yield in the main-channel subcatchments in the lower Avon Valley below Northam. In comparison with the high ZOE case, the low ZOE case produces slightly lower water yields for most subcatchments by the end of the century. In the 102-year period of artificial drainage from 1999 to 2100, cumulative streamflow for the Avon River at Great Northern Highway increases by 29 % for the high ZOE case and by 18 % for the low ZOE case. This increase occurs in both high and low flow years and is greater towards the end of the simulation period as drainage densities increase. Although artificial drainage has only moderate impact on annual peak flows at Great Northern Highway, there is more significant impact at most of the sites further upstream. The general trend is for peak flows in the drainage scenarios to exceed those in the baseline scenario. In Wakeman Creek, the only first-order subcatchment among the eight key sites, peak flows in the low-flowing years are substantially greater under the drainage scenarios, but there is little change in the high-flow years. At all other sites, the largest events in the simulation period have drainage peaks that are at least twice as large as the baseline scenario.

The high ZOE scenario yields substantial quantities of salt (in excess of 300 t km–2 y–1) from much of the catchment and more than 200 t km–2 y–1 from practically the entire densely-drained, agricultural region above Northam. The largest sources are in the lower East Mortlock and in the Avon between Northam and Beverley, where yields exceed 500 t km–2 y–1. Predicted salt yield has increased (in comparison with the baseline case) in all

A regional drainage evaluation for the Avon Basin Page xxv

subcatchments except for some mainstream subcatchments below Northam. Salt yields for the low ZOE scenario are generally less than those for the high ZOE scenario, but most substantially exceed the baseline case.

For the eight featured subcatchments, the proportional increases in cumulative salt load under drainage are much greater than the corresponding increases in streamflow. Predicted annual salt load shows a strong increasing trend in annual loads associated with the artificial drainage scenarios. These trends appear to be stronger, relative to the baseline cases, than the corresponding annual streamflow predictions. However, there is some evidence that this rate of increase begins to slow in the latter part of the century.

The streams will generally be more saline under artificial drainage than under the baseline, no-drainage case. Stream salinity is limited either by groundwater salinity or by lake salinity. Flow-weighted stream salinities at Qualandary Crossing are dictated entirely by the salinity of the Yenyening Lakes, and can exceed 250 g L–1 for the artificial drainage scenarios during some low-flow years early in the twenty-first century. Such levels are not predicted for later in the century as lake discharges increase and lake residence times decrease. At other locations, maximum flow-weighted salinities are influenced both by groundwater salinity and the effects of any upstream lakes.

The modelled lakes tend to discharge more frequently, at greater volumes and with greater salinity under artificial drainage than in the baseline scenario. All overflowing lakes discharge more water in the low ZOE scenario than the baseline scenario. Discharges increase further for the high ZOE scenario. The high salinity of the artificial drainage discharges lead to substantial increases in the salinity of lake storages, particularly those in the Yilgarn and Camm Rivers. Elsewhere, salinity increases are relatively small.

Impacts of subcatchment and arterial drainage and storage of drainage discharge in salt lakes up to elevated outlet heights (Drainage SSr) and full heights (Drainage SSf)

Results of two modelling strategies are reported here. In the first strategy, all 20 modelled lakes are made terminal (Drainage SSf) by preventing all discharge of water downstream. In the second modelling strategy, the discharge height for all lakes is raised by 30 cm (Drainage SSr) to have an increased dead storage capacity. In both cases, simulations and analysis presented here are limited to open drains with high zones of effectiveness. However, the results and interpretations are virtually identical for leveed drains.

The scenario with permanent retention of lake storage has reduced flows by about 30–50 % at most of the key subcatchments, although this figure is strongly dependent on the proximity and discharge characteristics of any upstream lakes in the drained scenario. At most sites, the retention of what are normally fairly saline lake discharges, leads to reductions in salt load that exceed those of streamflow. As a consequence, stream salinities under the full retention scenario are only 30–60 % of those without retention.

The impact of lake retention on peak flows is less marked at most sites. For the two Avon River sites (Great Northern Highway and Northam), peak flows in the retention scenario remain as high as 85 % of those in the drainage scenario. This implies that the bulk of the water involved in the drainage peak flows at these sites is sourced from downstream of the Yenyening Lakes. In the Lockhart River, the ratio of peak flows is less than the ratio of mean annual flows. The implication here is that in the drainage scenario, much of the peak flow is derived from overtopping of Lake Kurrenkutten.

In contrast, the peak load ratios are substantially less than the mean annual load ratios. This is not unexpected, given that it is the highly saline lake discharges that have been removed in the retention scenario. Even in cases where the lakes contribute little water to a peak event, their contributions of salt could be quite significant.

The scenario with raised discharge heights has little effect in reducing the frequencies and volumes of lake overflows. Discharge volumes decrease by less than 10 % for most lakes. However, the slightly increased detention times lead to increases in discharge salinity for all

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discharging lakes. The combination of slightly decreased discharge and slightly increased salinity means that for all 12 lakes with substantial discharges, the discharge of salt changes by less than 0.7 % between the partially retained and unretained scenarios. This scenario has considerably less impact than the full retention scenario on mean annual flows at the eight main sites and practically no influence on mean annual salt loads. The reason for the lack of impact on loads is that, despite the slightly increased lake losses of water to evaporation, the salt remains in storage in the lake and when the lake eventually overfills this slightly more concentrated salt discharges a similar mass, but dissolved in less water, than in the drainage scenario. Inevitably, this means that streamflows at the key sites are slightly more saline than in the drainage scenario.

Impacts of subcatchment drainage and storage of drainage discharge in subcatchment scale evaporation basins (Drainage EB)

All lakes experience substantial reductions in predicted discharge frequency in the subcatchment retention scenario. Three lakes cease to discharge altogether, while two are reduced from discharging every year to discharging just once in 28 years. Annual discharge volumes also decrease substantially. The diversion of the saline drainage water away from the lakes also results in large decreases in the salinity of lake storages.

This scenario also has substantial impacts on streamflows and salt loads. Mean annual flow is reduced by more than half at all sites, with the greatest reductions occurring in the eastern subcatchments. The impacts on salt load are even greater; loads are reduced to between 2 % and 10 % of their comparative rates for the unretained case. With the magnitude of the load reductions exceeding the magnitude of the flow reductions, stream salinities are reduced at all sites by at least 60 %.

The subcatchment retention scenario leads to reductions in peak flows of 35–80 % and reductions in peak salt loads of at least 50 %. In the Lockhart River there is a 98 % decrease in the 10-year return peak load, due to the elimination of saline discharge from Lake Kurrenkutten.

In comparison with the two regional arterial with salt lake storage options, it is clear that this subcatchment retention scenario has by far the biggest desirable impacts in reducing mean annual streamflows and salt loads, flow-weighted stream salinity, peak flows and loads (except for the Yilgarn and Lockhart Rivers), and (with the obvious exception of the full regional retention scenario), the frequency, volume and salinity of lake discharges.

Impacts of subcatchment drainage and arterial channels and no storage of drainage discharge in salt lakes (Drainage LB)

In the absence of lakes, mean annual flow is greater at all locations than for the drained scenario. This is because lake evaporation represents a significant mechanism for loss of water from the catchment. Salt loads also increase in the absence of the lakes, but these increases are not as large as the flow increases. The combination of large increases in mean annual flow and smaller increases in mean annual salt load, mean that the flow-weighted salinity decreases at most sites. Increases in the ten-year return period flows suggest that there is an increased propensity for flooding when the lakes are removed. At most sites, the increases are less substantial than the increases in mean annual flow.

Impacts of revegetation strategies (woody perennials)

Revegetation has a major impact on groundwater depths in all subcatchments that are currently comprised primarily of pasture and cropping land. The assumption that the revegetated forests are fully mature in 2001 leads to immediate reductions in groundwater levels in all affected subcatchments. For the 50 % revegetation scenario, the rates of fall of the watertables are slightly less than the pre-existing rates of rise prior to 2001, while the rates of fall for the 100 % revegetation scenario are significantly greater. By 2100, the 100 % scenario has predicted watertables that are about 4.4 m below those of the untreated scenario for all three subcatchments.

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Predicted groundwater salinities for the same three subcatchments show significant increases for the revegetated scenarios, increases that are substantially greater than those for any other scenario assessed in this report. It is interesting to note that while the clearing of native vegetation over the past 160 years has resulted in increased salinisation, so too does the reintroduction of similar land cover.

Revegetation causes reductions in predicted cumulative streamflow at Great Northern Highway of 27 % and 38 % for the partial and full revegetation scenarios, respectively, over the period 2001–2100. Further upstream the impact of revegetation on streamflows is even greater. By the end of the century, streamflows in the Salt River, Lockhart River and Wakeman Creek for the full revegetation option are less than 10 % of those without revegetation.

The predictions of annual flow indicate that for all sites other than the two on the Avon River, revegetation leads to a greater preponderance of zero-flow years than are present in the baseline scenario. For the Avon and perhaps to a lesser extent, the Mortlock Rivers, revegetation appears to have little influence on peak flows, except for a slight reduction in the extreme peaks. However, further upstream, revegetation leads to significant reductions in all peak flows after the first 20 years.

For both scenarios, there is a substantial reduction in salt loads, and for all eight key subcatchments, the reductions are greater than the corresponding reductions in streamflow. Like the streamflow impacts, the salt loads predicted for the revegetation scenarios decrease over time. By the end of the twentieth century, salt loads discharging from all eight subcatchments for the full revegetation scenario are less than 1 % of the discharges in the baseline case.

The predicted annual salt loads confirm the massive impact of revegetation on salt loads and, in particular, show the contrast between the increasing loads of the baseline scenario and the decreasing loads of the revegetation scenarios. The substantial decreases in salt load are also reflected in the predictions of flow-weighted salinity.

In comparison with the baseline scenario, all modelled lakes in the revegetation scenarios have greatly reduced discharge frequencies as a consequence of the widespread reductions in inflow. Three lakes in the Lockhart system (Kurrenkutten, Jilakin and Gulsen) become terminal under the partial revegetation scenario, while a further three (Hinds, Kondinin and Grace North) do so under full revegetation. Typically the lake’s mean annual discharge volumes under full revegetation are about half of those under partial revegetation.

Impacts of climate change In general, greater amounts of rain lead to greater recharge and therefore higher watertables. By 2100, the predicted difference in groundwater levels between the wettest and driest scenarios is greater for subcatchments in the east of the basin than it is for those in the west. For much of the twenty-first century, the wetter climates exhibit larger groundwater salinities. However, there is a slow, long-term reversal of salinities to a situation in which the drier climates have more saline watertables by the end of the century.

Comparison of the predicted distribution of streamflow sources for the wetter climate and the baseline case indicate that there is a general increase in streamflow generation in the wetter climate in all subcatchments with non-zero net flows, but the relative increases tend to be greater in the east than in the west. In contrast, the three dry scenarios result in streamflow generation rates that are progressively smaller than the baseline, and again the impacts are greater in the east. A clear conclusion is that the dryer, eastern parts of the catchment are more sensitive to climate change than the wetter, western parts.

At all sites and in all climate scenarios there is evidence that predicted annual streamflows continue to rise slowly throughout the simulation period in response to generally rising watertables. In contrast to the rising trends in annual flow, the graphs of peak streamflow generally indicate little change over time for the four wettest catchments for all scenarios.

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However, a clear increasing trend is evident for the driest four subcatchments, particularly among the wetter scenarios. As is the case in the baseline scenario, the dominant sources of salt in the wetter climate are in the Avon valley between Toodyay and Qualandary Crossing. The increases in salt yield in the west are less than the increases in water yield, thus indicating a freshening in discharge. However, in the east, the salt increases tend to be greater than the water increases.

For the three dryer scenarios, the Avon Valley remains a significant source of salt. Downstream of Yenyening, salt yield reductions relative to the baseline scenario for the –10 % climate are typically 20–30 % and are less than the reductions in water yield. Further east, the salt decreases are greater than the water decreases, indicating a freshening of discharges. This regional pattern of increasing salinity with decreasing rainfall downstream of Yenyening and decreasing salinity with decreasing rainfall upstream of Yenyening is repeated in the –20 % and –30 % climate scenarios.

All lakes discharge more frequently for the wet climate than for the baseline scenario. In the wet climate, three lakes discharge every year (compared to none for the unchanged climate), and three (Walyormouring, O’Connor and Varley) are no longer terminal. Only three lakes remain terminal in the wet climate. The differences between the unchanged and dry scenarios are even more stark. Only three lakes discharge at all in the 28 year period and only one of those (Yenyening) does so more than once.

With flow-weighted discharge salinity, the general trend is for the wet climate to have the freshest discharges. The combination of substantial flow increases and modest salinity decreases means that the lakes discharge considerably more salt in the wet climate than in the unchanged climate.

Cost of regional drainage discharge management The length of artificial drainage is 4,540 km in 2000 which is estimated from the salt-affected area and assumed ZOE. The total artificial drainage length, required to treat the salt-affected area by 2100, is estimated as 22,710 km. The artificial drainage length is assumed to linearly increase from 4540 km in 2000 to 22710 km in 2100. The drainage discharge management options are evaluated assuming the implementation of these drainage lengths in LASCAM.

Feasibility of subcatchment and regional scale drainage systems is assessed on the basis of their construction costs alone. These include subcatchment scale leveed and open drainage with drainage discharge management via natural creek system, leveed arterial channel (with salt lake storage), leveed arterial channel (without salt lake storage), subcatchment scale evaporation basins, and open arterial channels.

Cost of subcatchment leveed drainage and natural creek discharge management The indicative construction costs of installing the leveed drainage system are estimated assuming a low, medium and high unit cost of $8,000, $10,000 and $12,000 per kilometre of drain length. An indicative construction cost estimate (using medium unit construction costs) is about $45 million for 4540 km long drains and $227 million for 22701 km long drains. These costs of farm scale and subcatchment scale drains exclude any drainage discharge management, disposal and treatments costs.

Cost of subcatchment open drainage and natural creek discharge management The low, medium and high unit construction costs of farm and subcatchment scale open drains are assumed as $6,000, 8,000 and 10,000 per kilometre of drain. An indicative construction cost estimate of the open drainage system in 2000 is about $36 million assuming a medium unit construction cost. In 2100, about $182 million will be required for the construction of an open drainage system based on current medium unit cost. An open drainage system is expected to cost less than a leveed system. However a much higher maintenance cost is likely for an open system.

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The release of drainage discharge from most drainage systems is highly saline and some times acidic and since streamflow is mostly drainage discharge during summer, its continuous and uncontrolled release into the downstream natural environment may cause degradation. For this reason it seems an infeasible option for regional drainage management.

Leveed and open arterial channels An arterial or conveyance channel is defined as an engineered regional channel designed and constructed to only convey drainage discharge and/or surface water received from subcatchment scale leveed and open drainage although it may also lower watertable in adjacent areas. Two arterial channel options are considered: leveed arterial channels to convey the drainage discharge from farm and subcatchment scale leveed drainage; and open arterial channels to convey the surface runoff and drainage discharge from farm and subcatchment scale open drainage.

Nine arterial channel sections or reaches are assumed: Lockhart East, Lockhart South, Lockhart Main, Yilgarn, Salt River, Mortlock North, Mortlock East, Upper Avon and Lower Avon, were assumed. The arterial channel sections are designed on the basis of daily peak flows from subcatchment scale leveed and open drainage systems. Generally smaller size arterial channels are required for the leveed drainage system with salt lake storage and than for the open arterial system due to its handling of both drainage discharge and surface runoff. The channel size increases as a result of the implementing more artificial drainage over time. However the increase in size is relatively small for the leveed arterial system. For an open arterial system, the largest channel is required for the lower Avon because of its location downstream to all arterial channels.

The construction cost estimates are based on the channel size. Costs other than construction are not included. The construction cost of leveed arterial channels with and without storage of drainage discharge in the salt lakes is estimated. Similarly the construction cost of an open arterial system with storage of the mixture of drainage discharge and surface runoff in salt lakes is also estimated. The construction cost per cubic metre of an arterial channel of 1.5 to 2.5 m depth is $1.50. It is $1.75 for an arterial channel of 2.5 to 3 m depth. The construction cost per cubic metre is increased by 50 % from $1.75 to $2.6 if the top width was greater than 10 m due to double handling of the soil.

Cost of leveed arterial channels with and without salt lake storage of drainage discharge

An indicative estimate of the total construction cost of a leveed arterial system is about $27 million. This arterial system will be required to manage the drainage discharge from subcatchment scale leveed drainage systems during first 25 years of the twenty-first century. The estimated construction cost is about $29 million if these channels are constructed new during 2075–2100.

Larger leveed arterial channels are required if the drainage discharge is not allowed to enter in the salt lakes. The total estimated construction cost of this leveed arterial system is about $53 million. This arterial system is likely to be enough to manage the drainage discharge from subcatchment scale leveed drainage systems during first 25 years of the twenty-first century. To manage the drainage discharge during 2075–2100 larger size leveed arterial system will be required and its construction cost will be about $93 million assuming the construction of new channels. The cost is likely to be much less if size of previously constructed channels is increased to accommodate increased drainage discharge during 2075–2100.

Cost of open arterial channels with salt lake storage of drainage discharge An open arterial system will require larger channels than a leveed arterial system because it entertains both surface runoff and drainage discharge. Significantly larger channels are required during 2075–2100 than during 2000–2025 due to increased surface runoff and drainage discharge during later part of the twenty-first century. Total indicative construction

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cost of an open arterial system will be about $118 million during 2000–2025. About $188 million will cost to construct a larger open arterial system for managing a mixture of drainage discharge and surface runoff during 2075–2100 assuming the construction of a brand new system. Remodelling of an old system, built during 2000–2025, is likely to incur less cost.

Cost of subcatchment retention system (evaporation basins)

The evaporation basin sizes are determined using mean annual inflows (drainage discharge rates from subcatchment scale leveed drainage system) of 1 in 10 recurrence interval mean annual rainfall, mean annual evaporation and drainage discharge salinity levels. The construction cost per ha of evaporation basin area was assumed as $10,000.

An indicative construction cost estimate of evaporation basins in whole of the Avon Basin during first quarter of the twenty-first century is about $340 million. For managing the drainage discharge during last quarter of the twenty-first century their construction will cost about $540 million assuming the construction of new evaporation basins. Another option is the re-engineering of evaporation basins constructed during first quarter of the twenty-first century to increase their capacity to accommodate additional flows during last quarter of the twenty-first century. This cost is not estimated but is expected to be much less than the construction cost of new basins.

Based on indicative construction cost estimates it seems infeasible to construct evaporation basins at the outlet of each subcatchment. If other costs such as site investigation, planning, design, surveying, operation and maintenance, salt disposal and management are also included the drainage discharge management via subcatchment scale retention system (evaporation basins) will become even more cost prohibitive.

The most cost effective system is leveed and open drainage with discharge management through natural channels. However it is technically infeasible to allow the continuous discharge of drainage water into the natural creek, river and wetland systems. The leveed subcatchment and leveed arterial is next most feasible (based on their constructions costs alone) system if the drainage discharge is allowed to enter and be stored in salt lake system. The open arterial system is most expensive of all arterial systems evaluated in this study. Subcatchment retention system (evaporation basins) has prohibitive construction costs.

A regional drainage evaluation for the Avon Basin Page 1

1. AVON BASIN

1.1. Introduction Nearly twice the size of Tasmania, the Avon Basin (AB) is the largest surface water drainage basin in the southwest region of Western Australia and covers about 118,000 km2 (about 11.8 million hectares). The region has a population of around 46,000. There are 150 communities within 43 Local Government Areas (LGAs) that are partly or entirely within the ARB. Its boundary extends from the Darling Range in the west (about 70 km inland) to the western goldfields in the east about 500 km inland (Figure 1.1). Mean annual rainfall ranges from 830 mm in the west to 300 mm in the east. The groundwater resources of the region are generally unfit for domestic and agricultural use. The groundwater salinity ranges from 2,700 mg L-1 in the west to over 55,000 mg L-1 in the eastern parts of the basin. Numerous rural towns and associated infrastructure and heritage and cultural sites lie within the Avon Basin. A network of around 25,000 km of roads and 1,900 km of railways exists in the basin.

Preclearing vegetation in the Avon Basin included five major formations – heathland, samphire shrubland, tall mallee shrubland, tall open shrubland and extensive areas of woodland (O’Connor et al., 2004). Agro-pastoral activities commenced along the western margins of the basin following the establishment of Perth in 1829 but remained relatively stagnant during the nineteenth century. It was early in the twentieth century when a change in the government policy aimed at increasing agricultural production, resulted in a wider take-up of farmland. The land clearing essentially progressed along the transport corridors, to the north, south and lastly to the east. Since 1900 around 80,000 km2 of native vegetation has been cleared for agriculture in the lower reaches (Avon Catchment Council, 2004). Now the remnant vegetation is highly fragmented except in subcatchments east of the rabbit proof fence line (Figure 1.2). Around 65 % of the Avon Basin was cleared for agriculture most of which occurred between 1940 and 1970. However many upper catchments have clearing proportions of 85 to 95 % (Hatton and Ruprecht, 2001). The land used for agriculture amounts to 7.4 million hectares. The Avon Basin accounts for 39 % of all farmland in the state. The main agricultural activities in the cleared land include grazing and cereal crops (barley, wheat, lupins, field peas, canola and lentils).


Figure 1.1. Map of the Avon Basin in the wheatbelt of Western Australia

Figure 1.2. Distribution of remnant vegetation in the Avon Basin


1.2. Hydrology The hydrological and hydrogeochemical impacts of clearing of native vegetation for agriculture in the Avon Basin are profound and enduring (Hatton et al., 2003). Changes in the underlying hydrological and hydrogeological processes on a grand scale in space and time caused excessive accessions to groundwater resulting in the development of shallow watertables, increased waterlogging and flooding, and salinisation. The hydrological impacts of land clearing for agriculture have been studied and reviewed in detail in numerous studies (Farrington et al. 1992; McFarlane et al., 1993; Nulsen, 1993; Bettenay et al., 1964; Schofield, 1990; Williamson and Bettenay, 1979). The impacts were due to the replacement of native sclerophyll vegetation with annual crops and pastures over a major portion of the Avon Basin. Some of these impacts were highlighted relatively very early by Bleazby (1917), Teakle and Burvill (1938), and Wood (1924) but had no influence on land policy.

The water use patterns of agricultural systems vary dramatically from the original preclearing vegetation. The seasonal evaporation from winter cropping systems together with wet conditions and low evaporation demand usually exceeds that of native systems but annual evaporation falls significantly short of that under native vegetation (Farrington et al., 1992; Greenwood and Beresford, 1982; Greenwood et al., 1985). One of the major consequences of clearing was a dramatic increase, typically two orders of magnitude, in diffuse and localised groundwater recharge (George, 1992, Peck and Hurle, 1973; Salama et al., 1993). Principal changes in the water balance due to clearing included reduced annual evaporation and interception in the agricultural catchments and increased runoff and recharge. Preclearing average annual streamflow in the Avon was estimated to be 18 % of the current water yield (Ruprecht and Schofield, 1991; Viney and Sivapalan, 2001).

Prior to clearing there were no defined creek lines in most of the area where well developed creeks now exist (Salama et al., 1993). The recently developed creek systems were originally low-lying depressions covered by native vegetation the transpiration from which kept the groundwater levels deep. Excessive groundwater recharge caused the development of saline shallow watertables and capillary flux and groundwater discharge from low lying area mobilised salts by bringing them on or closer to the soil surface. This caused increased salt outflows and the development of secondary salinity. In the Avon Basin much of the salt redistributes within the upper reaches and most of the salt that reaches the ocean outlet is sourced from areas between Yenyenning Lakes and Northam, and from the Mortlock North River except during rare rainfall events when the whole system contributes runoff and salt into the Swan River (Viney and Sivapalan, 2001).

1.3. Rivers and lakes Three major catchments of the basin include the Avon (most western reaches), the Yilgarn (northern and eastern most reaches) and the Lockhart (southern and eastern most reaches) as shown in Figure 1.3. The Avon River becomes the Swan River from the lower slopes of the Darling Scarp into the Swan–Canning estuary in Perth. The Avon River extends from Yenyenning Lakes to the confluence of Wooroloo Brook at Walyunga including the south branch through the town of Brookton. The Upper Swan River extends from the confluence of Wooroloo Brook to the junction of the Brockman River including Ellen Brook and Helena River. The Avon River becomes Swan River from the lower slopes of the Darling Scarp into the Swan-Canning estuary in Perth.

The landscape and surface drainage of the basin vary considerably. In the east, a large network of ancient salt lakes exists across broad flat valleys. Most of this network is internally drained during normal rainfall years. They tend to link and flow only following widespread and significant rainfall or flood events. The landscape becomes slightly undulated in the middle parts of the basin with defined drainage lines and scattered salt lakes. However, in the west the river systems are younger with valleys varying from broad and gentle to incised and deep near the Darling Range. This rejuvenated part of the basin drains areas of forest and agriculture and represents about 10 % of the basin but contributes nearly 90 % of the total annual flow in the Avon River (Pen, 1999).


Figure 1.3. Main catchments and drainage system of the Avon Basin

1.4. Soils and geology According to Galloway (2004) there are 6 major soil types covering a majority (82 %) of the Avon Basin. These include deep and shallow sandy duplexes, shallow loamy duplexes, sandy and loamy earths, and ironstone gravels (Figure 1.4). The remaining area is covered by eigth other soil types. Soil related land degradation (secondary salinity, waterlogging) estimates are continuously being revised. Secondary salinity is a major issue which was first reported in a seminal paper by Wood (1924). There are estimates of around 450,000 ha on 2,279 farms being affected by salinity (O’Connor et al., 2004). Over 63 % of the salt-affected land is unproductive at present.


Figure 1.4. Main soil types of the Avon Basin


2. BACKGROUND AND OBJECTIVES It is not known how much of the Avon Basin was affected by primary salinity prior to clearing but it was most likely less than 1 %. However, the secondary salinity, caused by clearing, has increased significantly over time and about 5 % of the basin was affected by 2004 (McFarlane et al., 2004; Ali et al., 2004a). Many ephemeral freshwater lakes have salinised. The concentration of salt in the runoff is increasing with time. Many of the valley flats are degraded. It is predicted that about 25–30 % of the basin may be at risk of salinisation when a new hydrological equilibrium is reached (McFarlane et al., 2004) some time in the twenty-first century.

Salinisation due to the development of saline shallow watertables in vast areas of the Avon Basin has not only increased the groundwater discharge but also increased the total flow and flood peaks (Hatton et al., 2003). Changes in river geomorphology and structure are occurring due to changing hydrology of the river systems. Previously ephemeral waterways have now become rivers and creeks and wet valleys are now observed which have not occurred in recent history. Additional impacts are being observed, such as a toxic algal bloom in the Swan Estuary in 2000 which occurred, in part, due to flooding from the Avon River. A review of the effect of revegetation by George et al., (1999) found that in all but three of the 80 sites reviewed, revegetation of small proportions of the land area has little or no effect on the watertables more than 10–30 m from the planted area. This review changed the view that revegetation was likely to recover the land and rivers of the wheatbelt from secondary salinity. Increasing concern about the impacts of salinity and waterlogging, combined with an understanding that the current level of catchment revegetation is inadequate for salinity control, is driving many landowners to consider engineering options. Groundwater pumping and deep open drains to reduce land salinisation have been widely practised at scales from managing local discharges (Ali et al., 2004a; Ali et al., 2004b; Ali et al., 2004c; Bettenay, 1978; George and Frantom, 1990; Salama et al., 1994) to regional systems (Ali and Coles, 2001; Otto and Salama, 1994). Siphons and other methods are also being tested or adopted. Some are implemented at a small scale, others are proposed for implementation on a very large scale. Salinity abatement schemes that include engineering options are being implemented to reduce the impact of salinity on some rural towns. Some engineering options have potential to cause deleterious impacts locally, i.e. to neighbours or within the local catchment, or off-site (e.g. significant wetlands, waterways and possibly the city of Perth). Unless managed properly the drainage discharge from artificial drainage systems can potentially cause adverse downstream ecological and hydrological impacts.

This project evaluates regional drainage and water management strategies for the Avon Basin and assesses the feasibility of various subcatchment and regional scale drainage discharge management strategies. The specific objectives of this project are to:

develop analytical tools to assess the impacts of do-nothing, plant-based and engineered interventions on on-site groundwater levels and off-site impacts on flows, salt loads and salinity of the creeks, river systems and lakes of the Avon Basin; and

evaluate the hydrological impacts and economic feasibility of various subcatchment and regional scale drainage discharge management options.

3. PROJECT DESCRIPTION The success of deep open drains in various parts of the wheatbelt and scientific evaluations of their effectiveness (Ali et al., 2004a, 2004b, 2004c) led to the groundswell of support for catchment and regional scale drainage. Regional scale drainage proposals (such as arterial drainage of the Yilgarn, Lockhart, and Salt rivers through Yenyenning to the Swan, arterial drainage of the upper Blackwood through to Lake Dumbleyung, and drainage along the length of the Yarra Yarra system into the chain of lakes), along with drivers such as the limited influence of trees and the success of farm scale drainage led to the investment by the State Government of $4 million and establishment of the Engineering Evaluation Initiative. Since no suitable regional drainage model was available to evaluate the feasibility of regional


drainage schemes in the wheatbelt it was not possible to assess these regional drainage proposals. This lack of a credible analytical framework for assessing the design, benefits, cumulative impacts and costs associated with these proposals was creating uncertainty and concern in both rural and urban communities and led to the development of this project. The project aims to develop a framework for informed decisions about engineering options for salinity, waterlogging and flooding control. The impacts of current and possible future situation with no drainage or do-nothing, subcatchment scale drainage and various subcatchment and regional scale drainage discharge management options, revegetation and climate change on groundwater levels, streamflows and salt loads were assessed through modelling and analysis. The following are the main research stages of this project.

Stage 1 – Review and develop analytical tools to assess landscape change, and ongoing project management

Stage 2 – Establish catchment inventory of hydrological data of the Avon Basin

Stage 3 – Assess impacts of current land use and future climate

Stage 4 – Evaluate subcatchment and regional scale water management options

Stage 5 – Assess the economic feasibility of various drainage systems.

The steps in each of these stages are outlined below.

Stage 1 – Develop analytical tools to assess landscape change This stage developed the basic tools by which the non-intervention and engineered futures of the river basins can be modelled at the regional scale. A suitable model was selected by evaluating various catchment models. The selected model was modified to:

estimate the present and potential increase in surface water runoff at the full extent of

salinity (at the subcatchment and regional scale);

estimate the current and future salt load and flow at the subcatchment and regional

scales;

simulate the implementation of various engineering interventions (artificial drainage at

subcatchment level, arterial channels, lake storage, subcatchment evaporation

basins) and predict mean annual flows, peak flows and salt loads; and

deliver clear visualisations of future regional drainage management strategies.

After modification, the selected model was calibrated using historical hydrological and climatic data.

Stage 2 – Establish catchment inventory All the relevant data for the Avon Basin were collated and reviewed and a GIS database was established. The DEM of the basin was developed including the determination of basin boundaries, subcatchments and their boundaries, natural drainage network, flow direction, subcatchment outlet nodes, major lakes and arterial channels. After defining subcatchments of the basin, baseline information of each was collated. Field data were collected to check and reconfirm the existing data from some of the precincts.

Stage 3 – Assess impacts of current landuse The Wheatbelt rivers are still in transitional states. It is crucial to quantify the possible future state of the river systems without engineering intervention and present it to the wider community. The lack of such a baseline handicaps proposed interventions under the likely misconception that the current character and values of the rivers, lakes and valleys will be retained if no engineering is done. The model developed and calibrated in Stage 1 was used to predict the future likely impacts of doing nothing to ameliorate current and future salinisation. This stage of the project predicted the flow rates, volumes and flooding risks at the subcatchment and regional scale levels in future years under the ‘do–nothing’ or baseline scenario.


Stage 4 – Assess regional water management options This stage of the project delivered the analyses of engineering and revegetation interventions. Various water management options and scenarios were developed in consultation with project partners and community and modelled. These interventions were evaluated using the regional model and changes in flows, salt loads and groundwater levels were assessed. The impacts of these interventions were compared to the non–intervention baseline developed in Stage 3. There were four steps of this stage:

Clearly define engineering and other water management options in a way that is relevant and acceptable in the Wheatbelt River Basins

Subcatchment (evaporation basins) and regional scale (arterial channels) drainage discharge management options by simulating the implementation of farm and subcatchment scale open and leveed artificial drainage systems.

Assess the impacts of non–engineering salinity control options (revegetation)

Assess the impacts of wet and dry climates on flows and loads and how they compare with the current climate.

Stage 5 – Assess the economic feasibility of drainage systems The work in the above stages demonstrated what could be done with respect to regional engineering, and the likely impact on asset protection. The project then carried out detailed analysis of technical and economic feasibility of various drainage discharge management options and drainage systems as a whole.


4. MODEL DEVELOPMENT

4.1. Model selection To select a suitable model for this project the available modelling platforms were reviewed. The generation of runoff from each subcatchment and routing the flow along streams and through storages, such as salt lakes, were the two main tasks identified for modelling. Salinity modelling was considered part of the flow generation and routing models. Three large–scale hydrological models/modelling platforms were evaluated because of their potential availability. They are:

Environmental Management Support System (EMSS)

The Integrated Quantity and Quality Model (IQQM), and

LArge Scale CAtchment Model (LASCAM)

The EMSS uses the lumped conceptual catchment scale model to estimate daily runoff and pollutant load from catchments. It consists of a number of models connected together to create a network. The IQQM is a hydrologic modelling tool developed by the NSW Department of Infrastructure, Planning and Natural Resources (DIPNAR) for use in planning and evaluating water resource management policies at river basin scale. It is designed to address water quality and quantity issues. LASCAM is a large scale conceptual hydrologic model that uses daily input data. LASCAM has previously been used in the Avon Basin (Viney and Sivapalan, 2001).

These models were reviewed for the purposes of doing the whole job, both subcatchment runoff, and flow and salt routing including salt lakes, and as a subset doing either of the main jobs. An objective analysis was carried out to select the best model. The issues such as the ability of the model to do the task with least modifications, credibility, national and international reputation, ease to use and modify, and model performance in a data poor environment were considered. Based on these evaluations it was determined that LASCAM was the most suitable and complete candidate to perform both tasks concurrently. For further details about the model selection procedure the reader is referred to Appendix 1.1.

4.2. Model description The hydrological model LASCAM (Sivapalan et al., 2002) was used for this project. LASCAM was developed to predict the impact of climate and land use changes on fluxes of water, salt, sediment and nutrients in forested and agricultural catchments in Western Australia. It operates on a daily time step and relies on calibration of model parameters against one or more observed records of streamflow and load. Gridded topographic information is used to divide a catchment into a number of subcatchments and to delineate a stream network. LASCAM is applied separately to each of these subcatchments and the resulting flows are routed along the stream network. At the subcatchment scale, the model is based on four inter-connected stores of soil water representing the near-stream perched aquifer, the upper soil layers, the deeper regional groundwater and the unsaturated zone. Streamflow is generated from infiltration-excess and saturation-excess overland runoff and from a baseflow discharge from the near-stream perched aquifer store. The hydrological processes and subcatchment properties influencing them are assumed to be lumped at the subcatchment scale, but are allowed to vary between subcatchments. A global set of model parameters is used (i.e. all subcatchments use the same parameter set). Routing is achieved through a simple but efficient scheme in which bulk stream velocity is dependent on streamflow volume. A detailed description of LASCAM is given in Appendix 1.2.

4.3. Model modifications LASCAM was modified to enable its use for all modelling tasks of this project. The modifications include changes to algorithms describing lake storage and discharge, natural channel storage and flow, lake and natural channel evaporation and artificial drainage.


The existing LASCAM model has a rudimentary lake routing algorithm. The relative lack of complexity of this algorithm is dictated by the general paucity of information on lake dimensions and discharge characteristics. However, it is considered too simple to adequately describe lake processes in a region such as the upper parts of the Avon Basin where playas exert a dominant control on stream routing. The LASCAM code was modified to enable the prediction of discharge of water from lakes, evaporation from surface of salt lakes and salt balance of lakes. For further details about lake modifications the reader is referred to Appendix 1.3.2.

The Avon Basin is comprised of broad and relatively flat natural creeks, channels and rivers of enormous storage capacity. To simulate storage of surface water and/or drainage water and evaporation of water within these creek and river systems the LASCAM code was modified. The expressions were developed for all 108 major streams and rivers and a list of parameters developed based on these expressions. One of the main advantages of this approach is the ability to change a natural channel into an engineered channel by just changing relevant parameters in the input table. Channel modifications are described in detail in Appendix 1.3.3.

One of the main engineering options assessed in this study was the simulation of the impacts of subcatchment scale artificial drainage on flow rates, volumes and quality. Modification of LASCAM was required to incorporate this capability in the model. Two options were considered: open drains that include both surface and groundwater; and leveed drains that handle only groundwater. For open and leveed drains at the subcatchment scale, a modified form of the Hooghoudt drainage equation (Ritzema, 1994) was used. This equation enables the prediction of groundwater discharge volume and rate based on the length of drainage systems in any subcatchment. The reader is referred to Appendix 1.3.4 for further details about modifications of LASCAM for artificial drainage.

5. INPUT DATA

5.1. GIS and DEM data A GIS database was established to provide spatially distributed parameters in a consistent fashion as input into LASCAM. This involved identifying a series of points of interest where LASCAM would be required to calculate flows and loads. These points would become subcatchment outlets and would coincide with stream gauge locations, major lake outlets and subcatchments directly upstream of major confluences. The subcatchment is the minimum resolution of the LASCAM model hence input parameters were computed at this scale. Two key datasets were required to derive the subcatchments: digital elevation data and existing topographic feature data. Existing GIS datasets for catchment, drainage, lake systems, etc. were sourced from Department of Water (DoW) and processed into the GIS. Other datasets were derived from existing GIS data or generated from non-spatial data.

A Digital Elevation Model (DEM) was constructed for the Avon Basin and processed to produce the hydrological network and subcatchments to be used for the LASCAM. Flow direction and flow accumulation were computed to derive the drainage network. The flow direction data set was modified at a number of locations where the DEM derived drainage varied significantly from the existing mapped stream lines. Due to the extremely flat nature of some of the valley systems, small errors of 0.5 meters or less in the DEM can create significant errors in the flow directions computed. Corrections were only made where these errors impacted the catchment boundaries in the vicinity of major confluences, stream gauges and lake outlets. Stream ordering provided a convenient and consistent method to identify an initial set of approximately 400 subcatchment outlets. The locations of stream gauges and a set of 33 lakes were added to the initial set of subcatchment outlets. These were vetted to remove very small subcatchments and others not deemed necessary for LASCAM modelling to arrive at a final count of 108 subcatchments with subcatchment areas ranging from 5 km2 to almost 10,000 km2 (Figure 5.1). The basin outlet coincides with the Walyunga gauging station on the Great Northern Highway which is approximately 30 km north-east of the Perth Central Business District (CBD). The development of the basin


hydrological network showed that a 3500 km2 region (shown in grey in Figure 5.1) including Lake Magenta did not contribute flow to the Avon Basin as previously thought. An area totalling 116,660 km2 was defined as contributing to the Avon above the Walyunga gauging station. For further details about the GIS and DEM the reader is referred to Appendix 2.1.

Figure 5.1. Subcatchment delineation for the Avon Basin

5.2. Rainfall data The hydrological modelling requires daily rainfall fields distributed in space across the catchment. In general, each subcatchment will have a unique daily rainfall input series which will encompass the modelling period 1965–2003. To generate these data fields without any gaps in either the spatial or temporal domains, high quality observed rainfall data from a large number of rainfall stations within and adjacent to the catchment is required.

Rainfall data used in this project has been sourced from the rainfall networks of the Bureau of Meteorology (BoM) and the Department of Water (DoW). The BoM rainfall network includes over 6,000 stations nationwide, all of which record daily rainfall using standardised equipment and observing protocols. The observations are made at 0900 hours each day, with the 24-hour rainfall total being recorded against the day of observation. In the main, these gauges are operated by volunteers, often at workplaces like post offices, local


government offices and farms. As a consequence, the quality of the data is quite variable, even for different time periods at a single station.

Model calibration is carried out using the rainfall and other observed data. If the quality of the rainfall data used to calibrate the model is poor it impacts on its calibration. Some of the rainfall data quality issues have been discussed by Lavery et al. (1992) and Viney and Bates (2004). They include observer inconsistencies (e.g. ignoring small rainfall catches; rounding rainfall to the nearest millimetre) and exposure changes (changes in the height or structure of the gauge; changes in the windfield associated with growing trees or the construction of nearby buildings) and missing observations.

In this project, any rainfall station whose record was suspected of containing untagged accumulations (Viney and Bates, 2004) was eliminated from analysis. For the remaining stations, any days with missing data or tagged accumulations were ignored, but the valid, single-day observations were included. This elimination of some observations does not significantly affect the calculated daily rainfall fields.

In contrast to the BoM network, most rainfall stations operated by the DoW use automated tipping bucket gauges and are therefore not usually affected by observer error and bias. In most cases the only data quality issue is that of missing observations, which are dealt with in the same way as the BoM data. For compatibility with the BoM sites, the DoW data was accumulated to 24 hour totals to 9.00 am.

The geographical domain chosen for the rainfall data set encompasses the Avon Basin and adjacent areas within about 50 km of the basin boundary. In excess of 450 rainfall stations are located within this region. However, many of these stations have quite short or discontinuous records, while others have been decommissioned altogether. This yielded a final set of 358 rain gauges: 294 operated by BoM and 64 by DoW. This final set of stations provides good coverage of the Avon Basin, except for the far east and far northeast areas. These areas are uncleared and largely unpopulated and therefore contain few candidate rain gauges.

For use in LASCAM, the rain gauge data are pre-processed to generate daily time series of precipitation for each subcatchment. This is done by a weighted inverse-distance interpolation scheme that uses only the three nearest gauges in each directional quadrant (northeast, southeast, southwest and northwest) from the subcatchment centroid. For each quadrant, only the three nearest gauges with valid precipitation observations (i.e. no missing data and no accumulated data) on the day in question are considered.

5.3. Streamflow data The Avon Basin above the Great Northern Highway contains many streamflow gauging stations. For this study, only those with catchment areas of greater than about 50 km2 are used. This yields a set of 26 gauges operated by DoW and one operated by CSIRO. However, not all have operated simultaneously and some have been decommissioned.

The subcatchment delineation (Figure 5.1; Appendix 2.1) was done in such a way that all 27 gauging locations coincide with subcatchment outlets. A further three DoW gauging stations on the Avon River (at Toodyay, York and Qualandary Crossing) have river stage data but as yet have not been rated. These too, coincide with subcatchment outlets, with a view to using them in the modelling at a later date if streamflow volumes become available following the development of rating curves. Data from all stations were accumulated to give daily streamflows for the 24 hours to 9.00 am.

5.4. Leaf Area Index (LAI) Leaf Area Index (LAI) for the Avon Basin was estimated from remote sensing data. The average value of LAI was derived for the entire basin on a monthly basis from July 1981 to December 2000 and then redistributed across the 108 subcatchments based on actual annual rainfall and proportion of native vegetation. Figure 5.2 shows the inferred distribution of LAI for 1975. Further details of LAI data used in this study are given in Appendix 3.


Figure 5.2. Estimated annual average subcatchment LAI for the Avon Basin in 1975

5.5. Soil data LASCAM requires detailed information on subcatchment-averaged soil properties. These properties include: ‘topsoil’ depth (subcatchment mean and minimum), regolith depth, porosity, field capacity, specific yield, Brooks-Corey texture index and bubbling pressure. Three principal sources were used to establish these data values. Some were available directly from the distributed dataset Soil Hydrologic Properties of Australia (SHPA) developed by the Cooperative Research Centre for Catchment Hydrology. Some properties were established by deferring to expert knowledge of staff members of Department of Agriculture and Food Western Australia (DAFWA) specifically the Natural Resource Assessment Group (NRAG). The remaining soil properties were estimated using simple pedotransfer functions of the data in the SHPA and were based on functions presented by Williams et al. (1989) and Saxton et al. (1986).

Initially the SHPA dataset was used to determine solum depth within each subcatchment (Figure 5.3). This data set is based on the Atlas of Australian Soils and provides soil hydrologic properties for A and B horizons derived from soil mapping, a database of typical soil properties and pedotransfer functions.


Figure 5.3. Area-weighted average solum depth for Avon subcatchments

The spatial resolution of these data is approximately 0.01 degree or 1 km. Soil properties provided include:

solum depth

solum plant available water holding capacity

A horizon thickness

A horizon saturated hydraulic conductivity

uncertainty estimate for A horizon saturated hydraulic conductivity

A horizon porosity

A horizon field capacity

A horizon wilting point

A horizon plant available water holding capacity

Note: Most of these properties are also available for the B Horizon.

These properties were deemed important as input parameters for LASCAM so area-weighted averages were determined within each subcatchment polygon. However, feedback from the project management team suggested that these data were too general to determine the soil depth parameter and that the NRAG soil-landscape data may be able to provide better estimates. The soil-landscape mapping attempts to delineate repeating patterns of landscapes and associated soils throughout the south west (See: http://www.agric.wa.gov.au/pls/portal30/docs/FOLDER/IKMP/LWE/LAND/TR280.PDF). These data were obtained and processed to determine an area-weighted average value for the Unrestricted Rooting Depth (URD) in each subcatchment. For most soil units this was


equivalent to the A-horizon depth. The area-weighted averages for each catchment were used as the soil depth input parameter for the LASCAM model (Figure 5.4).

Figure 5.4. NRAG soil attribute Unrestricted Rooting Depth used to determine soil depth for each

subcatchment for input into the LASCAM.

5.6. Evaporation data Evaporation data were mainly from the pan evaporation network of Bureau of Meteorology (BOM) and that from the patched point dataset (PPD). The PPD is a dataset containing daily rainfall, minimum and maximum temperatures, radiation, evaporation and vapour pressure. It combines original Australian BOM measurements for a particular meteorological station with infilling of any gaps in the record using an interpolation method (see http://www.nrm.qld.gov.au/silo/ppd for further details).

Thirty years of pan evaporation data for five geographically representative stations within the Avon Basin were obtained. Figure 5.5 shows a plot of the 30-year daily pan evaporation (in mm d–1) for Beacon, at the northern boundary of the basin. A curve given by:

ET = f0+f1sin(t+f2cos(t

was fitted through the plot using multiple linear regression. Here, for t in Julian days, the angular frequency is equal to 2/365.25 d–1. The regression coefficients f0, f1 and f2 are used to calculate and c in the following equation used in LASCAM:

ET = f0 [1+csin((t+))] where the phase (in days) is equal to:

tan–1(f2/f1)/��

and c is equal to:

(1/f0) sqrt(f12+f2

2)


Table 5.2 lists the computed parameters for evaporation for five representative stations. The variable f0 represents the mean daily pan evaporation.

0

5

10

15

20

25

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

Julian Day

Eva

po

rati

on

(m

m)

Figure 5.5. Thirty years of daily pan evaporation values for Beacon (each unique symbol is a separate

year, not listed)

Table 5.1. Evaporation parameters computed for five representative stations in the Avon Basin

Station (number) f0 f1 f2 c Mean annual potential

evaporation*, (mm) Merredin (10001) 5.90 0.798 4.47 81.0 0.770 2155

Beacon (10004) 5.88 0.803 4.46 80.9 0.770 2375

Toodyay (10063) 5.56 0.916 4.10 78.5 0.756 2035

Kulin (10607) 5.00 0.613 3.78 81.9 0.765 1825

Southern Cross (12048) 6.03 0.550 4.51 84.2 0.754 2200* From SILO Patched Point Data set of Queensland Department of Natural Resources and Water

5.7. Lake area and volume To model the volumes of water that can be stored in various salt lakes the height-area-volume-discharge relations of those lakes were required. Lakes were selected based on LASCAM requirements and points of interest. Lake boundaries were extracted from topographic data supplied by DoW. Data were “cleaned” (dangles and undershoots removed) before lake polygons were created. Initially several lakes were examined to identify if lake volume could be determined from the DEM. A number of lakes were field surveyed. The purpose was to compare DEM derived 3D sections with those constructed from the field survey.

Area volume curves were derived for each of the 20 lakes via 3D modelling in ArcGIS. DEM data were extracted for the mapped lake area plus a buffer of 500 m outside the lake perimeter. The elevation data were converted to a Triangulated Irregular Network (TIN). The outlet height was determined by finding the lowest elevation value at the outlet of each lake. The elevation at the next point of overflow on the lake perimeter was determined and assumed as height corresponding to lake capacity (Figure 5.6; Table 5.2). Area and volume were calculated between minimum and maximum lake elevation (capacity) in 0.1 m increments. These data enabled an area-volume curve to be plotted for each lake. The area volume curve for Lake Walyormouring is shown in Figure 5.7.


Figure 5.6. Schematic of outlet height and next overflow height used for determining dead storage

and maximum volume of a salt lake

Table 5.2. Dead storage, outlet height and next overflow height for selected lakes.

Lakes Dead

storage (GL)

Outlet height (m AHD)

Next overflow height (m AHD)

Lakes for which 3D modelling was used Brown North 4.89 277.6 278.4 Brown South 3.11 265.7 265.9 Ace 3.55 320 321.6 Baandee 8.13 244.2 244.8 Campion 3.74 277.9 278.5 Gulsen 9.91 308.6 310.4 Hinds 33.27 256.9 259.2 Hurlestone 39.60 308.6 310.4 Jilakin 29.83 264.3 267.3 King 25.64 310.7 311.5 Kurenkutten 29.18 255 255.2 Ninan 19.34 239.1 247.5 O'conner 17.53 306.4 307.1 Varley 13.61 309.6 310.0 Walyormouring 51.49 269.3 273.5 Yealering 5.49 272.3 273 Yenyenning 2.93 210.9 ? Cowcowing 675.50 289 290 Kondinin 20.23 259 260.3 Dowerin 19.33 255.7 256

Lakes for which simple method was used Camm 64.80 313.67 314.7 Carmody 3.01 303.79 307.5 Chinocup 18.41 278.58 281.25 Fox 72.76 315.34 316 Grace North 19.10 280 280.5 Grace South 275.24 282.05 282.5 Wallambin 207.79 289.32 290.0


Figure 5.7. Area-volume curve for Lake Walyormouring obtained by constructing a 3D model of the lake from DEM data

To determine height-discharge relationships, cross-sections of the outlets of these lakes were extracted from the DEM to capture the shape of the outlet channel. These passed through the minimum elevation point of the outlet and extended far enough to ensure the next overflow elevation was reached. Again these data were compiled in a spreadsheet and cross sections plotted. The outlet cross-section for the Lake Walyomouring is shown in Figure 5.8. Creek elevation to 5 km downstream was also extracted from DEM to find out the longitudinal gradient required for flow calculations.

268

269

270

271

272

273

274

275

0 20 40 60 80 100 120 140

Distance (m)

Hei

gh

t (m

AH

D)

Figure 5.8. Cross-section of the outlet at Lake Walyomouring extracted from DEM

The 3D modelling procedure was not applied to all selected lakes due to either time constraints or their location being outside the higher resolution DEM coverage. For the remaining lakes a quicker and simpler method of estimating the dead storage and maximum volume was developed (Table 5.2). The mapped boundary of the lakes (from the DoW drainage layer) was used to define the area of the lake and DEM was extracted. Average depth of lake was calculated by subtracting the lake DEM from a plane of the outlet height (bed) and averaging the result. Volume was determined by multiplying average depth and area of the lake. This was assumed as dead storage of the lake. The next overflow elevation was determined by examining the DEM around the lake margins for assessing lake capacity. The elevation of this point was used in the same way as the outlet height to determine average depth below a plane which was then used to calculate average lake volume (Figure 5.9). A potential source of error in this method is that the lake area remains constant in both calculations whereas area of the lake at its full capacity level should be greater than the area of the lake at dead storage. However, there were not any definite ways to determine the magnitude of these differences. Below the dead store level there will be an

0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 Volume (GL)

Are

a (h

a)


underestimation of the volume and conversely above the dead store there will be an underestimation of volume. Given the flatness of the terrain in the vicinity of the lakes it is unlikely that these errors are significant. The reader is referred to Appendix 1.3.2 and 4 for further details about lake area and volume.

Figure 5.9. Method for the rough estimation of dead storage (DS) and maximum volume (Vmax) for

lakes without a 3D model

Lake volume and discharge We calculated volume-discharge and volume-area curves for major lakes. The lake outlet cross-section coordinates, stage-plan area and stage-volume data were extracted from DEM of the basin as described above. A stage-discharge curve was computed for each lake outlet using Manning’s equation. The main objective was to derive normalised volume-discharge curves and volume-area curves for 15 main lakes (modelled lakes) of the Avon Basin for which the detailed topographic and other data are available. These volume-discharge and volume-area relations were then used to derive the same for the remaining (non-modelled) lakes of the basin for which detailed data were not available.

The steps used to calculate volume-discharge and area volume relations are described below:

1. Calculate cross-sectional area and wetted perimeters for a number of incremental stages above the lake outlet using the outlet cross-sectional data.

2. Calculate outlet discharge values for those stages using the Manning’s equation with a roughness coefficient of 0.03.

3. Derive the relation from the given stage-volume data.

4. Calculate the lake volume for stages in (2) using the relation derived in (3)

5. Generate the volume-discharge curve using values from (2) and (4).

6. Repeat the process for all modelled lakes.

From the relations developed for each lake and normalised paramters estimated in Table 5.3, the general relationships were developed as listed in Appendix 1.3.2. These relations were then used to estimate area, volume and discharge rates for all lakes.

Lake boundary

Flat bottom elevation

Idealised bank geometry

b

c

Estimated depth below

A

DS = A x (b + c) Vmax = DS +(A x a)

a

Height of next overflow


Table 5.3. The normalisation parameters for salt lakes in the Avon Basin

Lake Subcat. Amax V′max Qmax Vmax Vo (km2) (GL) (GL d-1) (GL) (GL) Ninan 26 9.2 19.0 93.4 40.2 3.5 Hinds 27 14.6 33.0 47.8 68.4 33.0 Dowerin 36 7.8 20.1 25.4 22.5 20.1 Walyormouring 37 10.8 51.3 195.3 100.1 51.3 Cowcowing 38 187.4 795.4 2361.1 1537.4 795.4 Brown (south) 54 2.5 3.0 1.2 6.7 3.0 Yealearing 57 9.4 6.4 43.9 14.5 6.4 Yenyening 58 28.9 9.5 2.6 38.4 16.8 Kurrenkutten 66 15.2 28.6 0.1 31.8 28.6 Kondinin 67 18.2 20.2 16.5 50.1 20.2 Jilakin 69 13.9 27.2 122.7 72.8 41.2 Grace North 74 76.4 38.2 1532.4 85.0 38.2 O'Connor 77 11.7 17.2 5.1 25.2 17.2 Varley 78 9.6 14.0 0.7 18.0 14.0 Gulsen 79 14.1 9.1 15.6 49.4 9.1 King 80 44.8 24.3 3.8 78.5 24.3 Ace 83 1.6 3.5 28.3 5.9 1.0 Baandee 91 7.9 7.9 3.3 13.9 8.1 Brown (Yilgarn) 99 33.1 4.9 1.3 30.6 6.2 Campion 100 7.5 3.1 0.5 7.9 3.7

5.8. Natural channels cross-sectional data and curve fitting The creek/stream/river cross sectional data were extracted at three locations for each of the 88 natural streams/rivers. A maximum flooding stage of 5 m from the lowest point of stream cross–section was assumed for calculating the stage-discharge relation.

5.8.1. Channel characterisation

To account for the storage within the natural channel system, modification of LASCAM was required. To modify LASCAM for this purpose, there was a need to develop height-area-volume-discharge relationships for all major natural creeks and rivers in the Avon Basin. This was achieved by constructing cross-sections of creeks and rivers through the use of DEM data, field survey of representative channel cross sections, comparison of observed and DEM generated cross sections, and development of height-area-volume-discharge relationships for the main channel stream in all 108 subcatchments within the Avon Basin. Eight RTK GPS survey sites were selected for cross-sections through the main natural channel systems and cross section surveys were completed (Table 5.4). These were compared with DEM derived cross-sections to validate that the DEM was suitable to derive cross-sections for all 108 subcatchments. At each survey site a Standard Survey Mark (SSM) or Benchmark was used to correct GPS heights. These points were extracted from the DEM to determine a correction factor for the DEM within the survey area.

Table 5.4. List of surveyed natural channel cross sections

Shire Cross-section location

Nungarin Baird Rd / Mcglinn Rd Johnson Rd

Kellerberrin Kellerberrin Yotting Rd Gardener Glenluce Rds Victoria Rd

Quairading Doodlakine Bruce Rock Rd (Bandee cross-section)

Bruce Rock Yad Yadin Rd Eujinyn Rd


The DEM is known to have a vertical accuracy of 2 m; however, it may be relatively more accurate on a cell-to-cell basis so applying the correction factor is a reasonable approach. The comparison showed that the corrected DEM heights compared favourably to the surveyed heights at some sites (Figure 5.10) and less favourably at others (Figure 5.11). This is most likely a combination of the variable quality of the DEM generation and a known problem involving vegetation effects in the automated photogrammetry technique. When spot heights fall on a medium to high canopy vegetation it is more likely that the crown top will be recorded as ground surface.

243.0

244.0

245.0

246.0

247.0

0 500 1000 1500 2000Distance (m)

Ele

vati

on

(m

AH

D)

Surveyed cross section

Corrected DEM

Figure 5.10. Comparison between surveyed and DEM based natural channel cross-section at Baird

Road

262.0

263.0

264.0

265.0

0 200 400 600 800 1000 1200 1400

Distance (m)

Ele

vati

on

(m

AH

D)

Surveyed cross section

Corrected DEM

Figure 5.11. Comparison between surveyed and DEM based natural channel cross-section at

Doolakine – Baandee cross-section


Since it was not possible to obtain ground surveyed cross-section data for all of the catchments, the DEM based cross-sections were used for developing the relationships. For each catchment covered by higher resolution DEM data, three cross-sections were digitised and the elevations extracted from the DEM. The locations of the cross-sections were determined by first applying the Multi Resolution Valley Bottom Flatness (MRVBF) approach developed by Gallant and Dowling (2002). The MRVBF index operates on varying scales and identifies areas that are both relatively flat and low in the landscape, which is interpreted as a map of valley bottom areas (Gallant and Dowling, 2002). These were then classified into different classes to identify the typical width of the flat valleys within each catchment. The location of the cross-sections was based on a visual interpretation of the typical width of the channel for the catchment. These were then normalised and averaged.

5.8.2. Velocity-discharge calculations in streams

The flow velocities in the streams were calculated using Manning’s equation with stream gradients derived from DEM data. The Manning’s n or roughness coefficient was determined by considering several factors given in the following equation.

n = m5(n0 + n1 + n2 + n3 +n4) Various parameters in this equation are defined in Table 5.5 along with their values for a range of conditions. Cross-sectional area and other channel parameters were based on surveyed and/or DEM derived cross-sections of the streams. The discharge and velocity for all three cross sections of a stream were determined and plotted and the procedure repeated for all cross-sections of 88 streams in 88 subcatchments.

Table 5.5. Parameters for determining roughness coefficient in Manning’s equation

Channel characteristics General conditions n type Value

Material involved

Earth

n0

0.020 Rock cut 0.025 Fine gravel 0.024 Coarse gravel 0.028

Degree of irregularity

Smooth

n1

0.000 Minor 0.005 Moderate 0.010 Severe 0.020

Variation of channel x-section

Gradual

n2

0.000 Alternating occasionally

0.005

Alternating frequently

0.010–0.015

Relative effect of obstruction

Negligible

n3

0.000 Minor 0.010–

0.015 Appreciable 0.020–

0.030 Severe 0.040–

0.060

Vegetation

Low

n4

0.005–0.010

Medium 0.010–0.025

High 0.025–0.050

Very high 0.050–0.100

Degree of meandering Minor

m5 1.000

Appreciable 1.150 Severe 1.300


A curve was fitted through the velocity-discharge data points to calculate the coefficients a and b of the power function V = aQb for all streams of the 88 subcatchment stream sections. The values of a and b for all subcatchments are given in Appendix 5.

5.9. Landmonitor salinity data A method for salinity mapping and monitoring has been developed through the Landmonitor project (Allen and Beetson, 1999) for the Western Australian wheatbelt. The mapping method involves stratification and separation of groundcover types from selected dates of Landsat-TM images combined with DEM attributes to calculate the probability of areas being salt-affected (Evans, 1998). Salinity maps have been produced from historical Landsat-TM data between 1988 and 1998 and are available for most of the wheatbelt. To identify the current extent of salinity in each subcatchment for LASCAM modelling, the 1998 salinity map was overlaid on the subcatchment polygons and the area of salinity in each subcatchment determined. A total of approximately 400,000 ha or 3.5 % of the basin was mapped as saline at 1998 (Figure 5.15). Most of the unprocessed area in Figures 5.12 and 5.13 lies within the pastoral zone where widespread clearing and secondary salinity has not occurred over time.

Figure 5.12. The extent of mapped salinity for the Avon Basin

Evans and Caccetta (2000) refined a method to predict the watertable position at hydrological equilibrium and applied it throughout the wheatbelt. The method aims to replicate the hydrologists’ opinions about the future extent of valley (salinity) hazard (where watertables are within 2 m of the surface and when hydrological equilibrium is reached for a set of training areas). A number of variables thought to control the development of salinity are used to predict the likelihood of shallow groundwater occurring for any given part of the catchment. Some of these variables are derived from the DEM (e.g. slope, flow accumulation, etc.) whilst others are readily available thematic coverages (e.g. geology, hydrogeology, etc.). The result is the most reliable thematic coverage of salinity hazard


available although experts now believe the hazard area to over-estimate risk by a considerable degree and probably to be overly pessimistic (McFarlane et al., 2004).

The valley hazard maps were overlaid on the subcatchment polygons to determine the area of salinity hazard for each subcatchment. Given that not all of the hazard area is expected to become saline it was not possible to imply salinity risk to the entire hazard area. Hence the valley hazard data was used as a surrogate for salinity risk and it was assumed that only 50% of the hazard area within any given subcatchment would become salinised at equilibrium. This assumption may not hold true if climate during the twenty-first century is different to what has been assumed in this study. These risk areas were used as basis for determining the length of drains required to protect or reclaim saline land as explained in the following section. Figure 5.13 depicts the extent of valley hazard areas at the new hydrological equilibrium for the Avon Basin. The clearing line roughly follows the unprocessed area boundary in Figures 5.12 and 5.13.

Figure 5.13. Landmonitor valley hazard predictions for the Avon Basin at hydrological equilibrium

The percent mapped salinity area in 1998 is shown in Figure 5.14 along with the Landmonitor valley hazard area. The salinity mapped area and valley hazard varies significantly among various catchments depending upon their location, landuse, rainfall and available relief. Catchments located in uncleared areas will be less affected by salinity over time while those located in the flat areas of the lower eastern and central regions of the Avon Basin are likely to be significantly affected when new equilibrium is reached. Catchments in the western region of the basin with high rainfall and relief are likely to be less affected over time. Within each of the 108 subcatchments, the area that the model assumes is at risk of salinity at the reach of new equilibrium is shown in Figure 5.15.


Figure 5.14. Percent area of the Avon Basin mapped as saline in 1998 and valley hazard area at

equilibrium

Figure 5.15. Area of subcatchments of the Avon Basin with valley hazard at equilibrium


5.10. Artificial drainage The artificial drainage length required to reclaim salinised land in each subcatchment depends on the extent of its salt-affected area (estimated above) and drainage effectiveness. Table 5.6 lists the total length of drains that will be required in the Avon Basin to reclaim salt-affected areas for a range of zones of effectiveness (ZOE). The ZOE is defined as the width in metres on each side of the drain where the watertables are declined due to drainage and land is returned to full productivity. If the salt-affected area at the reach of new equilibrium is assumed to be 20 % of the Avon Basin, then 46,800 km of drains will be required in the Avon Basin to reclaim that area assuming zone of effectiveness (ZOE) of drains of 500 m (i.e. 250 m either side of the drain). About 23,400 km of drains will be required if 10 % of the basin area is salinised and ZOE is 500 m. For this project the total artificial drainage length, in each subcatchment of the Avon Basin, was based on the assumed ZOE of the drains and the Landmonitor valley hazard.

Table 5.6. Drainage length required for various proportions of the salt-affected area for a range of ZOEs in the Avon Basin

Affected area Drainage effectiveness (ZOE) distance (m) both sides of a drain

Percent km2 100 200 300 400 500 600

Drainage length required to reclaim the affected area (km)

10 11700 117000 58500 39000 29250 23400 1950020 23400 234000 117000 78000 58500 46800 3900025 29250 292500 146250 97500 73125 58500 4875030 35100 351000 175500 117000 87750 70200 58500

5.10.1. Zone of effectiveness (ZOE) of artificial drainage

The ZOE is influenced primarily by soil type, topography, geological features, watertable depth and permeability. Because data on these controls is not available at the scale that groundwater drains operate, it was necessary to assume a ZOE for the recovery of cropping and pastures in the Avon Basin. Evidence from previous farm scale drainage studies and evaluations in various parts of the Avon Basin were considered while making assumptions for the ZOE of artificial drainage. The Avon Basin was divided into three regions (Figure 5.16) in which the ZOE was assumed to be a constant value. In each region two ZOEs (high and low) were defined. The ZOE values in each region reflect the topography and hydrogeology of the region with western more incised country with shallower regolith having a smaller ZOE and the deeper and flatter eastern regions having higher ZOE. For the Western catchments the low and high ZOE were 50 and 100 m, respectively, on either side of the drain (Figure 5.16).


Figure 5.16. Drainage effectiveness regions of the Avon Basin

5.10.2. Determination of current and future artificial drainage lengths

An estimate of the current and future artificial drainage lengths in each subcatchment of the Avon Basin was needed as input to LASCAM. At the time of the analysis, no complete and verified GIS data were available to determine the current extent of deep drains in the basin. However some subcatchments had reasonably accurate vector representations of existing artificial drainage as a result of previous and current investigations into the effectiveness of such drainage schemes. One of these was the Wakeman subcatchment near Narembeen which had been the focus of a 5 year investigation by CSIRO Land and Water (Ali et al., 2004a; Ali et al., 2004b; Ali et al., 2004c).

A methodology for deriving representative drainage lengths for all subcatchments in the Avon Basin was developed based on the Wakeman subcatchment. The stream ordering technique used to define LASCAM subcatchments was customised to reflect the existing deep drainage network. The technique involved setting a threshold for the contributing area to define streamlines, then stream ordering to identify sections likely to be in the 1998 Landmonitor mapped salinity areas and consequently within the existing drainage network. Several thresholds for streamline derivation were tested and compared with the length and extent of the existing drainage network. It was found that higher order subcatchments had little or no artificial drainage using this approach (because they had no streams of order 4 or 5) and some first order subcatchments had much more than their valley hazard area. Similar problems were found with the future drainage numbers. For this reason the approach was abandoned.

Instead a simpler method was used to estimate the current and future required artificial drainage lengths in each subcatchment of the Avon Basin by utilising the Landmonitor valley hazard mapping. This method was applied uniformly across all subcatchments of the basin. The current and future required artificial drainage lengths were estimated using this method as described below.

Current required artificial drainage An estimation of the length of deep open drains already constructed (until 2000) within the Avon Basin was required as a starting point for the drainage scenarios run in LASCAM. As


stated previously, no complete and verified GIS data were available at the time of the analysis so the Landmonitor valley hazard data set was used as a basis to estimate the extent of current required drainage within each subcatchment. It was assumed that 10 % of the Landmonitor valley hazard area at the reach of a new equilibrium (assumed to occur by the year 2100) in each subcatchment required artificial drainage currently. The artificial drainage lengths required to drain that area were determined using the appropriate ZOE. Using this method 4,540 km of drains are assumed to exist for the drainage scenario runs. A full listing of subcatchments, valley hazard areas and drainage lengths is given in Table (5.7).

Whilst the majority of drainage schemes presently constructed in the Avon basin are small, isolated, unconnected and concentrated in relatively few subcatchments, the lack of information on their locations and extent required the development of a consistent method to estimate the lengths in each subcatchment. It is acknowledged that only a few subcatchments will have more extensive drainage currently (e.g.: Wakeman, Bodallin) and many may have very little or no current artificial drainage. Furthermore, existing drainage schemes tend to have no disposal mechanism and the connectivity of drains and associated drainage discharge is varied and uncertain.

Future drainage To assess future artificial drainage requirements the only spatial data available across the whole Avon Basin was the Landmonitor valley hazard maps. Most hydrologists now consider the valley hazard data set as a worst case scenario at best and an extremely pessimistic prediction at worst (McFarlane et al., 2004; George pers comm., 2006). McFarlane et al. (2004) points out that the presence of a shallow watertable hazard does not imply the development of salinity and, in the context of a drying climate, the development of shallow watertables may not be a rapid nor widespread as was believed at the time the Landmonitor maps were developed. Fifty percent of valley hazard areas represent the most likely full expression of salinity when new hydrological equilibrium is reached. Whilst the final extent of salinity (or at least the potential for it) is debateable, the 50 % of valley hazard used here is the best guess for the extent of salinity risk at 2100. Using this method 22710 km of drains were required at 2100 to treat 50 % of the valley hazard areas assuming that drains had the high ZOE. Subcatchment drainage lengths assumed for 2100 are presented in Table 5.7.


Table 5.7. Drainage lengths based on mapped 1998 salinity and Landmonitor valley hazard

Reg

ion

SU

BC

AT

ID

To

tal

Are

a (k

m2)

Landmonitor (LM)

1998 salinity

Landmonitor (LM) valley hazard (VH)

Target 50% of VH - future drainage 2100

Target 10% of VH - current drainage 2000

LM

ma

pp

ed

S

alin

ity

199

8 (k

m2 )

sub

catc

h w

ith

m

app

ed 1

998

Are

a w

ith

ou

t va

lley

haz

ard

(V

H)

(km

2)

Val

ley

haz

ard

(V

H)

area

(km

2 )

Per

cen

t o

f su

bca

tch

wit

h V

H

To

tal

len

gth

of

dra

ins

(km

) fo

r 50

% o

f V

H

Are

a d

rain

ed -

lo

w

ZO

E s

cen

ario

(h

a)

Are

a d

rain

ed -

h

igh

ZO

E s

cen

ari

o

(ha)

To

tal

len

gth

of

dra

ins

(km

) fo

r 10

% o

f V

H

Are

a d

rain

ed -

lo

w

ZO

E s

cen

ario

(h

a)

Are

a d

rain

ed -

h

igh

ZO

E s

cen

ari

o

(ha)

Wes

tern

6 292 1 0 270 22 7 55 546 1093 11 109 219

12 230 0 0 218 12 5 29 292 584 6 58 117

13 192 0 0 184 8 4 20 201 402 4 40 80

14 393 1 0 365 28 7 70 697 1394 14 139 279

15 845 17 2 712 133 16 332 3322 6643 66 664 1329

16 161 1 1 149 12 7 30 301 602 6 60 120

17 130 1 1 118 12 9 31 306 612 6 61 122

18 44 0 1 39 5 12 13 126 253 3 25 51

19 114 3 3 93 21 18 52 520 1041 10 104 208

20 7 0 0 6 1 19 3 33 67 1 7 13

21 5 0 0 4 1 19 2 24 48 0 5 10

22 57 2 3 46 11 19 27 272 544 5 54 109

23 130 5 4 103 27 21 68 681 1363 14 136 273

30 238 5 2 196 42 18 105 1051 2102 21 210 420

31 115 5 4 89 26 23 65 654 1308 13 131 262

32 746 33 4 568 177 24 443 4429 8859 89 886 1772

39 48 0 1 43 5 11 13 129 259 3 26 52

40 366 4 1 332 34 9 85 855 1709 17 171 342

41 375 5 1 322 53 14 132 1316 2632 26 263 526

42 75 0 0 66 9 12 22 221 441 4 44 88

43 595 26 4 479 116 19 289 2895 5789 58 579 1158

44 174 2 1 149 24 14 61 607 1214 12 121 243

45 7 0 1 6 1 16 3 27 54 1 5 11

46 21 0 0 18 4 17 9 90 179 2 18 36

47 603 10 2 492 110 18 276 2758 5517 55 552 1103

48 836 10 1 752 84 10 210 2098 4195 42 420 839

49 282 5 2 222 59 21 148 1481 2963 30 296 593

50 286 3 1 248 37 13 93 933 1865 19 187 373

51 77 0 0 64 13 17 33 333 666 7 67 133

52 1157 54 5 856 301 26 752 7519 15039 150 1504 3008

Ce

ntr

al

24 853 49 6 629 224 26 280 5602 11203 56 1120 2241

25 1502 114 8 1076 426 28 532 10649 21298 106 2130 4260

26 813 115 14 535 278 34 347 6946 13893 69 1389 2779

27 69 8 12 40 29 42 37 732 1465 7 146 293

28 1363 159 12 1003 359 26 449 8981 17962 90 1796 3592

29 2126 172 8 1489 638 30 797 15945 31891 159 3189 6378

33 1532 132 9 1074 458 30 573 11451 22902 115 2290 4580

34 1501 86 6 1046 455 30 568 11370 22739 114 2274 4548

35 1134 52 5 770 364 32 455 9098 18197 91 1820 3639

36 311 24 8 197 115 37 143 2864 5728 29 573 1146

37 923 78 8 651 273 30 341 6813 13625 68 1363 2725

38 3393 369 11 2250 1143 34 1428 28567 57133 286 5713 11427

53 1459 85 6 1110 349 24 436 8722 17445 87 1744 3489

56 346 19 5 257 89 26 112 2236 4472 22 447 894 58 454 41 9 290 163 36 204 4080 8160 41 816 1632


Table 5.7 (Continued). Drainage lengths based on mapped 1998 salinity and Landmonitor valley hazard

Reg

ion

SU

BC

AT

ID

To

tal

Are

a (k

m2)

Landmonitor (LM)

1998 salinity

Landmonitor (LM) valley hazard (VH)

Target 50% of VH - future drainage 2100

Target 10% of VH - current drainage 2000

LM

ma

pp

ed

S

alin

ity

199

8 (k

m2)

Per

cen

t o

f su

bca

tch

wit

h

map

ped

199

8 sa

linit

y

Are

a w

ith

ou

t V

alle

y h

azar

d

(VH

) (k

m2)

Val

ley

haz

ard

(V

H)

area

(k

m2 )

Per

cen

t o

f su

bca

tch

wit

h V

H

To

tal

len

gth

of

dra

ins

(km

) fo

r 50

% o

f V

H

Are

a d

rain

ed -

lo

w Z

OE

sce

nar

io

(ha)

Are

a d

rain

ed -

h

igh

ZO

E

scen

ario

(h

a)

To

tal

len

gth

of

dra

ins

(km

) fo

r 10

% o

f V

H

Are

a d

rain

ed -

lo

w Z

OE

sce

nar

io

(ha)

Are

a d

rain

ed -

h

igh

ZO

E

scen

ario

(h

a)

Ce

ntr

al

59 1218 52 4 951 267 22 334 6682 13365 67 1336 2673

60 1606 109 7 1097 509 32 637 12736 25471 127 2547 5094

61 194 14 7 134 59 31 74 1487 2975 15 297 595

86 238 29 12 138 101 42 126 2523 5046 25 505 1009

89 85 4 4 70 14 17 18 355 710 4 71 142

Eas

tern

54 483 26 5 398 85 18 71 2121 4241 14 424 848

55 10 1 13 5 5 46 4 114 227 1 23 45

57 885 49 5 712 172 19 143 4305 8609 29 861 1722

62 1898 93 5 1299 599 32 499 14971 29942 100 2994 5988

63 759 28 4 519 240 32 200 5991 11982 40 1198 2396

64 2808 48 2 2112 696 25 580 17412 34824 116 3482 6965

65 618 22 4 408 210 34 175 5240 10481 35 1048 2096

66 1152 80 7 836 316 27 263 7900 15800 53 1580 3160

67 836 63 7 642 194 23 161 4841 9681 32 968 1936

68 591 26 4 468 122 21 102 3060 6120 20 612 1224

69 757 66 9 599 158 21 132 3960 7919 26 792 1584

70 1995 121 6 1564 431 22 359 10778 21557 72 2156 4311

71 334 32 9 254 80 24 67 2000 4000 13 400 800

72 1948 58 3 1597 350 18 292 8756 17511 58 1751 3502

73 623 41 7 480 143 23 119 3568 7135 24 714 1427

74 3539 342 10 2358 1182 33 985 29542 59085 197 5908 11817

75 2166 110 5 1699 466 22 389 11660 23320 78 2332 4664

76 2707 67 2 2361 346 13 289 8659 17317 58 1732 3463

77 1876 81 4 1591 284 15 237 7108 14216 47 1422 2843

78 246 11 5 190 56 23 47 1402 2803 9 280 561

79 1383 68 5 1094 289 21 241 7224 14449 48 1445 2890

80 1085 124 11 750 335 31 279 8382 16763 56 1676 3353

81 81 0 0 81 0 0 0 0 0 0 0 0

82 181 15 8 99 82 45 68 2045 4090 14 409 818

83 77 1 2 59 18 23 15 449 897 3 90 179

84 483 1 0 453 31 6 26 767 1535 5 153 307

87 6 1 12 1 4 75 4 111 221 1 22 44

88 18 1 6 11 7 39 6 170 340 1 34 68

90 638 33 5 435 203 32 169 5070 10140 34 1014 2028

91 237 19 8 146 91 39 76 2286 4572 15 457 914

92 2203 53 2 1641 562 26 468 14051 28103 94 2810 5621

93 1426 62 4 965 461 32 384 11532 23064 77 2306 4613

94 1997 76 4 1270 727 36 606 18177 36353 121 3635 7271

95 1927 7 0 1268 659 34 549 16471 32942 110 3294 6588

96 604 34 6 365 239 40 199 5973 11945 40 1195 2389

97 2945 12 0 1973 972 33 810 24305 48611 162 4861 9722

99 286 52 18 149 137 48 114 3422 6843 23 684 1369

100 693 49 7 434 259 37 216 6478 12957 43 1296 2591

101 1863 23 1 1315 549 29 457 13715 27429 91 2743 5486

102 2318 20 1 1702 616 27 513 15401 30802 103 3080 6160

104 4612 60 1 3372 1240 27 1033 30988 61976 207 6198 12395


This methodology also assumes that hydrological equilibrium will be realised by 2100. Hatton et al. (2002) showed that for the lower rainfall regions of the wheatbelt, it may be several centuries before salt and water balances equilibrate given the current climate. However in the western catchments hydrographs suggest that water levels may have already come to equilibrium and salinity in these catchments is not growing significantly. The effect of climate change over the next 100 years is also not known so there is considerable uncertainty in this assumption.

5.11. Arterial channels To accommodate large flow volumes and rates that will be likely from some artificial drainage scenarios an arterial channel through the main-stream subcatchments of the Avon Basin will be required. The purpose of this channel is to convey water, not to drain shallow groundwater (although that may occur over much of its length), nor principally to recover salinised land for agricultural production. Stream ordering was used to identify the major reaches of the drainage network and therefore an arterial channel. Streams of order 6 or higher were extracted from the drainage network derived from the DEM. Some sections were removed as they were deemed unnecessary given the presence of terminal lakes or largely uncleared subcatchments. Two types of arterial channels were modelled: open arterial channels and leveed arterial channels. The open arterial channel is defined as an engineered channel that accommodates both surface runoff and drainage discharge from an artificial drainage system. The leveed arterial channel is defined as a channel that conveys only drainage discharge from leveed deep drains. The open arterial channel is assumed to be located coincident with the natural drainage in the broad valley systems of Lockhart. Yilgarn, Salt River, Mortlock (North and East) and as such would occupy a valley floor that is highly salinised and degraded to a width greater than the extent of drawdown that the channel may provide. Therefore the physical isolation of the ZOE from adjacent farmland would make it impractical for that land to be returned to agricultural production even if the land could be rehabilitated. There may be other benefits and costs of open arterial channels to factors such as biodiversity, roads, etc., however these were not examined.

The sole purpose of such an arterial channel is to convey saline drainage discharge from subcatchments to either the nearest salt lake (as in the regional retention scenario) or ultimately to the ocean via the Swan/Avon river system. Furthermore it was assumed that the natural channel from Yenyenning to Northam and from Northam to Walyunga (Avon River and Lower Avon respectively in Figure 5.17) would be sufficient to cope with peak flows without the need for additional engineering and channel maintenance. A leveed arterial channel is assumed to be located parallel to the natural creek system at some distance away so that it does not cause any obstruction to the surface water flow in the natural system. Again it is expected that such a channel will be constructed to convey the drainage discharge from various subcatchments to either various salt lakes or the main river system rather than reclaiming the landscape. Its depth will be dictated by the available gradient, roughness, flow rates and extent of artificial drainage and may result in very shallow depths along its course.


Figure 5.17. Modelled arterial channel network showing various reaches


6. MODEL CALIBRATION

6.1. Calibration procedure LASCAM is a conceptual model and some of its driving equations include variables whose values are unknown and must be calibrated. The optimisable parameters of LASCAM are assumed to apply identically in all subcatchments. That is, the parameters are universal for a given catchment.

The calibration process involves assessing the quality of model predictions for different sets of parameter values. For LASCAM, this is achieved by a semi-automated procedure that is based on the Shuffled Complex Evolution method (Duan et al., 1993). The calibration procedure seeks to find a set of model parameters that optimises the value of a prescribed objective function. Typically, such a function will involve some method of minimising the difference between observed and predicted streamflows and salt loads. In this case, the model efficiency (Nash and Sutcliffe, 1970) is chosen. Since streamflow and salt load observations are available for several of the modelled subcatchments, we can construct an objective function that is a linear combination of the model efficiencies of the daily streamflows and salt loads at several locations in the catchment.

Rainfall, streamflow and salt load data are available from 1965 to 2003, although many streamflow stations operate for only part of this period. The first five years of this time series is a spin-up period used to allow the initial storages time to reach equilibrium. The actual calibration period is taken from 1970 to 2003, and model efficiencies are calculated only during this period. Since the aim is to develop a model with many new components, it was decided to use the entire period for calibration in preference to splitting the record into calibration and validation periods. This allows the entire record (and the temporal heterogeneities in climate and land cover that are contained within it) to be utilised to the fullest possible extent in establishing optimal parameter values. But it means the model outputs are not validated using independent data.

The calibration technique yields a single set of parameters that has wide applicability across the catchment and which has taken advantage of any spatial heterogeneities existing in the observed data. This provides some confidence in the quality of predictions on the ungauged subcatchments, as well as on the gauged subcatchments.

6.2. Calibration data and processing

6.2.1. Streamflow data

The Avon Basin above the Great Northern Highway contains many streamflow gauging stations. For this study, only those with catchment areas of greater than about 50 km2 (except subcatchment 85) were used. This yielded a set of 26 gauges operated by DoW and one operated by CSIRO. However, not all have been operated simultaneously and some have been decommissioned (Table 6.1). In one case, Salt River at Qualandary Crossing, while the gauge has continued operating to 2003, data recorded since 1992 are considered to be of doubtful quality and have not been used for calibration.


Table 6.1. Streamflow data used in this study

The subcatchment delineation was carried out in such a way that all 27 gauging locations coincide with subcatchment outlets. A further three DoW gauging stations on the Avon River (at Toodyay, York and Qualandary Crossing) have stage data but as yet have not been rated. These too, coincide with subcatchment outlets, with a view to using them in the modelling at a later date if streamflow volumes become available. Data from all stations are accumulated to give daily streamflows for the 24 hours to 9 am.

6.2.2. Salt load data

Where available, observations of salt concentration at the 27 stream gauging sites listed in Table 6.1 were used to create time series of salt load. In some cases, the salinity observations consist of irregular grab samples, while in others they involve continuous sampling. Some sites have a combination of both sampling techniques. On days where both concentration and flow data are available, daily salt load is calculated by multiplying the two. Where one or both observations are missing on a particular day, no salt load is calculated. No attempt is made to temporally interpolate observed salt load. Those gauging stations that have concentration observations are listed in Table 6.2, along with details of data duration and occurrence.

Table 6.2. Salt load data used in this study (R stands for River; and Ck stands for creek))

Subcat. Gauge Location Area (km2)

Years Number of obs.

1 616076 Avon R at Great Northern Hwy 116657 1996–2003 2516 2 616011 Avon R at Walyunga 116624 1970–2002 11570 4 616001 Wooroloo R at Karls Ranch 514 1970–2003 12114 5 616081 Wooroloo R at Weilling Pool 450 1970–1975 1937 6 616005 Wooroloo R at Noble Falls 292 1980–1999 6653 8 616019 Brockman R at Yalliawirra 1522 1975–2003 9879 9 616179 Brockman R at Glen Darran 1503 1970–1979 3178

11 616006 Brockman R at Tanamerah 961 1980–2003 8302 13 616192 Julimar Brook 192 1970–1974 1529 16 615026 Avon R at Toodyay 112872 unrated 17 615021 Avon R at Dunbarton Bridge 112711 1977–1981 1033 19 615030 Wongamine Bk 114 1997–2001 1078 23 615013 Mortlock R North 6857 1975–2003 10001 31 615020 Mortlock R East 9655 1975–2003 10173 39 615062 Avon R at Northam Weir 95605 1977–2003 9196 40 615028 Spencers Brook 366 1995–2000 1327 42 615024 Avon R at York 94815 unrated 45 615014 Avon R at Brouns Farm 93972 1975–2001 9469 47 615027 Dale R at Waterhatch 2006 1995–2003 2257 50 615222 Dale R at Jelcobine 286 1970–1999 10430 52 615025 Avon R at Beverley 91861 1995–2003 2888 53 615029 Avon R at Qualandary 3183 unrated 58 615022 Salt R at Qualandary 87521 1982–1992 3829 62 615012 Lockhart R at Kwolyn 28163 1976–2003 9386 64 CSIRO Wakeman Ck at Narembeen 2809 2000–2003 1038 81 615018 Lake King Creek 81 1976–1999 8344 84 615016 Lake Ace Ck at Spencers Farm 505 1978–2002 8763 85 615017 Lake Ace Ck at Hatters Hill 21 1976–2002 9552 87 615015 Yilgarn R at Gairdners Crossing 55648 1976–2003 9728 89 615011 Mooranoppin Ck 85 1974–2003 9939


6.3. Calibration assumptions and targets

6.3.1. Calibration assumptions

There is just a single set of model parameters for the entire catchment. This means that the same calibrated parameter set must be capable of predicting streamflows and loads in all parts of the catchment. Inevitably in such a large and hydrologically diverse catchment, this will entail some degree of compromise. This strategy does however, allow the model to take advantage of any spatial heterogeneities in the observed data and provides some confidence in the quality of predictions on the ungauged subcatchments, as well as on the gauged subcatchments.

Calibration of water and salt parameters is carried out simultaneously. This means that what might have been the best water flux parameters for predicting streamflows may not necessarily be the best parameters for predicting both water and salt. Again, some compromise is inevitable. The objective function is comprised of daily flow and load efficiencies only. No account is taken during calibration of monthly or annual prediction efficiencies or of the overall prediction biases. No measures of soil moisture (including watertable depths), soil salinity, lake stage or lake volume are employed in the calibration process. The calibration assumes no artificial drainage. This assumption is necessary because no information on the location and size of existing artificial drainage in various subcatchments was available at the time of model calibration. Subcatchment attribute data used by the model (e.g., land cover, soil depths, soil physical properties, climate, lake and channel characteristics) are all prescribed a priori. There has been no local tweaking of these attributes to improve fits except reducing the soil depths at Mooranoppin (as recommended in the project external review).

6.3.2. Calibration targets

Before calibration, a set of performance targets was developed. These targets specify the quality of the model predictions that the calibration process should aim to achieve. For streamflow predictions, the targets for each of the major flow gauging sites are that the total flow over the calibration period should be within 30 % of the observed total, and prediction efficiencies should at least be 0.5 for daily flows, 0.6 for monthly flows and 0.7 for annual flows. For salt load predictions the targets for efficiency are the same, but the total load (or

Sub cat.

Gauge Location Area (km2)

Years No. of obs.

1 616076 Avon R at Great Northern Hwy 116657 2000–2003 1166 2 616011 Avon R at Walyunga 116624 1973–1999 4091 4 616001 Wooroloo R at Karls Ranch 514 1970–2003 5189 5 616081 Wooroloo R at Weilling Pool 450 1970–1975 64 6 616005 Wooroloo R at Noble Falls 292 1980–1999 1885 8 616019 Brockman R at Yalliawirra 1522 1975–2003 7174

11 616006 Brockman R at Tanamerah 961 1980–2003 5619 16 615026 Avon R at Toodyay 112872 unrated 17 615021 Avon R at Dunbarton Bridge 112711 1977–1981 564 23 615013 Mortlock R North 6857 1975–2003 1804 31 615020 Mortlock R East 9655 1975–2003 1755 39 615062 Avon R at Northam Weir 95605 1977–2003 4206 42 615024 Avon R at York 94815 unrated 45 615014 Avon R at Brouns Farm 93972 1976–2000 3372 47 615027 Dale R at Waterhatch 2006 2000–2003 920 50 615222 Dale R at Jelcobine 286 1970–1999 4122 52 615025 Avon R at Beverley 91861 1999–2003 1144 53 615029 Avon R at Qualandary 3183 unrated 58 615022 Salt R at Qualandary 87521 1983–1992 359 62 615012 Lockhart R at Kwolyn 28163 1977–2003 2031 87 615015 Yilgarn R at Gairdners Crossing 55648 1976–2000 97 89 615011 Mooranoppin Ck 85 1975–2003 2302


bias) target is relaxed to fall within 40 % of the observed total. For most of the gauged subcatchments, these targets greatly exceed the prediction accuracy achieved by Viney and Sivapalan (2001) in a similar modelling exercise in the Avon Basin.

The targets are not designed to be absolute or prescriptive. The expectation is that the predictions at most sites will meet or exceed most of the targets. However, it is not realistic to expect that all targets will be met at all sites, or even that every site will meet some targets. The targets are intended as a gauge on model performance across the catchment. Finally, it should be reiterated that the objective function used for calibration involves only the daily efficiencies and does not include monthly or annual efficiencies or bias. The last three measures are included in the targets to provide extra information on model accuracy.

6.4. Calibration results

6.4.1. Streamflow

Although semi-automated, the calibration process is extremely time consuming. The semi-stochastic nature of the optimiser algorithm, together with the large number of model parameters that must be calibrated, means that a large number of model runs are required. In calibrating LASCAM for the Avon basin, in excess of 400000 model runs have been conducted. The calibration performance of the model for the major gauging sites is shown in Table 6.3. All but one of the sites meet the water balance target for bias, with Wooroloo Brook missing only narrowly. All four targets are achieved by seven of the 16 listed streamflow sites. All the others meet at least one target. Several sites (e.g. Brockman, Mortlock East, Mooranoppin) have substantially poorer daily than monthly and annual efficiencies. This suggests that the overall volumes of predicted streamflows are accurate, but their timing is not. At the daily time step, the predicted flows are either too fast or too slow.


Table 6.3. Calibration performance of the model for the major gauging stations

Gauging stations Bias (%) Efficiency

Daily Monthly Annual

WATER (30) (>0.5) (>0.6) (>0.7) Great Northern Highway –12 0.65 0.77 0.79 Walyunga –14 0.68 0.79 0.73 Wooroloo –34 0.46 0.74 0.20 Brockman –1 –0.85 0.74 0.79 Dunbarton Bridge –7 0.75 0.90 0.90 Mortlock North –9 0.58 0.74 0.85 Mortlock East 22 0.40 0.71 0.73 Northam Weir –6 0.63 0.73 0.73 Spencers Brook –22 0.42 0.56 0.82 Brouns Farm –1 0.58 0.66 0.68 Dale –11 0.68 0.79 0.72 Beverley 12 0.51 0.59 0.57 Yenyening 23 0.74 0.89 0.88 Lockhart –26 0.40 0.47 0.77 Yilgarn 24 0.50 0.63 0.35 Mooranoppin –5 0.19 0.90 0.89 SALT (40) (>0.5) (>0.6) (0.7) Great Northern Highway 14 0.71 0.88 0.83 Walyunga –22 0.67 0.82 0.82 Wooroloo –56 0.34 0.50 0.19 Brockman –18 –0.61 0.69 0.45 Dunbarton Bridge –24 0.59 0.77 0.82 Mortlock North –66 0.23 0.42 0.35 Mortlock East –13 0.49 0.88 0.91 Northam Weir 2 0.61 0.76 0.85 Brouns Farm –3 0.48 0.69 0.87 Dale –38 0.70 0.75 0.29 Beverley –26 0.53 0.66 0.80 Yenyening 27 0.76 0.65 0.56 Lockhart –45 0.56 0.72 0.75 Yilgarn –38 0.53 0.50 0.32 Mooranoppin 16 –0.05 –0.45 –0.42

Note: Box colours indicate results that fail, meet or exceed the targets; results that meet or exceed targets are in green coloured boxes; results that just fail to meet the targets by less than 10 % for bias or by less than 0.1 for efficiency are in orange; and results that fail to get close enough to the target are in pink coloured boxes.

Examples of the predicted daily hydrographs for two of the streamflow gauges (one in the wetter part and one in the dryer part of the Basin) are shown in Figures 6.1 and 6.2. Figure 6.1 for the Walyunga gauging station shows an overprediction of the February peak, but an underprediction of the spring recession flows. Winter flows are well-predicted. Figure 6.2 for the Yilgarn gauging station shows evidence of overprediction of autumn and spring events. There is also a slight timing error with the winter peaks, where the June peak is predicted one day early and the July peak two days late. Although daily model predictions are modest in the more arid parts of the catchment, when aggregated to monthly or annual timesteps the predictions are much more satisfactory and suggest that the model is predicting long-term yields quite well right across the Basin (Figures 6.3 to 6.13).


Figure 6.1. Observed and predicted daily streamflow for the Avon River at Walyunga, 1986

Figure 6.2. Observed and predicted daily streamflow for the Yilgarn River at Gairdner’s Crossing, 1986


Figure 6.3. Observed and predicted monthly streamflow for the Avon River at Walyunga

Figure 6.4. Observed and predicted monthly streamflow for Wooroloo Brook at Karl’s Ranch


Figure 6.5. Observed and predicted monthly streamflow for the Brockman River at Yalliawirra

Figure 6.6. Observed and predicted monthly streamflow for the North Mortlock River


Figure 6.7. Observed and predicted monthly streamflow for the East Mortlock River

Figure 6.8. Observed and predicted monthly streamflow for the Avon River at Northam


Figure 6.9. Observed and predicted monthly streamflow for the Avon River at Broun’s Farm

Figure 6.10. Observed and predicted monthly streamflow for the Dale River at Waterhatch


Figure 6.11. Observed and predicted monthly streamflow for the Salt River at Qualandary Crossing.

Figure 6.12. Observed and predicted monthly streamflow for the Lockhart River


Figure 6.13. Observed and predicted monthly streamflow for the Yilgarn River

6.4.2. Salt load

A statistical summary of the salt load calibration appears in Table 6.3. There is an overall tendency towards underprediction (negative bias) of salt load (Table 6.3). In some cases, such as Wooroloo and Lockhart, this underprediction stems, at least in part from the underpredictions in the streamflow. However, in other subcatchments (most notably Mortlock North) it is clear that insufficient salt is being generated. The calibration statistics for salt load are compromised to some extent by the limited amounts of observed data for some gauging sites. For example, on the Yilgarn River at Gairdner’s Crossing, there are just 97 days with observed salt load over a 25-year period, an average of fewer than four per year. This means that the monthly totals of observed load are comprised of perhaps just one or two observations. As such, the monthly and annual efficiencies are more heavily influenced by timing issues than they would be at sites with continuous salt load data.

To a large extent, biases in salt load prediction at individual subcatchments reflect the biases in streamflow. Thus, for example, the overprediction of salt load at Qualandary Crossing is of a similar magnitude to the overprediction of streamflow. The Mortlock North River has the worst salt predictions, with a bias of –66 % despite a streamflow bias of –9 % and with all statistics substantially below the targets. It is not clear why the predicted salt concentrations are so low here, given that Lake Ninan is regularly contributing saline water to the river (see next section). There is also a marked difference in water and salt biases for the Yilgarn River, but its significance is uncertain due to the relative paucity of observed salt load data. This paucity of observations could also be part of the reason why monthly and annual efficiencies are less for salt than for water in the Yilgarn.

Time series of monthly observed and predicted salt load are shown in Figures 6.14 to 6.24. Recall that for comparison purposes, monthly totals include only those days within the month that have observed salt loads. The magnitude of these totals depends not only on the magnitude of the flows during the month, but also on the prevalence of the observations. Thus little significance should be ascribed to changes from month to month or from year to year in Figures 6.14–6.24. For example, the apparent reduction is salt loads at Walyunga from 1996 onwards is entirely due to a reduction in sampling frequency, rather than to a reduction in actual loads. The main purpose of these figures is to assess how well the


observed and predicted hydrographs compare with each other. The underprediction of salt load peaks is most likely due to some unreliable observed data and model errors.

Figure 6.14. Observed and predicted monthly salt load for the Avon River at Walyunga

Figure 6.15. Observed and predicted monthly salt load for Wooroloo Brook at Karl’s Ranch


Figure 6.16. Observed and predicted monthly salt load for the Brockman River at Yalliawirra

Figure 6.17. Observed and predicted monthly salt load for the North Mortlock River


Figure 6.18. Observed and predicted monthly salt load for the East Mortlock River

Figure 6.19. Observed and predicted monthly salt load for the Avon River at Northam


Figure 6.20. Observed and predicted monthly salt load for the Avon River at Broun’s Farm

Figure 6.21. Observed and predicted monthly salt load for the Dale River at Jelcobine


Figure 6.22. Observed and predicted monthly salt load for the Salt River at Qualandary Crossing

Figure 6.23. Observed and predicted monthly salt load for the Lockhart River


Figure 6.24. Observed and predicted monthly salt load for the Yilgarn River

6.4.3. Lake storage and discharge

The lake routing algorithm of the model appears to be working satisfactorily. A typical time series of lake volume is shown in Figure 6.25. In this case, Lake Kondinin is predicted to overflow five times in 39 years, during those periods when its storage volume exceeds its dead storage level. During most summer periods it recedes to comparatively small volumes by evaporation and seepage.

Figure 6.25. Predicted storage volume in Lake Kondinin. The dashed line shows the assumed dead storage level

There is little observational information available in reports and databases to validate lake storages and discharges. Some lakes have limited amounts of depth data, but there are


difficulties in converting this into volumes for comparison with the predicted storages. Only one lake, Yenyenning, has gauged discharges. However these discharges, despite being well predicted (Table 6.3, Figure 6.11), are measured at a regulated weir whose operation is not modelled. Nonetheless, with the exception of a small predicted discharge in 1988, it is clear in Figure 6.11 that the model is correctly predicting that there were discharges in all years except 1985 and 1987, for which it correctly predicts no discharges.

Prior to calibration of the model, expert estimates were sought on the number of times, in a 39 year period, each of the modelled lakes would be expected to discharge (Muirden, pers comm). These numbers are compared with the frequency of modelled lake discharges in Table 6.4. Of the twenty modelled lakes, seven failed to fill to the dead storage level during the calibration period and may therefore be considered as terminal for streamflow during this period. These seven lakes are Dowerin, Walyormouring and Cowcowing in the Mortlock East River, and O’Connor, Varley, Gulsen and King in the Camm River. All other lakes discharged at least once during the calibration period. Some lakes, notably Ninan, Brown (Avon South) and Yenyenning are predicted to discharge significantly more frequently than the expert estimates, although in the case of Lake Brown, the discharges are mostly relatively small. Interestingly, the overpredicted discharge frequency for Lake Ninan is associated with a slight underprediction of flow at Mortlock North (Table 6.3).

Table 6.4. Comparison of expert estimates of the frequency of lake discharges with those predicted by the model, 1965–2003. The model predictions are divided into events greater than and less than

1 GL discharge

Lake Expert

estimates Model predictions

q > 1 GL q < 1 GL Ninan 10 13 5 Hinds 2 1 0 Dowerin 0 0 0 Walyormouring 0 0 0 Cowcowing 0 0 0 Brown (Avon South) 5 7 11 Yealearing 10 13 3 Yenyening 15 24 3 Kurrenkuttten 2 1 0 Kondinin 2 5 0 Jilakin 1 1 0 Grace North 1 1 1 O'Connor 2 0 0 Varley 0 0 0 Gulsen 3 0 0 King 3 0 0 Ace 5 0 3 Baandee 15 6 1 Brown (Yilgarn) 5 3 1 Campion 5 7 2

Overall, the model predictions for lake discharge and storage appear to match reasonably well with the expert data given the extreme difficulty in establishing reliable and robust a priori accumulation and discharge characteristics for the modelled lakes. In most cases these characteristics were estimated without access to any reliable bathymetry data and in some cases without any survey data.


7. BASELINE CONDITIONS

7.1. Current and future hydrological drivers In simulation mode, all LASCAM runs are for the period 1965 to 2100. For the years after 2003, no observed rainfall and potential evaporation data were available to drive the hydrological responses. To overcome this, the 28-year period of observed weather from 1976 to 2003 was repeated about three and a half times to extend the meteorological forcing data to 2100. This means that the observed hydrological data (rainfall and evaporation, LAI, etc) from 1976 to 2003 were repeated in the model to represent the future weather and hydrological conditions.

The rainfall patterns of southwest Western Australia have shown some evidence of change over the past 30 years. Beginning about 1975, a regime of reduced winter rainfall commenced over the region. Some climate projections suggest that this reduction will continue to intensify over coming decades. The baseline conditions used in this project do not account for these predictions and there has been no attempt to incorporate any projections of possible future climate change into the future weather data. The 28-year weather cycle is sourced entirely from the dryer period from the mid-1970s onwards. As such, the baseline and drainage scenario predictions in this project are based on an implicit assumption that the climate of the region will remain similar to that of the past 30 years. Whether this climate eventuates or not is not particularly important since we are mainly concerned with assessing differences between scenarios and not so much with establishing absolute predictions. However, we do evaluate the impacts of different climate assumptions in Section 8.3.

The LASCAM predictions also assume that land use, vegetation cover and lake characteristics will remain static in the catchment at their 2003 levels. Whilst it is likely that such changes might occur over the twenty-first century, there is insufficient data (let alone agreement) on possible directions and magnitudes of change and such modelling is beyond the scope of this project.

7.2. Current and future predictions of baseline scenario This section reports on a set of baseline hydrological predictions that are made on the assumption that there is no effective or hydrologically significant artificial drainage in the catchment (previously constructed artificial drainage is ignored in this scenario). The purpose of these predictions is to provide a benchmark against which we can assess the impacts of all other management and climatic scenarios.

Current and future predictions of subcatchment average groundwater trends, mean annual flows and annual peak flows from eight main catchments (Mortlock North, Mortlock East, Yilgarn, Lockhart, Yenyenning, Wakeman Creek at Narembeen and two locations along the Avon) will be discussed here. For all other subcatchments predictions of mean annual flow rates, peak annual flows and subcatchment average groundwater trends are given in Appendix 6.

7.2.1. Impacts on subcatchment-average groundwater trends

The groundwater depths predicted by LASCAM are averages over an entire subcatchment; they are not intended to be representative of any specific point in the subcatchment. Indeed, it follows that most of the area of a subcatchment—and especially the lower slopes and valley bottoms—would have watertables considerably shallower than the subcatchment average.

Modelling indicates that most of the wheatbelt subcatchments would exhibit continuing rises in average groundwater level, but at a slower rate than is currently occurring. Typical examples are shown in Figure 7.1 for six major subcatchments in the basin. There is considerable variability in groundwater response within, as well as between regions. The six subcatchments in Figure 7.1 have average rates of modelled watertable rise of between 20


mm y–1 and 26 mm y–1 in 2006. Over the course of the twenty-first century these rates of rise decline at all sites. By 2100, the rates of rise in the Lockhart and Yilgarn subcatchments reduce to 4 mm y–1 and 6 mm y–1, respectively, while the other four subcatchments are essentially at equilibrium. As well as being slower to reach equilibrium, there is also a general tendency for subcatchments in the east of the Basin to reach a higher equilibrium watertable than subcatchments in the west.

Figure 7.1. Predicted groundwater depth in the absence of artificial drainage for representative

subcatchments in each major region of the Avon Basin

7.2.2. Impacts on streamflow and water yield

Cumulative streamflow A consequence of the rising watertables is that streamflows increase over the simulation period. Figure 7.2 shows cumulative streamflows for the eight subcatchments, comprising several of the main regions of the catchment, some key points along the main river and one example first-order subcatchment (Wakeman Creek at Narembeen). The increasing streamflows over time are indicated by the increasing gradients of the curves in Figure 7.2, especially for Narembeen. Of the eight subcatchments shown in Figure 7.2, the largest flows per unit of upstream catchment area are in the North Mortlock and East Mortlock Rivers. In the latter case this is despite the area above Lake Dowerin (48 % of the catchment) not contributing to the flow. The lowest flows are for Yilgarn, a catchment that includes substantial areas of uncleared land which produce little flow and the presence of salt lakes with large dead storage capacity. This low discharge rate is also translated downstream to Yenyening.

Annual streamflow Eastern and central regions of the Avon Basin contribute very little flow to the basin outlet during first quarter of the twenty-first century (Figure 7.3). At the basin outlet the mean annual streamflow is increased by about 10 % from 298 GL y–1 to 328 GL y–1 during the first quarter of the twenty-first century (Table 7.1). The streamflow at Yenyening increases by about 45 % from 18 GL y–1 to 26 GL y–1. For the Avon River at Northam, mean annual streamflow increases by about 50 % from 128 GL y–1 to 193 GL y–1 over the course of the


twenty-first century and the proportion of days with flow increases from 46 % to 60 %. By contrast, for the Lockhart, mean annual streamflow increases from 6 GL y–1 to 30 GL y–1 over the same period and the proportion of days with flow increases from 29 % to 94 %. The rate of increase in streamflow varies across the Basin and is greatest for the eastern subcatchments due most probably to saturation excess processes (Table 7.1). Rising streamflows can also be seen in most of the examples in Figures 7.4 where the mean annual streamflows increase for each subsequent occurrence of the years in the 28-year weather cycle. The increase in streamflow occurs due to the development of shallower watertables in the eastern subcatchments. These shallower watertables are able to discharge more water to the perched aquifers, which, in turn, are able to discharge more water to the streams. In the eastern and central subcatchments, increasing streamflows are also influenced by the increasing frequency and volume of lake discharges.

Annual peak flows The increased moisture in the surface layers also increases surface saturation and leads to increased saturation-excess runoff, but the trend in annual peak flow rates is not as pronounced (Figure 7.5) and in some cases (e.g. Northam) is non-existent. This indicates that the increases in annual total streamflows are caused primarily by increases in baseflow, rather than in surface runoff.

Water Yield Figures 7.6 and 7.7 show the water yield from each subcatchment for two time periods, 1976–2003 and 2004–2031. Here, the term “water yield” refers to the annual average net amount of water that is generated within a subcatchment and discharged from it. It is calculated by subtracting tributary inflows from outflows. As such, this net water yield implicitly includes losses due to evaporation, seepage and storage increases in soil and lakes. For those subcatchments that include lakes, this calculation can sometimes result in a negative water yield, but such occurrences have been depicted as zero in Figures 7.6 and 7.7.

In Figure 7.6 it can be seen that the largest sources of streamflow are in the wetter subcatchments in the far west, with annual streamflow generation exceeding 40 mm in Wooroloo Brook and part of Brockman River. The parts of the catchment that exceed 25 mm are all below Dunbarton Bridge. Above Northam, the most productive areas are Spencers Brook and the western branch of the Dale River. There is very little streamflow generated in the more than 50 % of the catchment east of Kellerberrin. In the main, this distribution of water yields reflects the spatial patterns of rainfall.

It should be noted that most of the subcatchments with lakes are indicated in Figure 7.6 as having no water yield. It does not necessarily follow though, that these subcatchments do not discharge water to downstream subcatchments; merely that whatever water they do discharge is less than the volume of their tributary inflows. These subcatchments are therefore acting as net sinks of water, but may still be discharging. Of course, some of the lakes really don’t discharge at all—these are identified as terminal in Section 7.2.4.

During first quarter of the twenty-first century the largest sources of streamflow are still western parts of the basin where the annual rainfall is high (Figure 7.7). By the end of the twenty-first century, there are small increases in streamflow generation in almost all subcatchments. The greatest absolute increases (by more than 5 mm) are in the region between Northam and Beverley, and in the subcatchments closest to the basin outlet. In percentage terms, the largest increases (by more than 100 %) are in the relatively low-yielding subcatchments in the upper Mortlock North, Avon South, Wakeman Creek, the Lake Grace region and the area between Lakes Baandee and Brown. It is to note that the predictions are more reliable during first quarter than during last quarter of the twenty-first century. The reason for higher reliability during first quarter is its close proximity to the calibration period; it is not likely that the weather and other conditions will be significantly different from calibration period during the first quarter.


Figure 7.2. Predicted cumulative flows in the absence of artificial drainage for eight subcatchments

Figure 7.3. The relative flow contributions (percentage of total flow) from the eight key subcatchments

for the baseline scenario during the period 2004–2031

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Figure 7.6. Mean annual water yield of subcatchments in the Avon Basin, 1976–2003

Figure 7.7. Mean annual water yield of subcatchments in the Avon Basin, 2004–2031


Table 7.1. Mean annual streamflow and mean annual salt load for eight selected subcatchments over three time periods

7.2.3. Salt outflow and yield

Cumulative salt loads The increasing discharge of deeper groundwater into shallower aquifers also has a significant impact on stream salinities and salt loads, since the source of that discharge is saline. This increased salinity is exacerbated by the rising watertables dissolving some of the substantial amounts of salt stored in the unsaturated zone. In comparing cumulative salt loads (Figure 7.8) with cumulative streamflows (Figure 7.2), it is clear that the rate of increase in gradients is greater for the former, thus indicating that salt loads are increasing faster than streamflows. In the Mortlock East River, for example, flow-weighted salinities increase from 8 g L–1 to 21 g L–1 (g L–1 = mg L–1 divided by 1000) during the twenty-first century, while for the Lockhart, they increase from 8 g L–1 to 74 g L–1. These large increases in salinity in the eastern wheatbelt are associated with increases in the discharge of saline groundwater and from increased volumes and salinities of lake discharges.

Annual salt loads and flow-weighted salinity Relative contributions of annual salt load from various regions of the Avon basin are shown in Figure 7.9. At the basin outlet, the annual salt load almost doubles (2699 kty-1) by end of first quarter of the twenty-first century (2004–2031). Similar to the 1976–2003 period, most of the salt load during the first quarter of the twenty-first century is generated from the lower parts of the basin. Very little is produced by Lockhart and Yilgarn mainly because of little streamflow generation due to low rainfall.

Plots of predicted salt loads for the key sites are presented in Figure 7.10 and plots of predicted stream salinity are presented in Figure 7.11. For all sites the predicted flow-weighted salinity increases, thus indicating that salt discharge is increasing at a greater rate than water discharge throughout the catchment. This increase is particularly significant at sites just downstream of discharging lakes (e.g., Yenyening, Lockhart and Yilgarn), where flow-weighted salinities typically exceed 40 g L–1 by the end of this century.

Salt yield Areally-weighted mean annual salt yields may be calculated in the same way as the mean annual water yields of Figures 7.6 and 7.7. These are shown in Figures 7.12, 7.13 and 7.14 for the three time periods, 1976–2003, 2004–2031 and 2073–2100. For the period 1976–2003, the largest predicted salt yields (more than 60 t km–2 y–1) are from parts of the

Location (subcatchment number)

Water (GL y–1) Salt (kt y–1) 1976–2003

2004–2031

2073–2100

1976–2003

2004–2031

2073–2100

Great Northern Hwy (1) 298 328 399 1366 2619 7400Mortlock North (23) 18 22 33 128 290 926Mortlock East (31) 22 27 37 178 345 804Northam (39) 128 147 193 770 1603 5100Yenyening (58) 18 26 57 237 599 3336Lockhart (62) 6 10 30 49 194 2208Narembeen (64) 1 2 6 7 23 218Yilgarn (87) 8 9 17 50 102 520


Brockman River and some of the subcatchments near Northam. Areas producing more than 40 t km–2 y–1 include the subcatchments along the Avon River between Toodyay and Yealering, and in the lower Mortlock East. Interestingly, one of the lake subcatchments (Lake Campion) that produces no net water yield in Figure 7.6, has a small net yield of salt. This is because the water that it discharges, although less than its tributary inflows, is more saline as a consequence of lake evaporation.

By 2004–2031 areas further east, to those yielding salt during the calibration period, start to yield salt as well (Figure 7.13). By 2073–2100 (Figure 7.14), there are predicted to be substantial increases in salt yield across most of the catchment. The largest increases (by more than 200 t km–2 y–1) are between Northam and Beverley and make some of these subcatchments the largest contributors of salt (per unit area) in the catchment. There are now also significant salt sources in the cleared parts of the Lockhart and Yilgarn catchments, and downstream of the main lakes in the two Mortlock branches. Apart from Lake Brown (Yilgarn), all the lake subcatchments in the lower Yilgarn, lower Lockhart, upper Avon and Salt Rivers now discharge more salt than they receive from upstream subcatchments.

Figure 7.8. Predicted cumulative areally-weighted salt loads in the absence of artificial drainage for

eight subcatchments.


Figure 7.9. The relative salt load contributions (percent of total salt load) from the eight key subcatchments for the baseline scenario during the period 2004–2031

Mean Annual Flows for Eight Subcatchments

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12000

15000

18000

g. Narembeen

Figure 7.10. Predicted annual salt loads at major gauging sites for the baseline case


1965 1985 2005 2025 2045 2065 2085

01020304050607080

e. Yenyening

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)

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60b. Mortlock North

05

1015

20253035

c. Mortlock East

Figure 7.11. Predicted annual flow weighted salinity at major gauging sites for the baseline case.

Gaps in the salinity curves correspond to years when no flow is predicted


Figure 7.12. Mean annual salt yield of subcatchments in the Avon Basin, 1976–2003




7.2.4. Lake volumes and loads

Increased flows have also manifested as increased frequencies and volumes of lake discharge. Table 7.2 compares the occurrences of lake discharges in two 28-year periods, 1976–2003 and 2073–2100. Discharge frequencies increase for all but one of the non-terminal lakes, although only one of the previously identified terminal lakes (Lake Gulsen) discharges in the final 28 years of the simulation period. Several of the lakes that are near-terminal in the twentieth century fill quite frequently by the late twenty-first (e.g. Kurrenkutten and Jilakin) century.

Since the lakes discharge more frequently, it is no surprise that the mean annual discharge is also greatly increased. Interestingly, this increase is also quite substantial for the one lake (Ace) that doesn’t increase its discharge frequency. There are also significant increases in flow-weighted discharge salinity, which reflect the increased salinity of lake inflows.

As can be seen in Table 7.2, Lake Dowerin, the next modelled lake upstream of the Mortlock East flow gauge, remains terminal throughout the simulation. This means that the flow and salinity increases predicted at the gauge site are unaffected by increases in flow and salinity from the lake system, and are entirely due to increased flow generation downstream of Lake Dowerin. In contrast, other key gauging sites are affected to a far greater extent by increases in upstream lake discharges. For example, Lockhart River flows increase from 6 GL y–1 to 30 GL y–1, with flow-weighted salinities increasing from 8 g L–1 to 74 g L–1 under the influence of the frequent and highly saline discharges from Lake Kurrenkutten.


Table 7.2. Comparison of predicted lake discharge characteristics for two different 28-year time periods

Lake 1976–2003 2073–2100

Freq. Discharge Salinity Freq. Discharge Salinity (GL y–1) (g L–1) (GL y–1) (g L–1)

Ninan 13 2.41 13.8 25 7.58 57.3 Hinds 1 0.75 17.4 8 3.18 86.7 Dowerin 0 0.00 — 0 0.00 — Walyormouring 0 0.00 — 0 0.00 — Cowcowing 0 0.00 — 0 0.00 — Brown (Avon South) 14 0.65 16.4 25 2.54 39.9 Yealearing 12 1.24 19.0 23 4.00 49.7 Yenyening 20 18.43 12.8 26 57.00 58.5 Kurrenkuttten 1 0.21 23.6 10 11.64 140.0 Kondinin 3 1.41 15.9 16 14.67 99.3 Jilakin 1 0.33 14.1 9 8.22 117.2 Grace North 2 0.20 8.3 5 3.38 138.3 O'Connor 0 0.00 — 0 0.00 — Varley 0 0.00 — 0 0.00 — Gulsen 0 0.00 — 1 0.01 11.9 King 0 0.00 — 0 0.00 — Ace 2 0.01 1.2 2 0.05 23.5 Baandee 4 5.18 5.2 19 12.74 34.0 Brown (Yilgarn) 3 0.92 1.7 4 1.59 13.1 Campion 5 2.44 1.4 8 3.69 9.9

An example of the predicted lake storage levels is shown in Figure 7.15 for Lake Kondinin. Storage levels average 5 GL between 1976 and 2003 and rise to an average of 14 GL between 2073 and 2100. Also noticeable in Figure 7.15 is a rise in the annual minimum storage volume, which, by the end of this century is usually greater than the current average storage. Note that some one-day discharge events reduce the end-of-day volumes below the dead storage level and give the misleading appearance, in this figure, of having not reached the dead storage level.

Figure 7.15. Predicted daily lake storages for Lake Kondinin (green line). The thin red line is a

smoothed average storage volume and the broken black line is the dead storage level.


7.2.5. Summary of the baseline scenario

The foregoing sections have highlighted that the current hydrological state of the Avon Basin is not static. The basin is not at equilibrium. We are presently in a phase of steadily increasing watertables, streamflows, lake storages and salt loads. The model predictions show that most of the eastern catchments will reach near-equilibrium conditions by 2100 and western catchments will reach equilibrium before the turn of the century. The groundwater levels and groundwater salinity will continue to rise and streamflows will continue to increase over time. Increasing amounts of salt will discharge from the basin over time as the salt-affected area increases. Similarly the overflow frequency of salt lake systems will increase over time because of increased flows from larger saturated areas and subsequent increases in baseflow and saturation–excess runoff. These increases have been occurring in some parts of the basin since European settlement and are in response to the widespread replacement of native vegetation with shallow-rooted crops and pasture. Viney and Sivapalan (2001) conclude that streamflows in the Avon catchment increased five-fold between the onset of European settlement and the end of the twentieth century. The results presented in this chapter suggest that while these increases continue, the rates of increase are now slowing and that most parts of the catchment will—in the absence of further land management changes—reach equilibrium some time during the current century.

A consequence of this non-stationary catchment behaviour is that in assessing the impacts of any future management or climatic changes, it would be misleading to compare those impacts with the current state of the catchment. Instead, we should strive to make comparisons against the non-stationary benchmark scenario that has been established in this chapter.


8. WATER MANAGEMENT SCENARIOS This section assesses the impacts on streamflows, salt loads, stream salinities, lake discharge rates and salinities and groundwater levels and salinities of the following scenarios.

Leveed subcatchment drainage assuming high and low zone of effectiveness with

leveed arterial channels and storage of drainage discharge in salt lakes up to existing

outlet heights (Drainage SSe)


leveed arterial channels and storage of drainage discharge in salt lakes up to

elevated outlet heights (Drainage SSr). The lake outlet heights are raised by 0.3 m in

this scenario.


leveed arterial channels and storage of drainage discharge in salt lakes by making

them terminal (Drainage SSf)

Leveed subcatchment drainage assuming high zone of effectiveness and storage of

drainage discharge in the evaporation basins at the outlet of each subcatchment

(Drainage EB)

Open subcatchment drainage assuming high and low zone of effectiveness with open

arterial channels and storage of drainage discharge in salt lakes up to existing outlet

heights (Drainage SSe)


arterial channels and storage of drainage discharge in salt lakes up to elevated outlet

heights (Drainage SSr). The lake outlet heights are raised by 0.3 m in this scenario.


arterial channels without storage of drainage discharge in salt lakes. The lakes are

bypassed in the scenario (Drainage LB).

Revegetation of 50 % of the area of each subcatchment of the Avon Basin with woody

perennials

Revegetation of 100 % of the area of each subcatchment of the Avon Basin with

woody perennials

Changed future climate with 10 % more rainfall

Changed future climate with 10 %, 20% and 30% less rainfall

8.1. Artificial drainage

8.1.1. Background to artificial drainage scenarios

The impacts of implementing subcatchment-scale open and leveed artificial drainage were separately assessed. Open drains transport drainage water and natural flows generated through surface and subsurface runoff. The leveed drains transport drainage water from artificial drainage systems. The leveed drainage scenario is modelled as a dual-channel system, with drainage discharge and some subsurface runoff transported in the artificial


channel and the remaining flow in the natural creek channel. This dual channel system crosses subcatchment boundaries, but is discontinued when both channels deliver flow into a lake. The total discharge from a leveed subcatchment is thus the sum of water being carried in the artificial and natural channels.

For each type of drain, the drain dimensions and lengths for a given subcatchment are the same. The equations governing the discharge of groundwater to the drains are also the same (Chapter 5). As a consequence, the predicted rates of groundwater discharge from the two types of drain are identical. Although the groundwater dynamics are affected slightly by the different treatments of surface water, the impacts of these differences on subcatchment and catchment discharges is extremely small.

The resulting predictions of streamflow, salt load and stream salinity are practically identical for the two scenarios (Table 8.1), as are predictions of groundwater level and lake storages. For all eight key sites within the basin, the predicted mean annual streamflows and salt loads for the open drain scenario are slightly less than those for the total discharges of the leveed drain scenario, with maximum reductions of 1.7 % for water 1.1 % for salt. As a consequence, predicted salinities differ by a maximum of just 0.6 %. Peak flows and loads also tend to be slightly lower for the open drain scenario, although for some (e.g. flows at Wakeman Creek and Yilgarn River) the reductions are more substantial.

In the light of the predicted differences between the two scenarios being so slight, especially for mean discharges, we present results in this chapter for leveed drains only. It may be assumed that for most fluxes these results are appropriate for open drains too.

Table 8.1. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the open drain scenario expressed as a proportion of the corresponding quantities for

the leveed drain scenario. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness

8.1.2. Impacts of Drainage SSe

The artificial drainage modelling strategy called Drainage SSe involves farm and subcatchment open and leveed artificial drains with open and leveed arterial channels and storage of drainage discharge in the salt lakes up to their dead storage capacity. Similar to the benchmark case the simulations for the artificial drainage scenarios commence in 1965 and end in 2100 and include the same initial values and model parameters. Artificial drainage is assumed to commence in 1999 and the length of drainage in each drained subcatchment is assumed to increase annually throughout the next 102 years.

Comparisons of the predicted impacts of artificial drainage on watertables with those of the baseline case are shown in Figures 8.1–8.3 for selected subcatchments.

Location Mean annual 10-year-return peak flow load salinity flow load

Great Northern Highway 0.99 0.99 1.00 0.99 1.00Mortlock North 0.99 0.99 1.00 1.00 0.96Mortlock East 0.99 1.00 1.00 0.99 1.02Avon at Northam 0.99 0.99 1.00 1.01 0.93Qualandary Crossing 0.99 0.99 1.00 0.97 1.01Lockhart River 0.99 0.99 1.00 0.99 0.97Wakeman Creek 0.99 1.00 1.01 0.91 1.01Yilgarn River 0.98 0.99 1.01 0.86 0.94


Figure 8.1. Predicted groundwater depths for subcatchment 54 (Avon South River); drainage density

0.15 km–1

Figure 8.2. Predicted groundwater depths for subcatchment 61 (lower Lockhart River); drainage

density 0.47 km–1


Figure 8.3. Predicted groundwater depths for subcatchment 87 (lower Yilgarn River); drainage density

1.20 km–1. Note that the red and blue lines overlap

In all subcatchments with artificial drainage, watertables are lower and reach an earlier maximum than in the baseline case. The watertables then begin to decline towards a new, lower equilibrium value. The impact of artificial drainage on groundwater levels is greater for the high ZOE than for the low ZOE. Watertable response rates vary from subcatchment to subcatchment, with those subcatchments having greater drain densities showing the fastest responses. By 2100, the maximum difference between subcatchment-averaged groundwater depths in the drained and non-drained cases is 1.1 m (for subcatchment 87).

Groundwater salinities (Figures 8.4–8.6) are slightly reduced by artificial drainage over the course of the simulations. This is because the increased groundwater abstractions are more saline than the (relatively unchanged) recharge, thus leading to a net freshening of the groundwater. Once again, the impact is greatest for subcatchments with large drainage densities and is greater for high ZOE than low ZOE. By 2100, the biggest drainage-induced reduction in groundwater salinities is 1.6 g L–1 for subcatchment 87.


Figure 8.4. Predicted groundwater salinities for subcatchment 54 (Avon South River); drainage

density 0.15 km–1

Figure 8.5. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River); drainage

density 0.47 km–1


Figure 8.6. Predicted groundwater salinities for subcatchment 87 (lower Yilgarn River); drainage

density 1.20 km–1. Note that the red and blue lines overlap

The impact of artificial drainage on water yields is shown in Figures 8.7–8.8. In comparison with Figure 7.6 for the baseline case, it can be seen in Figure 8.7 that artificial drainage leads to substantial increases in subcatchment water yields in the heavily drained areas upstream of Beverley. The largest increase is by 7.8 mm y–1 from the baseline case in subcatchment 82, near Lake King. There are also, however, some substantial decreases in water yield in the main-channel subcatchments in the lower Avon Valley below Northam, especially in subcatchments 1, 2, 7 and 20. These decreases occur because the subcatchments themselves are not artificially drained and the increased year-round inflows from upstream partially reinfiltrate and then evaporate during otherwise dry periods of the year.

In comparison with the high ZOE case, the low ZOE case (Figure 8.8) produces slightly lower water yields for most subcatchments by the end of the century, although most of these decreases are imperceptible at the resolution of the colour scales in Figures 8.7 and 8.8. However, there are also slightly increased yields for some subcatchments (e.g. subcatchment 60).

The impacts of artificial drainage on cumulative, annual and peak streamflows are shown in Figures 8.9–8.32 for eight selected subcatchments. The model assumes there is no leveed arterial channel on the Avon River below Northam. Nor are there leveed drains discharging from the modelled lakes, so the plots for the Avon River at Great Northern Highway and the Yenyening Lakes discharge at Qualandary Crossing, unlike those for the other six sites, do not include the components of the streamflow carried in the leveed channels. For the other six sites, these fluxes are included (the gold and purple lines), but it should be remembered that they are merely constituents of, and not supplements to, the total drainage streamflow (the blue and red lines, respectively).


Figure 8.7. Predicted net water yields for open drains with high zone of effectiveness, 2073–2100

Figure 8.8. Predicted net water yields for open drains with low zone of effectiveness, 2073–2100


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

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40

50

60

Cum

ulat

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flow

(T

L)

No drainageDrainage: high ZOEDrainage: low ZOE

Figure 8.9. Predicted cumulative streamflow for the Avon River at Great Northern Highway.

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

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No drainageDrainage: high ZOEDrainage: low ZOELeveed channel only: high ZOELeveed channel only: low ZOE

Figure 8.10. Predicted cumulative streamflow for the North Mortlock River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

1000

2000

3000

4000

5000

6000

Cum

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flow

(G

L)


Figure 8.11. Predicted cumulative streamflow for the East Mortlock River


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

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30

Cum

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flow

(T

L)


Figure 8.12. Predicted cumulative streamflow for the Avon River at Northam

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

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(G

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Figure 8.13. Predicted cumulative streamflow for the Salt River at Qualandary Crossing

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

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Figure 8.14. Predicted cumulative streamflow for the Lockhart River


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

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(G

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Figure 8.15. Predicted cumulative streamflow for Wakeman Creek at Narembeen

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

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6000

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Figure 8.16. Predicted cumulative streamflow for Yilgarn River

Although there are no artificial drains below Northam, the Avon River at Great Northern Highway still experiences increased flows as a result of drainage works farther up the catchment (Figure 8.9). In the 102-year period of artificial drainage from 1999 to 2100, cumulative streamflow increases by 29 % for the high ZOE case and by 18 % for the low ZOE case. In Figure 8.17 it can be seen that this increase occurs in both high and low flow years and is greater towards the end of the simulation as drainage densities increase. However, artificial drainage has little impact on peak flow rates (Figure 8.25). This suggests that peak flow rates are mostly governed either by surface processes or by processes occurring in the undrained subcatchments near the catchment outlet.

In proportional terms, the differences between non-drained and drained cumulative discharges increase considerably upstream of Northam. For the high ZOE drainage case, there is a doubling of total discharge between 1999 and 2100 for the Salt and Lockhart Rivers (Figures 8.13 and 8.14), a trebling of discharge for Wakeman Creek (Figure 8.15) and almost a quadrupling for Yilgarn River (Figure 8.16).


The proportion of the flow carried by the leveed channels at any location in the catchment depends, in part, on the distance downstream of lakes. In Mortlock North River (Figure 8.10) and the Avon River at Northam (Figure 8.12), the leveed channel carries a small proportion of the total leveed drainage flow. This proportion increases for Mortlock East (Figure 8.11), while for the first-order Wakeman Creek (Figure 8.15), almost all the subcatchment discharge is carried by the leveed channel. The leveed channel proportions for Lockhart and Yilgarn Rivers (Figures 8.14 and 8.16) are less than this because they are downstream of lakes, but their leveed channels still carry more water than the natural channel in the undrained case.

For all eight featured subcatchments, the cumulative drainage for the low ZOE scenario is intermediate between those for the high ZOE and baseline scenarios. In the East Mortlock River, the effects of drainage are similar to those for the Avon catchment overall. Artificial drainage increases predicted cumulative flows by 44 % and 28 % for the two ZOEs (Figure 8.11).

Plots of annual streamflow (Figures 8.17–8.24) show that predicted discharges for the drained scenarios exceed those of the baseline case in both high and low flow years. There is a general tendency for discharges from the high ZOE case to consistently exceed those from the low ZOE case. However, for the two Mortlock branches (Figures 8.18 and 8.19) and the Yilgarn (Figure 8.24), while high ZOE discharges clearly exceed low ZOE discharges for most of the predicted time series, there is little difference by 2100.

A persistent feature of the annual streamflow predictions (Figures 8.18–8.20 and 8.22–8.24) is the steady increase over time of flow in the leveed channels. This is mainly a result of increasing drainage lengths across the catchment, but may also be attributed partly to continuing increases in watertables in many subcatchments, especially in the first half of the twenty-first century. The year-round nature of the artificial drainage discharges means that the four subcatchments upstream of Qualandary Crossing no longer experience any zero-flow years once drainage is installed.

Although artificial drainage has only little impact on annual peak flows at Great Northern highway, there is more significant impact at most of the sites further upstream (Figures 8.26–8.32). The general trend is for peak flows in the drainage scenarios to exceed those in the baseline scenario, although the overlapping red and blue lines indicate that there is usually little difference between the two zones of effectiveness. In Wakeman Creek, the only so called first-order subcatchment among the eight key sites, peak flows in the low-flowing years are substantially greater under the drainage scenarios, but there is little change in the high-flow years. At all the other sites, the largest events in the simulation period have drainage peaks that are at least twice as large as the baseline scenario and the impacts of drainage on peak flows are seen in all years.

There are two main reasons for the increased peak flows in the drainage scenarios. One is that the artificially drained scenarios are characterised by larger lake storages, and therefore larger and more frequent lake overflow events. This is clearly the dominant reason for increased peak flows at Qualandary Crossing where flow is governed by lake discharge processes, rather than hillslope or channel process. Figure 8.29 also shows that while there are years without any discharge at Qualandary Crossing persisting into the 2090s for the baseline case, there are no such years after 2008 for the high ZOE case or after 2036 for the low ZOE case. The Lockhart and Yilgarn Rivers (Figures 8.30 and 8.32) also have peak drained flows that greatly exceed the peaks of the baseline case. Again, this is due, at least in part, to their location downstream of lakes that fill more frequently and discharge in greater volumes in the artificially drained scenarios. In contrast, Wakeman Creek, which has no upstream lake, has similar peak flows for all scenarios. However, even under artificial drainage, none of the Wakeman Creek peaks in the twenty-first century are close to matching the event of early 1966 which resulted from record, but relatively localised, rainfall in subcatchment 64. Note that this rain event does not form part of the 28-year repeating cycle of weather that is used to generate streamflows beyond 2003.


The second reason for increased peak flows stems from the increased stream velocities in the engineered conveyance channels. These increased velocities result in less stream evaporation and less attenuation through temporal spreading of the peak flow. In fact, tests of the drainage scenarios show that this second reason has greatest impact on the increases in peak flows in most subcatchments.

1980 2000 2020 2040 2060 2080 21000

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Ann

ual t

otal

flow

(G

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Figure 8.17. Predicted annual streamflow for Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

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Ann

ual t

otal

flow

(G

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Figure 8.18. Predicted annual streamflow for North Mortlock River


1980 2000 2020 2040 2060 2080 21000

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120


Ann

ual t

otal

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(G

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Figure 8.19. Predicted annual streamflow for East Mortlock River

1980 2000 2020 2040 2060 2080 21000

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ual t

otal

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(G

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Figure 8.20. Predicted annual streamflow for the Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

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ual t

otal

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(G

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Figure 8.21. Predicted annual streamflow for Salt River at Qualandary Crossing


1980 2000 2020 2040 2060 2080 21000

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300


Ann

ual t

otal

flow

(G

L)

Figure 8.22. Predicted annual streamflow for Lockhart River

1980 2000 2020 2040 2060 2080 21000

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25

30

35

40


Ann

ual t

otal

flow

(G

L)

Figure 8.23. Predicted annual streamflow for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

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150

200

250


Ann

ual t

otal

flow

(G

L)

Figure 8.24. Predicted annual streamflow for Yilgarn River


1980 2000 2020 2040 2060 2080 21000

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20

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40

50

60

70


Ann

ual p

eak

flow

(G

L)

Figure 8.25. Predicted annual peak streamflow for Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6

7


Ann

ual p

eak

flow

(G

L)

Figure 8.26. Predicted annual peak streamflow for North Mortlock River

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6


Ann

ual p

eak

flow

(G

L)

Figure 8.27. Predicted annual peak streamflow for East Mortlock River


1980 2000 2020 2040 2060 2080 21000

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25

30

35

40


Ann

ual p

eak

flow

(G

L)

Figure 8.28. Predicted annual peak streamflow for Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25


Ann

ual p

eak

flow

(G

L)

Figure 8.29. Predicted annual peak streamflow for Salt River at Qualandary Crossing

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12

14

16

18

20


Ann

ual p

eak

flow

(G

L)

Figure 8.30. Predicted annual peak streamflow for Lockhart River


1980 2000 2020 2040 2060 2080 21000

0.2

0.4

0.6

0.8

1

1.2

1.4


Ann

ual p

eak

flow

(G

L)

Figure 8.31. Predicted annual peak streamflow for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12


Ann

ual p

eak

flow

(G

L)

Figure 8.32. Predicted annual peak streamflow for Yilgarn River

Predicted source areas for salt are mapped in Figures 8.33 and 8.34 for the two zones of effectiveness. These may be compared with Figure 7.14, which represents the same time period for the baseline case. The high ZOE scenario (Figure 8.33) yields in excess of 300

t km–2 y–1 of salt from much of the catchment and more than 200 t km–2 y–1 from practically the entire densely-drained, agricultural region above Northam. The largest sources are in the lower East Mortlock and in the Avon between Northam and Beverley, where yields exceed 500 t km–2 y–1. Predicted salt yield has increased in comparison with the baseline case in all subcatchments except for some mainstream subcatchments below Northam.

Salt yields for the low ZOE scenario (Figure 8.34) are generally less than those for the high ZOE scenario, but most substantially exceed the baseline case. The areas with highest yield (greater than 400 t km–2 y–1) are in the two Mortlock branches and in subcatchments 71 (just below Lake Grace), 82 (just below Lake Ace) and 86 (lower Yilgarn).

For both drainage scenarios, the salt yields from the agricultural areas upstream of Northam exceed those of many of the forested subcatchments in the far west. This contrasts with the corresponding map of water yield (Figure 8.7) and indicates that the most significant salt yield areas do not correspond with the most significant water yield areas under the artificially drained scenarios.


For the eight featured subcatchments, the proportional increases in cumulative salt load under drainage (Figures 8.35–8.42) are much greater than the corresponding increases in streamflow (Figures 8.9–8.16). For example, while the cumulative streamflow increases at Great Northern Highway are 29 % and 18 % for the two zones of effectiveness, the corresponding increases in cumulative salt load are 210 % and 120 %. For the Yilgarn River (Figure 8.42) the predicted salt loads for the two drained cases are fourteen times and ten times the baseline levels. For East Mortlock (Figure 8.37) and Wakeman Creek (Figure 8.41) almost all of the salt is carried in the leveed channels in the leveed drainage scenarios, and the proportions of salt carried in the leveed channels are greater than the corresponding proportions of water (Figures 8.11 and 8.15).

The graphs of predicted annual salt load (Figures 8.43–8.50) show a strong increasing trend in annual loads associated with the artificial drainage scenarios. These trends appear to be stronger, relative to the baseline cases, than the corresponding annual streamflow predictions (Figures 8.17–8.24). However, there is some evidence for Mortlock East (Figure 8.45), Wakeman Creek (Figure 8.49) and Yilgarn River (Figure 8.50) that this rate of increase begins to slow in the latter part of the century.

The strength of the increasing trends in salt load, relative to those of streamflow, is shown in the graphs of annual flow-weighted salinity (Figures 8.51–8.58). All suggest that streams will generally be more saline under artificial drainage than under the baseline, baseline case. An exception to this is for some low-flow years in the Salt River (Figure 8.55). In these cases, the low baseline flows are concentrated by evaporation to a greater degree than the significantly larger drainage flows.

Stream salinity is limited either by groundwater salinity or by lake salinity. The former is best shown in Figures 8.53 and 8.57, where annual salinities often reach, but never exceed, about 63 g L–1. On the other hand, flow-weighted stream salinities at Qualandary Crossing (Figure 8.55) are dictated entirely by the salinity of the Yenyening Lakes, and can exceed 250 g L–1 for the artificial drainage scenarios during some low-flow years early in the twenty-first century. Such levels are not predicted for later in the century as lake discharges increase and lake residence times decrease. At other locations, maximum flow-weighted salinities are influenced both by groundwater salinity and the effects of upstream lakes. For the Lockhart River, which receives occasional discharges from Lake Kurrenkutten, both governing factors are evident (Figure 8.56).

Figure 8.33. Predicted net annual salt yields for open drains with high zone of effectiveness, 2073–

2100


Figure 8.34. Predicted net annual salt yields for open drains with low zone of effectiveness, 2073–

2100

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

200

400

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1200

1400

1600

Cum

ulat

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salt

load

(M

t)


Figure 8.35. Predicted cumulative salt load for Avon River at Great Northern Highway


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

20

40

60

80

100

120

140

160

180

200

Cum

ulat

ive

salt

load

(M

t)


Figure 8.36. Predicted cumulative salt load for Mortlock North River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

20

40

60

80

100

120

140

160

180

Cum

ulat

ive

salt

load

(M

t)


Figure 8.37. Predicted cumulative salt load for Mortlock East River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

200

400

600

800

1000

1200

Cum

ulat

ive

salt

load

(M

t)


Figure 8.38. Predicted cumulative salt load for Avon River at Northam


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

100

200

300

400

500

600

700

800

900

Cum

ulat

ive

salt

load

(M

t)


Figure 8.39. Predicted cumulative salt load for Salt River at Qualandary Crossing

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

300

350

400

Cum

ulat

ive

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load

(M

t)


Figure 8.40. Predicted cumulative salt load for Lockhart River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

Cum

ulat

ive

salt

load

(M

t)


Figure 8.41. Predicted cumulative salt load for Wakeman Creek at Narembeen


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

300

350

400

Cum

ulat

ive

salt

load

(M

t)


Figure 8.42. Predicted cumulative salt load for Yilgarn River

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35

40

45


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.43. Predicted annual salt load for Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

1000

2000

3000

4000

5000

6000

7000

8000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.44. Predicted annual salt load for Mortlock North River


1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.45. Predicted annual salt load for Mortlock East River

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.46. Predicted annual salt load for Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.47. Predicted annual salt load for Salt River at Qualandary Crossing


1980 2000 2020 2040 2060 2080 21000

2000

4000

6000

8000

10000

12000

14000

16000

18000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.48. Predicted annual salt load for Lockhart River

1980 2000 2020 2040 2060 2080 21000

200

400

600

800

1000

1200

1400


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.49. Predicted annual salt load for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

2000

4000

6000

8000

10000

12000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.50. Predicted annual salt load for Yilgarn River


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.51. Predicted annual flow-weighted salinity for Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70

80


Ann

ual s

alin

ity (

g/L)

Figure 8.52. Predicted annual flow-weighted salinity for Mortlock North River

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70


Ann

ual s

alin

ity (

g/L)

Figure 8.53. Predicted annual flow-weighted salinity for Mortlock East River


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70

80


Ann

ual s

alin

ity (

g/L)

Figure 8.54. Predicted annual flow-weighted salinity for Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250

300


Ann

ual s

alin

ity (

g/L)

Figure 8.55. Predicted annual flow-weighted salinity for Salt River at Qualandary Crossing

1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250


Ann

ual s

alin

ity (

g/L)

Figure 8.56. Predicted annual flow-weighted salinity for Lockhart River


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70


Ann

ual s

alin

ity (

g/L)

Figure 8.57. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120

140


Ann

ual s

alin

ity (

g/L)

Figure 8.58. Predicted annual flow-weighted salinity for Yilgarn River

The modelled lakes tend to discharge more frequently, at greater volumes and with greater salinity under artificial drainage than in the baseline scenario (Tables 7.2 and 8.2). In the baseline scenario, no lakes overflow every year between 2073 and 2100. However, for the low ZOE case, five lakes are predicted to overflow every year, and for the high ZOE case, this number increases to nine. Two of the six lakes that are predicted to be terminal without drainage discharge once in the high ZOE scenario.

All overflowing lakes discharge more water in the low ZOE scenario than the baseline scenario. This increase is relatively small for Lakes Brown (Avon South) and Yealearing, but ranges up to 400 % for Lake Brown (Yilgarn). Discharges increase further for the high ZOE scenario.

The high salinity of the artificial drainage discharges lead to substantial increases in the flow-weighted salinity of lake discharges for many lakes, particularly those in the Yilgarn and Camm Rivers. Elsewhere, salinity increases are relatively small. For some lakes, the discharge is more saline in the low ZOE case than in the high ZOE case. This is due to the increasing frequency and amounts of discharge causing a reduction in lake residence time, thereby reducing the potential for evaporative concentration of lake waters. However, for all discharging lakes (except for Lake Grace North, where the difference is less than 0.2 %) the total mass of salt discharged is greater in the high ZOE scenario.


Table 8.2. Comparison of predicted lake discharge characteristics for two different drainage scenarios for the period 2073–2100

Lake High ZOE Low ZOE


Ninan 28 20.70 85.3 28 17.80 83.2 Hinds 28 14.68 97.1 25 11.66 98.1 Dowerin 1 0.07 237.8 0 0.00 — Walyormouring 1 0.18 210.7 0 0.00 — Cowcowing 0 0.00 — 0 0.00 — Brown (Avon South) 27 3.09 48.3 26 2.81 42.9 Yealearing 23 4.60 60.5 23 4.34 54.8 Yenyening 28 162.02 92.2 28 136.43 90.0 Kurrenkuttten 28 38.09 124.5 21 25.70 145.0 Kondinin 28 43.56 100.3 26 30.11 111.1 Jilakin 28 25.28 108.5 26 18.07 123.3 Grace North 14 6.13 176.7 14 6.00 180.8 O'Connor 0 0.00 — 0 0.00 — Varley 0 0.00 — 0 0.00 — Gulsen 1 0.13 68.2 1 0.11 55.8 King 0 0.00 — 0 0.00 — Ace 17 0.19 113.5 4 0.09 84.7 Baandee 28 73.30 87.7 28 64.95 82.0 Brown (Yilgarn) 28 10.29 246.0 28 8.34 232.0 Campion 28 19.06 133.7 28 15.35 127.8

8.1.3. Impacts of Drainage SSr and Drainage SSf

These two scenarios involve subcatchment scale artificial drainage with arterial channels and storage of drainage water in the salt lakes. To reduce the downstream flows of water and salt the storage capacity of major salt lakes is increased in two separate modelling strategies.

In the first artificial drainage modelling strategy called Drainage SSf, all 20 modelled lakes are made terminal by preventing all discharge of water downstream. The physical means of preventing discharges is not specified. Nor are we concerned with the feasibility of achieving this outcome: merely in assessing the impacts of its notional implementation.

In the second modelling strategy called Drainage SSr, the discharge height for all lakes is raised by 30 cm. This standard height is chosen because for some lakes, it is the upper limit that can possibly be used. The raised discharge heights mean that the lakes will have an increased dead storage capacity, but it remains possible that they may exceed this capacity and therefore discharge some water downstream. For most lakes then, this second strategy will not be as severe as the first. It will also be more feasible from an engineering viewpoint.

In both cases, simulations and analysis presented here are limited to open drains with high zones of effectiveness. However, the results and interpretations are virtually identical for leveed drains.

Drainage SSf

In Table 8.3 it can be seen that the scenario with permanent retention of lake storage has reduced flows by about 30–50 % at most of the key subcatchments. However, three predictions lie far outside this range and require detailed inspection.

The streamflow at Qualandary Crossing can of course only be zero, since it is immediately downstream of the Yenyening Lakes. This location is included in Table 8.3 only for consistency with other similar tables in this section.


Mortlock East produces the same streamflows for the regional retention scenario as for the corresponding scenario without retention. This is because Lake Dowerin discharges just once (Table 8.2) and by a relatively small amount (around 0.1 % of the mean annual discharge) in the high ZOE drainage scenario. The retention of this small volume makes no practical difference to the Mortlock East discharge characteristics.

Streamflows in the Yilgarn River under the full retention scenario are a small fraction of those without retention. The reason for this is that the nearest upstream lake (Baandee) frequently discharges under the drainage scenario (Table 8.2) and provides almost all of the streamflow at the Yilgarn River site (subcatchment 87). Retention of this large fraction then leaves little water flowing in the Yilgarn below Lake Baandee.

At all sites except Mortlock East, the retention of what are normally fairly saline lake discharges leads to reductions in salt load that exceed those of streamflow. As a consequence, stream salinities under the full retention scenario are only 30–60 % of those without retention.

The impact of lake retention on peak flows is less marked at most sites. For the two Avon River sites, Great Northern Highway and Northam, peak flows in the retention scenario remain as high as 85 % of those in the drainage scenario. This implies that the bulk of the water involved in the drainage peak flows at these sites is sourced from downstream of the Yenyening Lakes. In the Lockhart River, the ratio of peak flows is less than the ratio of mean annual flows. The implication here is that in the drainage scenario, much of the peak flow is derived from overtopping of Lake Kurrenkutten.

Table 8.3. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the regional retention scenario (Drainage SSf with open drains) with no lake discharge, expressed as a proportion of the corresponding quantities for open drains without retention (Drainage

SSe). Predictions are for the 2073–2100 period and drainage is assumed to have a high ZOE

In contr

ast, the peak load ratios are substantially less than the mean annual load ratios. This is not unexpected, given that it is the highly saline lake discharges that have been removed in the retention scenario. Even in cases where the lakes contribute little water to a peak event, their contributions of salt could be quite significant.

Drainage SSr Discharge characteristics for the second regional retention scenario—elevated lake discharge heights—are presented in Tables 8.4 and 8.5. In comparison with the high ZOE case in Table 8.2, the raised discharge heights have little effect in reducing the frequencies and volumes of lake overflows (Table 8.4). Discharge is less frequent at only seven of the lakes, including the four smallest (Dowerin, Brown (South Avon), Yealearing and Ace). Discharge volumes decrease by less than 10 % for most lakes. However, the slightly increased detention times lead to increases in discharge salinity for all discharging lakes. The combination of slightly decreased discharge and slightly increased salinity means that for all 12 lakes with discharges of more than 0.2 GLy–1 on average, the discharge of salt load changes by less than 0.7 % between the partially retained and unretained scenarios.

This scenario has considerably less impact than the full retention scenario on mean annual flows at the eight main sites and practically no influence on mean annual salt loads (Table 8.5). The reason for the lack of impact on loads is that despite the slightly increased lake

Location Mean Annual 10-Year-Return PeakFlow Salt load Salinity Flow Salt load

Great Northern Highway 0.67 0.28 0.42 0.91 0.16 Mortlock North 0.63 0.40 0.63 0.82 0.16 Mortlock East 1.00 1.00 1.00 1.00 0.92 Avon at Northam 0.47 0.14 0.30 0.85 0.13 Qualandary Crossing 0.00 0.00 — 0.00 0.00 Lockhart River 0.53 0.32 0.61 0.25 0.05 Yilgarn River 0.08 0.04 0.43 0.16 0.02


losses of water to evaporation, the salt remains in storage in the lake and when the lake eventually overfills this slightly more concentrated salt discharges a similar mass, but dissolved in less water, as in the drainage scenario. Inevitably, this means that streamflows at the key sites are slightly more saline than in the drainage scenario.

Only in the Salt and Yilgarn Rivers are there any significant impacts on peak flows under the partial retention scenario. At Qualandary Crossing, the decrease in peak flows is readily interpreted as a direct consequence of the increased height of the Yenyening Lakes discharge level. The mechanisms for the increase in peak flows in the Yilgarn River are unclear, but it appears that under the partial retention scenario, the modelled lakes in the Yilgarn system tend to have shorter discharge durations, but with larger peak magnitudes than in the unretained scenario. The combination of equivalent or increased peak flows and increased salinity of lake discharges means that there are significant increases in peak salt load at all sites except Mortlock East.

Table 8.4. Predicted lake discharge characteristics for the regional retention scenario with raised discharge heights for the period 2073–2100

Lake Freq. Discharge Salinity

(GL y–1) (g L–1)

Ninan 28 18.821 93.9 Hinds 28 13.811 103.6 Dowerin 0 0.000 — Walyormouring 1 0.076 209.7 Cowcowing 0 0.000 — Brown (Avon South) 26 2.857 52.1 Yealearing 21 3.280 82.4 Yenyening 28 153.930 96.6 Kurrenkuttten 28 34.481 135.9 Kondinin 28 39.835 108.2 Jilakin 28 23.351 114.6 Grace North 11 4.884 207.7 O'Connor 0 0.000 — Varley 0 0.000 — Gulsen 0 0.000 — King 0 0.000 — Ace 7 0.124 133.6 Baandee 28 70.707 90.6 Brown (Yilgarn) 24 8.633 291.0 Campion 28 17.653 144.0

Table 8.5. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the regional retention scenario (open drains) with lake discharge heights raised by 30

cm, expressed as a proportion of the corresponding quantities for open drains without retention. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of

effectiveness

Location Mean Annual 10-Year-Return PeakFlow Salt load Salinity Flow Salt load

Great Northern Highway 0.98 1.00 1.02 1.00 1.33Mortlock North 0.97 1.00 1.04 1.00 1.22Mortlock East 1.00 1.00 1.00 1.00 0.92Avon at Northam 0.97 1.00 1.03 0.98 1.39Qualandary Crossing 0.95 1.00 1.05 0.72 1.03Lockhart River 0.95 0.99 1.04 1.06 1.16Yilgarn River 0.97 1.00 1.03 1.29 1.12


In summary, the Drainage SSr scenario appears to have limited potential in mitigating streamflows and lake discharges, and causes increases in flow-weighted stream salinity and lake salinity. The height of the assumed rise (30 cm) is quite small for some lakes and we might expect the impact of this scenario to have been greater for greater rises in discharge height. However, it is difficult to conceive of any scenario of this nature—short of preventing all lake discharges—causing significant reductions in salt loads and salinity.

8.1.4. Impacts of Drainage EB

The Drainage EB option involves retaining all leveed channel flows in the subcatchments in which they are generated by constructing an evaporation basin at the outlet of each drained subcatchment. Once again, we are not overly concerned with the physical mechanism by which this is achieved. This option applies only to the leveed channel scenario, and not the open channel scenario. The retention does not impact on flows in the natural creek channels, which continue to discharge to downstream subcatchments as before. Simulations and analysis are limited to leveed drains with high zones of effectiveness.

The effects of subcatchment retention on lake discharges are summarised in Table 8.6. In comparison with the high ZOE case in Table 8.2, it can be seen that all lakes experience substantial reductions in predicted discharge frequency. Three lakes (Dowerin, Walyormouring and Gulsen) cease to discharge altogether, while another two (Hinds and Kurrenkutten) are reduced from discharging every year to discharging just once in 28 years. Annual discharge volumes also decrease substantially, with no lake discharging more than 30 % of the discharge in Table 8.2. The diversion of the saline drainage water away from the lakes also results in large decreases in the salinity of discharges for most lakes. Exceptions to this are Brown (South) and Yealearing, where the salinity reductions are minimal, and Kurrenkutten, where discharge salinity increases. This increase is a result of the greatly increased residence time for water in Lake Kurrenkutten and the associated increases in evaporation-driven concentration. Of particular note are the extremely small discharge salinities for the three Yilgarn lakes: Baandee, Brown and Campion.

This scenario also has substantial impacts on streamflows and salt loads (Table 8.7). Mean annual flow is reduced by more than half at all sites, with the greatest reductions occurring in the four eastern subcatchments. The impacts on salt load are even greater; loads are reduced to between 2 % and 10 % of their comparative rates for the unretained case. With the magnitude of the load reductions exceeding the magnitude of the flow reductions, stream salinities are reduced at all sites by at least 60 %. The largest reduction in salinity is 84 % in the Yilgarn River, just downstream of the significantly freshened Lake Baandee.

The subcatchment retention scenario leads to reductions in peak flows of 35–80 % and reductions in peak salt loads of at least 50 %. In the Lockhart River there is a 98 % decrease in the 10-year return peak load, thanks to the elimination (at the level of the 10-year return peak) of saline discharge from Lake Kurrenkutten.

In comparison with the two regional retention options discussed in Section 8.1.3, it is clear that this subcatchment retention scenario has by far the biggest desirable impacts in reducing mean annual streamflows and salt loads, flow-weighted stream salinity, peak flows and loads (except for the Yilgarn and Lockhart Rivers), and (with the obvious exception of the full regional retention scenario), the frequency, volume and salinity of lake discharges.


Table 8.6. Predicted lake discharge characteristics for the subcatchment retention scenario for the period 2073–2100

Lake Freq. Discharge Salinity

(GL y–1) (g L–1)

Ninan 14 1.941 37.1 Hinds 1 0.651 73.9 Dowerin 0 0.000 — Walyormouring 0 0.000 — Cowcowing 0 0.000 — Brown (Avon South) 19 0.922 46.0 Yealearing 16 1.291 58.2 Yenyening 19 18.022 30.3 Kurrenkuttten 1 1.138 139.2 Kondinin 4 2.742 81.1 Jilakin 3 1.054 93.7 Grace North 1 0.088 41.3 O'Connor 0 0.000 — Varley 0 0.000 — Gulsen 0 0.000 — King 0 0.000 — Ace 2 0.018 4.9 Baandee 10 6.768 14.4 Brown (Yilgarn) 4 1.135 4.3 Campion 6 2.944 3.3

Table 8.7. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for the subcatchment retention scenario, expressed as a proportion of the corresponding quantities for the total discharge from the leveed drainage scenario without retention. Predictions are

for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness

8.1.5. Impacts of Drainage LB (lake bypass)

This section assesses the impacts of subcatchment artificial drains and arterial channels on catchment flows and loads without the storage of drainage discharge in salt lakes (i.e., the complete removal or bypassing of all lakes). All flows and loads generated within the catchment are routed through the stream network without impediment. The analysis presented here is for leveed drains with high ZOE only. When lakes are included in the model along with a leveed drainage system, the leveed arterial channels stop at the lake inflow. In contrast, when the lakes are removed, the arterial leveed channel is fully connected down to the junction of the Mortlock and Avon Rivers. Thus, as far as channel flows and loads are concerned, the scenario without lakes is equivalent to having a leveed drainage system in which the arterial channels bypass the lakes. This provides us with a

Location Mean annual 10-year-return peak flow salt load salinity flow salt load

Great Northern Highway 0.44 0.088 0.20 0.63 0.157 Mortlock North 0.27 0.072 0.26 0.56 0.104 Mortlock East 0.33 0.102 0.31 0.65 0.500 Avon at Northam 0.33 0.074 0.23 0.59 0.113 Qualandary Crossing 0.11 0.037 0.33 0.17 0.068 Lockhart River 0.11 0.040 0.38 0.36 0.015 Wakeman Creek 0.12 0.044 0.36 0.64 0.358 Yilgarn River 0.12 0.019 0.16 0.36 0.113


means of assessing the maximum possible leveed arterial channel flows at all points in the catchment.

Clearly, flows and loads from subcatchments that are upstream of the modelled lakes will be unaffected by the presence or absence of the lakes. We therefore limit the analysis of the impact of lakes to seven of the eight key sites (excluding Wakeman Creek) that are studied in detail in Section 8.1.2.

Table 8.8 expresses the flows and loads in these subcatchments as a proportion of the corresponding fluxes for the open drain scenario with lakes. In the absence of lakes, mean annual flow is greater at all locations. This is because lake evaporation represents a significant mechanism for loss of water from the catchment. The increases range from 33 % at Mortlock North to 102 % at the Lockhart River. The large increase for Lockhart River is largely due to the removal of the three large lakes (Kurrenkutten, Kondinin and Jilakin) immediately upstream, although other lakes further upstream will also have contributed.

Salt loads also increase in the absence of the lakes, but with the exception of Mortlock East, these increases are not as large as the flow increases. That they are increasing at all suggests that under high ZOE drainage, some lakes are still net accumulators of salt at the end of the current century. This, of course, will be true of the lakes that are terminal or near-terminal. The large increase in annual salt load for Mortlock East results from the fact that even under high ZOE drainage, there is very little discharge from Lake Dowerin—the next lake upstream—when the lakes are included (Table 8.2). In contrast, the lakes above the Mortlock North location discharge every year (Table 8.2), so there is negligible increase in annual salt load when the lakes are removed.

The combination of large increases in mean annual flow and smaller increases in mean annual salt load, means that the flow-weighted salinity decreases at most sites. For Qualandary Crossing and Lockhart River, the salinity in the absence of lakes is little more than half the salinity with lakes. The increase in salinity for Mortlock East is a consequence of the flows generated upstream of Lake Dowerin being more saline than the flows generated downstream, which constitute almost all the flow when lakes are included.

Increases in the ten-year return period flows suggest that there is an increased propensity for flooding when the lakes are removed. At most sites, the increases are less substantial than the increases in mean annual flow. However, for Mortlock East and, to a lesser extent, the Avon at Great Northern Highway, the increases in peak flow are greater than those in annual flow. For Mortlock East, this increase is presumably due to the removal of the terminal or near-terminal lakes in the upper part of the catchment. However, it is not clear why this increase should be so much greater than the 92 % increase in effective catchment area. Nonetheless, it is likely that the downstream propagation of this peak event is also the chief cause of the increased peak flow at Great Northern Highway.

Subcatchment 103, in the upper Yilgarn has no artificial drainage, but its tributary, subcatchment 104, does. This means that in the artificial drainage scenarios, subcatchment 103, which is normally dry and discharges quite infrequently, receives a continuous supply of water and salt from upstream. The volume of water, however, is insufficient to sustain flow through the entire length of subcatchment 103 and is usually entirely reinfiltrated. This reinfiltrated water quickly evaporates, but the salt that it once carried continues to accumulate in the subcatchment. When a discharge event finally occurs in subcatchment 103, this accumulated salt is dissolved in the flow and results in extremely high salinities. Thus the peak loads predicted in subcatchment 103 are extremely high and these peak events propagate downstream (Appendix 6). In artificial drainage scenarios with lakes intact, these peak loads dissipate in Lakes Campion, Brown and Baandee before reaching the Gairdner’s Crossing gauging station in the lower Yilgarn River. However, when the lakes are removed, these salt peaks propagate through the entire river network to the Great Northern Highway. This is why there is a substantial increase in peak salt loads in the absence of lakes at all four key locations downstream of subcatchment 103. It is important to note that this phenomenon has little impact on annual or cumulative salt loads. It does not represent a new source of salt, merely a redistribution in time of the daily salt load. There are also parts


of the lower Avon River where artificially drained subcatchments also discharge into non-drained subcatchments. However, this reinfiltration phenomenon has little direct impact there (other than the already-discussed propagation of peaks from subcatchment 103 in the scenario without lakes) because the flows are greater and the receiving subcatchments smaller than in the upper Yilgarn. As a consequence, complete reinfiltration does not occur in the lower Avon.

The peak salt loads in Table 8.8 also increase for Mortlock East, and again this is due to the release in this scenario of salt that is trapped by and above Lake Dowerin in any scenario that includes lakes. However, there are substantial decreases in peak salt loads at the other two sites (Mortlock North and Lockhart) when their upstream lakes are removed. Presumably this is because when lakes are included the peak events are typically augmented by saline discharge from overtopping lakes. One might speculate that this reduction in peak loads would also apply at the other locations (except Mortlock East) if a continuous flow could be maintained in subcatchment 103.

Table 8.8. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for leveed drains (total discharges) without lakes, expressed as a proportion of the

corresponding quantities for leveed drains with lakes. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness

Table 8.9 shows the proportions of the total leveed discharges (i.e., the sum of discharges in the leveed and natural channels) that occur in the leveed channel alone. At least 65 % of the total streamflow and 91 % of the total salt load are carried in the leveed channels. The proportions are greatest in the east of the catchment, where natural streamflows are relatively small. The peak flows in the leveed channels are, however, a much smaller proportion (typically about one-third) of the total peak flows. The impact of the extreme peak salt discharge from subcatchment 103 is evident in the large peak salt load proportions for Yilgarn, Qualandary Crossing and Northam.

Table 8.9. Streamflows, salt loads and salinities for mean annual discharges and 10-year-return peak discharges for leveed channels without lakes, expressed as a proportion of the corresponding

quantities for total leveed drains without lakes. Predictions are for the 2073–2100 period and drainage is assumed to have a high zone of effectiveness

Location Mean Annual 10-Year-Return Peak Flow Salt load Salinity Flow Salt load

Great Northern Highway 1.37 1.14 0.83 1.48 1.85Mortlock North 1.33 1.02 0.76 0.95 0.28Mortlock East 1.76 1.85 1.05 3.04 3.13Avon at Northam 1.48 1.06 0.73 1.24 2.41Qualandary Crossing 1.90 1.09 0.58 1.04 1.98Lockhart River 2.02 1.15 0.57 1.14 0.18Yilgarn River 1.37 1.04 0.76 1.31 3.40

Location Mean annual 10-year-return peak

flow salt load salinity flow salt load Mortlock North 0.69 0.91 1.32 0.36 0.55Mortlock East 0.69 0.92 1.33 0.33 0.49Avon at Northam 0.65 0.91 1.41 0.31 0.99Qualandary Crossing 0.76 0.94 1.23 0.31 1.00Lockhart River 0.77 0.93 1.21 0.34 0.81Wakeman Creek 0.87 0.96 1.09 0.37 0.65Yilgarn River 0.82 0.98 1.19 0.28 1.00


8.2. Woody perennials

8.2.1. Background to revegetation scenarios

One strategy that has been mooted as having potential to remediate salinity in the wheatbelt is revegetation with woody perennials. The thinking behind this strategy is that before the conversion of native woodlands to pasture and cropping lands, the streamflows and stream salinities in the Avon Basin were smaller than they are now and the watertables and groundwater salinities were lower.

Two revegetation scenarios are modelled. In one, the entire basin is revegetated to 100% deep-rooted perennials. In the second scenario, the area of deep-rooted perennials is increased to a minimum of 50 % of every subcatchment. In modelling these scenarios, we recognise that revegetation on this scale is likely to be impractical throughout most of the Avon Basin.

Revegetation is assumed to occur at the beginning of 2001, and is implemented in the model as fully-mature forests from the outset. That is, there is no gradual transition of the vegetation cover from shallow-rooted to deep-rooted and the model does not “grow” the trees over a period of time. The model also takes no account of any biological growth limitations that may be imposed by the salinity of the landscape.

8.2.2. Impacts of revegetation on groundwater

Revegetation has a major impact on groundwater depths in all subcatchments that are currently comprised primarily of pasture and cropping land. Three examples are shown in Figures 8.58–8.60. The assumption that the revegetated forests are fully mature in 2001 leads to immediate reductions in groundwater levels in all affected subcatchments. For the 50 % revegetation scenario, the rates of fall of the watertables are slightly less than the pre-existing rates of rise prior to 2001, while the rates of fall for the 100 % revegetation scenario are significantly greater. By 2100, the 100 % scenario has predicted watertables that are about 4.4 m below those of the untreated scenario for all three subcatchments in Figures 8.58–8.60.

Predicted groundwater salinities for the same three subcatchments are shown in Figures 8.61–8.63. All show significant increases for the revegetated scenarios, increases that are substantially greater than those for any other scenario assessed in this report. These increases are caused by the substantial reductions in the volume of water held in the groundwater stores, reductions that are primarily attributable to the increased transpiration of the revegetated landscapes.

1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−10.5

−10

−9.5

−9

−8.5

−8

−7.5

−7

−6.5

−6

−5.5

Gro

undw

ater

dep

th (

m)

No Change50 %100 %


Figure 8.58. Predicted groundwater depths for subcatchment 54 (Avon South River) for different revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−11

−10.5

−10

−9.5

−9

−8.5

−8

−7.5

−7

−6.5

−6

Gro

undw

ater

dep

th (

m)

No Change50 %100 %

Figure 8.59. Predicted groundwater depths for subcatchment 61 (lower Lockhart River) for different

revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−12

−11.5

−11

−10.5

−10

−9.5

−9

−8.5

−8

−7.5

−7

Gro

undw

ater

dep

th (

m)

No Change50 %100 %

Figure 8.60. Predicted groundwater depths for subcatchment 87 (lower Yilgarn River) for different



1965 1980 1995 2010 2025 2040 2055 2070 2085 210058

60

62

64

66

68

70

72

74

76

Gro

undw

ater

sal

inity

(g/

L)

No Change50 %100 %

Figure 8.61. Predicted groundwater salinities for subcatchment 54 (Avon South River) for different revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 210060

62

64

66

68

70

72

74

76

78

80

Gro

undw

ater

sal

inity

(g/

L)

No Change50 %100 %

Figure 8.62. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River) for different



1965 1980 1995 2010 2025 2040 2055 2070 2085 210060

62

64

66

68

70

72

74

76

78

80

Gro

undw

ater

sal

inity

(g/

L)

No Change50 %100 %

Figure 8.63. Predicted groundwater salinities for subcatchment 87 (lower Yilgarn River) for different


8.2.3. Impacts of revegetation on streamflow

Revegetation of the catchment with woody perennials causes reductions in predicted cumulative streamflow at Great Northern Highway of 27 % and 38 % for the partial and full revegetation scenarios, respectively, over the period 2001–2100 (Figure 8.64). The rates of these reductions, however, are not constant: over the first 28 years of the century the corresponding reductions are 13 % and 27 %, while over the last 28 years they are 36 % and 45 %. Further upstream the impact of revegetation on streamflows is even greater (Figures 8.65–8.71). By the end of the century, streamflows in the Salt River, Lockhart River and Wakeman Creek for the full revegetation option are less than 10 % of those without revegetation. The impacts are not as severe for the Yilgarn River (Figure 8.71) because much of its catchment area is already woodland. A similar reason may be advanced to explain the relatively smaller impact of revegetation at Great Northern Highway: that is, that a significant proportion of the flow there is generated from forested parts of the far west of the catchment.

Similar spatial and temporal patterns are evident in the graphs of annual streamflow for the revegetation scenarios (Figures 8.72–8.79). None of the sites show any evidence that the proportional reductions vary with the magnitude of the annual flow, but there is a gradual widening over time of the gap between annual discharges of the three scenarios. This widening gap is a consequence of the watertables becoming lower over time, thus reducing the influence of the regional groundwater on streamflow generation through groundwater discharge to the surface aquifers and possibly through reduced surface saturation areas. The predictions of annual flow indicate that for all sites other than the two on the Avon River, revegetation leads to a greater preponderance of zero-flow years than are present in the baseline scenario.

Annual peak flows for the revegetation scenarios are presented in Figures 8.80–8.87. For the Avon (Figures 8.80 and 8.83) and perhaps to a lesser extent, the Mortlock (Figures 8.81–8.82) Rivers, revegetation appears to have little influence on peak flows, except for a slight reduction in the extreme peaks. However, further upstream (Figures 8.84–8.87), revegetation leads to significant reductions in all peak flows after the first 20 years.


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

10

15

20

25

30

35

40

45

50

Cum

ulat

ive

flow

(T

L)

No change50 % revegetation100 % revegetation

Figure 8.64. Predicted cumulative streamflow for the Avon River at Great Northern Highway for

various revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

2000

2500

3000

3500

Cum

ulat

ive

flow

(G

L)


Figure 8.65. Predicted cumulative streamflow for the North Mortlock River for various revegetation

scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

2000

2500

3000

3500

4000

Cum

ulat

ive

flow

(G

L)



Figure 8.66. Predicted cumulative streamflow for the East Mortlock River for various revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

10

15

20

25

Cum

ulat

ive

flow

(T

L)


Figure 8.67. Predicted cumulative streamflow for the Avon River at Northam for various revegetation

scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Cum

ulat

ive

flow

(G

L)


Figure 8.68. Predicted cumulative streamflow for the Salt River at Qualandary Crossing for various


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

2000

2500

Cum

ulat

ive

flow

(G

L)



Figure 8.69. Predicted cumulative streamflow for the Lockhart River for various revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

300

350

400

450

Cum

ulat

ive

flow

(G

L)


Figure 8.70. Predicted cumulative streamflow for Wakeman Creek at Narembeen for various


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

Cum

ulat

ive

flow

(G

L)


Figure 8.71. Predicted cumulative streamflow for the Yilgarn River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600

700

800

900

1000


Ann

ual t

otal

flow

(G

L)


Figure 8.72. Predicted annual streamflow for the Avon River at Great Northern Highway for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120

140


Ann

ual t

otal

flow

(G

L)

Figure 8.73. Predicted annual streamflow for the North Mortlock River for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120


Ann

ual t

otal

flow

(G

L)

Figure 8.74. Predicted annual streamflow for the East Mortlock River for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600


Ann

ual t

otal

flow

(G

L)


Figure 8.75. Predicted annual streamflow for the Avon River at Northam for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250

300

350


Ann

ual t

otal

flow

(G

L)

Figure 8.76. Predicted annual streamflow for the Salt River at Qualandary Crossing for various


1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250


Ann

ual t

otal

flow

(G

L)

Figure 8.77. Predicted annual streamflow for the Lockhart River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35


Ann

ual t

otal

flow

(G

L)


Figure 8.78. Predicted annual streamflow for Wakeman Creek at Narembeen for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120

140


Ann

ual t

otal

flow

(G

L)

Figure 8.79. Predicted annual streamflow for the Yilgarn River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual p

eak

flow

(G

L)

Figure 8.80. Predicted annual peak streamflow for the Avon River at Great Northern Highway for


1980 2000 2020 2040 2060 2080 21000

0.5

1

1.5

2

2.5

3


Ann

ual p

eak

flow

(G

L)


Figure 8.81. Predicted annual peak streamflow for the North Mortlock River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

0.5

1

1.5

2

2.5

3


Ann

ual p

eak

flow

(G

L)

Figure 8.82. Predicted annual peak streamflow for the East Mortlock River for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25


Ann

ual p

eak

flow

(G

L)

Figure 8.83. Predicted annual peak streamflow for the Avon River at Northam for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6


Ann

ual p

eak

flow

(G

L)


Figure 8.84. Predicted annual peak streamflow for the Salt River at Qualandary Crossing for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6

7


Ann

ual p

eak

flow

(G

L)

Figure 8.85. Predicted annual peak streamflow for the Lockhart River for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

0.2

0.4

0.6

0.8

1

1.2

1.4


Ann

ual p

eak

flow

(G

L)

Figure 8.86. Predicted annual peak streamflow for Wakeman Creek at Narembeen for various


1980 2000 2020 2040 2060 2080 21000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


Ann

ual p

eak

flow

(G

L)


Figure 8.87. Predicted annual peak streamflow for the Yilgarn River for various revegetation scenarios

8.2.4. Impacts of revegetation on salt load

Figures 8.88–8.95 show predictions of cumulative salt loads for the revegetation scenarios. For both scenarios, there is a substantial reduction in salt loads, and for all eight subcatchments, the reductions are greater than the corresponding reductions in streamflow. Like the streamflow impacts, the salt loads predicted for the revegetation scenarios decrease over time. By the end of the twentieth century, salt loads discharging from all eight subcatchments for the full revegetation scenario are less than 1 % of the discharges in the baseline case, with the lowest being 0.03 % for Lockhart River and Wakeman Creek. The low salt loads are a consequence of the lower watertables being less able to discharge highly saline water to the surface, and of the reduced discharge frequencies and volumes from the modelled lakes.

The predicted annual salt loads (Figures 8.96–8.103) confirm the massive impact of revegetation on salt loads and, in particular, show the contrast between the increasing loads of the baseline scenario and the decreasing loads of the revegetation scenarios. The substantial decreases in salt load are also reflected in the predictions of flow-weighted salinity (Figures 8.104–8.111). Except for Salt River, predictions of flow-weighted salinity for the full revegetation case are less than 0.6 g L–1 for the last 28 years of the simulation period. Predictions of salinity for the Salt River (Figure 8.108) remain above 1.7 g L–1 (flow-weighted) for the last 28 years—largely because they represent direct lake discharge—but are still significantly less than the predictions (12.8 g L–1) for the latter part of the twentieth century, prior to the implementation of revegetation. Indeed, they are significantly less than the 4.6 g

L–1 predicted for the catchment outlet between 1976 and 2003 in the baseline scenario.

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

100

200

300

400

500

600

Cu

mu

lativ

e s

alt

loa

d

(Mt)


Figure 8.88. Predicted cumulative salt load for the Avon River at Great Northern Highway for various



1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

Cum

ulat

ive

salt

load

(M

t)


Figure 8.89. Predicted cumulative salt load for the North Mortlock River for various revegetation

scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

Cum

ulat

ive

salt

load

(M

t)


Figure 8.90. Predicted cumulative salt load for the East Mortlock River for various revegetation

scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

300

350

Cum

ulat

ive

salt

load

(M

t)


Figure 8.91. Predicted cumulative salt load for the Avon River at Northam for various revegetation

scenarios


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

20

40

60

80

100

120

140

160

180

Cum

ulat

ive

salt

load

(M

t)


Figure 8.92. Predicted cumulative salt load for the Salt River at Qualandary Crossing for various


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

80

90

100

Cum

ulat

ive

salt

load

(M

t)


Figure 8.93. Predicted cumulative salt load for the Lockhart River for various revegetation scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

2

4

6

8

10

12

Cum

ulat

ive

salt

load

(M

t)


Figure 8.94. Predicted cumulative salt load for Wakeman Creek at Narembeen for various



1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

10

15

20

25

30

Cum

ulat

ive

salt

load

(M

t)


Figure 8.95. Predicted cumulative salt load for the Yilgarn River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.96. Predicted annual salt load for the Avon River at Great Northern Highway for various


1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.97. Predicted annual salt load for the North Mortlock River for various revegetation scenarios


1980 2000 2020 2040 2060 2080 21000

200

400

600

800

1000

1200

1400

1600

1800


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.98. Predicted annual salt load for the East Mortlock River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12

14

16

18

20


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.99. Predicted annual salt load for the Avon River at Northam for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

2000

4000

6000

8000

10000

12000

14000

16000

18000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.100. Predicted annual salt load for the Salt River at Qualandary Crossing for various



1980 2000 2020 2040 2060 2080 21000

2000

4000

6000

8000

10000

12000

14000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.101. Predicted annual salt load for the Lockhart River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600

700

800


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.102. Predicted annual salt load for Wakeman Creek at Narembeen for various revegetation

scenarios

1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.103. Predicted annual salt load for the Yilgarn River for various revegetation scenarios


1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35


Ann

ual s

alin

ity (

g/L)

Figure 8.104. Predicted annual flow-weighted salinity for the Avon River at Great Northern Highway for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.105. Predicted annual flow-weighted salinity for the North Mortlock River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35

40


Ann

ual s

alin

ity (

g/L)


Figure 8.106. Predicted annual flow-weighted salinity for the East Mortlock River for various revegetation scenarios

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.107. Predicted annual flow-weighted salinity for the Avon River at Northam for various


1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250


Ann

ual s

alin

ity (

g/L)

Figure 8.108. Predicted annual flow-weighted salinity for the Salt River at Qualandary Crossing for


1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250


Ann

ual s

alin

ity (

g/L)


Figure 8.109. Predicted annual flow-weighted salinity for the Lockhart River for various revegetation scenarios. Gaps in the salinity curves correspond to years when no flow is predicted

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.110. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen for various

revegetation scenarios. Gaps in the salinity curves correspond to years when no flow is predicted

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70

80


Ann

ual s

alin

ity (

g/L)

Figure 8.111. Predicted annual flow-weighted salinity for the Yilgarn River for various revegetation

scenarios. Gaps in the salinity curves correspond to years when no flow is predicted

8.2.5. Impacts of revegetation on lake storages and discharges

The impacts of the two revegetation scenarios on lake storages and discharges are shown in Table 8.10. In comparison with the baseline scenario (Table 8.2), all modelled lakes have greatly reduced discharge frequencies as a consequence of the widespread reductions in inflow. Three lakes in the Lockhart system (Kurrenkutten, Jilakin and Gulsen) become terminal under the partial revegetation scenario, while a further three (Hinds, Kondinin and Grace North) do so under full revegetation. Only Yenyening discharges more frequently than one year in three under either scenario. Except for Baandee, where the frequency is unchanged, all the discharging lakes flow more often and with greater volumes under partial revegetation than under full revegetation. Typically their mean annual discharge volumes under full revegetation are about half of those under partial revegetation

The salinity of the revegetated discharges is also substantially less than those in the baseline scenario. Reduced discharge frequencies mean longer residence times and more


opportunity for evaporative concentration, but this is clearly more than offset by the large reductions in salt inflow.

Table 8.10. Comparison of predicted lake discharge characteristics for two different revegetation scenarios for the period 2073–2100

Lake 50 % revegetation 100 % revegetation


Ninan 9 1.22 5.77 8 0.53 0.71 Hinds 1 0.20 9.17 0 0.00 — Dowerin 0 0.00 — 0 0.00 — Walyormouring 0 0.00 — 0 0.00 — Cowcowing 0 0.00 — 0 0.00 — Brown (Avon South) 6 0.33 13.90 4 0.13 3.00 Yealearing 8 0.70 15.85 3 0.37 3.26 Yenyening 17 9.78 7.45 11 5.07 1.74 Kurrenkuttten 0 0.00 — 0 0.00 — Kondinin 2 0.36 14.15 0 0.00 — Jilakin 0 0.00 — 0 0.00 — Grace North 2 0.10 10.38 0 0.00 — O'Connor 0 0.00 — 0 0.00 — Varley 0 0.00 — 0 0.00 — Gulsen 0 0.00 — 0 0.00 — King 0 0.00 — 0 0.00 — Ace 2 0.02 2.10 1 0.00 0.26 Baandee 4 3.02 1.85 4 1.63 0.37 Brown (Yilgarn) 3 0.82 0.58 2 0.48 0.37 Campion 5 2.28 0.47 3 1.62 0.28

Of all the modelled lakes, the impacts of revegetation are less marked for Lakes Campion and Brown (Yilgarn). This is most likely due to their location just downstream of large areas of uncleared vegetation, where little replanting can be undertaken.

8.3. Climate impacts

8.3.1. Background to climate scenarios

The impacts of five climate scenarios are assessed for the baseline case. These include the benchmark scenario, one wetter climate and three drier climates. The wetter climate has rainfall that is 10 % greater than the benchmark or baseline scenario and is typical of what might be expected if the climate of the region returns to pre-1975 conditions. The three dryer climates have rainfall that are 10 %, 20 % and 30 % less than the benchmark. They give an indication of what might happen in the catchment if the current trend of climate change continues. In all cases, the changed climate is applied from the beginning of the simulation period, 1965.

A relatively simple approach is taken in generating the climate data. Each of the modified climates was achieved by simply scaling each daily rainfall input by the required percentage. This means that the overall frequency of rainy days remains unchanged across all five climate scenarios. There were also no changes to potential evaporation among the scenarios, despite the fact that potential evaporation should increase slightly. These two factors are likely to lead to compensating biases in streamflow predictions for the various climate scenarios.

8.3.2. Climate impacts on groundwater

The impacts of different climate regimes on groundwater levels in three subcatchments are shown in Figures 8.112–8.114. In general, a wetter climate leads to greater recharge and


therefore higher watertables. By 2100, the predicted difference in groundwater levels between the wettest and driest scenarios is greater for subcatchments in the east of the basin than it is for those in the west. This is exemplified by spreads in groundwater levels of 0.4 m and 0.5 m for subcatchments 54 and 61, but a spread of 1.6 m for a drier subcatchment 102. This wide range in the impact of climate differences is partly caused by the slow rate of response for the dry climates in what is already a dry subcatchment. However, it is also clear from Figures 8.112–8.114 that the different climate scenarios lead to different equilibrium groundwater levels.

The salinity responses for subcatchments 54 and 61 (Figures 8.115 and 8.116) are typical of the subcatchments in the western half of the basin. For much of the twenty-first century, the wetter climates exhibit larger groundwater salinities. However, towards the end of the century, the groundwater salinities associated with the wettest climate appear to settle at a level below those of some of the drier scenarios. There appears to be a slow, long-term reversal of salinities to a situation in which the drier climates have more saline watertables. The short to medium term rises in salinity for the wetter climates are due to the rises in water level being more rapid than those for drier climates. This leads to greater dissolution of mineral salt from the soil profile and results in increasing groundwater salinities. However, by 2100, the rates of watertable rise in the wetter scenarios are less than those in the drier scenarios, so it is the latter that are now dissolving the most mineral salt. This reversal is not yet evident in subcatchment 102 (Figure 8.117) because its slow response means that at the end of the simulation period, the watertable for the wettest scenario is still rising as fast as, or faster than, the watertables of the other scenarios.

1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−8

−7.5

−7

−6.5

−6

−5.5

Gro

undw

ater

dep

th (

m)

Wet (+10%)No changeDry (−10%)Dry (−20%)Dry (−30%)

Figure 8.112. Predicted groundwater depths for subcatchment 54 (Avon South River) for different

climate scenarios


1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−8.5

−8

−7.5

−7

−6.5

−6

Gro

undw

ater

dep

th (

m)


Figure 8.113. Predicted groundwater depths for subcatchment 61 (lower Lockhart River) for different

climate scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 2100−7

−6.5

−6

−5.5

−5

−4.5

Gro

undw

ater

dep

th (

m)


Figure 8.114. Predicted groundwater depths for subcatchment 102 (upper Yilgarn River) for different

climate scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 210059

59.5

60

60.5

61

61.5

62

62.5

63

63.5

Gro

undw

ater

sal

inity

(g/

L)


Figure 8.115. Predicted groundwater salinities for subcatchment 54 (Avon South River) for different

climate scenarios


1965 1980 1995 2010 2025 2040 2055 2070 2085 210061.5

62

62.5

63

63.5

64

64.5

65

65.5

Gro

undw

ater

sal

inity

(g/

L)


Figure 8.116. Predicted groundwater salinities for subcatchment 61 (lower Lockhart River) for different

climate scenarios

1965 1980 1995 2010 2025 2040 2055 2070 2085 210061

61.5

62

62.5

63

63.5

64

64.5

Gro

undw

ater

sal

inity

(g/

L)


Figure 8.117. Predicted groundwater salinities for subcatchment 102 (upper Yilgarn River) for

different climate scenarios

8.3.3. Climate impacts on streamflow

Comparison of the predicted distribution of streamflow sources for the wetter climate and the baseline case (Figures 8.118 and 8.20, respectively) indicate that there is a general increase in streamflow generation in the wetter climate in all subcatchments with non-zero net flows. The relative increases tend to be greater in the east than in the west, and reach in excess of 250% in the upper Yilgarn. The relative increase is smallest in the lower Yilgarn (subcatchment 87; 17 %), but is typically 30–35 % for the wetter subcatchments further downstream. This increasing trend is accompanied by an eastward displacement of the colour bands in Figure 8.118. Two of the lake subcatchments (Walyormouring and O’Connor) become net dischargers.

By contrast, the three dry scenarios result in streamflow generation rates that are progressively smaller than the baseline and have colour bands being displaced to the west. For the –10% scenario (Figure 8.119), the smallest reduction in yield is 21 % (subcatchment 20), while the reduction is typically 25–30 % downstream of Yenyening and around 60 % upstream. For the –20 % and –30 % scenarios (Figures 8.120 and 8.121), the smallest


relative reductions are 43 % and 61 %, respectively (subcatchment 8), while the reductions are typically 50–55 % and 70–75 % downstream of Yenyening and about 85 % and 95 % upstream, respectively. Despite these extreme reductions in water yield upstream of Yenyening, only two subcatchments cease to be net dischargers: those containing Lake Brown (Yilgarn) and Lake Grace North. For the driest scenario (Figure 8.121), the largest yield is 23 mm y–1 (subcatchment 8). The largest yields for the baseline and wet scenarios are 65 mm y–1 and 85 mm y–1 (subcatchment 4).

By comparison with the high ZOE drainage scenario, the yield predictions of the wetter climate are typically greater by 15–40 % below Yenyening, but less (by amounts varying between 10 % and 80 %) for the artificially drained subcatchments above Yenyening.

Figure 8.118. Predicted net annual water yields for a climate that is 10 % wetter than present, 2073–

2100


Figure 8.119. Predicted net annual water yields for a climate that is 10 % drier than present, 2073–2100

Figure 8.120. Predicted net annual water yields for a climate that is 20 % drier than present, 2073–

2100

Figure 8.121 Predicted net annual water yields for a climate that is 30 % drier than present, 2073–

2100

Cumulative predicted streamflows for the eight key sites are presented in Figures 8.122–8.129. In terms of their relative rate of increase, the cumulative streamflow graphs fall into two categories. In the first, the wettest four subcatchments (Figures 8.122–8.125) show significant streamflow even for the driest scenario, for which the level is about half that of the


second driest scenario. In respect of the benchmark scenario, the 10 % increase in rainfall yields changes in cumulative streamflow in the order of 50–60 %, while the 10 % reduction in rainfall leads to 35–40 % reduction in streamflow. In the second group of subcatchments (Figures 8.126–8.129), streamflows in the driest scenario are a small proportion of those in the second driest. A clear conclusion is that the drier, eastern parts of the catchment are more sensitive to climate change than the wetter, western parts.

Also evident for the four driest subcatchments is the episodic nature of the discharge events. This is characterised in Figures 8.126–8.129 by the discrete steps in the cumulative flow curves. In the Salt, Lockhart and Yilgarn Rivers in particular, most of the cumulative flow occurs during these relatively few high-flow years, and is almost certainly dictated by lake discharge events. In contrast, the cumulative flow curves for the four wettest subcatchments (Figures 8.122–8.125) are much smoother, thus indicating less volatility in flows from year to year.

Predicted annual streamflows for the five climate scenarios are shown in Figures 8.130–8.137. At all sites and in all scenarios there is evidence that flow rates continue to rise slowly throughout the simulation period in response to generally rising watertables. In proportional terms, the differences between scenarios are greater in low-flow years than in high-flow years. The differences between scenarios are also greater for the four drier catchments than for the four wetter catchments. The drier catchments also have an increased prevalence of zero-flow years, particularly for the two driest scenarios.

In contrast to the rising trends in annual flow, the graphs of peak streamflow (Figures 8.138–8.145) generally indicate little change over time for the four wettest catchments for all scenarios. However, a clear increasing trend is evident for the driest four subcatchments, particularly among the wetter scenarios.

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

80

Cum

ulat

ive

flow

(T

L)


Figure 8.122. Predicted cumulative streamflow for the Avon River at Great Northern Highway


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

1000

2000

3000

4000

5000

6000

Cum

ulat

ive

flow

(G

L)


Figure 8.123. Predicted cumulative streamflow for the North Mortlock River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

1000

2000

3000

4000

5000

6000

Cum

ulat

ive

flow

(G

L)


Figure 8.124. Predicted cumulative streamflow for the East Mortlock River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

10

15

20

25

30

35

Cum

ulat

ive

flow

(T

L)



Figure 8.125. Predicted cumulative streamflow for the Avon River at Northam

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

2000

4000

6000

8000

10000

12000

Cum

ulat

ive

flow

(G

L)


Figure 8.126. Predicted cumulative streamflow for the Salt River at Qualandary Crossing

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

1000

2000

3000

4000

5000

6000

Cum

ulat

ive

flow

(G

L)


Figure 8.127. Predicted cumulative streamflow for the Lockhart River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

100

200

300

400

500

600

700

800

900

Cum

ulat

ive

flow

(G

L)



Figure 8.128. Predicted cumulative streamflow for Wakeman Creek at Narembeen

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

500

1000

1500

2000

2500

3000

3500

4000

Cum

ulat

ive

flow

(G

L)


Figure 8.129. Predicted cumulative streamflow for the Yilgarn River

1980 2000 2020 2040 2060 2080 21000

500

1000

1500


Ann

ual t

otal

flow

(G

L)

Figure 8.130. Predicted annual streamflow for the Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120

140

160

180

200


Ann

ual t

otal

flow

(G

L)

Figure 8.131. Predicted annual streamflow for the North Mortlock River


1980 2000 2020 2040 2060 2080 21000

20

40

60

80

100

120

140


Ann

ual t

otal

flow

(G

L)

Figure 8.132. Predicted annual streamflow for the East Mortlock River

1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600

700

800

900

1000


Ann

ual t

otal

flow

(G

L)

Figure 8.133. Predicted annual streamflow for the Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

100

200

300

400

500

600

700


Ann

ual t

otal

flow

(G

L)

Figure 8.134. Predicted annual streamflow for the Salt River at Qualandary Crossing


1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250

300

350

400


Ann

ual t

otal

flow

(G

L)

Figure 8.135. Predicted annual streamflow for the Lockhart River

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual t

otal

flow

(G

L)

Figure 8.136. Predicted annual streamflow for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250

300


Ann

ual t

otal

flow

(G

L)

Figure 8.137. Predicted annual streamflow for the Yilgarn River


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70

80

90


Ann

ual p

eak

flow

(G

L)

Figure 8.138. Predicted annual peak streamflow for the Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6


Ann

ual p

eak

flow

(G

L)

Figure 8.139. Predicted annual peak streamflow for the North Mortlock River

1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6


Ann

ual p

eak

flow

(G

L)

Figure 8.140. Predicted annual peak streamflow for the East Mortlock River


1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35

40

45

50


Ann

ual p

eak

flow

(G

L)

Figure 8.141. Predicted annual peak streamflow for the Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12

14


Ann

ual p

eak

flow

(G

L)

Figure 8.142. Predicted annual peak streamflow for the Salt River at Qualandary Crossing

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12

14

16

18


Ann

ual p

eak

flow

(G

L)

Figure 8.143. Predicted annual peak streamflow for the Lockhart River


1980 2000 2020 2040 2060 2080 21000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


Ann

ual p

eak

flow

(G

L)

Figure 8.144. Predicted annual peak streamflow for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

2

4

6

8

10

12


Ann

ual p

eak

flow

(G

L)

Figure 8.145. Predicted annual peak streamflow for the Yilgarn River

8.3.4. Climate impacts on salt loads

Maps of annual salt yields for the four climate scenarios are shown in Figures 8.146–8.149, and may be compared with the baseline scenario (Figure 8.39) and with the corresponding water yields (Figures 8.118–8.121). As is the case in the baseline scenario, the dominant sources in the wetter climate (Figure 8.146) are in the Avon valley between Toodyay and Qualandary Crossing. The greatest annual yield is 420 t km–2 for subcatchment 40 (Spencer’s Brook). All the subcatchments that are net dischargers of salt in the baseline scenario have increased net salt loads in the wetter climate. In the western half of the catchment these increases are typically between 15 % and 30 %, but for some subcatchments in the east, they exceed 400 %. The increases in salt yield in the west are less than the increases in water yield, thus indicating a freshening in discharge. This freshening is presumably a consequence of increased proportions of surface runoff in the streamflow. However, in the east, the salt increases tend to be greater than the water increases. A possible explanation for this is that raised watertables associated with the enhanced recharge increase the discharge of saline groundwater into the surface waters, and that in the drier parts of the catchment, this increase in salinity is not offset by increases in fresh surface runoff.

For the three drier climate scenarios (Figures 8.147–8.149), the Avon Valley remains a significant source of salt, but the greatest yield comes from subcatchment 8 in the Brockman River. Downstream of Yenyening, salt yield reductions relative to the baseline scenario for


the –10 % climate are typically 20–30 % and are less than the reductions in water yield. This is indicative of a less saline discharge. Further east, the salt decreases are greater than the water decreases, indicating a freshening of discharges. This regional pattern of increasing salinity with decreasing rainfall downstream of Yenyening and decreasing salinity with decreasing rainfall upstream of Yenyening is repeated in the –20 % and –30 % climate scenarios (Figures 8.148 and 8.149).

Figure 8.146. Predicted net annual salt yields for a climate that is 10 % wetter than present, 2073–

2100

Figure 8.147. Predicted net annual salt yields for a climate that is 10 % drier than present, 2073–2100




The plots of cumulative salt load for the climate scenarios (Figures 8.150–8.157) are quite similar to those for streamflow (Figures 8.122–8.129). Like the latter, they tend to fall into two categories: four show significant salt discharge for even the driest climate, and four show negligible salt discharge for the two driest scenarios. All curves, however, tend to have a more distinct upward inflection over time than their streamflow counterparts. The cumulative salt load curves for the Salt, Lockhart and Yilgarn Rivers appear to be more episodic than that for Wakeman Creek, again suggesting that the flow events are driven by saline lake discharges.


The upward inflections in the cumulative salt load curves are reflected in the plots of mean annual salt load (Figures 8.158–8.165) by the consistent increases in discharge over time. That these rates of increase are greater than the corresponding rates of increase in streamflow means that stream salinities also increase with time (Figures 8.166–8.173). Interestingly, for the first four subcatchments, stream salinities are initially greater for the dry scenarios than for the wet scenarios. However, after about 2015, the order of salinities reverses, with the wetter climates having greater salinities than the drier climates. In contrast, the three easternmost subcatchments of Lockhart, Narembeen and Yilgarn tend to have greater salinities for the wetter climates throughout the simulation period, but the differences in predicted salinity between climates increases with time. At Qualandary Crossing (Figure 8.170), discharge from the Salt River is governed entirely by the discharge from Yenyening Lakes and the flow-weighted salinity is equal to the lake salinity. As a consequence of the increased residence times, discharge salinity at Qualandary Crossing is inversely proportional to rainfall and this ordering persists throughout the simulation period. For the drier climates, annual flow-weighted discharge salinity at Qualandary Crossing regularly exceeds 300 g L–1 by the end of the twenty-first century, although such high values tend to correspond to low flow years.

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

100

200

300

400

500

600

700

800

900

Cum

ulat

ive

salt

load

(M

t)


Figure 8.150. Predicted cumulative salt load for the Avon River at Great Northern Highway

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

80

90

100

Cum

ulat

ive

salt

load

(M

t)


Figure 8.151. Predicted cumulative salt load for the North Mortlock River


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

80

90

Cum

ulat

ive

salt

load

(M

t)


Figure 8.152. Predicted cumulative salt load for the East Mortlock River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

100

200

300

400

500

600

Cum

ulat

ive

salt

load

(M

t)


Figure 8.153. Predicted cumulative salt load for the Avon River at Northam

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

300

350

400

Cum

ulat

ive

salt

load

(M

t)


Figure 8.154. Predicted cumulative salt load for the Salt River at Qualandary Crossing


1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

50

100

150

200

250

Cum

ulat

ive

salt

load

(M

t)


Figure 8.155. Predicted cumulative salt load for the Lockhart River

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

5

10

15

20

25

30

Cum

ulat

ive

salt

load

(M

t)


Figure 8.156. Predicted cumulative salt load for Wakeman Creek at Narembeen

1965 1980 1995 2010 2025 2040 2055 2070 2085 21000

10

20

30

40

50

60

70

Cum

ulat

ive

salt

load

(M

t)


Figure 8.157. Predicted cumulative salt load for the Yilgarn River


1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.158. Predicted annual salt load for the Avon River at Great Northern Highway

1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.159. Predicted annual salt load for the North Mortlock River

1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.160. Predicted annual salt load for the East Mortlock River


1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.161. Predicted annual salt load for the Avon River at Northam

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25


Ann

ual t

otal

sal

t loa

d (

Mt)

Figure 8.162. Predicted annual salt load for the Salt River at Qualandary Crossing

1980 2000 2020 2040 2060 2080 21000

5000

10000

15000


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.163. Predicted annual salt load for the Lockhart River


1980 2000 2020 2040 2060 2080 21000

200

400

600

800

1000

1200

1400


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.164. Predicted annual salt load for Wakeman Creek at Narembeen

1980 2000 2020 2040 2060 2080 21000

500

1000

1500

2000

2500

3000

3500

4000

4500


Ann

ual t

otal

sal

t loa

d (

kt)

Figure 8.165. Predicted annual salt load for the Yilgarn River

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35


Ann

ual s

alin

ity (

g/L)

Figure 8.166. Predicted annual flow-weighted salinity for the Avon River at Great Northern Highway


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.167. Predicted annual flow-weighted salinity for the North Mortlock River

1980 2000 2020 2040 2060 2080 21000

5

10

15

20

25

30

35

40

45


Ann

ual s

alin

ity (

g/L)

Figure 8.168. Predicted annual flow-weighted salinity for the East Mortlock River

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60


Ann

ual s

alin

ity (

g/L)

Figure 8.169. Predicted annual flow-weighted salinity for the Avon River at Northam


1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250

300

350

400


Ann

ual s

alin

ity (

g/L)

Figure 8.170. Predicted annual flow-weighted salinity for the Salt River at Qualandary Crossing

1980 2000 2020 2040 2060 2080 21000

50

100

150

200

250


Ann

ual s

alin

ity (

g/L)

Figure 8.171. Predicted annual flow-weighted salinity for the Lockhart River. Gaps in the salinity

curves correspond to years when no flow is predicted

1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70


Ann

ual s

alin

ity (

g/L)

Figure 8.172. Predicted annual flow-weighted salinity for Wakeman Creek at Narembeen. Gaps in

the salinity curves correspond to years when no flow is predicted


1980 2000 2020 2040 2060 2080 21000

10

20

30

40

50

60

70

80

90


Ann

ual s

alin

ity (

g/L)

Figure 8.173. Predicted annual flow-weighted salinity for the Yilgarn River. Gaps in the salinity curves

correspond to years when no flow is predicted

8.3.5. Climate impacts on lake storages and discharges

Lake discharge characteristics for the two most extreme climates appear in Table 8.11. All lakes discharge more frequently for the wet climate than for the baseline (i.e., unchanged climate) scenario (Table 8.2). In the wet climate, three lakes discharge every year compared to none under the baseline scenario, and three lakes, Walyormouring, O’Connor and Varley, are no longer terminal. Only three lakes remain terminal under the wet climate. The differences between the baseline and dry scenarios are even more stark. Only three lakes discharge at all in the 28 year period and only Yenyening does so more than once.

All lakes have much greater discharges in the wet climate than in the unchanged climate or baseline, some by as much as 200 %. For many of the lakes, the wet climate discharges are of a similar magnitude to those of the high ZOE drainage scenario (Table 8.2). The three lakes that remain discharging in the dry climate, only do so in extremely small volumes.

In the case of flow-weighted discharge salinity, the general trend is for the wet climate to have the freshest discharges. Exceptions are Lakes Gulsen and Ace, for which salinity is lower in the unchanged climate, but in both cases, the discharges are extremely small. The combination of substantial flow increases and modest salinity decreases means that the lakes discharge considerably more salt in the wet climate than in the unchanged climate. One of the dry climates has a salinity of lake discharge that is greater than in the unchanged climate, but again, it represents an extremely small, single event, so no meaningful interpretation can be ascribed.


Table 8.11. Comparison of predicted lake discharge characteristics for two different climate scenarios for the period 2073–2100

Lake Wet (+10 %) climate Dry (–30 %) climate


Ninan 28 16.99 43.8 1 0.075 30.2 Hinds 16 9.91 52.7 0 0.000 — Dowerin 0 0.00 — 0 0.000 — Walyormouring 1 0.11 146.4 0 0.000 — Cowcowing 0 0.00 — 0 0.000 — Brown (Avon South) 28 4.29 31.0 0 0.000 — Yealering 25 7.03 36.2 1 0.051 73.9 Yenyening 28 133.55 47.1 3 0.520 94.4 Kurrenkuttten 20 41.29 74.8 0 0.000 — Kondinin 24 45.52 62.2 0 0.000 — Jilakin 23 26.17 70.6 0 0.000 — Grace North 10 8.32 101.4 0 0.000 — O'Connor 1 0.13 11.3 0 0.000 — Varley 1 0.13 68.0 0 0.000 — Gulsen 2 0.37 20.8 0 0.000 — King 0 0.00 — 0 0.000 — Ace 9 0.25 28.5 0 0.000 — Baandee 25 36.41 30.4 0 0.000 — Brown (Yilgarn) 12 9.07 19.4 0 0.000 — Campion 17 14.49 15.0 0 0.000 —

8.4. Summary of catchment scenarios This report has assessed the impacts on streamflows, salt loads, salinities, lake discharge rates and salinities and groundwater levels and salinities for 17 different catchment scenarios:

No drainage; unchanged climate

Leveed drainage, high zone of effectiveness

Leveed drainage, low zone of effectiveness

Leveed drainage, high ZOE, elevated lake discharge heights

Leveed drainage, high ZOE, no lake discharge

Leveed drainage, high ZOE, subcatchment retention

Open drainage, high zone of effectiveness

Open drainage, low zone of effectiveness

Open drainage, high ZOE, elevated lake discharge heights

Open drainage, high ZOE, no lake discharge

Open drainage, high ZOE, all lakes removed

Revegetation to at least 50 %

Revegetation to 100 %


Changed climate with 10 % more rainfall

Changed climate with 10 % less rainfall



Key conclusions from this study include:

Within subcatchment there is negligible difference in flows and loads between the open drainage scenario and the sum of the leveed and natural channel flows in the leveed drainage scenarios.

The artificial drainage scenarios lower catchment average groundwater levels and salinities in the drained subcatchments; the degree and rapidity of these responses being largely dependent on drain density and zone of effectiveness.

Artificial drainage leads to modest increases (up to 29 %) in flow at the basin outlet and to more substantial increases (up to 210 %) in salt load. The impacts on both streamflow and salt load increase with distance upstream.

The scenarios with high zones of effectiveness typically produce more streamflow and salt load than the scenarios with low zones of effectiveness.

The proportion of the total leveed flow that is carried by the leveed channel depends on the presence and distance of upstream lakes. In first-order subcatchments in the more arid, eastern parts of the catchment, almost all of the total leveed flow is carried by the leveed channel.

Peak streamflows are considerably larger for the drained scenarios than for the baseline scenario in arid subcatchments downstream of lakes, but in first-order subcatchments and in subcatchments below Qualandary Crossing, drainage has little impact on peak streamflows.

Peak salt loads can become extremely large in undrained, infrequently discharging subcatchments with artificially-drained tributaries.

Artificial drainage increases the frequency, volume and salinity of lake discharges.

Mitigation of the impacts of artificial drainage by preventing the lakes from discharging is effective in reducing streamflows, salt loads and salinities in most downstream subcatchments except for those that are already downstream of terminal lakes. This 100 % retention scheme also leads to modest reductions in peak flows in lower parts of the catchment and larger peak flow reductions in the Yilgarn and Lockhart Rivers.

Mitigation of the impacts of artificial drainage by elevating the discharge height of the lakes by 0.3 m has little effect in reducing streamflows, has no effect on salt loads and leads to small increases in flow and/or salinity in most downstream subcatchments. It has similar impacts on lake discharges.

Mitigation of the impacts of leveed drainage by retaining the leveed channel flows within the subcatchment in which they are generated in evaporation basins is highly effective in reducing flows, loads and salinities in both streams and lake discharges. It is also effective in reducing peak streamflows and salt loads.

The impacts of artificial drainage whereby lakes are removed or bypassed leads to significant increases in streamflow including peak flow events, and slight increases in salt load, but significant decreases in stream salinity in downstream subcatchments.

For a leveed drainage system without lakes, about 65 % of the mean annual streamflow and more than 90% of the mean annual salt load are carried by the engineered arterial channel.

Revegetation of the catchment with woody perennials causes substantial falls in watertables, but substantial increases in groundwater salinity.


Revegetation reduces streamflows substantially and the magnitude of these changes increases with time. However, it has little impact on peak flows at the catchment outlet.

Revegetation leads to enormous reductions in stream salt load and salinity.

Revegetation reduces the frequency, volume and salinity of lake discharges.

The five climate scenarios for the Avon basin over the next 100 years create an enormous variability in hydrological response in the catchment.

The drier, eastern subcatchments are more sensitive to climate change than the wetter, western subcatchments.

Drier climates lead to lower watertables and slower watertable responses than wetter climates.

In the short to medium term, the wetter climates give rise to greater groundwater salinities than the drier climates, but towards the end of the simulation period, this ordering begins to reverse in the more responsive subcatchments.

Streamflows and salt loads are greater for the wetter climates than for the drier climates.

For the first 50 years of the climate simulations, the wetter climates generate less saline streamflows than the drier climates. However, later in the simulation, the order is reversed.

The wet climate leads to increases in lake discharge occurrences and volumes, and to decreases in discharge salinity.

The driest climate leads to an almost complete cessation of discharge from all lakes.


9. REGIONAL DRAINAGE DISCHARGE MANAGEMENT

9.1. Introduction In this chapter, the size of various regional and subcatchment scale drainage discharge management options is estimated using outputs from LASCAM. Their construction/excavation cost is estimated using their size. This helps assess the feasibility of various water management strategies.

Of the subcatchment and regional scale drainage discharge management strategies assessed in this study, a few are selected for their size and construction cost estimation. It is of note that the subcatchment scale leveed and open drainage size was an input to the LASCAM for simulating the implementation of artificial drainage over time. The lengths of subcatchment scale drainage systems are estimated from Landmonitor valley hazard maps and assumed zones of effectiveness (ZOE) for various regions. The artificial drainage lengths, determined using various ZOEs given in Figure 5.16, are estimated as 4540 km in 2000. By 2100 about 50 % of the valley hazard area is assumed to require artificial drainage. To treat this area, about 22,710 km long artificial drains are estimated based on ZOEs given in Figure 5.16. The artificial drainage length was assumed to linearly increase from 4,540 km in 2000 to 22,710 km by 2100. The excavation cost of these management options are estimated from their size which in are determined from LSACAM outputs. These costs are then compared to assess their feasibility.

The discharge management options assessed include subcatchment scale leveed and open drains with drainage discharge management via:

natural creek system,

leveed (open) arterial channel with salt lake storage (Drainage SSe),

leveed (open) arterial channel without salt lake storage (Drainage LB), and

subcatchment scale evaporation basins (Drainage EB).

9.2. Leveed and open drainage with natural channel discharge management

9.2.1. Leveed drainage with natural creek discharge management

The main assumptions for subcatchment scale leveed drains are listed in Table 9.1. An indicative excavation cost estimate, of about 4540 km long artificial drains in various subcatchments of the Avon Basin in 2000, is about $45 million assuming medium unit excavation costs (Table 9.2). By 2100, it will cost about 227 million to excavate 22710 kilometre long drains. About $91 million will be required to excavate around 9080 km drains by 2025. These are excavation cost estimates of the farm scale and subcatchment scale drains only. No drainage discharge management, disposal or treatments costs are included. For this scenario it is assumed that the natural creeks that receive drainage discharge have enough buffering capacity to neutralise any adverse ecological and hydrological impacts.


Bottom width of open drain is 1.1 m. Drain is 2.5 m deep every where. Side slopes are 1:1 on both sides. No spoil banks are constructed to prevent the entry of surface water into these

drains. Surface water can enter these channels via properly designed and designated

inlets. The construction cost of these inlets is not included. All open drains follow natural drainage lines. Dimensions of the farm and subcatchment scale open drains remain constant

Bottom width of the farm and subcatchment scale leveed drains is 1.1 m. An artificial drain is 2.5 m deep everywhere. Side slopes are 0.5:1 on both sides of the drain. Distance between the drain edge and spoil bank is 3 m on both sides of the

drain. Spoil bank 1 m high on field side and 1.5 m high on the valley bottom or

creek side. Drains run parallel to the natural creek lines (higher side of the valley floors

to guard against the entry of surface runoff). Size of deep drains is uniformly applicable across all subcatchments of the

Avon Basin.

Table 9.1. Main assumptions for farm and subcatchment scale leveed artificial drains

Table 9.2. Excavaion cost of farm and subcatchment scale leveed drainage

Note: The low, medium and high unit construction (excavation) costs are $8000, $10,000 and $12,000 per kilometre, respectively, of farm and subcatchment scale leveed drainage.

9.2.2. Open drainage with natural creek discharge management

Open farm and subcatchment scale artificial drainage differs from leveed drainage in that the drainage discharge is mixed with surface runoff, if any, at the source instead of at the subcatchment outlet for the leveed system. The discharge to the downstream natural creeks and rivers is a mixture of surface and groundwater (drainage discharge) in both cases.

The main assumptions about the size and other characteristics of farm and subcatchment scale open drains are given in Table 9.3. Indicative excavation cost estimates of the farm and subcatchment scale open drainage system are listed in Table 9.4. Unit excavation costs are lower for the open drainage system and higher for the leveed system due to the construction of embankments on both sides of leveed drains to prevent the entry of surface water. However the maintenance costs, not included in these estimates, are expected to be higher for the open drainage system than for the leveed system.

An indicative excavation cost estimate of the farm and subcatchment scale open drainage system in 2000 is about $36 million based on a medium construction cost of $8,000 per kilometre drain length (Table 9.4). In 2100, the excavation will cost about $182 milllion. The excavation cost is lower for the open drainage system than for the leveed system but the maintenance cost for the open system is expected to be much higher over time.

Table 9.3. Main assumptions for farm and subcatchment scale leveed artificial drains

Year

Leveed drainage costs (excavation costs only)

Drainage length (km)Total cost ($million)

Low Medium High

2000 4540 36 45 54

2025 9080 73 91 109

2050 13620 109 136 163

2075 18160 145 182 218

2100 22700 182 227 272


Table 9.4. Farm and subcatchment scale open drainage excavation costs

Note: The low, medium and high unit construction costs are $6,000, 8,000 and 10,000 per kilometer, respectively of farm and subcatchment scale open drainage.

9.2.3. Leveed and open drainage with arterial channel discharge management

Introduction

To manage the drainage discharge from leveed and open farm and subcatchment scale artificial drainage systems, two arterial channel scenarios are considered:

a) leveed arterial channel to convey drainage discharge from subcatchment scale leveed artificial drainage systems; and

b) open arterial channel to convey both surface runoff and drainage discharge from subcatchment scale open drainage systems.

For the leveed arterial channels and drainage system (subcatchment and arterial) several options were simulated in LASCAM and their impacts on flows and loads evaluated. Two of these options are considered here for estimating their size and cost. In the first option the drainage water or discharge from leveed arterial channels was allowed to enter and be stored in the salt lakes up to their existing outlet heigths (Drainage SSe). Outflow from such lakes was a mixture of the surface and drainage water to downstream natural or engineered arterial channels. In the second option the drainage discharge from the leveed arterial channel was not allowed to enter and be stored in the salt lakes (Drainage LB). No loss of drainage discharge from leveed arterial channels occurred except evaporation and seepage. In this option the two Mortlocks and Salt River leveed arterial channels eventually deliver the drainage discharge into the Avon River.

Arterial channel reaches

In total, nine arterial channel sections were assumed, the length and location of each section is shown in Figure 9.1. The length of each arterial channel varies depending on the size and number of subcatchments involved and the alignment and meandering of the natural creeks/streams/rivers within these subcatchments. Here the term ‘subcatchments involved’ means that an arterial channel traverses through these subcatchments and receives drainage discharge and surface runoff (open) or only the drainage discharge (leveed) from these subcatchments. An arterial channel also entertains/receives or conveys artificial drainage discharge, if any, or a mixture of drainage discharge and surface runoff from all upstream subcatchments that are part of the same drainage network.

The Lockhart East section of the arterial channel is 114 km long and starts at the outlet of subcatchment 77 and ends at the out let of subcatchment 75 passing through subcatchment 76 and 75 (Table 9.5). About 63 km long Lockhart South section of the arterial channel starts at the outlet of subcatchments 73 and 72 and ends at the outlet of subcatchments 68 and 75. Two relatively large lakes, Lake Grace and Lake Jilakin, form part of the upstream subcatchments and have large capacity to accommodate significant volumes of drainage water. The Lockhart Main section of the arterial channel is 167 km long. It starts at the

Year

Open drainage costs (excavation costs only)

Drainage length (km)Total cost ($million)

Low Medium High

2000 4540 27 36 45

2025 9080 54 73 91

2050 13620 82 109 136

2075 18160 109 145 182

2100 22700 136 182 227


outlets of subcatchments 68 and 75, receives drainage discharge from both Lockhart east and south sections of the leveed arterial channels and ends at the outlets of subcatchments 61 and 86. Two relatively large salt lakes, Lake Kondinin and Lake Kurrenkutten, form part of this section (Table 9.5). A 177 km long Yilgarn section of the arterial channel starts at the outlets of subcatchments 101 and 102 and ends at the outlet of subcatchment 86. Two salt lakes (Lake Deborah South and Lake Baladjie) are part of the upstream subcatchments and two lakes, Lake Campion and Lake Brown, form part of those subcatchments through which the arterial channel passes. The Salt River arterial channel section of about 89 km starts at the outlet of subcatchments 61 and 86 and ends at the outlet of subcatchment 58 at Qualandary Crossing. The Salt River leveed arterial channel receives drainage discharge from Lockhart and Yilgarn sections of the arterial channels. About 169 km long the Mortlock north section of 169 km starts at the outlet of subcatchment 27 (which is also outlet of Lake Hinds) and ends at the outlets of subcatchment 22 and 30. About 295 km long the Mortlock east section of the arterial channel starts at the outlet of subcatchment 38 just downstream of Cowcowing Lakes and ends at the outlets of subcatchments 30 and 22. It is the longest of all sections (Table 9.5).

The area drained by these arterial channels and contributing artificial drainage lengths in 2000 and 2100 are also listed in Table 9.5. The last two arterial channel sections, the Upper Avon and the Lower Avon, are not formally within the scope of this study. Any drainage discharge generated from artificial drainage in these parts of the Avon Basin is assumed to discharge into natural system or be managed locally.

An arterial channel size, in various sections, was estimated using LASCAM outputs as described below. The indicative costs were then estimated from their required size.

Size of arterial channels Daily peak flows considering drainage discharge from subcatchment scale leveed drainage and a mixture of surface runoff and drainage discharge from open drainage system,were obtained by simulating the implementation of above described artificial drainage lengths in LASCAM. The daily flows were available at the outlets of all 108 subcatchments of the Avon Basin. To estimate design flows two periods were considered: 2000–2025 and 2075–2100. The daily peak flows from these periods for both leveed and open subcatchment and arterial systems were used to estimate daily peak flows of 1 in 10 year return period. These were then used as design flows for deteriming the size of arterial channels (Table 9.6).

As expected the design flows in the leveed arterial channels, both with and without the salt lake storage option, are significantly smaller than in the open arterial systems. For the leveed arterial channels the design flow rates for the Drainage LB are higher than those for the Drainage SSe and Drainage SSr. In general the drainage discharge and/or surface runoff in the leveed and open arterial channels from the subcatchment scale leveed and open drainage systems are smaller during first quarter than during last quarter of the twenty-first century because of increasing artificial drainage lengths over time.

It is important to note that the drainage discharge or design flow rates in the leveed arterial channels from subcatchment scale leveed drainage systems are based on daily peak flows of 1 in 10 year recurrence interval to enable handling of large flow events by the drainage system, however it is not designed to cope with extreme flood events. The drainage discharge in L sec-1 km-1 of drainage length based on 1:10 year recurrence interval daily peak flow is expected to be significantly larger than that estimated on the basis of mean annual drainage discharge rates or design flows.

The salt lakes play big role in managing drainage discharge from some subcatchments in the case of leveed arterial channels with salt lake storage option. For example, the drainage discharge rates into the Lockhart Main leveed arterial channel from subcatchments 67 and 66 that are also recipients of drainage discharge from a number of upstream subcatchments (63, 64 and 67 to 85) are zero for the leveed drainage system (Drainage SSe) because of the presence of Lake Kondinin and Kurrenkutten accommodating all drainage discharge from upstream subcatchments (Table 9.6). Therefore the Lockhart Main section of the leveed


arterial channel with salt lake storage option will handle only the drainage discharge generated from subcatchments downstream of Lake Kurrenkutten (61 to 65).

The Mortlock East section of both leveed (Drainage SSe) and open arterial channels will handle drainage discharge and/or surface runoff from subcatchments downstream of Lake Dowerin (30 to 35). The design flow rates into this section from subcatchments 36, 37 and 38 are zero for both leveed with salt lake storage and open drainage systems because of the terminal salt lakes at their outlets (Table 9.6).

In the Mortlock North section of the leveed arterial channel (Drainage SSe) no drainage discharge is expected from subcatchment 26 and all upstream subcatchments because of the Lake Ninan and Hinds accommodating all drainage discharge from subcatchment scale leveed drainage systems. In the Yilgarn leveed arterial channel section, no drainage discharge is expected from subcatchment 99 and all upstream subcatchments (Table 9.5 and 9.6) due to presence of a number of relatively large salt lakes in the region.

For both open and leveed arterial systems (both Drainage SSe and Drainage LB) the design flow rates generally increase between 2000–2025 and 2075–2100 due to increasing artificial drainage over time (Table 9.6). The ratio between flow rates in 2000–2025 and 2075–2100 varies between 0 and 2 except in few subcatchments in the Lockhart south, Lockhart Main and Yilgarn sections where they increase by greater proportions. As expected the design flow rates are relatively large for the far western subcatchments near the basin outlet because they receive drainage discharge and/or surface runoff from the entire basin and have higher average annual rainfall contributing a major fraction of the total flow.

During first quarter of the twenty-first century the average flow rates (an average of all arterial channels) from the leveed drainage system with Drainage LB option are almost double from those with Drainage SSe option. During last quarter of the twenty-first century this ratio is around 2.7. The ratio of average flow rates between open and leveed system (Drainage SSe) is above 9.


Figure 9.1. Layout of arterial channels in the Avon Basin

Table 9.5 Arterial channel lengths, subcatchments involved, upstream subcatchments and contributing drainage lengths

Arterial channel

Subcatchments with arterial

channel

Subcatchments contributing discharge to

arterial channel

Area drained (km2) by arterial channel

Contributing artificial drainage

lengths (km)

Section

Length (km)

2000 2100

1 Lockhart East 114 75,76 77 to 85 10306 530 1589

2 Lockhart Main 167

61 and 62 65 to 67

68 to 85 except 70 63 and 64 28357 1140 5598

3 Lockhart South 63

68 to 71 except 70 72 to 74 9786 275 2055

4

Yilgarn 177

86, 87, 90, 91, 93, 96, 99, and

100 92 to 97 except 93 and 96

101 to 108 55887 1129 5748 5

Salt River 89 58,60

59 to 85 except 60, 63, 70, 75 and 76

92 94 to 97 except 96 101 to 108 87521 2312 12522

6 Mortlock East 294

30 to 38 except 32 32 9893 222 4121

7 Mortlock North 169

22 to 26 except 24 24 to 29 except 25 and 26 6916 188 5238

8 Upper Avon 100

39 to 45 except 40 and 43 51 and 52

9

Lower Avon 122

1, 2, 7, 12, 14, 16, 17, 20, and

21


Table 9.6. Daily peak flows of 1 in 10 year recurrence interval used as design flows (calculated at end points) for various sections of the leveed and open arterial channels

Subcatch ID

Design flow (ML day-1) Leveed drainage with

salt lake storage

Design flow (ML day-1) Leveed drainage without

salt lake storage Design flow (ML day-1)

Open drainage Arterial Channel Section

2000–2025

2075–2100 Ratio

2000–2025

2075–2100 Ratio

2000–2025

2075–2100 Ratio

75 127 231 1.82 170 286 1.68 795 921 1.16 Lockhart east 76 66 114 1.73 123 226 1.84 261 329 1.26

68 29 43 1.48 340 631 1.86 287 2254 7.85

Lockhart south

69 0 0 0.00 322 599 1.86 246 2170 8.8271 129 235 1.82 295 510 1.73 935 1895 2.0361 313 517 1.65 790 1492 1.89 2664 3760 1.41

Lockhart main

62 238 464 1.95 793 1472 1.86 1928 3764 1.9565 61 92 1.51 672 1202 1.79 931 3026 3.2566 0 0 0.00 591 1169 1.98 88 3101 35.2467 0 0 0.00 546 1114 2.04 730 2622 3.5986 392 475 1.21 780 2188 2.81 1934 4777 2.47

Yilgarn

87 262 322 1.23 747 2517 3.37 1754 5952 3.3990 248 302 1.22 728 2524 3.47 1706 6038 3.5491 0 0 0.00 626 2450 3.91 1560 6693 4.2993 352 1712 4.86 518 2067 3.99 1723 5289 3.0796 134 341 2.54 238 760 3.19 603 1221 2.0299 0 0 0.00 174 564 3.24 187 464 2.48100 0 0 0.00 173 481 2.78 1304 1808 1.3958 0 0 0.00 2001 3976 1.99 5890 12235 2.08 Salt

River 60 1037 1687 1.63 1884 3544 1.88 9369 11866 1.2722 806 859 1.07 930 1136 1.22 2841 3236 1.14

Mortlock north

23 758 805 1.06 899 1090 1.21 2719 3154 1.1625 558 576 1.03 661 848 1.28 2045 2131 1.0426 0 0 0.00 384 627 1.63 273 1163 4.2630 1068 1350 1.26 2578 4182 1.62 4580 4567 1.00

Mortlock east

31 1102 1348 1.22 2496 4704 1.88 4675 4658 1.0033 1014 1211 1.19 2324 4877 2.10 4270 4244 0.9934 784 822 1.05 2297 3952 1.72 2984 2955 0.9935 387 435 1.12 1769 3797 2.15 1336 1357 1.0236 0 0 0.00 1663 4059 2.44 0 0 0.0037 0 0 0.00 1807 3607 2.00 0 0 0.0038 0 0 0.00 1093 2717 2.49 0 0 0.0039 4622 4847 1.05 6844 9115 1.33 20612 24286 1.18

Upper Avon

41 4331 4674 1.08 6675 8826 1.32 19476 22959 1.1842 3866 4482 1.16 6194 7874 1.27 16091 19733 1.2344 3525 4023 1.14 5927 7505 1.27 14776 18573 1.2645 3372 3941 1.17 5674 7061 1.24 13326 17313 1.3051 2136 2587 1.21 5167 6094 1.18 9773 14179 1.4552 2159 2620 1.21 4947 5828 1.18 8964 13556 1.511 0 0 0.00 0 0 0.00 28672 32039 1.12

Lower Avon

2 0 0 0.00 0 0 0.00 28303 31956 1.137 0 0 0.00 0 0 0.00 27564 31186 1.1312 0 0 0.00 0 0 0.00 25413 29034 1.1414 0 0 0.00 0 0 0.00 26155 29180 1.1216 0 0 0.00 0 0 0.00 25660 28975 1.1317 0 0 0.00 0 0 0.00 25289 28960 1.1520 5987 6574 1.10 8775 11424 1.30 24063 28355 1.1821 1979 2283 1.15 3059 4834 1.58 7234 7293 1.01


The design flows (daily peak flows of 1 in 10 year recurrence interval) were used in the Manning’s equation to estimate channel design parameters. The longitudinal channel gradient was derived from the DEM data. To design arterial channels a number of assumptions were made (Table 9.7). A trapezoidal section was assumed for the leveed and open arterial channels. The size of an arterial channel varies in various sections depending on the design flow rates and management option as discussed below.

The Lockhart East section of the leveed arterial channel (both Drainage SSe and Drainage LB) is likely to require relatively small size arterial channels and their size is not expected to increase over time (Table 9.8). The channel depth is defined as the flow depth plus a free board of 0.5 m. Increased artificial drainage over time will increase drainage discharge. However the presence of a number of salt lakes (Lake Ace, Lake King, Lake Gulsen, Lake Varley and Lake O’Connor) accommodate the bulk of drainage discharge and therefore the channel size is likely to remain relatively constant over time. The size of an open arterial channel is not expected to increase significantly over time since the bulk of increased drainage discharge and surface water runoff over time will continue to be accommodated by a number of salt lakes (Lake Ace, Lake King, Lake Gulsen, Lake Varley and Lake O’Connor) (Table 9.10). Therefore a relatively constant channel section is likely to be enough to handle both the surface runoff and drainage discharge over the next 100 years.

The Lockhart South section of the leveed arterial channel (Drainage SSe) requires a small channel of relatively fixed size to manage the drainage discharge from this region. Its flow depth increases from 1.3 to 1.45 m over a period of 100 years (Table 9.8) which is mainly due to handling of the bulk of drainage discharge, generated in upper reaches of the Lockhart South system, by two relatively large lakes. For Drainage LB a smaller size leveed arterial channel can be constructed to handle the drainage discharge rates during first quarter of the twenty-first century. This channel can be remodelled later to increase its size to handle additional drainage discharge during last quarter of the twenty-first century (Table 9.9). The size of an open arterial channel (Drainage SSe) increases significantly over time due to increased flows from subcatchments 71, 69 and 68 (Table 9.10). The ratios between design flows in 2100 and 2000 are around 7.85, 8.82 and 2.03 for subcatchments 68, 69 and 71, respectively. It means that major increases originate from subcatchment 68 and 69. A lower ratio for subcatchments 71 (this subcatchment also receives drainage discharge and surface runoff from upstream subcatchments) suggests that the lakes (Lake Grace and Lake Jilakin) continue to accommodate the bulk of flows from upstream subcatchments over time.

A relatively stable size Lockhart Main section of the leveed arterial channel is likely to be enough for handling the arterial drainage discharge over the next 100 years for Drainage SSe option (Table 9.8). Its size is unlikely to increase due to the presence of two relatively large lakes and accommodating the bulk of the drainage discharge from two arterial channels (Lockhart east and south). The size of the Lockhart Main section of the leveed arterial channel increases considerably over time for Drainage LB (Table 9.9). However, a smaller size channel, constructed to handle the flows during first quarter of the twenty-first century, can be remodelled later to accomodate additional flows during 2075–2100.

The Lockhart Main section of the open arterial channel receives drainage discharge and surface water runoff from two upstream open arterial channel sections, namely Lockhart East and Lockhart South. It also handles drainage discharge and surface runoff from subcatchments 67, 66, 65, 62 and 61. During 2075–2100 the flow contributions from subcatchments 67, 66 and 65 increase 3.6, 35.2 and 3.3 times the flow rates during 2000–2025 (Table 9.6). The flow rates from subcatchment 62 to open arterial channel almost double and from 61 they increase by almost 50 % by 2075–2100. The most dramatic increase is from subcatchment 66 due to the presence of Lake Kurrenkutten which reaches its storage capacity more often leading to an increased overflow frequency over time. To accommodate large flows a large size open arterial channel is required in the Lokhart Main section during the first quarter of the twenty-first century. Even a larger size arterial channel will be reuired during last quarter of the twenty-first century due to increase in flows.

The Yilgarn leveed arterial channel section for Drainage SSe option is of relatively small size (Table 9.8) due to accommodation of bulk of the drainage discharge by the lakes


encompassing this region. The Lake Deborah South and Lake Baladjie form part of the upstream subcatchments and two smaller lakes (Lake Campion and Lake Brown) are part of subcatchments through which the arterial channel passes. Since its size is not expected to increase significantly over time a fixed size channel can be constructed to manage the drainage discharge during the next 100 years (Table 9.8). For Drainage LB option, a significantly larger size leveed arterial channel is required during first quarter of the twenty-first century (Table 9.9). This channel can be remodelled later to increase its size to accommodate additional flows during last 25 years of the twenty-first century (Table 9.9). The bed width of the Yilgarn section of the open arterial channel during last quarter of the twenty-first century increase almost four times its bottom width during first quarter of the twenty-first century (Table 9.10).

The flow depth of the Salt River section of the leveed arterial channel remains almost constant during the next 100 years for Drainage SSe option. Its bottom width however increases during last quarter of the twenty-first century. For Drainage LB option a large size leveed arterial channel is required to manage the drainage discharge during the 2000–2025 since it drains relatively large area. Even larger size channel will be required to manage the drainage discharge during 2075–2100. A very large open channel will be required to handle a mixture of drainage discharge and surface runoff during first quarter of the twenty-first century. Its size will increase further during last quarter of the twenty-first century to handle larger flows during last quarter of the twenty-first century.

The size of the Mortlock North arterial channel section of the leveed arterial channel is not expected to increase significantly over time due to the presence of Lake Hinds and Lake Ninan (Table 9.8). Thus, a fixed size channel can be constructed to handle the drainage discharge during the next 100 years. Due to larger flows expected under the Drainage LB option, a larger channel will be required than for the Drainage SSe option as listed in Table 9.9. A very wide but shallow open arterial channel will be required to handle the drainage discharge and surface water runoff during first 25 years of twenty-first century (Table 9.10). Its bottom width and flow depth increases further during 2075–2100.

The bottom width and depth of Mortlock East section of the leveed arterial channel for the Drainage SSe option remains constant during the next 100 years (Table 9.8). This is due to zero discharge from subcatchments 36, 37 and 38 to downstream subcatchments because of the presence of three terminal lakes: Lake Cowcowing, Lake Walyormouring and Lake Dowerin. A wide but shallow leveed channel will be required for the Drainage LB option (Table 9.9). Its size is not expected to increase over time. A very wide but shallow open arterial channel will be required to manage the mixture of drainage discharge and surface water runoff during 2000–2025 (Table 9.10). The channel size is not expected to change during the next 100 years.

It is to note that the above leveed and open arterial channel size assessments are first-order estimates only and based on many assumptions. Further studies and detailed assessments will be required for the detailed technical feasibility, planning, design (size) and construction of these arterial systems.


Table 9.7. Main assumptions for channel design parameters

Minimum bottom width of an arterial channel irrespective of its type is 1.1 m due to excavator size limitations.

Flow velocity is restricted to a maximum value of 1.2 m sec-1 to minimise erosion and scouring of channels.

Ratio between bed width (above 1.1 m) and drain depth depends on the available longitudinal gradient, roughness factor and velocity.

A trapezoidal section is assumed for both leveed and open arterial channels.

Side slopes for the open channels are 1:1 on both sides.

Side slopes for the leveed channels are 0.5:1 both sides.

Roughness factor or Manning’s n is 0.022 and applicable everywhere for unlined engineered earthen arterial channels.

Table 9.8. Size and excavation cost of leveed arterial system with salt lake storage option (Drainage SSe)

Table 9.9. Size and excavation cost of leveed arterial system without salt lake storage option (Drainage LB)

Leveed arterial channel without salt lake storage (LB)

Channel section Total

length (km)

Channel size (2000–2025) Total

cost ($ million)


cost ($ million)

Bed width (m)

Channel depth

(m)

Bed width (m)

Channel depth

(m) Lockhart East 114 1.1 2.4 0.9 1.1 2.4 0.9Lockhart South 63 1.9 2.7 0.9 3.3 2.8 1.4Lockhart Main 167 3.6 2.9 4.1 6.4 2.9 6.6Yilgarn 177 2.1 2.6 2.6 7.7 2.7 10.9Salt River 89 9.4 3.0 14.2 18.2 3.0 25.7Mortlock North 169 7.5 2.0 4.0 7.5 2.0 4.0Mortlock East 294 15.5 2.1 26.4 26.3 2.1 43.4Grand total 1073 53.1 92.9

Leveed arterial channel with salt lake storage (SSe)


length (km)

Size (2000–2025) Total

cost ($ million)

Size (2075–2100)

Total cost ($ million) Bed

width (m)

Channel depth

(m)

Bed width (m)

Channel depth



Table 9.10. Size and excavation cost of open arterial system

Open arterial channels


length (km)


cost ($ million)


cost ($ million)

Bed width (m)

Channel depth

(m)

Bed width (m)

Channel depth


Note: The construction cost during 2075–2100 is not applicable where the channels were constructed during 2000–2025 and their sizes remained unchanged over time. Excavation cost of arterial channels The indicative cost estimates of the leveed and open arterials were estimated from their size and a large number of assumptions. The excavation cost estimates are listed Tables 9.8 - 9.10 for two periods 2000–2025 and 2075–2100 and assumptions are listed in Table 9.11. The excavation cost of these individual systems was estimated to help assess their feasibility. The cost estimates for 2075–2100 assume the construction of brand new channels; the previously constructed channels (2000–2025) are ignored. The remodelling or re-engineering cost, to increase the size of previously constructed channels, is expected to be substantially less than the construction of new channels.

The arterial channel excavation cost of various sections varies according to their size and length. The larger and longer is the channel, the higher is its excavation cost. During 2000–2025, the Lockhart South section of the leveed arterial channel (Drainage SSe) has the lowest excavation cost ($0.7 million) because of its size and length (Table 9.8). For Drainage LB option, excavation cost of a leveed channel in this section will be higher ($0.9 million) due to its larger size. During 200–2025, excavation of the Mortlock East section (longest of all sections) of the leveed arterial will cost about $13 million (Table 9.8) with salt lake storage option (Drainage SSe) and $26.4 million (Table 9.9) without salt lake storage option (Drainage LB). Larger channels required to accommodate larger flows will incurr higher excavation costs.

In the case of open arterial channels, the Lockhart East section again has the lowest excavation costs during 2000–2025 (Table 9.10) due to smaller size channel. The Salt River section of an open arterial will have an excavation cost of $45.7 million during 2000–2025. A very high excavation cost of this section is due to very large size channel required to accommodate both surface runoff and drainage discharge during 2000–2025 (Table 9.10).

To manage the drainage discharge from subcatchment scale leveed drainage systems during first 25 years of the twenty-first century the construction cost of leveed arterial channels (with salt lake storage, i.e., Drainage SSe) in all sections will be about $27 million (Table 9.8). For managing the drainage discharge during 2075–2100, the estimated excavation cost is about $29 million if these channels are constructed new assuming that the previously constructed channels were filled-in completely. Because of the small differences in the size of channels required during first and last quarters of the twenty-first century the preferred option will be to construct these channels such that they have enough capacity to handle drainage discharge for the next 100 years.

The total estimated excavation cost of leveed arterial system without salt lake storage (Drainage LB) is about $53 million (Table 9.9) during first 25 years of twenty-first century (2000–2025). To manage the drainage discharge during 2075–2100, a larger size leveed


arterial system will be required and its excavation is estimated to cost about $93 million assuming the construction of new channels.

Total indicative excavation cost of an open arterial system will be about $118 million for managing the drainage discharge and surface runoff during 2000-2025 (Table 9.10). About $188 million will be required for excavating a larger size open arterial system for managing both drainage discharge and surface runoff during 2075–2100 assuming the construction of a brand new system.

Table 9.11. Main assumptions for the excavation cost of arterial channels

Cost estimates represent excavation/construction costs only.

No road and railway crossing (bridges and culverts) and maintenance costs are included.

Same unit excavation/construction costs are applied to both leveed and open arterial channels.

Construction costs are applied uniformly across all sections of the arterial channels.

Accessibility, mobilisation, soil types, waterlogging, etc are not considered in these cost estimates and therefore the actual construction costs may differ from these indicative estimates depending on the field conditions.

Extra protective banks, that may be required near towns, are not included in the cost estimates

Construction cost per cubic meter of an arterial channel of 1.5 to 2.5 m depth is assumed as $1.50.

It is $1.75 m-3 for an arterial channel of 2.5 to 3 m depth.

Excavation cost is increased by 50% from $1.75 m-3 to $2.63 per m-3 if the top width is greater than 10 m due to double handling of the soil.

Cost of shaping of batters, if required, is not included.

Cost impact of using the bulldozers and scrapers for larger channels depending on the field conditions and accessibility is not considered.

No blasting costs, that may be required to cut the channels through rock formations, are included

Site investigation costs (soil testing, drilling, monitoring, surveying, etc.) are not included.

9.2.4. Leveed drainage and subcatchment retention system (evaporation basins)

Introduction This scenario, called subcatchment retention system (Drainage EB), was undertaken to assess the feasibility of constructing evaporation basins at the outlet of each subcatchment for managing the drainage discharge from farm and subcatchment scale leveed artificial drainage systems. The following preliminary assessment will attempt to assess their size and indicative construction costs. Detailed investigations about the planning, design, construction and maintenance of the evaporation basins were beyond the scope of this project. In this project the aim was to provide a first-order estimate of their size using design inflow volumes and their excavation cost following the guidelines in JDA and WADA (2004).

Parameters affecting the evaporation basin size The basin size was based on annual design inflow, evaporative potential or potential net evaporative loss, and drainage water salinity.

Design inflow To estimate the size of evaporation basins during first quarter of the twenty-first century, the design inflow was based on the drainage discharge volumes from subcatchment scale leveed drainage during 2000–2025. Similarly to estimate their size during last quarter of


twenty-first century, the second design inflow was based on the drainage discharge volumes during 2075–2100. The annual drainage discharge volumes from 2000–2025 and 2075–2100 at the outlet of each subcatchment were then used to determine the annual drainage discharge volumes of 1 in 10 year recurrence interval (Table (9.13). These annual drainage discharge volumes were used as the design annual inflow rates for evaporation basins size estimation.

The design annual inflow rates varied among subcatchments depending on their size and salinity risk areas. They were zero for uncleared and forested subcatchments where drainage was not required. They were also zero for those subcatchments that contained terminal salt lakes accommodating all the drainage discharge. Generally the design annual inflow rates increased over time in those subcatchments where the salinity risk areas (drainage lengths) and therefore the drainage discharge increased over time (Table 9.13).

Net evaporative loss The evaporative potential or potential net evaporative loss is defined as the difference between annual Class A pan evaporation and rainfall. For a given salinity of the drainage water the evaporation basin size depends directly on the net evaporative loss. For a constant design annual inflow a smaller size evaporation basin is required where the net evaporative loss is relatively large and vice versa. The net potential evaporative loss curves for the wheatbelt area are available in JDA and WADA (2004). They vary from 2600 mm for the eastern and northern subcatchments to 1200 mm for the western parts of the wheatbelt. Approximations from these curves were used to estimate the net evaporative loss in all 108 subcatchments of the Avon Basin.

Since the annual pan evaporation and rainfall for each of the 108 subcatchments of the Avon Basin were previously estimated and used during calibration of LASCAM these values were also used for the estimation of net evaporative loss. These net evaporative loss values are given in the Table 9.13 for all 108 subcatchments of the Avon Basin. The net evaporative loss varies considerably among subcatchments. It ranges from over 2000 mm for the eastern subcatchments with high annual Class A pan evaporation and low annual rainfall to under 1300 mm for some of the western subcatchments with relatively lower annual pan evaporation and high annual rainfall.

Drainage water salinity The evaporation basin size also depends on salinity of the drainage water to be stored and evaporated in the basin. The evaporation basin guidelines by JDA and WADA (2004) developed relationships between potential net evaporative loss and basin area (ha) required per 100 ML year-1 inflow for three salinity levels of the inflow or drainage water: 5000 mg L-1 (909 mS m-1), 10,000 mg L-1 (1818 mS m-1) and 50,000 mg L-1 (9090 mS m-1). The curve used for estimating the evaporation basin size in this study is based on 50,000 mg L-1 (9090 mS m-1) salinity level (Figure (9.2). It was reasonable to assume this salinity level since groundwater or drainage discharge from most of the eastern and central regions of the Avon Basin is usually very saline and its evaporation within the drains increases its salinity even further. The salinity of drainage discharge from some subcatchments is lower than 50,000 mg L-1 and therefore will require relatively smaller evaporation basins.

Determining basin size A nonlinear equation approximating the curve in Figure 9.2 was developed to estimate the evaporation basin area for 100 ML year-1 inflow for any given evaporative loss. This equation was used to determine the evaporation basin area required in each of the 108 subcatchments to store and evaporate an annual design inflow rate of 100 ML year-1. To estimate the evaporation basin size for a subcatchment, the evaporation basin area, determined for 100 ML year-1 inflow rate, was multiplied by the design annual inflow of the subcatchment and divided by 100.

The evaporation basins vary in size in various subcatchments depending on their location or evaporative loss and design inflow (Figure 9.3). As expected the relationship between design inflow and basin area is similar for both design inflow periods (2000–2025 and 2075–2100). The impact of net evaporative loss on the required basin area is very evident when


design inflows are larger (> 5000 ML year-1). For example, the basin area varies from as low as 600 ha to as high as 1300 ha for a design inflow of 5000 ML year -1 due to different net evaporative losses in various subcatchments of the Avon Basin.

The evaporation basin size required during first quarter of twenty-first century varies from less than 25 ha to more than 3500 ha in various subcatchments depending on their salinity risk area (drainage length), annual rainfall and pan evaporation (Figure 9.4). Generally larger evaporation basins are required during last quarter of the twenty-first century due to larger salinity risk areas and larger artificial drainage lengths in various subcatchments (Figure 9.5). Relatively larger evaporation basins are required for the western subcatchments due to low net evaporative loss and high rainfall.

The evaporation basins are not required in subcatchments with terminal lakes which accommodate all the flow from subcatchment scale leveed drainage. Similarly no evaporation basins are required in forested and uncleared subcatchments that have no salinity risk areas (Figure 9.4 and 9.5).

9.2.5. Determining basin costs

The indicative construction/excavation cost of evaporation basins is estimated from their area and assumptions listed in Table 9.12. Since no salinity risk was assumed for the first 20 subcatchments comprising the western region, these subcatchments do not appear in Table 9.13.

The indicative construction cost of evaporation basins in subcatchments of the Avon Basin for the two design inflow periods is given in Figures 9.6 and 9.7. During 2000–2025 the costs are generally higher for the western region subcatchments and lower for the central and eastern regions. But this is not the case for the second design inflow period (2075– 2100). The costs during this period are rather randomly distributed because the differences between higher design inflows, from larger required drainage lengths over time in the eastern subcatchments and relatively steady design inflow in the western subcatchments are balanced by the differences in the net evaporative loss between the eastern and western region subcatchments.

The cost of evaporation basins during first quarter of the twenty-first century varies from $0.3 to about $40 million in various subcatchments depending on the design inflows, basin area and net evaporative loss (Figure 9.6). During last quarter of the twenty-first century it varies from $0.5 to $50 million among various subcatchments (Figure 9.7). For managing the drainage discharge during first quarter of the twenty-first century the total indicative excavation cost of evaporation basins is about $325 million. For managing the drainage discharge during last quarter of the twenty-first century the total indicative cost is about $540 million. The increased salinity risk areas requiring more artificial drainage over time increases the drainage discharge requiring larger evaporation basins over time. Therefore the total indicative construction cost of evaporation basins during last quarter of the twenty-first century is expected to be substantially higher than that during the first quarter. However evaporation basins previously constructed to manage the drainage discharge during 2000–2027 can be re-engineered to increase their capacity to accommodate additional flows during last quarter of the twenty-first century. The cost of re-engineering of previously constructed evaporation basins is not estimated but it is expected to be much less than the construction cost of new basins.


Table 9.12. Assumptions for cost estimation of evaporation basins

Indicative construction cost is $10,000 per ha of basin area.

This cost is uniformly applicable across all subcatchments that require evaporation basins.

Cost does not depend on soil type, landscape, groundwater level, permeability, allowable leakage, size, etc. Actual construction cost may vary considerably from the assumed cost depending on the field conditions and a multitude of other factors.

Construction cost does not include any planning, design, operation and maintenance costs.

No salt disposal costs are included.


Table 9.13. Design inflows and construction cost of subcatchment scale evaporation basins

Su

b-

catc

h I

D

Mea

n

ann

ual

p

an

evap

or-

atio

n

Mea

n

ann

ual

ra

infa

ll

Net

ev

apo

r-at

ive

loss

(2

-3)

Bas

in

area

per

10

0 M

L

yr-1

in

flo

w

Des

ign

fl

ow

(L

evee

d

dra

ina

ge)

To

tal

bas

in

area

To

tal

cost

(mm) (mm) (mm) (ha) ML yr-1 ML yr-1 (ha) (ha) ($ 000) ($ 000)

1 2 3 4 2000-2025

2075-2100

2000-2025

2075-2100

2000-2025

2075-2100

21 1979 420 1559 18 36 26 6 5 64 46

22 1979 406 1573 17 684 700 120 122 1197 1225

23 2045 400 1645 16 1415 1537 230 250 2301 2500

24 2049 357 1692 16 3588 6056 561 947 5609 9468

25 2162 386 1776 15 9868 12336 1456 1820 14562 18203

26 2206 375 1831 14 0 0 0 0 0 0

27 2206 361 1845 14 0 0 0 0 0 0

28 2268 339 1929 14 2534 5860 346 799 3457 7995

29 2301 317 1984 13 3740 9410 498 1253 4979 12528

30 2001 396 1605 17 2262 2594 382 439 3824 4385

31 2001 378 1623 17 848 999 141 166 1408 1659

32 1990 377 1613 17 4271 6561 716 1100 7162 11002

33 2052 358 1694 16 6926 9627 1081 1503 10811 15027

34 2118 321 1797 15 3818 5973 556 871 5565 8706

35 2173 319 1854 14 2999 4694 424 664 4240 6636

36 2136 350 1786 15 0 0 0 0 0 0

37 2202 332 1870 14 0 0 0 0 0 0

38 2260 307 1953 13 0 0 0 0 0 0

39 1979 439 1540 18 549 552 100 100 999 1005

40 1950 535 1415 22 5059 5921 1106 1294 11055 12939

41 1903 440 1463 20 4278 4964 866 1005 8657 10045

42 1903 436 1467 20 987 1060 199 213 1985 2132

43 1913 377 1536 18 3972 5157 726 943 7264 9432

44 1833 425 1408 22 2002 2316 443 512 4427 5122

45 1833 422 1411 22 70 77 15 17 154 169

46 1833 417 1416 22 219 247 48 54 478 539

47 1833 478 1355 24 6988 9020 1700 2194 16997 21939

48 1742 560 1182 35 10699 11030 3752 3869 37524 38685

49 1767 442 1325 26 3003 3822 774 985 7739 9850

50 1716 473 1243 31 2996 3359 916 1027 9160 10270

51 1837 400 1437 21 673 816 142 172 1418 1720

52 1800 416 1384 23 15821 23013 3649 5307 36486 53072

53 1775 359 1416 22 7706 10891 1681 2376 16811 23760

54 1829 347 1482 20 0 0 0 0 0 0

55 1804 349 1455 20 60 59 12 12 123 121

56 1709 373 1336 25 2081 2780 525 701 5249 7012

57 1760 348 1412 22 0 0 0 0 0 0

58 1833 373 1460 20 0 0 0 0 0 0

59 1855 336 1519 19 5535 7914 1035 1479 10347 14794

60 1961 332 1629 17 7707 8907 1272 1470 12723 14704

61 2060 316 1744 15 662 652 100 98 997 982

62 2038 324 1714 15 4489 8304 690 1277 6903 12769

63 2052 335 1717 15 960 2761 147 424 1473 4236

64 2114 335 1779 15 1877 12164 276 1792 2765 17917

65 2027 333 1694 16 1701 3133 266 489 2655 4890

66 1921 326 1595 17 0 0 0 0 0 0

67 1866 332 1534 18 0 0 0 0 0 0


Table 9.13 (Continued). Design inflows and construction costs of subcatchment scale evaporation basins

Su

b-c

atch

ID

Mea

n a

nn

ual

p

an e

vap

or-

atio

n

Mea

n a

nn

ual

ra

infa

ll

Net

ev

apo

r-at

ive

loss

(2-

3)

Bas

in

are

a/10

0 M

L/y

r in

flo

w

Des

ign

flo

w

(Lev

eed

d

rain

age)

To

tal b

asin

ar

ea

To

tal c

ost

(mm) (mm) (mm) (ha) ML/yr ML/yr (ha) (ha) ($ in 000) ($ 000)

1 2 3 4 2000–2025

2075–2100

2000–2025

2075–2100

2000–2025

2075–2100

68 1829 327 1502 19 865 3000 165 574 1654 5738

69 1829 330 1499 19 0 0 0 0 0 0

70 1829 342 1487 20 1411 6868 276 1342 2756 13416

71 1859 334 1525 19 1028 1426 191 264 1907 2645

72 1808 344 1464 20 2472 6888 499 1392 4995 13918

73 1767 338 1429 21 813 2658 174 567 1736 5675

74 1698 349 1349 25 0 0 0 0 0 0

75 1946 334 1612 17 3129 6346 525 1065 5252 10652

76 1972 347 1625 17 2762 9097 458 1508 4577 15075

77 1972 351 1621 17 0 0 0 0 0 0

78 1928 346 1582 17 0 0 0 0 0 0

79 1899 345 1554 18 0 0 0 0 0 0

80 1829 362 1467 20 0 0 0 0 0 0

81 1829 357 1472 20 0 0 0 0 0 0

82 1862 344 1518 19 292 885 55 166 547 1657

83 1829 345 1484 20 0 0 0 0 0 0

84 1829 343 1486 20 109 173 21 34 213 338

85 1829 343 1486 20 0 0 0 0 0 0

86 2060 321 1739 15 957 1182 145 179 1446 1786

87 2118 317 1801 15 33 19 5 3 48 28

88 2118 315 1803 15 48 76 7 11 70 110

89 2118 329 1789 15 274 398 40 58 401 583

90 2147 319 1828 14 6185 11815 886 1693 8862 16929

91 2155 319 1836 14 0 0 0 0 0 0

92 2125 334 1791 15 1650 9471 241 1385 2413 13852

93 2239 313 1926 14 1564 4101 214 560 2137 5603

94 2275 312 1963 13 4224 7659 568 1029 5676 10291

95 2374 312 2062 13 2086 7920 268 1019 2683 10188

96 2257 294 1963 13 1125 1555 151 209 1512 2090

97 2374 293 2081 13 1716 6229 219 795 2189 7945

98 2618 315 2303 12 0 0 0 0 0 0

99 2301 296 2005 13 0 0 0 0 0 0

100 2239 304 1935 14 0 0 0 0 0 0

101 2198 329 1869 14 978 3964 137 556 1372 5562

102 2249 321 1928 14 972 4059 133 554 1327 5540

103 2337 312 2025 13 0 0 0 0 0 0

104 2268 331 1937 14 1730 9626 235 1309 2352 13085

105 2337 301 2036 13 0 0 0 0 0 0

106 2206 333 1873 14 0 0 0 0 0 0

107 2553 305 2248 12 0 0 0 0 0 0

108 1928 352 1576 17 0 0 0 0 0 0

32569 54019 325692 540186


Figure 9.2. Relationship between potential net evaporative loss and basin area

Figure 9.3. Design inflow and basin area relationship for the Avon Basin


Figure 9.4. Evaporation basin size required in various subcatchments of the Avon Basin during first

quarter of the twenty-first century

Figure 9.5. Evaporation basin size required in various subcatchments of the Avon Basin during last



Figure 9.6. Indicative evaporation basin costs in various subcatchments of the Avon Basin during first


Figure 9.7. Indicative evaporation basin construction costs in various subcatchments of the Avon

Basin during last quarter of the twenty-first century


9.3. Summary The arterial channels are more feasible than evaporation basins based on their construction costs. Among arterial channels, the leveed arterial system which allows the entry and storage of drainage discharge in the salt lake system is the most cost effective of all arterial systems assessed in this study. To meet regional drainage management requirements of first 25 years of the twenty-first century the total excavation cost of this arterial system is around $27 million. The excavation cost is almost double ($53 million) for the leveed arterial without salt lake storage for the same period. The excavation cost for an open arterial system is more than 4 times the cost of leveed arterial with salt lake storage option.

The leveed arterial system with salt lake storage option assumes that the drainage discharge will be allowed to enter and be stored in all salt lakes encompassing the region. It is unlikely that all salt lakes of the Avon Basin will be declared as sacrificial lakes. If some salt lakes or at least parts of some already degraded salt lakes are classified as sacrificial lakes and others, which have significant environmental, biodiversity, ecological, recreational values, are classified as protected salt lakes then a combination of the two leveed arterial systems (with and without salt lake storage) can be implemented. The determination of which lakes are to be protected or sacrificed is beyond the scope of this project and requires a comprehensive environmental impact assessment and conservation strategy for any proposed regional drainage scheme.

On the basis of excavation costs alone a leveed arterial system without salt lake storage option is feasible if the drainage discharge is not allowed to enter and be stored in the salt lakes. However, such a system will require an effective disposal or evaporative or treatment system at its outlet. An equivalent treatment or disposal cost of piping of about 57 GL per annum mean annual drainage discharge to ocean ranges between $97 and $110 million.

Open arterial system require larger channels and therefore more expensive and least feasible. This system also requires very wide and relatively shallow, practically infeasible, channels in some sections due to constraints on maximum velocity limits, roughness and longitudinal gradients. The evaporation basins are not feasible because they require large basin areas and have prohibitive construction costs.


10. CONCLUSIONS The prediction quality of the calibrated model is satisfactory and provides good confidence in the ability of the model to predict spatial and temporal patterns of flows and loads in ungauged subcatchments and in the scenario predictions. Model predictions for the no-drainage case have highlighted that the current hydrological state of the Avon Basin is not static. The catchment is not at equilibrium. We are presently in a phase of steadily increasing watertables, streamflows, lake storages and salt loads. These increases have been occurring in some parts of the basin since European settlement and are in response to the widespread replacement of native vegetation with shallow-rooted crops and pasture. The results indciate that while these increases continue, the rates of increase are now slowing and that most parts of the catchment will—in the absence of further land management changes—reach equilibrium some time during the current century. A consequence of this non-stationary catchment behaviour is that in assessing the impacts of any future management or climatic changes, it would be misleading to compare those impacts with the current state of the catchment. Instead, we should strive to make comparisons against a non-stationary benchmark scenario.

Key conclusions from analysis of 17 different scenarios, conducted to assess the impacts on streamflows, salt loads, salinities, lake discharge rates and salinities and groundwater levels, are described. The difference in flows and loads between the open drainage scenario and the sum of the leveed and natural channel flows in the leveed drainage scenarios is negligible. The artificial drainage scenarios lower groundwater levels and salinities in the drained subcatchments. Artificial drainage leads to modest increases of up to 29 % in flow at the basin outlet and to more substantial increases up to 210 % in salt load. Peak streamflows are considerably larger for the drained scenarios than for the baseline scenario in arid subcatchments downstream of lakes, but in first-order subcatchments and in subcatchments below Qualandary Crossing, drainage has less impact on peak streamflows. Peak salt loads can become extremely large in undrained, infrequently discharging subcatchments with artificially-drained tributaries. Artificial drainage increases the frequency, volume and salinity of lake discharges.

Mitigation of the impacts of artificial drainage by preventing the lakes from discharging is effective in reducing streamflows, salt loads and salinities in most downstream subcatchments, except for those that are downstream of terminal lakes. Mitigation of the impacts of artificial drainage by elevating the discharge height of the lakes by 0.3 m has little effect in reducing streamflows, has no effect on salt loads and leads to small increases in flow salinity in most downstream subcatchments.

Mitigation of the impacts of leveed drainage by retaining the leveed channel flows within the subcatchment in which they are generated is highly effective in reducing flows, loads and salinities in both streams and lake discharges. It is also effective in reducing peak streamflows and salt loads. Mitigation of the impacts of artificial drainage by complete elimination of the lakes leads to significant increases in streamflow including peak flow events and slight increases in salt load, but to decreases in stream salinity in downstream subcatchments.

Revegetation of the catchment with woody perennials causes substantial falls in watertables, but substantial increases in groundwater salinity. Revegetation reduces streamflows substantially and the magnitude of these changes increases with time. However, it has little impact on peak flows at the basin outlet. Revegetation leads to enormous reductions in stream salt load and salinity. Revegetation reduces the frequency, volume and salinity of lake discharges.

The five climate scenarios assessed in this report are all possible for the Avon Basin over the next 100 years. Despite this, they give rise to enormous variability in hydrological response in the catchment. The drier, eastern subcatchments are more sensitive to climate change than the wetter western subcatchments. Drier climates lead to lower watertables and slower


watertable responses than wetter climates. Streamflows and salt loads are greater for the wetter climates than for the drier climates. The driest climate leads to an almost complete cessation of discharge from all lakes.

The estimated construction cost of various drainage options was used to assess and rank these systems. An indicative construction cost estimate of the farm and subcatchment scale leveed drainage system with drainage discharge management via natural channels for about 4540 km drains in various subcatchments of the Avon Basin in 2000 is around $46 million. By 2100 the total estimated construction cost of the subcatchment scale leveed drainage system is about $227 million. Around $91 million will be required to construct around 9080 km drains by 2025.

An indicative estimate of the total construction cost of the farm and subcatchment scale open drainage system with drainage discharge management via natural channels in 2000 is about $36 million based on medium unit construction cost of $8,000 per kilometre of drain. In 2100 an estimate of the total construction costs is around $182 million.

Various sections of the leveed and open arterial channels were designed on the basis of daily peak flows from subcatchment scale leveed and open drainage systems. The cost estimates for constructing the arterial channels were based on their size.

For the subcatchment leveed drainage with leveed arterial system and salt lake storage option the size of all sections of the leveed arterial channels remains unchanged except the Salt River section. It means that the channels constructed to manage the drainage discharge during 2000–2025 will also be enough to handle the drainage discharge during 2075–2100 provided their required maintenance is carried out regularly. An indicative estimate of the total construction cost of all sections of the leveed arterial channels with salt lake storage option is about $27 million. These channels will be required to manage the drainage discharge from subcatchment scale leveed drainage systems during first 25 years of the twenty-first century.

Larger leveed arterial channels are required for the leveed subcatchment drainage with leveed arterial system if the drainage discharge is not allowed to enter and be stored in the salt lakes. The total estimated construction cost of this leveed arterial system is about $53 million. To manage the drainage discharge during 2075–2100 a larger size leveed arterial system will be required and its total construction is likely to be about $93 million assuming the construction of new channels.

An open arterial system will require larger channels than leveed arterial system because it entertains both surface runoff and drainage discharge. The Salt River and Mortlock East sections require very large channels. Significantly larger channels are required during 2075–2100 than during 2000–2025 because of increased surface runoff and drainage discharge rates during later part of the twenty-first century. Total indicative construction cost of an open arterial system will be about $118 million for managing the drainage discharge and surface runoff during 2000–2025. About $188 million will be required to construct a larger open arterial system to manage both drainage discharge and surface runoff during 2075–2100 assuming the construction of a brand new system. Remodelling of the old system built during 2000–2025 is likely to incur less cost.

To manage the drainage discharge from subcatchment scale leveed drainage system the feasibility of constructing the evaporation basins at the outlet of each subcatchment was assessed. The construction of evaporation basins in whole of the Avon Basin during first quarter of the twenty-first century is likely to cost about $340 million. For managing the drainage discharge during last quarter of the twenty-first century the construction of evaporation basins is likely to cost about $540 million assuming the construction of new evaporation basins. The cost of re-engineering of the previously constructed evaporation basins is expected to be much less than the cost of constructing the new basins. The construction of evaporation basins seems infeasible because of prohibitive construction costs. If other costs such as site investigation, planning, design, surveying, operation and maintenance, salt disposal and management are also included the total cost of managing the


drainage discharge by subcatchment scale retention system via evaporation basins will become even more prohibitive.

Of all systems assessed in this study the leveed and open drainage with natural channels discharge management is the most cost effective. However it is infeasible due to likely adverse hydrological and environment impacts downstream. The leveed subcatchment and leveed arterial is the next most feasible system based on the constructions costs alone if the drainage discharge is allowed to enter and be stored in the salt lake system. Open arterial system is the most expensive of all arterial systems. Subcatchment retention system using evaporation basins has prohibitive construction costs.


APPENDIXES

APPENDIX 1. MODEL DEVELOPMENT

Appendix 1.1. Model selection Three modelling platforms were reviewed for this project. The generation of runoff from each subcatchment and routing the flow along streams and through storages, such as natural creeks and river systems, and salt lakes were the two main tasks identified for modelling. Salinity modelling was considered part of the flow generation and routing models. Large-scale hydrological models/modelling platforms, evaluated because of their potential availability, were:

a. Environmental Management Support System (EMSS)

b. The Integrated Quantity and Quality Model (IQQM), and

c. LArge Scale CAtchment Model (LASCAM)

a) The Integrated Quantity Quality Model (IQQM) The IQQM is a hydrologic modelling tool developed by the NSW Department of Infrastructure, Planning and Natural Resources (DIPNAR) for use in planning and evaluating water resource management policies at river basin scale. It is designed for addressing water quality and quantity issues. For rainfall runoff model, IQQM uses the Sacramento Model. It uses daily input and produces output at a daily time step. It consists of other utilities such as an optimisation model and statistical tools.

b) Environmental Management Support System (EMMS) EMSS uses the lumped conceptual catchment scale model to estimate daily runoff and pollutant load from catchments. It consists of a number of models connected together to create a network. The three main models used in the EMSS are:

the catchment model – Colobus;

stream routing model – Marmoset; and

the storage reservoir/dam model – Mandrill.

Colobus uses daily input data.

c) Large Scale Catchment Model (LASCAM)

LASCAM is a large scale conceptual hydrologic model. It uses daily input data. This model has previously been used in the Avon Basin.

These models were reviewed for the purposes of doing the whole job, both subcatchment rainfall-runoff, and flow and salt routing including salt lakes, and doing a subset of the main jobs. Initial study suggested that all these models have been used in large scale hydrologic modelling, and have relative strengths and weaknesses in different areas such as pre and post processing capabilities.

Objective criteria An objective analysis was done to select the best model by seeking to maximise the objective function Z given by:

Z = w1a1+ w2a2+w3a3+w4a4+w5a5+w6a6


Where:

a1 = past track records of the model to do the core tasks

a2 = local familiarity of the model

a3 = national reputation of the model

a4 = useability, easy to learn, versatility

a5 = data requirements

a6 = other factors, and

w1..6 is the relative weight given to each a according to its importance in the final

outcome.

Each a was given a score between 1 and 3, with 3 being the most favourable mark. The issues considered in deciding a1 to a6 are listed below.

1. How much of the job does the model already do? Which model may require the least

modification?

2. How familiar are the West Australian water resource professionals with the model? What

is its credibility/profile among the public?

3. How well is the model known nationally? How widely is the model used outside Western

Australia?

4. How useable is the model for modellers of the project? Is it (relatively) easy to

learn/master? What is the range and quality of result presentation? Is there a pre or

post-processor, and graphics user interface (GUI) for results?

5. How well is the model capable of being used in a data poor environment?

6. Other extenuating factors, e.g. reservations/doubts, modeller’s experience with the

model? Model availability without any strings attached? User support? Source code

availability? (this was a major factor as the model almost certainly would need

modification to meet modelling requirements of the project).

Determination of decision variables

After a detailed study we determined that:

all models are capable of carrying out river network and catchment modelling.

data requirements are a big challenge for all models.

Issues with the IQQM

While evaluating IQQM, the issue of lack of proper salinity modelling and instability of IQQM came up. While negotiating these issues with DIPNAR, we became aware that IQQM would no longer be supported by them. Given salinity and other issues still unresolved and non-availability of a contact person at DIPNAR, it was decided to explore other alternatives.

Issues with the EMSS

EMSS has been used in 175 catchments of south east Queensland to estimate daily runoff and pollutant loads. It was written in C++ under the Tarsier framework, developed and supported by the CRCCH (Cooperative Research Centre for Catchment Hydrology). The model codes (Colobus, Marmoset, Mandrill) themselves are large, and the supporting code is


much larger (more than 500,000 lines of code). There would be a great penalty in learning the code from our end and modifying it as necessary.

As it became clear that DIPNR would not support IQQM into the future, CRCCH looked towards TIME (The invisible Modelling Environment) instead of Tarsier; new E2 model of eWater CRC is set to take the lead as the daily integrated water model for them. Given the model was 12 to 18 months away, it was not appropriate to select either EMSS or E2.

Issues with LASCAM

LASCAM is available to use with full access to its code. It has previously been used in the Avon Basin. On the negative side, it was found that LASCAM does not have well developed pre and post processing capabilities which makes it less user friendly.

Based on the above considerations, values of decision parameters and relative weights were assigned. Table A1.1 shows the model suitability maximisation matrix with all parameter values.

Table A1.1. Model maximisation matrix for the evaluation of various models

Already Does (core tasks)

Local Familiarity

National Reputation

Useability Data need

Others

a1 a2 a3 a4 a5 a6EMSS 1 1 2 2 1 1IQQM 2 2 3 2 2 0.5LASCAM 3 3 1 2 2 2

W 0.25 0.1 0.15 0.25 0.05 0.2

Z calculation z1 z2 z3 z4 z5 z6 ZEMSS 0.25 0.1 0.3 0.5 0.05 0.2 1.4IQQM 0.5 0.2 0.45 0.5 0.1 0.1 1.85LASCAM 0.75 0.3 0.15 0.5 0.1 0.4 2.2

Model Name

Based on the above maximisation exercise it was determined that LASCAM (with highest Z value) was probably the most suitable and complete candidate to perform both tasks concurrently.


Appendix 1.2. Model description LASCAM (Sivapalan et al., 2002) hydrological model was used to predict flows and loads in the Avon Basin. LASCAM was developed to predict the impact of climate and land use changes on fluxes of water, salt, sediment and nutrients in forested and agricultural catchments in Western Australia. It operates on a daily time step and relies on calibration of model parameters against one or more observed records of streamflow and load.

Gridded topographic information is used to divide a catchment into a number of subcatchments and to delineate a stream network. LASCAM is applied separately to each of these subcatchments and the resulting flows are routed along the stream network. At the subcatchment scale, the model revolves about four inter-connected stores of soil water representing the near-stream perched aquifer (the A store), the upper soil layers (D store), the deeper, regional groundwater (the B store) and the unsaturated zone (the F store). The conceptual arrangement of stores in LASCAM is shown in the Figures A1.1 and A1.2. Streamflow is generated from infiltration-excess and saturation-excess overland runoff and from a baseflow discharge from the A store. The hydrological processes and subcatchment properties influencing them are assumed to be lumped at the subcatchment scale, but are allowed to vary between subcatchments. A global set of model parameters is used, i.e. all subcatchments use the same parameter set. Routing is achieved through a simple but efficient scheme in which bulk stream velocity is dependent on streamflow volume.

Figure A1.1. Schematic of a conceptual hillslope in LASCAM and the arrangement of the conceptual

stores


Figure A1.2. Flowchart of water fluxes in LASCAM

The inputs to the water balance component of the model are subcatchment specific daily rainfall, pan evaporation and land use information, e.g. leaf area index, which is allowed to vary with time, while topographic data are needed to define the subcatchments and the stream network. For calibration purposes, measured streamflow records are also required at one or more points in the catchment's stream network. The outputs from the model, for each subcatchment and for the total catchment, are surface and subsurface runoff, actual evaporation, recharge to the permanent watertable, baseflow and measures of soil moisture.

The version of LASCAM used in this project was modified by Zammit et al. (2003) to include explicit representation of spatial heterogeneity in soil and topographic characteristics and to reduce the number of model parameters that require calibration. This revised model uses the concept of a variable infiltration capacity to model throughflow and baseflow processes as a function of a notional distribution of soil depths. This allows greater physical realism in the conceptualisations of subsurface flow processes and allows for explicit prediction of watertable depths. Among the topographic variables that must be specified for each subcatchment is a measure of drainage density. This variable will be exploited in this project to facilitate prediction of the impacts of artificial drainage.


Appendix 1.3. Model modifications To make LASCAM suitable for the study and fulfil project requirements the model needed modifications. These modifications included lake storage, natural channel storage and flow, lake and natural channel evaporation, and artificial drainage.

Appendix 1.3.1. Description of modifications

There is an extensive network of salt lakes in the Avon Basin. Most of these lakes are large. They rarely overflow and most of them remain dry for significant periods in a year. One of the obvious options therefore is to assess the feasibility of storing drainage water in these lakes and estimate the volumes of water that can be stored both with and without their training or engineering. The selected model lacked lake storage simulation ability and modification of the computer code was required to include this capability. The Avon Basin is also comprised of broad and relatively flat natural creeks, channels and rivers. They have huge storage capacity. These natural creeks and channels hold back a significant portion of surface runoff under normal rainfall conditions and prevent it from entering the Avon River. To assess and simulate the volumes of water that remain stored within these natural channels the LASCAM code needed modification. The model also needed modifications to include lake and natural channel evaporation. Ignoring evaporation from simulations was likely to produce unreasonable results. One of the main engineering options to be assessed in this study was to simulate the impact of subcatchment scale artificial drainage on the flow rates, volumes and quality. Modification of LASCAM was required to incorporate this capability in the model. A detailed description of these modifications follows.

Appendix 1.3.2. Lake modifications

The existing LASCAM model has a rudimentary lake routing algorithm. The relative lack of complexity of this algorithm is dictated by the general paucity of information on lake dimensions and discharge characteristics. However, it is considered too simple to adequately describe lake processes in a region such as the upper parts of the Avon Basin where playas exert a dominant control on stream routing. With this project including a comprehensive lake survey campaign, coupled with some detailed GIS based lake modelling, the opportunity arose to revise the lake algorithm in LASCAM, while still retaining the original lake modelling framework.

The hydrological and river routing modules of LASCAM are capable of generating volumes of inflows into the lakes. The basic requirements of a lake model then enable predictions of the discharge of water from the lakes and the evaporation of water from the surface. In general, discharge is a function of lake volume, provided the volume exceeds the dead storage level of the lake. Furthermore, we would expect the rate of increase of discharge to increase with increasing volumes. We therefore model this discharge by an empirical expression:

Q = Qmax ((V – Vo) / (Vmax – Vo)) αq

where Vo is the dead storage volume; Vmax is some characteristic maximum lake volume; Qmax is the discharge rate at the maximum volume level; and αq is an optimisable parameter whose value is expected to be greater than one. The evaporation from a lake surface is a function of its area, which, in turn, depends on the volume. In this case, while we expect surface area to increase with volume, we expect the rate of increase in surface area to decrease with increasing volume. Surface area is modelled by:

A = Amax (V / V′max) αa

where Amax is the surface area corresponding to the characteristic lake volume V′max; and αa is an optimisable parameter whose value is expected to be less than one. The prime in V′max indicates that this volume is not necessarily the same as the characteristic volume in the expression for lake discharge.

The lake modelling equations outlined above require the specification of up to five characteristic dimensions and rates (Qmax, Vo, Vmax, Amax and possibly V′max) for each modelled lake. The development of these datasets is described in Section 5.7. The


equations also introduce two new (global) model parameters that must be optimised during the calibration process.

Appendix 1.3.3. Channel modifications

To account for the storage within the natural channel system, modification of LASCAM was required. The expressions were developed for all 108 major streams and rivers as explained in Section 5.8. A list of parameters was developed based on these expressions. The LASCAM code was modified to calculate channel velocities based on this list of parameters. Velocities calculated by this method were compared with those computed by LASCAM based on the original set of parameters. If velocity at any location of the regional channel was less than a fixed minimum, a threshold velocity value was assigned instead. The discharge rates were then calculated from velocity-discharge relations.

One of the main advantages of this approach was the ability to change a natural channel into an engineered channel by just changing relevant parameters in the input table. To assess the impact of an engineered channel on flow velocities and volumes any where within the Avon Basin, it was only required to change a few parameters in the input table. A partial regional drainage as well as complete regional drainage of the Avon was made possible through this modification of LASCAM.

Appendix 1.3.4. Artificial drainage modifications

Modifications for regional scale artificial drainage have been explained in the above section. For the subcatchment scale artificial drainage two options were considered: open drains that include both surface and groundwater; and leveed drains that handle only groundwater. The artificial drainage length was estimated from Landmonitor hazard maps. For open and leveed drains at the subcatchment scale, a modified form of the Hooghoudt drainage equation (Ritzema, 1994) was used. The Hooghoudt equation can be given as:

q = (8K2drh+4K1h2)/L2

where L is the drain spacing (m); K1 and K2 are weighted hydraulic conductivities above and below the drains, respectively (mm/hr); h is the watertable head above drain at midpoint between drains (m); dr is the reduced depth to impermeable layer (m); and q is the drainage rate (mm/hr). This equation was used to determine the groundwater discharge volume and rate based on the length of leveed drainage in the subcatchment. The artificial drainage length, required in each subcatchment of the Avon Basin under a range of scenarios and conditions, is detailed in Section 5.10.


APPENDIX 2. GIS DATA PREPARATION

Appendix 2.1. GIS and DEM A GIS database was established to provide spatially distributed parameters in a consistent fashion as input into LASCAM. This involved identifying a series of points of interest where LASCAM would calculate flows and loads. These points would be subcatchment outlets coincided with stream gauge locations, major lake outlets and subcatchments directly upstream of major confluences. Two key datasets were required to derive the subcatchments: digital elevation data and existing topographic feature data. Existing GIS datasets for catchment and subcatchment boundaries, drainage, lake systems, gauging stations, etc. were sourced from Department of Water (DoW) and processed into the GIS.

Appendix 2.1.1. Streamlines and catchment boundaries

Two datasets describing the hydrological features and drainage patterns for the Avon Basin were obtained for specific purposes. The Geodata TOPO 250K hydrological features dataset (http://www.ga.gov.au/nmd/products/digidat/250k.htm) available through Geoscience Australia (GA - Formerly Auslig) was used to calculate drainage density in each subcatchment, an input parameter for LASCAM (Figure A2.1). Streamlines for each derived LASCAM subcatchment were extracted and the total length of streams calculated to determine the drainage density.

Figure A2.1 Geoscience Australia Geodata 250K Hydrological features for the Avon Basin

The other drainage dataset was acquired from GIS database of the Department of Water. These data show mapped drainage/streamlines within the catchment and were used to guide the drainage network determination as well as an overlay for map production. The mapped subcatchment boundaries data were also sourced from DoW for comparison to derive subcatchment boundaries and splicing of boundaries in areas outside the Landmonitor DEM. These boundaries have been determined through visual assessment of contour data (5 m contours in the western and central areas and 10–20 m contours in the eastern extremities). When combined with the DEM derived catchment boundaries the resulting subcatchment map is the best representation of surface water catchments (watersheds) for the region. Figure A2.2 depicts the DoW’s subcatchments and mapped drainage for the Avon Basin. There are 378 subcatchments in the Avon Basin according to this data set, however the


position of many of these subcatchment outlets were not suitable for LASCAM. LASCAM assumes only one inflow point for each subcatchment so where stream confluences occur, extra outlets are required. For this reason a customised set of outlets was developed and the DEM used to define subcatchment boundaries.

Figure A2.2. Department of Water subcatchments and hydrology for the Avon Basin

Appendix 2.1.2. Digital Elevation Model (DEM) construction and processing

A high resolution DEM is available for most of the southwest agricultural region. This was used to derive hydrological features such as drainage lines and catchment boundaries for the Avon Basin. The Landmonitor DEM has an absolute vertical accuracy of approximately 2 m and is produced as a 10 m grid interpolated from spot heights derived by automated photogrammetry techniques (Allen and Beetson, 1999). Landmonitor DEM tiles were acquired from DoW covering most of the Avon Basin. Since Landmonitor DEM did not cover the entire study area, a lower resolution contour data were acquired from Geoscience Australia (GA) for the remainder of the basin and used to construct a DEM for the eastern uncleared zones, east of the Rabbit Proof Fence, and the western most uncleared catchments for which high resolution Landmonitor DEM was not available.

Appendix 2.1.3. Mosaicing, resampling and patching of coarser DEM

More than 70 Landmonitor DEM tiles were converted from ERMapper format to ESRI grid format and compiled into the GIS database. The DEM tiles were “mosaiced”, i.e. joined with edges matched, to produce several regional datasets. These were re-sampled from a 10 m grid to a 50 m grid to reduce file size and processing time. These were then “mosaiced” together with the Geoscience Australia DEM data to construct a DEM for the entire basin


(Figure A2.3). The Landmonitor DEM contains many artefacts and discontinuities mostly resulting from joining of DEM tiles from different contractors that produced them. This results in errors when determining contributing areas and flow directions. Most of these have been removed by a smoothing algorithm applied to the final product however it still does not produce a hydrologically sound DEM. That is, there will still be regions in the DEM that are internally draining, either erroneously or naturally, that need to be removed to calculate contributing areas and delineate subcatchment boundaries.

Appendix 2.1.4. Pit/sink filling, flow direction and flow accumulation

The DEM was interrogated to locate and fill localised sinks to enforce drainage to the catchment outlet. Whilst this does not strictly reflect reality, this process is required to derive subsequent datasets. Catchments in the eastern wheatbelt often flow into essentially terminal playa systems, which only link up to the regional drainage system in exceptionally wet years and major flood events. These playas and other spurious sinks were identified and elevations modified to allow drainage to flow from the lake into the downstream drainage line. This includes all lakes in the region. In such flat features the changes in elevation required to force drainage are small, typically 0.05 to 0.5 m, and determined by the minimum elevation of the neighbouring cells. The process of filling sinks is iterative until no sinks are found in the DEM. ARC/INFO's (Environmental Systems Research Institute, 1997) hydrological terrain modelling tools (based on the work of Jenson and Domingue, 1988) were used to derive the catchment boundary and drainage lines by computing the direction of surface flow and the accumulation of flow from the elevation data. Essentially, this calculates the number of cells which contribute to any given cell assuming gravity driven flow in one direction, also known as flow accumulation. The outlet point for the Avon Basin delineated from the filled DEM was chosen as the Walyunga gauging station near the Great Northern Highway, north east of Perth City.

Appendix 2.1.5. Avon Basin delineation

Hydrological modelling tools ARC/INFO (Environmental Systems Research Institute, 1997) were used to derive the catchment boundary from the elevation data. The watershed function calculates the contributing area from the flow direction grid given an outlet point. The Walyunga gauging station on the Avon River, approximately 30 km northeast of the Perth CBD, just upstream of where the Avon becomes the Swan River, was chosen as the basin outlet. The extent of the Avon Basin was derived from the filled DEM which resulted in significant modification of the existing basin boundaries held by DoW. An early finding from this process was that approximately 3500 km2 in the Lake Magenta region is likely to drain south to the Fitzgerald River catchment and not into the Avon as previously believed. Lake levels in this region confirmed the flow direction was most likely to the south and a ground truthing of the catchment divide convinced project team and collaborators at the Department of Water that the Lake Magenta region did not contribute surface water to the Avon. An extensive DGPS (Differential Global Positioning System) survey of the flowlines and divides would be required to confirm catchment boundaries for the region. Given the flatness of the area it is possible that during major flood events, floodwaters link up across the assumed boundary. This region can be seen in grey in Figure 5.1 and most other subsequent figures.


Figure A2.3. Digital Elevation Model of the Avon Basin

Appendix 2.1.6. Stream delineation and stream ordering

The drainage network for the catchment can be determined by setting a threshold number of contributing cells which is determined by the flow accumulation algorithm. A threshold of 50,000 cells (12,500 ha) was used to define drainage lines. Whilst this figure is essentially an arbitrary number arrived at via trial and error, it produced a drainage network similar to the stream network found on topographic maps. The length and connectivity of the drainage network is an important input to LASCAM (Figure A2.4). Stream ordering was conducted on the major subcatchment DEMs using the Strahler method, in which stream order increases when streams of the same order intersect (Strahler and Strahler, 1997). Figure A2.5 shows the DEM derived and Strahler stream ordering for the study area. Note that outside the Landmonitor DEM the courser DEM data is apparent with valley bottoms producing straight line drainage. This is also a problem in the vicinity of lakes and extremely flat valleys inside the Landmonitor region and is an artefact of the drainage enforcement during flow modelling.


Figure A2.4. The DEM derived drainage lines for the Avon Basin

Figure A2.5. DEM derived drainage and stream ordering for the Avon Basin

Appendix 2.1.7. Subcatchment delineation and manual editing of the flow direction grid

Subcatchments were determined by one of two methods. Outside the Landmonitor DEM region the existing subcatchments (sourced from Department of Water) were accepted as the best available. Where high resolution DEM was available, subcatchments were derived via hydrological modelling techniques within the GIS. Where Strahler stream orders changed from five to four, a minor subcatchment outlet was identified. Initially nearly 400 subcatchments were identified using this technique, however many were removed due to their small size. The stream gauge locations were added to the stream order determined points to incorporate outlets where actual flow and salt load measurements exist. Thirty three lakes were also identified as being significant hydrological features where predictions would be made. Subcatchment outlets were located at the outflow points of these lakes so that LASCAM could model the lake discharge frequency and characteristics. All of these points were used for hydrological modelling to determine the catchment areas for each. Again a number of very small subcatchments were amalgamated with neighbouring subcatchments to reduce the overall number. A number of places were identified where the mapped drainage dataset clearly showed the streamlines in significantly different locations to that derived from the DEM. Whilst this is not unexpected, it did lead to significant errors in determining catchment boundaries where major confluences were incorrectly located according to the DEM. In the flat broad valley systems especially in and around salt lakes where elevation differences are small across large areas, the flow direction algorithm used to determine flow paths can locate flow lines at some distance from the actual mapped streamlines when drainage is enforced. To correct these errors some manual editing of the flow direction grid was required. Mapped streamlines were used as a guide to determine flow directions needed to produce streamlines and therefore subcatchment boundaries that reflected more closely the actual flow paths for the areas identified.


A number of iterations of the above processes were undertaken before the final subcatchment delineation was determined. In total 108 subcatchments were identified by combining the existing DOW subcatchments in areas outside the DEM with the DEM derived subcatchments (Figure A2.6).

Figure A2.6. Final subcatchment delineation for the Avon Basin

Appendix 2.1.8. Drainage density

For each subcatchment the drainage density was determined by calculating the total length of natural drainage defined by the Geoscience Australia GEODATA TOPO-250K Series 2 data set (http://www.ga.gov.au/nmd/products/digidat/250k.htm). The level of detail in the drainage features was greater than that in the DoW drainage dataset. Although connectivity of drainage lines is poor in flat regions of the Geoscience Australia data; this may reflect reality better than an idealised drainage network.


APPENDIX 3. LEAF AREA INDEX (LAI) In August 2004, AVHRR (Advanced Very High Resolution Radiometer) data that calculated Leaf Area Index (LAI) over the whole of Australia was made available electronically on the web for the period from July 1981 to December 2000, covering most of the proposed modelling period (1975 to 2000). It was undertaken to extract the relevant data for the Avon Basin and estimate actual monthly LAI for all 108 identified subcatchments using the following procedure.

1. Average native vegetation LAI was estimated by using the method of Ellis et al.

(1999) for the Australian sclerophyll forests given as:

LAI = 2.9 (P/Eo)

where P is defined as mean annual precipitation (mm); and Eo is defined as total

annual potential evapotranspiration (mm). This value was estimated for each

subcatchment and used where a clear estimate of native vegetation cover was

available (east of the vermin-proof fence).

2. Native cover was modified so that it reflected more the temporal pattern of the

observed AVHRR data. It was felt that this represented how much rainfall occurred

during the growing season and might therefore affect the annual peak of dryland

native vegetation. The (arbitrary) function chosen was the 4th root of the ratio of peak

to long term average crop LAI and produced a scaling factor from 0.851 to 1.122.

3. The LAI for crops was obtained from AVHRR estimates of average NDVI over the

Avon Basin and picking of the peak values (Lovell and Graetz, 2001;

http://www.eoc.csiro.au/cats/bpal/AVHRR_GAC/index.eoc). The peak NDVI value

was converted into LAI using the equation of McVicar et al. (1996) and then dividing

by 3 for a more meaningful average annual (rather than peak) value.

4. The crop LAI was also modified, being that the AVHRR data was an average for the

entire Avon Basin. Each subcatchment value was modified by an amount dependent

on the ratio of subcatchment rainfall to basin average rainfall, so that the average

value of all modifiers remained exactly 1. The values ranged from 0.905 to 1.344 with

most slightly less than 1.0.

5. Total subcatchment LAI was then the sum of native proportion multiplied by native

LAI and modifier, and cropping proportion multiplied by AVHRR LAI and modifier.

With no better information to work with, the proportions of cropped and native land in

each subcatchment were held constant for the entire period. Figure A3.1 shows the

distribution of estimated LAI over the Avon Basin for 1975. LASCAM code requires

monthly LAI updates. So from the supplied annual values, a further monthly factor is

applied independently to the crop and native vegetation LAI and then recombined on

a subcatchment basis dependent on the actual proportion of the cropped land.

Therefore, while the LAI in the far east catchment, containing primarily native scrub,

will remain relatively constant throughout the year; for the cropping regions in the

central Avon Basin there will be a large variation in the active LAI throughout the

year.


Figure A3.1. Estimated annual average subcatchment LAI for the Avon Basin in 1975


APPENDIX 4. LAKE BATHYMETRY AND VOLUME To model the volumes of water that can be stored in various salt lakes the height-area-volume-discharge relations of those lakes were required. Lakes were selected based on LASCAM requirements and points of interest. Lake boundaries were extracted from topographic data supplied by DoW. Data were “cleaned” by removing dangles and undershoots before lake polygons were created. Initially several lakes were examined to identify if lake volume could be determined from the DEM. A number of lakes were field surveyed. The purpose was to compare DEM derived 3D sections with those constructed from the field survey. Cadastral surveying was carried out for 15 lakes by taking a few representative cross sections of the lakes. The system used to obtain cadastral data was the Pro Mark 2 GPS surveying system utilizing four receiver units consisting of two base stations and two walking or “Rover” units. This system requires Australian Height Datums (AHDs) given in ellipsoidal height and northing/easting of Standard Survey Marks (SSM) and/or Bench Marks (BM). The datum points located near the selected lakes (for survey) were obtained from the Department of Land Information (DLI) and used in the surveying of the selected lakes.

Surveys were conducted by setting up a Pro Mark 2 Base station over an SSM or BM and then by using a hand held Pro Mark 2 Rover receiver on a survey pole with the antenna set at approximately 2.4 m above the ground surface. If an SSM or BM was not available within a few hundred meters of a lake a new BM was installed near the site. After setting up the main base station over an SSM or BM nearest to the survey area a second based station was set up by installing a BM over a star picket or an existing fence post or permanent concrete structure.

After satellite initialisation of one or two Rover Premark 2 GPS units, surveying of the lakes commenced by either walking or by 4WD motor bikes. Survey was conducted by stopping at a point and taking a waypoint reading for 45 seconds before moving to the next desired point. Distances between waypoints varied according to the shape and irregularity of the perimeter, lake bed and bank or edge. The size of the lake also played a role in determining distances between survey points. The larger was the lake; the greater was the distance between points around or across the lake and vice versa. In general, the distance between waypoints varied between 50 and 100 m; occasionally 200 m was used for long transects and 0.5 to 1.0 m for capturing bank and edge geometry.

The 4WD motor bikes were equipped with another GPS unit to achieve accurate positioning between waypoints. The list of lakes surveyed is given in Table A4.1.

Table A4.1. List of salt lakes surveyed.

Lakes Surveyed Survey Status Lakes Surveyed Survey Status Kurrenkutten Complete Stubbs Complete Kondinin Complete O’Connor Complete Jilakin Complete Carmody Complete Grace North Partial complete Hurlstone Complete Grace South Partial complete Varley Complete Chinocup Partial complete Gulson Complete Pingarup Complete Fox Complete Alana Complete Camm Complete Tunney Complete King Partial complete Biddy Complete

Problems encountered due to the lack of specific equipment necessary for the accurate initialisation of the Pro Mark 2 Rover units resulted in discrepancies between elevations of the waypoints. After acquisition of the necessary initialisation bar data accuracy and reliability improved. There were still some unrelated discrepancies but generally the survey data fitted reasonably well with the DEM data. The vertical accuracy of the DEM is ±2m and the survey data, which should be accurate to approximately ±0.2 m, was generally within this range. Because a number of contractors were used in the creation of the DEM, the accuracy


of heights varies depending on which contractor derived the heights as well as other factors which affect accuracy, e.g. vegetation effects.

To calculate accurate elevation for each waypoint a correct ellipsoidal N value separation is required for each point. The data, in most instances, had full N value corrections attributed to waypoints. Where this did not occur, a broad-spectrum N value calculation for waypoints was obtained. This necessitated further processing of data files for calculating the ellipsoidal N value separation for waypoints. Due to problems with the required software, the values have to be calculated individually for all waypoints. The GA web site has provision for individual calculations using an online program that requires the input of latitude and longitude coordinates. The GA also has a ‘free to download’ program called WINTER with associated zone specific files used for N value calculations. This again proved to be problematic with files for zone 50 not being listed in accordance with the WINTER program specifications. Weather also caused access problems to some areas making it difficult to conduct complete surveys. Data obtained with the Pro Mark 2 receivers were processed using Ashtech Solutions software. Vector processing of surveyed waypoints was performed using AHDs obtained from DLI. Data output was in user-defined ASCII. These data were then imported into Excel and made available for use in further processing and comparing with DEM elevations.

When DEM elevations were compared to RTK GPS points determined through field survey significant non-systematic differences were identified. Detailed grid survey would be required to accurately obtain the necessary bathymetric data to construct representative 3D models of the lakes. It was beyond the scope of this project to obtain these data for 33 lakes, ranging from a few to hundreds of square kilometres in area. The GPS data did however allow some evaluation of the DEM elevations for the lakes.

It was apparent that there were several problems associated with the DEM data in the lakes. Firstly many lakes had significant areas where no elevation change was present. This is an artefact of the DEM generation process and is most likely caused by the presence of standing water in the lakes at the time aerial photographs were acquired, although this remains unsubstantiated. The extent of this flat topography varies greatly among lakes. There are lakes with no flat zones and there are other lakes that have a complete flatness across the mapped boundary. It is impossible to accurately gauge the depth of the lakes below this plane. However, it was generally assumed that if the flat area was less than 50 % of the total lake area then the average depth below the flat plane was 0.25 m. Where the area of flatness was greater than 50 % of the lake area then the average depth below the flat plane was 0.5 m. These assumptions were based on observations of lakes with good quality data and information collected during GPS survey of selected lakes. Not withstanding these shortcomings, the DEM represents the best available data to examine bathymetry and storage characteristics of the lakes modelled within LASCAM.


APPENDIX 5. DISCHARGE VELOCITY CURVE A AND B VALUES Table A5.1. Values of a and b for discharge velocity power curve relation of 88 subcatchments of the

Avon Basin

SNo Name a b SNo Name a b1 subcat13 1603.0 0.263 45 subcat59 224.6 0.3142 subcat14 626.0 0.283 46 subcat60 139.1 0.2943 subcat15 614.8 0.290 47 subcat61 96.6 0.3214 subcat16 585.9 0.277 48 subcat62 126.2 0.3065 subcat17 743.7 0.220 49 subcat63 144.0 0.3166 subcat18 1137.9 0.240 50 subcat64 233.8 0.3007 subcat19 984.9 0.264 51 subcat65 114.1 0.2728 subcat20 617.5 0.272 52 subcat66 124.2 0.2839 subcat21 919.2 0.191 53 subcat67 168.0 0.265

10 subcat22 489.6 0.268 54 subcat68 205.9 0.24511 subcat23 460.7 0.262 55 subcat69 225.1 0.21712 subcat24 696.6 0.241 56 subcat70 135.0 0.31513 subcat25 305.4 0.278 57 subcat71 243.7 0.25814 subcat26 218.8 0.285 58 subcat72 233.2 0.26715 subcat27 299.9 0.247 59 subcat73 583.2 0.17616 subcat28 180.7 0.321 60 subcat74 139.3 0.23617 subcat29 140.0 0.322 61 subcat75 134.9 0.31318 subcat30 807.0 0.215 62 subcat76 170.3 0.28919 subcat31 487.2 0.246 63 subcat77 172.3 0.25520 subcat32 495.5 0.272 64 subcat78 123.7 0.25821 subcat33 156.5 0.314 65 subcat79 239.9 0.23422 subcat34 255.1 0.256 66 subcat80 187.8 0.27723 subcat35 259.5 0.260 67 subcat81 574.9 0.26424 subcat36 239.6 0.270 68 subcat82 216.1 0.24525 subcat37 220.3 0.285 69 subcat83 370.8 0.26326 subcat39 565.5 0.263 70 subcat84 504.9 0.25427 subcat40 965.9 0.277 71 subcat85 724.0 0.27828 subcat41 724.8 0.239 72 subcat86 76.5 0.36229 subcat42 441.3 0.270 73 subcat87 409.0 0.25030 subcat43 1006.7 0.228 74 subcat88 190.4 0.32731 subcat44 556.0 0.285 75 subcat89 730.4 0.26432 subcat45 746.8 0.232 76 subcat90 146.1 0.28833 subcat46 651.4 0.248 77 subcat91 137.0 0.29434 subcat47 427.8 0.306 78 subcat92 250.5 0.28935 subcat48 653.5 0.290 79 subcat93 333.9 0.23236 subcat49 1690.6 0.200 80 subcat94 218.3 0.28737 subcat50 1648.5 0.199 81 subcat95 490.5 0.24238 subcat51 899.1 0.212 82 subcat96 253.6 0.27139 subcat52 1289.3 0.130 83 subcat97 241.9 0.26440 subcat53 508.8 0.227 84 subcat99 138.8 0.24941 subcat54 225.1 0.296 85 subcat100 285.4 0.22942 subcat55 134.9 0.330 86 subcat101 304.8 0.23343 subcat57 181.7 0.321 87 subcat102 205.1 0.26744 subcat58 145.2 0.254 88 subcat104 328.7 0.245

The a and b values of remaining 20 (of 108) streams of the subcatchments were assigned based on their similarity with one of the 88 analysed streams as given in Table A5.2. It is believed that the error introduced by this action fall within the scope of the accuracy of DEM data used in this project.


Table A5.2. The a and b values for the remaining streams

From subcatchment To subcatchment(s)

13 6 14 1 to 5 and 7 to 10, 12 15 11 28 38 57 56 97 98 102 103, 105, 107 104 106, 108


APPENDIX 6. FLOW AND QUALITY DATA The figures and tables in Chapters 8 and 9 detail predictions of flow and load for only selected subcatchments, these being the eight identified key subcatchments and the subcatchments containing the modelled lakes. In this Appendix, we present tabulated flows and salt loads for all 108 subcatchments for a number of scenarios. The scenarios and reporting periods are:

Baseline or do nothing and annual values reported for 1976–2003 and 2073–2100 periods.

Open drainage (SSe) assuming high ZOE (open high) and annual values reported for 2073–2100

Open drainage (SSe) assuming low ZOE (open low) and annual values reported for 2073–2100

Wet future climate (10 % more than historical period (1976–2003) rainfall) and mean values reported for 2073–2100 (Wet)

Dry future climate (10 % less than historical period (1976–2003) rainfall) and mean values reported for 2073–2100 (Dry: 10 %)

Wet future climate (20 % less than historical period (1976–2003) rainfall) and mean values reported for 2073–2100 (Dry: 20 %)

Wet future climate (10 % less than historical period (1976–2003 rainfall) and mean values reported for 2073–2100 (Dry: 30 %)

The tables include mean annual water and salt yields, mean annual streamflows and salt loads and ten-year return peak streamflows and salt loads.


Table A6.1. Predicted net water yields (mm y–1) for eight scenarios. Some small non-zero values may have been rounded down to zero.

Subcat. Baseline Baseline Open: high Open: low Wet Dry: 10 % Dry: 20 % Dry: 30 %

1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 1 22.72 30.64 4.09 5.35 42.73 20.41 11.53 4.46 2 14.21 19.90 0.00 0.00 28.98 12.38 5.64 0.32 3 31.79 38.87 38.81 38.85 53.74 26.04 15.44 6.87 4 63.95 64.91 64.89 64.90 85.81 46.92 31.79 19.41 5 58.78 60.07 59.29 59.83 79.94 43.07 28.74 17.13 6 51.25 54.15 54.87 54.47 71.00 39.55 27.04 16.64 7 37.37 35.55 13.72 15.62 47.86 25.05 15.82 8.15 8 56.32 60.37 60.37 60.37 76.21 46.29 33.94 23.45 9 34.30 35.12 35.12 35.12 47.99 24.14 14.96 7.73

10 34.85 36.85 36.85 36.85 48.47 26.67 17.89 10.62 11 25.08 24.86 24.86 24.86 33.74 17.17 10.66 5.53 12 26.26 28.75 30.09 30.67 40.46 18.99 10.77 4.16 13 23.99 25.97 26.12 26.12 36.67 16.98 9.72 4.26 14 33.00 34.60 35.08 35.19 45.17 25.47 17.37 10.58 15 20.76 22.63 25.45 23.66 30.18 16.07 10.51 5.91 16 25.19 25.61 27.56 27.64 33.86 18.63 12.25 6.94 17 23.29 23.54 25.35 25.68 30.83 17.43 11.97 7.23 18 23.56 25.66 28.11 27.38 33.52 18.79 12.83 7.92 19 14.05 17.11 20.93 18.75 23.05 11.90 7.47 3.92 20 9.52 8.43 1.71 3.75 10.20 6.61 4.56 2.52 21 12.90 13.91 10.27 9.85 18.35 10.29 7.04 4.20 22 14.46 17.32 20.66 21.35 22.99 12.43 8.16 4.65 23 13.62 15.77 19.34 18.54 21.09 11.27 7.37 4.11 24 4.08 8.55 12.08 10.85 12.11 5.34 2.63 0.80 25 7.08 10.76 14.43 14.07 14.91 7.10 3.98 1.69 26 2.04 5.41 7.41 7.53 8.70 2.69 0.89 0.09 27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 28 1.93 4.84 8.40 7.17 7.59 2.49 0.87 0.16 29 1.38 3.19 8.61 7.74 5.62 1.41 0.44 0.12 30 13.01 15.83 19.48 17.77 21.24 11.28 7.38 4.08 31 9.14 11.84 15.59 15.16 16.18 8.18 5.15 2.66 32 7.49 11.26 16.21 14.41 15.41 7.61 4.51 2.08 33 4.74 8.56 11.42 11.29 12.13 5.46 2.87 1.04 34 3.09 5.62 8.71 8.37 8.33 3.27 1.47 0.44 35 3.14 5.44 9.35 9.10 8.26 3.09 1.38 0.40 36 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 37 0.00 0.00 0.17 0.00 0.12 0.00 0.00 0.00 38 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 39 15.33 18.46 18.05 18.47 24.35 13.25 8.67 4.88 40 23.27 29.19 30.20 29.58 38.73 20.90 13.81 7.96 41 16.99 21.21 22.14 21.26 27.99 15.29 10.26 6.04 42 20.04 22.92 23.52 23.21 30.06 16.73 11.45 7.04 43 10.95 14.04 17.17 15.35 18.78 9.90 6.37 3.50 44 20.00 23.61 24.85 24.06 30.83 17.33 11.90 7.36 45 13.73 19.42 20.52 20.04 25.36 14.15 9.32 5.20 46 14.60 20.87 24.35 22.64 27.95 14.78 9.62 5.35 47 20.80 24.80 28.49 26.21 33.26 17.41 11.18 6.23 48 24.32 24.62 25.05 25.00 34.63 16.33 9.68 4.68 49 18.48 21.78 25.59 23.50 28.84 15.58 10.28 5.98 50 20.53 22.23 24.30 22.86 30.22 15.36 9.64 5.16 51 13.54 17.26 19.14 18.04 22.89 12.34 8.01 4.38 52 16.01 19.45 23.01 21.83 25.36 14.27 9.76 5.96 53 8.95 12.09 14.59 13.74 16.21 8.51 5.50 2.98 54 1.34 5.26 6.37 5.80 8.89 2.25 0.38 0.00 55 7.28 9.44 10.91 11.51 12.97 6.44 3.96 1.96 56 11.40 14.83 16.70 16.16 19.63 10.61 7.01 4.03 57 1.41 4.52 5.19 4.88 7.95 1.98 0.56 0.06 58 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 59 5.84 8.88 11.21 10.16 12.29 5.93 3.45 1.54 60 4.99 7.39 9.90 10.05 9.99 5.07 2.91 1.23 61 3.22 3.74 7.76 8.08 5.87 2.50 1.41 0.53 62 1.45 3.60 7.12 6.88 5.93 1.77 0.59 0.12 63 0.99 3.10 6.55 5.94 5.36 1.42 0.46 0.10 64 0.41 2.24 6.32 4.71 4.40 0.77 0.18 0.04 65 2.15 4.70 8.14 8.06 7.38 2.52 1.00 0.27 66 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 67 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 68 1.95 5.28 7.60 6.57 8.11 2.84 1.10 0.26 69 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 70 0.60 2.30 5.71 4.08 4.75 0.77 0.20 0.06


Table A6.1 (continued). Predicted net water yields (mm y–1) for eight scenarios. Some small non-zero values may have been rounded down to zero.


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 71 1.45 4.96 8.52 8.40 7.77 2.56 0.89 0.16 72 1.29 2.88 5.35 3.93 5.59 1.12 0.32 0.09 73 1.35 3.58 6.68 5.39 6.42 1.48 0.40 0.09 74 0.06 0.96 1.70 1.66 2.35 0.13 0.00 0.00 75 0.95 2.10 5.39 4.43 4.36 0.74 0.21 0.05 76 0.72 2.25 4.57 2.98 4.71 0.77 0.22 0.07 77 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 79 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 81 0.18 0.18 0.18 0.18 0.42 0.09 0.04 0.02 82 0.80 2.42 10.20 10.50 5.37 0.72 0.20 0.06 83 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 84 0.25 0.31 0.66 0.47 0.79 0.12 0.05 0.02 85 0.10 0.11 0.11 0.11 0.25 0.05 0.02 0.01 86 4.31 6.52 9.05 9.74 9.12 4.20 2.22 0.86 87 4.34 4.78 0.00 0.00 5.58 3.65 2.23 1.06 88 3.48 5.41 9.28 9.35 8.12 3.21 1.59 0.55 89 4.67 6.78 9.20 7.61 10.81 3.61 1.53 0.51 90 3.76 5.61 7.96 7.99 8.09 3.50 1.76 0.66 91 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 92 0.53 2.52 6.67 5.14 4.79 0.94 0.24 0.06 93 0.55 1.80 5.81 5.46 3.49 0.70 0.18 0.02 94 1.02 2.07 6.97 6.49 4.00 0.84 0.26 0.07 95 0.67 1.06 7.84 7.65 2.46 0.42 0.15 0.06 96 0.51 1.35 5.31 5.49 2.62 0.54 0.17 0.03 97 0.31 0.39 4.97 4.48 0.92 0.15 0.06 0.02 98 0.10 0.09 0.09 0.09 0.21 0.04 0.02 0.01 99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

100 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 101 0.10 0.34 4.82 3.89 1.25 0.09 0.02 0.01 102 0.10 0.35 4.86 3.91 1.23 0.08 0.01 0.00 103 0.01 0.00 0.00 0.00 0.04 0.00 0.00 0.00 104 0.15 0.26 4.58 3.46 0.75 0.10 0.04 0.02 105 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 106 0.16 0.18 0.18 0.18 0.45 0.08 0.03 0.02 107 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 108 0.34 0.49 0.49 0.49 1.32 0.16 0.05 0.02


Table A6.2. Predicted mean annual streamflows (GL) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 1 298.45 399.26 562.19 520.01 606.53 251.39 148.59 76.94 2 297.71 398.25 562.05 519.84 605.12 250.72 148.21 76.79 3 29.06 30.33 30.42 30.39 40.06 21.95 14.82 8.97 4 28.37 29.49 29.57 29.54 38.89 21.38 14.48 8.82 5 24.27 25.33 25.41 25.38 33.39 18.37 12.45 7.57 6 14.95 15.80 16.01 15.89 20.71 11.54 7.89 4.86 7 268.52 367.76 531.78 489.57 564.82 228.67 133.34 67.83 8 43.81 44.27 44.27 44.27 59.91 30.78 19.38 10.33 9 42.78 43.17 43.17 43.17 58.51 29.93 18.75 9.90

10 28.90 28.96 28.96 28.96 39.10 20.17 12.70 6.77 11 24.11 23.89 23.89 23.89 32.43 16.50 10.24 5.31 12 223.69 322.51 487.13 444.87 503.60 197.20 113.53 57.27 13 4.59 4.98 5.00 5.00 7.03 3.25 1.86 0.82 14 213.06 310.92 475.20 432.81 487.28 189.58 109.19 55.50 15 17.54 19.11 21.50 19.99 25.49 13.57 8.88 4.99 16 182.57 278.23 439.93 399.01 444.05 166.01 93.50 46.35 17 178.50 274.10 435.49 394.55 438.59 163.01 91.52 45.23 18 2.62 3.06 3.60 3.32 4.08 2.17 1.41 0.79 19 1.60 1.94 2.38 2.13 2.62 1.35 0.85 0.45 20 172.85 267.97 428.58 387.89 430.50 158.57 88.56 43.50 21 44.38 75.39 116.03 108.64 112.73 46.40 24.27 9.65 22 19.12 34.08 56.26 51.71 53.77 20.27 10.37 4.09 23 18.29 33.08 55.08 50.48 52.45 19.56 9.90 3.83 24 3.48 7.29 10.30 9.25 10.32 4.55 2.24 0.68 25 13.03 23.74 42.26 38.81 39.38 13.54 6.70 2.61 26 2.41 7.58 20.59 17.67 16.99 2.87 0.72 0.07 27 0.75 3.18 14.56 11.54 9.91 0.69 0.00 0.00 28 2.63 6.60 11.45 9.78 10.34 3.39 1.18 0.21 29 2.93 6.78 18.30 16.46 11.95 3.00 0.94 0.25 30 25.19 41.24 59.72 56.89 58.87 26.07 13.86 5.53 31 22.10 37.48 55.09 52.66 53.82 23.39 12.11 4.56 32 5.59 8.39 12.08 10.75 11.49 5.68 3.36 1.55 33 15.46 27.72 41.20 40.17 40.46 16.77 8.15 2.70 34 8.20 14.60 23.70 22.87 21.88 8.41 3.76 1.11 35 3.55 6.17 10.63 10.31 9.37 3.50 1.56 0.45 36 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 37 0.00 0.00 0.16 0.00 0.11 0.00 0.00 0.00 38 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 39 128.41 192.52 312.54 279.21 317.69 112.13 64.26 33.84 40 8.52 10.69 11.06 10.83 14.19 7.66 5.06 2.91 41 119.14 180.94 300.61 267.49 302.33 103.83 58.78 30.69 42 112.78 172.99 292.31 259.52 291.84 98.10 54.94 28.42 43 6.51 8.35 10.21 9.13 11.17 5.89 3.79 2.08 44 104.77 162.92 280.34 248.66 278.42 90.96 50.30 25.82 45 101.29 158.82 276.02 244.48 273.07 87.95 48.23 24.54 46 44.25 48.45 52.78 50.33 66.35 33.23 20.69 10.94 47 43.94 48.01 52.26 49.84 65.74 32.91 20.48 10.82 48 20.33 20.57 20.93 20.89 28.94 13.64 8.09 3.91 49 11.07 12.48 14.15 13.15 16.76 8.78 5.65 3.16 50 5.87 6.35 6.94 6.53 8.64 4.39 2.76 1.48 51 56.95 110.23 223.10 194.01 206.55 54.62 27.47 13.56 52 55.90 108.90 221.62 192.62 204.78 53.67 26.86 13.23 53 18.96 29.41 34.84 32.88 41.90 18.99 11.16 5.82 54 0.65 2.54 3.08 2.80 4.29 1.09 0.18 0.00 55 5.26 9.22 10.47 10.03 13.95 5.49 2.96 1.47 56 3.95 5.13 5.78 5.59 6.79 3.67 2.43 1.40 57 1.24 4.00 4.59 4.32 7.03 1.75 0.49 0.05 58 18.43 57.00 160.17 134.50 133.55 18.18 4.40 0.52 59 7.12 10.82 13.65 12.38 14.98 7.22 4.20 1.88 60 23.91 61.17 176.80 150.88 135.53 24.13 9.90 3.33 61 6.81 30.73 80.44 62.63 74.65 9.22 2.86 0.67 62 6.19 30.01 78.93 61.07 73.51 8.74 2.59 0.57 63 1.90 8.64 22.70 17.73 16.42 3.25 0.85 0.18 64 1.16 6.29 17.74 13.22 12.36 2.17 0.50 0.10 65 1.54 14.55 42.73 30.29 45.85 2.13 0.62 0.17 66 0.21 11.64 37.70 25.31 41.29 0.57 0.00 0.00 67 1.41 14.67 43.13 29.66 45.52 1.42 0.00 0.00 68 1.48 11.34 29.50 21.67 30.95 2.08 0.65 0.16 69 0.33 8.22 25.01 17.78 26.16 0.41 0.00 0.00


Table A6.2 (continued). Predicted mean annual streamflows (GL) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 70 1.19 4.58 11.39 8.15 9.47 1.54 0.39 0.11 71 4.03 12.88 23.46 19.70 25.81 4.40 1.17 0.28 72 2.50 5.61 10.43 7.65 10.89 2.17 0.62 0.17 73 1.04 5.61 10.19 9.24 12.32 1.37 0.25 0.06 74 0.20 3.38 6.03 5.89 8.32 0.45 0.00 0.00 75 4.01 10.64 24.04 17.65 22.32 3.69 1.05 0.28 76 1.96 6.09 12.37 8.06 12.89 2.09 0.59 0.18 77 0.00 0.00 0.00 0.00 0.13 0.00 0.00 0.00 78 0.00 0.00 0.00 0.00 0.13 0.00 0.00 0.00 79 0.00 0.01 0.13 0.11 0.37 0.00 0.00 0.00 80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 81 0.01 0.01 0.01 0.01 0.03 0.01 0.00 0.00 82 0.15 0.49 2.03 1.99 1.22 0.13 0.04 0.01 83 0.01 0.05 0.18 0.09 0.25 0.00 0.00 0.00 84 0.12 0.15 0.32 0.23 0.39 0.06 0.02 0.01 85 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 86 9.08 18.56 80.46 72.11 44.83 6.77 2.37 0.68 87 8.06 17.01 78.31 69.79 42.66 5.77 1.83 0.48 88 0.46 0.67 0.94 0.81 1.06 0.36 0.16 0.05 89 0.39 0.57 0.78 0.64 0.91 0.31 0.13 0.04 90 7.58 16.31 77.37 68.98 41.57 5.39 1.66 0.42 91 5.18 12.74 72.30 63.89 36.41 3.16 0.54 0.00 92 1.17 5.54 14.69 11.33 10.54 2.06 0.53 0.12 93 6.90 12.85 65.74 60.60 32.39 4.60 1.48 0.42 94 3.32 6.17 29.04 27.71 12.73 2.48 0.81 0.25 95 1.30 2.04 15.12 14.74 4.73 0.80 0.30 0.11 96 2.79 4.11 28.42 25.11 14.68 1.12 0.40 0.15 97 1.56 1.71 15.19 13.77 4.03 0.69 0.30 0.13 98 0.65 0.57 0.57 0.57 1.34 0.24 0.12 0.07 99 0.92 1.59 10.02 8.02 9.07 0.10 0.00 0.00

100 2.44 3.69 18.68 14.98 14.49 0.68 0.01 0.00 101 0.18 0.63 8.98 7.25 2.32 0.16 0.04 0.01 102 3.45 4.43 14.71 12.52 14.20 1.33 0.35 0.07 103 3.25 3.71 4.68 4.45 11.66 1.17 0.32 0.06 104 0.68 1.21 21.12 15.96 3.46 0.44 0.18 0.08 105 2.53 2.70 2.70 2.70 7.98 0.93 0.37 0.13 106 0.86 0.96 0.96 0.96 2.40 0.41 0.18 0.08 107 1.78 1.89 1.89 1.89 5.84 0.57 0.22 0.06 108 2.06 2.96 2.96 2.96 8.01 0.96 0.32 0.10


Table A6.3. Predicted ten-year return period peak streamflows (GL d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 1 23.68 24.90 32.04 32.11 31.74 18.40 12.23 6.97 2 23.16 23.89 31.96 31.77 30.24 17.51 12.09 6.97 3 3.85 3.88 4.82 4.82 4.80 2.89 1.79 1.25 4 4.08 4.13 4.60 4.60 5.14 3.10 1.94 1.12 5 3.91 3.95 4.35 4.35 4.88 3.04 1.93 1.10 6 2.72 2.75 2.90 2.90 3.38 2.13 1.55 0.86 7 20.53 21.33 31.19 31.00 28.60 16.20 10.72 6.03 8 6.98 7.00 7.00 7.00 8.96 5.35 3.92 1.79 9 6.75 6.77 6.77 6.77 8.83 5.29 3.88 1.87

10 4.04 4.04 4.04 4.04 4.87 3.45 2.56 1.64 11 3.68 3.67 3.67 3.67 4.86 2.99 2.20 1.14 12 17.50 18.74 29.03 28.86 27.81 13.23 8.09 4.94 13 1.01 1.05 1.11 1.11 1.60 0.74 0.44 0.24 14 18.64 19.77 29.18 29.13 25.63 12.64 7.89 4.85 15 3.58 3.84 4.36 4.37 4.82 2.51 2.02 1.28 16 16.02 17.26 28.98 28.75 22.20 12.57 7.11 4.25 17 15.21 16.45 28.96 28.64 21.64 12.17 7.07 4.30 18 0.51 0.52 0.68 0.68 0.65 0.39 0.29 0.17 19 0.40 0.41 0.46 0.47 0.52 0.33 0.23 0.16 20 14.18 15.85 28.36 27.94 22.44 10.96 6.62 4.30 21 3.65 4.40 7.29 7.40 6.71 2.61 1.55 0.88 22 1.66 2.10 3.24 2.90 2.94 1.27 0.78 0.46 23 1.71 2.12 3.15 2.80 2.95 1.30 0.78 0.46 24 0.63 0.75 0.63 0.66 1.09 0.45 0.24 0.12 25 1.22 1.60 2.13 2.11 2.18 0.97 0.57 0.34 26 0.22 0.72 1.16 1.26 1.08 0.19 0.09 0.00 27 0.00 0.59 0.89 0.99 0.90 0.00 0.00 0.00 28 0.34 0.44 0.44 0.48 0.67 0.26 0.13 0.05 29 0.58 0.63 1.16 1.17 0.91 0.40 0.23 0.10 30 2.38 2.70 4.57 4.84 3.69 1.44 0.96 0.51 31 2.36 2.62 4.66 4.90 3.71 1.34 0.90 0.50 32 0.84 0.90 1.03 1.07 1.20 0.62 0.36 0.19 33 2.09 2.25 4.24 4.43 3.19 1.21 0.70 0.40 34 1.99 2.18 2.95 2.98 2.99 1.11 0.68 0.35 35 1.14 1.25 1.36 1.40 1.79 0.78 0.46 0.24 36 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 37 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 38 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 39 12.27 12.54 24.29 23.81 17.90 9.23 6.01 3.60 40 0.60 0.63 0.64 0.64 0.84 0.45 0.30 0.19 41 11.75 12.37 22.96 22.51 17.79 8.65 5.47 3.45 42 11.97 12.53 19.73 19.13 17.90 8.51 5.39 3.45 43 1.52 1.62 1.78 1.80 2.25 1.13 0.58 0.37 44 11.13 11.74 18.57 17.85 16.50 7.91 4.84 3.10 45 10.11 10.65 17.31 16.57 14.77 7.44 4.69 3.05 46 5.79 5.85 5.08 5.09 7.77 4.12 2.52 1.33 47 5.55 5.60 5.17 5.23 7.32 4.02 2.46 1.38 48 1.71 1.72 1.79 1.79 2.39 1.12 0.68 0.34 49 2.39 2.44 2.82 2.84 3.51 1.58 0.93 0.57 50 1.13 1.15 1.29 1.30 1.68 0.73 0.43 0.25 51 5.78 6.27 14.18 13.45 10.95 4.14 2.50 1.50 52 5.44 5.95 13.56 13.32 10.70 3.91 2.17 1.41 53 4.58 5.02 7.49 7.27 7.14 2.57 1.34 0.84 54 0.13 0.21 0.29 0.26 0.50 0.10 0.04 0.00 55 1.19 1.75 2.21 2.11 3.24 0.89 0.59 0.32 56 0.88 0.92 1.43 1.45 1.24 0.69 0.48 0.29 57 0.59 1.47 1.75 1.65 2.05 0.41 0.14 0.00 58 1.99 3.26 12.24 12.29 9.28 0.99 0.36 0.13 59 1.91 2.08 2.36 2.45 2.71 1.44 0.83 0.42 60 3.41 4.81 11.87 10.98 9.11 1.81 0.77 0.39 61 0.65 2.51 3.76 3.80 5.16 0.88 0.29 0.10 62 0.63 2.53 3.76 3.78 5.15 0.88 0.27 0.09 63 0.35 0.56 0.55 0.57 0.84 0.25 0.15 0.07 64 0.25 0.37 0.35 0.37 0.58 0.21 0.11 0.03 65 0.60 2.17 3.03 2.91 4.36 0.40 0.06 0.03 66 0.00 2.11 3.10 2.77 4.28 0.00 0.00 0.00 67 0.16 2.03 2.62 2.37 4.11 0.05 0.00 0.00 68 0.16 1.43 2.25 2.26 2.02 0.15 0.04 0.03 69 0.00 1.41 2.17 2.17 2.03 0.01 0.00 0.00


Table A6.3 (continued). Predicted ten-year return period peak streamflows (GL d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 70 0.19 0.31 0.31 0.32 0.48 0.17 0.07 0.05

71 0.52 1.33 1.89 1.88 1.84 0.56 0.20 0.12 72 0.27 0.35 0.43 0.43 0.53 0.22 0.13 0.09 73 0.17 1.11 1.55 1.56 1.46 0.57 0.05 0.04 74 0.01 0.92 1.47 1.49 1.39 0.04 0.00 0.00 75 0.65 0.86 0.92 0.94 1.37 0.27 0.16 0.06 76 0.22 0.33 0.33 0.34 0.53 0.18 0.12 0.09 77 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 79 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 81 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 82 0.03 0.06 0.13 0.10 0.12 0.03 0.02 0.01 83 0.00 0.00 0.04 0.01 0.04 0.00 0.00 0.00 84 0.05 0.05 0.05 0.05 0.07 0.04 0.03 0.01 85 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 86 1.42 1.83 4.78 4.99 4.20 0.96 0.44 0.23 87 1.40 1.81 5.95 6.13 4.15 0.96 0.44 0.22 88 0.09 0.10 0.10 0.11 0.14 0.07 0.04 0.02 89 0.06 0.06 0.07 0.07 0.10 0.04 0.02 0.01 90 1.33 1.74 6.04 6.21 4.03 0.94 0.40 0.20 91 1.16 1.54 6.69 6.86 4.09 0.78 0.08 0.00 92 0.10 0.19 0.23 0.23 0.32 0.10 0.06 0.04 93 1.06 1.19 5.29 5.28 3.85 0.67 0.29 0.11 94 0.53 0.65 5.55 5.60 1.15 0.35 0.17 0.07 95 0.50 0.59 3.09 3.23 1.02 0.30 0.14 0.07 96 0.39 0.43 1.22 1.22 1.18 0.23 0.11 0.05 97 0.34 0.36 0.95 0.98 0.62 0.19 0.08 0.05 98 0.16 0.16 0.16 0.16 0.23 0.08 0.04 0.03 99 0.03 0.13 0.46 0.43 0.98 0.00 0.00 0.00

100 0.61 1.09 1.81 1.81 2.54 0.05 0.00 0.00 101 0.05 0.07 0.12 0.11 0.13 0.04 0.03 0.02 102 0.77 1.01 2.35 2.36 2.50 0.46 0.14 0.05 103 0.77 0.93 1.92 1.92 2.41 0.47 0.20 0.02 104 0.22 0.27 0.42 0.43 0.51 0.17 0.12 0.08 105 0.56 0.57 0.57 0.57 1.79 0.33 0.20 0.12 106 0.44 0.44 0.44 0.44 0.64 0.31 0.21 0.13 107 0.49 0.47 0.47 0.47 1.40 0.30 0.21 0.06 108 0.38 0.54 0.54 0.54 0.87 0.26 0.17 0.11


Table A6.4. Predicted net annual salt yields (t km–2) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–21001 16.0 7.2 8.8 7.1 9.3 4.4 3.7 2.2 2 18.4 7.1 10.8 7.2 9.0 5.4 3.7 2.2 3 21.8 6.9 6.9 6.9 8.7 5.2 3.6 2.1 4 100.7 110.7 110.7 110.7 149.0 76.8 48.6 26.9 5 49.5 27.0 26.5 26.5 37.9 18.3 11.6 6.5 6 79.2 216.9 245.6 219.7 258.6 171.6 124.7 78.7 7 88.4 97.4 227.6 224.4 139.8 63.1 37.9 20.2 8 164.2 356.0 356.0 356.0 384.1 320.5 275.5 222.8 9 46.0 26.5 26.5 26.5 37.1 17.8 10.8 5.4

10 127.8 226.6 226.6 226.6 269.0 179.9 131.0 82.6 11 37.5 39.2 39.2 39.2 52.0 27.3 16.8 8.2 12 23.7 9.3 7.2 6.6 12.9 6.3 4.0 2.1 13 23.9 10.7 10.6 10.6 15.5 6.7 3.7 1.5 14 103.4 248.5 238.8 218.2 290.9 200.1 151.3 101.3 15 68.4 156.2 319.5 193.5 185.7 122.1 86.4 50.4 16 93.8 212.0 167.7 143.5 246.3 173.8 135.3 90.6 17 124.8 278.2 193.9 154.6 321.6 229.1 181.5 131.4 18 106.8 231.3 234.9 172.3 259.8 199.7 161.7 112.7 19 56.4 175.0 408.5 241.4 214.2 131.8 87.8 45.2 20 138.2 351.7 303.6 185.5 398.4 295.8 231.3 169.7 21 92.3 236.5 391.8 262.8 289.4 186.4 132.5 80.3 22 70.7 230.0 386.7 376.3 281.2 177.6 122.9 71.1 23 79.9 221.0 431.7 341.8 271.8 167.8 113.6 63.9 24 27.9 201.7 465.2 355.7 274.7 126.2 58.4 14.7 25 40.4 194.0 458.6 412.5 254.6 132.3 73.8 29.5 26 24.8 194.6 413.1 406.9 272.8 118.6 43.6 2.8 27 0.0 0.0 274.5 678.8 121.2 0.0 0.0 0.0 28 16.0 120.2 381.7 279.5 190.5 57.0 16.6 1.8 29 6.4 61.1 410.8 330.8 119.6 21.9 4.4 0.7 30 70.5 231.7 446.3 301.1 282.5 175.6 118.3 66.6 31 63.8 215.0 455.6 382.2 267.9 157.7 100.3 51.1 32 52.5 213.8 516.2 363.0 272.6 152.7 92.1 41.0 33 40.6 195.9 444.5 414.1 261.2 128.6 65.8 21.2 34 29.5 130.1 371.7 331.8 188.3 74.4 30.4 7.4 35 22.7 109.8 399.7 366.7 165.1 59.9 23.8 5.2 36 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 37 0.0 0.0 35.7 0.0 16.9 0.0 0.0 0.0 38 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 39 88.9 283.2 308.5 245.0 329.7 229.2 169.0 110.4 40 93.3 347.7 395.5 354.7 419.6 271.3 190.7 111.8 41 102.3 304.1 412.4 318.7 361.4 241.8 176.8 110.4 42 123.3 288.5 343.5 283.6 338.8 233.9 177.7 118.4 43 78.9 222.1 422.7 282.7 270.5 171.1 118.1 67.1 44 111.6 298.8 393.4 304.0 349.8 243.3 184.8 123.3 45 66.9 350.2 500.4 358.0 411.6 280.4 204.7 124.9 46 73.7 311.5 493.1 332.8 368.1 249.0 183.3 113.1 47 70.9 242.5 460.3 298.9 301.3 180.9 120.8 68.2 48 37.7 30.2 34.6 31.5 51.9 16.3 7.9 3.0 49 74.7 226.2 457.7 302.8 272.2 176.4 125.2 76.3 50 61.0 145.7 266.0 168.5 190.8 102.9 64.7 33.0 51 95.3 304.4 449.9 313.3 363.2 240.2 172.6 106.0 52 85.8 259.7 499.1 386.2 306.8 207.5 153.5 99.4 53 64.2 210.4 376.1 298.1 256.8 160.8 109.2 60.4 54 22.0 209.9 308.3 248.9 275.5 137.4 39.5 0.0 55 72.6 200.7 348.3 359.3 241.3 156.3 110.9 65.2 56 88.3 244.4 393.7 339.0 291.8 192.6 137.8 82.9 57 26.7 224.5 313.5 267.2 288.0 151.8 60.7 4.3 58 0.0 85.1 205.0 259.1 253.8 115.2 38.3 0.0 59 47.9 176.8 351.7 260.1 227.8 123.6 71.9 29.6 60 50.0 180.5 392.0 376.4 242.0 121.6 67.3 25.9 61 36.8 155.9 371.5 366.6 222.9 92.7 41.8 12.8 62 10.5 106.8 377.7 349.4 185.9 45.9 12.5 1.8 63 8.6 103.6 355.2 303.4 184.7 41.4 9.7 1.3 64 2.5 77.5 351.0 239.5 169.4 19.9 2.5 0.2 65 17.5 130.7 375.9 359.8 205.9 66.6 22.8 4.2 66 0.0 150.3 317.6 326.7 224.4 0.0 0.0 0.0 67 0.0 165.1 298.2 286.3 236.7 0.0 0.0 0.0 68 11.9 165.2 327.2 249.7 255.2 80.2 24.1 3.4 69 0.0 148.6 339.4 311.3 239.0 0.0 0.0 0.0


Table A6.4 (continued). Predicted net annual salt yields (t km–2) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–210070 3.3 69.3 298.5 185.1 162.3 16.2 1.8 0.2 71 10.4 162.6 446.6 417.6 257.4 76.8 21.6 2.9 72 7.7 67.1 236.2 135.2 151.8 19.0 3.3 0.4 73 8.3 96.4 309.4 213.8 188.3 31.9 5.6 0.6 74 0.5 132.1 293.7 294.0 238.4 11.3 0.0 0.0 75 6.1 48.4 280.0 205.6 113.7 12.8 2.2 0.3 76 3.4 56.2 212.9 103.6 142.5 12.4 1.7 0.2 77 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 78 0.0 0.0 0.0 0.0 5.4 0.0 0.0 0.0 79 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 80 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 81 0.2 0.0 0.0 0.0 0.2 0.0 0.0 0.0 82 1.7 56.3 525.1 533.8 156.3 8.8 1.0 0.1 83 0.0 12.8 162.9 75.9 73.0 0.0 0.0 0.0 84 0.2 0.4 16.7 3.3 3.0 0.1 0.0 0.0 85 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 86 35.1 139.0 403.5 430.2 194.9 85.8 42.2 14.6 87 47.3 131.7 179.5 175.0 178.5 87.2 49.1 21.5 88 21.5 101.0 378.0 364.1 150.5 57.6 25.9 7.9 89 28.1 107.2 287.9 164.4 181.1 48.6 15.2 3.2 90 31.5 118.9 316.1 304.6 173.9 68.8 30.9 9.4 91 0.0 74.7 249.2 235.1 120.9 0.0 0.0 0.0 92 3.1 85.3 360.6 253.6 178.1 24.0 3.6 0.4 93 5.4 52.0 341.1 312.4 113.2 16.3 3.4 0.5 94 4.2 41.0 393.9 357.0 95.7 12.4 2.7 0.4 95 1.1 12.3 367.8 340.1 45.1 2.7 0.5 0.1 96 5.6 34.4 348.7 354.4 77.9 11.1 2.6 0.4 97 0.6 2.1 286.5 251.8 8.3 0.4 0.1 0.0 98 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 99 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

100 0.5 14.1 279.3 267.3 77.9 1.3 0.0 0.0 101 0.2 5.2 281.6 221.0 33.6 0.5 0.0 0.0 102 0.4 5.1 293.9 231.6 27.6 0.6 0.0 0.0 103 0.1 0.0 0.0 0.0 0.1 0.0 0.0 0.0 104 0.1 1.3 263.7 191.1 8.9 0.2 0.0 0.0 105 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 106 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 107 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 108 0.1 0.1 0.1 0.1 0.2 0.0 0.0 0.0


Table A6.5. Predicted mean annual salt loads (kt) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–21001 1365.64 7400.36 23514.41 19246.04 11646.54 3652.27 1972.51 880.71 2 1365.11 7400.12 23514.12 19245.81 11646.24 3652.13 1972.39 880.64 3 37.90 74.81 83.12 75.55 91.21 58.02 41.41 25.76 4 37.42 74.66 82.97 75.41 91.02 57.90 41.33 25.71 5 30.97 67.57 75.87 68.31 81.46 52.98 38.22 23.98 6 23.11 63.28 71.67 64.11 75.45 50.07 36.38 22.95 7 1327.06 7325.25 23430.91 19170.19 11554.96 3594.06 1930.95 854.87 8 75.19 86.10 86.10 86.10 108.98 64.03 43.53 25.47 9 72.18 79.57 79.57 79.57 101.93 58.15 38.48 21.39

10 53.59 68.87 68.87 68.87 86.94 50.95 34.13 19.21 11 36.01 37.69 37.69 37.69 49.95 26.21 16.11 7.84 12 1249.45 7236.48 23338.58 19077.95 11442.15 3528.31 1886.38 828.84 13 4.58 2.04 2.04 2.04 2.97 1.29 0.71 0.28 14 1239.41 7232.31 23334.89 19074.39 11436.21 3525.58 1884.76 828.08 15 57.79 131.97 269.88 163.48 156.90 103.14 73.02 42.59 16 1141.03 7002.79 22971.28 18825.25 11165.12 3343.88 1752.34 745.73 17 1125.90 6968.58 22944.21 18802.09 11125.36 3315.83 1730.51 731.11 18 11.06 29.96 56.63 34.93 35.65 23.67 17.02 10.04 19 6.41 19.87 46.39 27.42 24.33 14.97 9.97 5.13 20 1098.60 6902.43 22862.35 18747.05 11047.86 3262.35 1689.88 703.96 21 328.08 1800.06 5157.19 4387.05 2584.97 1047.74 500.45 173.58 22 132.16 939.57 2912.37 2451.19 1412.14 496.32 217.97 71.51 23 128.10 926.38 2890.20 2429.62 1396.02 486.14 210.92 67.43 24 23.77 172.00 396.65 303.27 234.25 107.64 49.82 12.57 25 93.91 725.56 2437.24 2081.77 1126.32 356.61 146.28 46.53 26 33.19 434.18 1748.49 1462.27 743.98 157.87 35.42 2.26 27 13.00 275.93 1412.54 1131.36 522.16 61.41 0.00 0.00 28 21.73 163.83 520.04 380.83 259.55 77.64 22.65 2.44 29 13.54 129.89 873.48 703.51 254.21 46.61 9.40 1.41 30 195.46 859.29 2242.82 1934.52 1171.36 550.46 281.80 101.66 31 178.69 804.19 2136.71 1862.93 1104.20 508.72 253.67 85.83 32 39.17 159.40 384.89 270.68 203.26 113.88 68.64 30.54 33 132.15 619.97 1699.21 1548.10 870.00 376.63 173.45 49.39 34 70.03 319.85 1018.19 913.62 469.77 179.67 72.59 16.99 35 25.71 124.53 460.30 415.72 187.15 67.94 26.97 5.88 36 0.00 0.00 7.17 0.00 0.00 0.00 0.00 0.00 37 0.00 0.00 32.99 0.00 15.57 0.00 0.00 0.00 38 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 39 769.54 5099.87 17703.01 14358.68 8460.06 2212.51 1187.79 529.18 40 34.17 127.37 144.85 129.93 153.67 99.36 69.86 40.94 41 731.09 4958.87 17543.30 14216.95 8290.52 2102.12 1109.79 482.93 42 692.72 4844.88 17388.71 14097.48 8155.03 2011.48 1043.51 441.54 43 46.93 132.06 251.40 168.11 160.87 101.73 70.24 39.90 44 636.57 4691.23 17111.61 13908.15 7968.83 1892.25 959.98 392.78 45 617.17 4639.31 17043.26 13855.32 7908.05 1849.98 927.87 371.35 46 114.27 283.46 521.87 347.03 364.08 207.11 137.08 77.01 47 112.69 276.78 511.28 339.89 356.18 201.76 133.14 74.58 48 31.46 25.27 28.92 26.28 43.36 13.65 6.58 2.54 49 38.48 105.34 204.91 133.43 131.19 79.09 53.73 30.94 50 17.44 41.64 76.01 48.16 54.52 29.40 18.48 9.44 51 502.43 4353.42 16517.93 13505.80 7541.11 1640.93 789.37 293.48 52 495.08 4329.93 16483.21 13481.63 7513.09 1622.40 776.05 285.30 53 159.15 693.60 1114.66 912.34 865.88 503.46 280.91 121.28 54 10.64 101.36 148.88 120.21 133.05 66.38 19.06 0.00 55 54.83 285.14 416.97 357.17 358.09 202.45 102.51 33.10 56 30.55 84.58 136.26 117.32 100.99 66.65 47.70 28.69 57 23.57 198.59 277.30 236.33 254.74 134.27 53.72 3.77 58 236.70 3335.94 14791.21 12122.59 6292.39 878.94 317.62 49.04 59 58.41 215.36 428.54 316.88 277.60 150.56 87.61 36.01 60 194.76 3081.96 14269.68 11688.18 5899.64 676.13 212.63 61.72 61 56.29 2238.61 6960.61 5528.87 4226.94 317.90 60.41 10.11 62 49.16 2208.38 6888.58 5457.78 4183.72 299.93 52.30 7.64 63 13.42 296.19 1255.20 902.72 615.92 87.31 14.49 1.59 64 6.94 217.60 985.73 672.56 475.82 55.90 7.13 0.64 65 15.82 1709.55 4916.78 3892.01 3215.05 125.53 14.06 2.60 66 4.98 1628.80 4684.50 3669.69 3087.86 84.38 0.00 0.00 67 22.40 1455.70 4318.67 3293.39 2829.38 147.15 0.00 0.00 68 11.72 1060.91 2886.96 2328.51 1997.93 87.66 14.25 1.99 69 4.71 963.33 2693.63 2181.01 1847.14 40.27 0.00 0.00


Table A6.5 (continued). Predicted mean annual salt loads (kt) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–210070 6.48 138.33 595.45 369.18 323.71 32.33 3.63 0.32 71 25.36 712.54 1841.26 1576.20 1342.54 122.25 17.04 2.06 72 15.03 130.68 460.07 263.21 295.55 36.95 6.35 0.74 73 6.85 527.60 1232.16 1173.63 961.08 59.68 3.47 0.37 74 1.67 467.54 1039.35 1040.39 843.76 39.83 0.00 0.00 75 22.33 256.86 1182.55 725.65 633.65 61.19 9.30 1.25 76 9.06 152.08 576.17 280.40 387.37 33.54 4.55 0.56 77 0.00 0.00 0.00 0.00 1.53 0.00 0.00 0.00 78 0.00 0.00 0.00 0.00 8.95 0.00 0.00 0.00 79 0.00 0.13 8.89 6.06 7.61 0.00 0.00 0.00 80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 81 0.02 0.00 0.00 0.00 0.01 0.00 0.00 0.00 82 0.32 11.37 115.75 104.12 35.39 1.59 0.18 0.02 83 0.01 1.18 20.64 7.45 7.09 0.00 0.00 0.00 84 0.09 0.20 8.07 1.59 1.45 0.02 0.00 0.00 85 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 86 58.20 553.50 6679.49 5554.77 1283.98 162.90 44.09 10.01 87 49.83 520.34 6583.28 5452.18 1237.51 142.44 34.04 6.53 88 2.75 10.84 30.99 20.31 17.96 5.13 1.74 0.41 89 2.38 9.06 24.35 13.90 15.32 4.11 1.28 0.27 90 46.80 508.72 6551.23 5430.84 1218.50 136.80 32.01 5.99 91 26.69 432.95 6349.71 5236.68 1107.65 92.91 12.30 0.00 92 6.79 187.88 794.53 558.81 392.46 52.84 7.87 0.95 93 24.99 227.36 5496.12 4622.16 686.54 62.08 12.97 2.01 94 10.48 105.62 1495.35 1368.19 278.08 29.97 6.27 0.98 95 2.13 23.79 708.70 655.32 86.91 5.15 0.95 0.17 96 6.82 47.58 3514.29 2808.43 247.05 8.85 1.83 0.31 97 1.86 6.14 843.63 741.72 24.56 1.31 0.25 0.07 98 0.08 0.05 0.05 0.05 0.13 0.02 0.01 0.01 99 1.58 20.70 2460.20 1852.84 175.50 0.86 0.00 0.00

100 3.47 36.60 2540.46 1952.18 216.88 3.98 0.06 0.00 101 0.40 9.75 524.65 411.77 62.57 0.87 0.04 0.00 102 2.72 17.12 1822.21 1355.19 100.28 2.21 0.25 0.06 103 1.81 6.61 1215.41 877.08 43.36 0.93 0.16 0.05 104 0.66 5.90 1216.10 881.34 41.11 0.72 0.08 0.01 105 0.89 0.56 0.56 0.56 1.83 0.16 0.06 0.03 106 0.14 0.07 0.07 0.07 0.25 0.02 0.01 0.01 107 0.73 0.48 0.48 0.48 1.55 0.14 0.05 0.03 108 0.54 0.36 0.36 0.36 1.25 0.09 0.03 0.01


Table A6.6. Predicted ten-year return period peak salt loads (kt d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–21001 53.77 230.11 797.62 807.57 410.07 112.81 77.36 46.08 2 53.05 224.41 784.41 804.46 405.34 111.93 78.96 45.66 3 2.87 5.83 7.33 6.61 6.34 5.04 3.86 2.97 4 3.06 5.45 8.20 6.97 5.89 4.61 3.47 2.62 5 2.90 5.67 8.05 6.72 6.39 4.54 3.54 2.42 6 2.61 6.96 7.03 7.25 7.64 6.27 4.44 3.08 7 52.12 221.37 782.21 802.60 400.70 110.50 77.83 44.37 8 8.13 9.02 9.02 9.02 10.54 6.94 4.92 3.07 9 8.18 9.15 9.15 9.15 10.20 6.87 4.97 3.16

10 4.50 5.50 5.50 5.50 6.23 4.68 3.56 2.38 11 3.58 3.80 3.80 3.80 4.23 3.09 2.21 1.22 12 49.39 219.34 777.40 799.08 384.38 111.74 73.37 43.65 13 0.60 0.23 0.24 0.24 0.32 0.18 0.12 0.06 14 48.31 224.40 763.60 790.35 423.93 114.64 68.75 42.71 15 6.30 13.48 14.03 16.75 14.91 11.81 10.86 7.60 16 45.56 220.73 724.78 752.13 449.59 105.68 67.85 40.66 17 46.51 222.56 725.18 745.70 439.55 102.13 67.61 39.69 18 1.15 2.63 2.76 3.53 2.75 2.21 1.91 1.35 19 1.13 2.87 2.29 3.05 2.99 2.46 1.99 1.18 20 46.56 220.37 656.29 690.78 388.44 104.18 65.36 40.39 21 13.90 68.73 106.97 114.95 80.16 29.68 18.85 10.38 22 6.90 54.14 93.99 100.19 66.24 19.92 10.49 4.44 23 6.93 54.47 94.22 99.72 66.08 20.05 10.41 4.29 24 1.62 6.70 4.31 5.38 8.83 4.16 2.58 1.43 25 6.21 52.20 95.01 97.74 64.77 17.08 9.06 3.23 26 2.96 50.50 90.79 93.92 64.62 9.81 4.26 0.00 27 0.00 70.67 95.19 110.77 78.76 0.00 0.00 0.00 28 1.77 6.17 5.51 6.31 8.12 4.14 2.07 0.61 29 1.73 7.97 10.44 13.70 10.92 4.32 1.53 0.30 30 8.74 21.31 28.90 32.63 23.76 16.73 10.61 5.63 31 8.21 20.13 27.14 30.83 23.38 15.92 9.77 5.10 32 2.21 6.04 5.56 7.03 7.49 5.13 4.05 2.57 33 7.03 16.78 23.06 26.31 21.49 12.99 8.43 4.08 34 5.32 12.93 16.60 19.03 17.68 9.97 6.34 3.33 35 2.93 7.54 5.99 7.36 10.04 5.25 3.13 1.49 36 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 37 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 38 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 39 40.06 213.47 631.15 640.95 345.37 78.33 51.69 31.52 40 1.11 3.06 2.95 3.06 3.53 2.44 1.91 1.40 41 42.16 217.98 649.03 661.19 323.90 73.31 49.03 30.30 42 37.70 217.00 692.05 705.75 411.92 69.37 47.44 26.91 43 3.60 6.86 5.42 7.85 9.77 5.29 4.43 3.52 44 33.75 217.25 699.53 712.18 422.93 67.84 47.01 26.00 45 30.53 223.27 724.59 736.88 417.32 68.46 47.17 25.87 46 7.96 19.05 13.64 16.02 25.37 15.03 12.36 9.01 47 7.73 17.70 13.53 15.71 21.92 13.87 11.55 8.75 48 1.43 1.13 1.05 1.05 1.70 0.70 0.36 0.16 49 3.08 8.53 6.72 9.67 11.82 6.44 4.85 3.76 50 1.19 2.87 2.78 3.05 4.56 2.24 1.66 1.16 51 27.90 222.44 719.25 728.69 410.21 65.45 46.10 22.54 52 27.51 220.71 735.35 744.94 377.57 65.43 45.51 22.99 53 13.10 69.45 99.32 79.66 87.18 33.60 20.27 11.05 54 2.13 10.11 16.85 13.66 17.29 6.03 3.71 0.00 55 13.22 66.31 93.72 81.31 87.68 34.21 17.18 3.26 56 2.69 5.74 4.94 5.48 5.91 4.91 3.81 2.77 57 11.67 67.49 93.93 81.86 86.53 34.80 16.58 0.00 58 21.62 218.61 729.17 752.53 402.53 43.76 29.49 12.94 59 5.51 13.52 10.41 12.74 17.02 9.42 7.15 4.41 60 14.14 296.55 582.56 585.09 247.15 50.56 10.05 5.14 61 2.52 279.71 298.73 337.56 234.85 7.49 3.80 1.58 62 2.44 279.59 288.34 321.51 233.94 6.08 2.37 0.63 63 0.98 6.15 6.63 6.80 9.63 2.97 0.91 0.23 64 0.56 4.74 4.98 5.17 7.58 1.95 0.55 0.12 65 0.57 268.32 280.63 301.86 225.58 1.40 0.71 0.25 66 0.00 267.28 293.16 299.40 249.69 0.00 0.00 0.00 67 2.59 175.73 174.38 203.40 156.48 7.31 0.00 0.00 68 0.28 168.57 246.93 288.55 159.43 1.82 0.54 0.18 69 0.00 167.15 238.27 279.04 170.73 0.56 0.00 0.00


Table A6.6 (continued). Predicted ten-year return period peak salt loads (kt d–1) for eight scenarios. Some small non-zero values may have been rounded down to zero


1976–2003 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–2100 2073–210070 0.37 2.45 2.93 2.76 4.24 0.98 0.22 0.03 71 1.03 111.93 297.58 306.58 214.52 5.92 0.83 0.18 72 0.69 3.43 3.50 3.55 5.20 1.45 0.36 0.07 73 0.30 115.02 223.97 234.02 199.69 4.68 0.22 0.05 74 0.05 117.99 286.83 297.07 198.66 3.89 0.00 0.00 75 1.36 8.07 8.58 8.74 13.17 3.46 0.98 0.18 76 0.64 5.02 5.20 5.20 8.26 2.08 0.56 0.11 77 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 79 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 81 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 82 0.05 0.47 6.34 2.20 2.26 0.15 0.03 0.01 83 0.00 0.04 5.49 1.38 1.58 0.00 0.00 0.00 84 0.02 0.03 0.05 0.04 0.18 0.01 0.00 0.00 85 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 86 9.13 43.08 241.25 221.73 73.92 22.88 4.80 1.89 87 8.92 42.63 290.51 271.27 74.69 22.29 4.79 1.60 88 0.21 0.61 0.50 0.57 0.88 0.34 0.15 0.07 89 0.16 0.46 0.41 0.47 0.70 0.24 0.11 0.04 90 8.74 42.43 292.49 272.69 73.57 21.90 4.41 1.56 91 8.05 42.33 314.70 288.65 74.47 20.40 1.57 0.00 92 0.40 3.19 3.69 3.63 5.38 1.42 0.39 0.07 93 2.76 11.76 88.15 80.02 23.67 5.38 1.85 0.70 94 1.30 6.11 27.12 32.72 10.83 2.84 1.01 0.24 95 0.49 2.64 16.53 20.43 6.69 1.10 0.27 0.06 96 0.70 2.73 82.25 73.14 16.55 0.79 0.32 0.11 97 0.33 0.90 5.32 5.19 2.29 0.27 0.08 0.03 98 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 99 0.10 2.41 79.10 70.29 15.06 0.00 0.00 0.00

100 1.06 6.31 320.30 203.20 28.36 0.45 0.00 0.00 101 0.08 0.53 1.82 1.59 1.63 0.13 0.02 0.00 102 0.57 4.26 1356.90 1144.50 11.49 0.86 0.20 0.04 103 0.52 3.70 1619.60 1211.60 12.27 0.86 0.20 0.04 104 0.21 0.84 4.59 3.93 2.77 0.23 0.04 0.02 105 0.43 0.20 0.20 0.20 0.61 0.10 0.07 0.05 106 0.04 0.03 0.03 0.03 0.05 0.01 0.01 0.01 107 0.39 0.21 0.21 0.21 0.63 0.10 0.05 0.06 108 0.13 0.06 0.06 0.06 0.11 0.02 0.01 0.01


GLOSSARY Water yield: Refers to the annual average net amount of water that is

generated within a subcatchment and discharged from it.

Basin: Basin represents all area of the Avon.

Brooks-Corey texture index: A parameter to account for the pore-size distribution index in their saturated hydraulic conductivity equation.

Bubbling pressure: Maximum pressure necessary for air entry into the saturated soil

Catchment: The catchment drains a particular region of the basin. For example Lockhart, Yilgarn, Mortlock East, etc.

Dangle: Situation where a digital line extends past the intended boundary line. This extension past the intended juncture point is called a dangle.

Leveed arterial channel: Channel that entertains only drainage discharge from subcatchment scale leveed drainage systems.

Open arterial channel: Channel that entertains both drainage discharge from subcatchment scale leveed drainage systems and surface runoff from rainfall.

Field capacity: The percentage of moisture remaining in a soil horizon 2–3 days after being saturated (by rainfall or irrigation) and after free drainage has ceased.

Porosity: Porosity is a measure of the void spaces in the soil

Regolith depth: The mantle or blanket of unconsolidated or loose rock material that overlies the intact bedrock and nearly everywhere forms the land surface.

Specific Yield: The percentage of water which will be yielded by a rock or soil (by gravity) after being saturated; computed in terms of the ratio of the volume of water retained to its own volume.

Topsoil depth: Depth of top layer of soil

Undershoot: Situation where a digital line does not meet up with its intended boundary line. The space between the two is called a gap.

Solum depth: The upper part of a soil profile above the parent material in which current processes of soil formation are active. This is where the living roots and other plant and animal life characteristics are exhibited.

Subcatchment: A number of farms form one subcatchment. For this study the Avon Basin was divided into 108 subcatchments.

Zone of effectiveness: Perpendicular distance on each side of the drain where the groundwater levels are lowered by the drain below a critical depth and land is returned to normal productivity.


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A regional drainage evaluation for the Avon Basin, Western Australia · 2011. 12. 6. · A Regional...

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