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C S I R O L A N D a nd WAT E R

Estimation of Groundwater Recharge to the

Parmelia Aquifer in the Northern Perth Basin

2001–2002

E B Bekele , R B Salama , D P Commander , C J Otto , W P Hick ,

G D Watson , D W Pollock and P A Lambert

CSIRO Land and Water

WA Water and Rivers Commission

CSIRO Petroleum

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Technical Report 10/03, March 2003

Estimation of Groundwater Recharge to the

Parmelia Aquifer in the Northern Perth Basin

2001–2002

E B Bekele , R B Salama , D P Commander , C J Otto , W P Hick ,

G D Watson , D W Pollock and P A Lambert


WA Water and Rivers Commission

CSIRO Petroleum

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1 1 1

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Technical Report 10/03, March 2003

ISSN 1446-6163

ISBN No. 0 643 06114 2 Centre for Groundwater Studies Report No. 105

© CSIRO Australia 2003

To the extent permitted by law, all rights are reserved and no part of this publication (including photographs, diagrams, figures and maps) covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

Important Disclaimer:

CSIRO Land and Water advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

Estimation of Groundwater Recharge to the Parmelia Aquifer in the Northern Perth Basin 2001-2002

TABLE OF CONTENTS

LIST OF TABLES ...................................................................................................................... ii

LIST OF FIGURES .................................................................................................................... ii

ABSTRACT............................................................................................................................... 1

INTRODUCTION ...................................................................................................................1

HYDROLOGIC BACKGROUND ............................................................................................ 2

GROUNDWATER SAMPLING AND GEOCHEMICAL ANALYSES ...................................... 8

Core and Rotary Air Blast Sample Analyses ....................................................................12

DESCRIPTION OF METHODS FOR ESTIMATING GROUNDWATER RECHARGE ........16

Hydrograph Technique Based on Specific Yield .............................................................16

Groundwater Recharge from Chloride Mass Balance ....................................................17

Soil Water Flux from Chloride Mass Balance..................................................................18

RESULTS: ESTIMATION OF GROUNDWATER RECHARGE............................................18

Hydrograph Method........................................................................................................18

Groundwater Chloride Mass Balance .............................................................................19

Soil Water Chloride Mass Balance ..................................................................................20

DISCUSSION .........................................................................................................................21

CONCLUSIONS .................................................................................................................... 26

ACKNOWLEDGEMENTS .....................................................................................................28

REFERENCES.........................................................................................................................28

APPENDIX.............................................................................................................................30

– i –


LIST OF TABLES

Table 1 (A) Groundwater chemistry data for 18 private bores sampled in October 2001 and (B) radiocarbon ages for a subset of the bores ..................................... 8

Table 2 Record of sites drilled and cored for the study and summary details .................. 14

Table 3 Percentages of silt, sand and clay obtained from sieve analysis and estimates of specific yield based on the relationships from Johnson (1967). Five sub-samples of the Tathra cores were analyzed........................................................ 16

Table 4 Concentrations of chloride in rainwater sampled near the weather station in Eneabba and the Arrowsmith River bores. ..................................................... 18

Table 5 Estimates of recharge (pre-clearing) based on groundwater chloride mass balance using A) initial water chemistry data from W&RC bores collected in 1966 and 1974, and B) data collected from private bores (October 2001). The latter are data from bores with well screens well below the water table; therefore they likely represent pre-clearing recharge. The average recharge rate based on both data sets is 16 mm/yr .............................. 21

Table 6 Summary of recharge estimates obtained using different methods ..................... 22

Table 7 Soil hydraulic parameters used for WAVES simulation ........................................ 30

LIST OF FIGURES

Figure 1 Map of the study area indicating the geologic boundaries and outcrops relative to the Parmelia Formation on the Dandaragan Plateau. The locations of cored sites and the weather station are shown. Sediment samples were collected with rotary air blast from the Alexander Morrison hole #3. The water table was reached in only the Clausen #3 hole. The average annual rainfall (1960-2001) is indicated for the town centers in the region .................................................................................................................... 3

Figure 2 (A) Groundwater hydrographs for the Eneabba Line bores, and (B) the Arrowsmith River bores with EL1C included for comparison. Changes in the rate of water level rise for AR21 and AR22 are large compared with the other Arrowsmith River bores, which appear to have adjusted to clearing before the Eneabba bores. Dashed line segments connect the last water level measurement from AQWABase and the data collected more recently by CSIRO................................................................................................. 4

Figure 3 The record of cumulative excess rainfall over evaporation as recorded at the CSIRO weather station over a period of 485 days (March 2001 to July 2002). The total excess rainfall is nearly 350 mm................................................ 5

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Figure 4 (A) Barometric pressure recorded at the CSIRO weather station near the Eneabba Line bore #2. Elevations of the water table measured in the Eneabba Line bores (B), and the Arrowsmith River bores (C) between March 2001 and July 2002. Barometric efficiencies of the Parmelia aquifer were calculated using several of the largest changes in barometric pressure ...... 6

Figure 5 Contour map of the water table based partly on water level data from CSIRO-logged bores in July 2002, and AQWABase data from 1994-1999 corrected to 2002 by extrapolating water level gradients.................................... 7

Figure 6 Relationship between groundwater age from radiocarbon dating and the depth of the sample below the water table. There are insufficient data to establish a reliable predictive relationship, but there is reasonable evidence to suggest groundwater age increases with depth for samples older than 5000 years. The sample depths are given relative to the pump, screen and total depth of the bore, if available ....................................................................... 9

Figure 7 Relationship between the δD and δ18O relative to the Perth meteoric water line. The majority of the groundwater samples from private bores are derived from a meteoric source. The artesian groundwater sample from the Culluton bore shows evidence of isotopic fractionation, most likely due to shallow circulation of water that has undergone evaporative concentration ...................................................................................................... 10

Figure 8 Stiff diagrams of groundwater in the Parmelia aquifer. Groundwater was sampled in October 2001 from 18 privately owned domestic, stock and aquaculture bores. The ratios of cations to anions are generally similar, suggesting similar rock-water interactions have occurred throughout the aquifer. The Read sample is an extreme case with a total ion concentration of 3900 mg/L. The high concentrations in the sample might be related to its proximity to a fault boundary and juxtaposition with other strata such as the Cattamarra Coal Member...................................................... 11

Figure 9 Piper plot showing that the Parmelia contains predominantly a Na-Mg-Cl type groundwater with low bicarbonate. The groundwater sample from the Baker bore is an exception with slightly higher concentrations of bicarbonate, carbonate and sulfate relative to chloride...................................... 12

Figure 10 Descriptions of sediment samples collected by CSIRO with a hollow stemmed auger and rotary air blast (Alexander Morrison #3 site). Sediment color is mainly influenced by iron content and the extent of weathering........................................................................................................... 13

Figure 11 Vertical profiles of measured soil water chloride and gravimetric water content, and computed soil water fluxes for (A) cleared and (B) uncleared sites from analysis of casing shoe samples. Soil water chloride is generally lower beneath the cleared sites, except Clausen #1, which contains fine-grained sediments that preserve the chloride bulge from former native vegetation. Higher water contents generally correspond to samples with greater clay content ............................................................................................ 15

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Figure 12 Computed specific yield values based on the analysis of core data from holes drilled in (A) Tathra National Park and (B) the Heitman paddock. Lower specific yield values generally correspond to very fine-grained, weathered sand and clay ..................................................................................... 16

Figure 13 Layered soil profile consisting of Spearwood B, a loamy sand, interbedded with clay for WAVES simulations. ....................................................................... 23

Figure 14 Comparison of moisture content computed with WAVES under Banksia and bare soil from two soil profiles at the end of a simulation ........................... 24

Figure 15 Comparison of average annual fluxes computed with WAVES under Banksia and bare soil from two soil profiles for a 10-year simulation ................ 24

Figure 16 Digital elevation map for the Parmelia aquifer and the locations of 11 proposed drill sites .............................................................................................. 27

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ABSTRACT

Quantifying the increase in groundwater recharge from the removal of native vegetation on the Dandaragan Plateau remains essential for predicting the sustainability of groundwater development. Several groundwater recharge estimation techniques were applied to the predominantly sandy Parmelia aquifer. Recharge estimates were obtained using groundwater hydrograph data and the mass balance of chloride in soil water and groundwater. Hydrologic monitoring, core analyses, and groundwater radiocarbon dating and geochemistry (stable isotopes and major ions) provide additional data to interpret recharge characteristics.

The thick unsaturated zone is a heterogeneous mixture of weathered sand interspersed with discontinuous lenses of clay, which complicates the estimation of groundwater recharge on a regional scale. Rising groundwater levels in the Parmelia aquifer, ranging from 30 to 55 cm/yr over the last decade, across the region suggest an increase in recharge. Assuming a uniform specific yield of 0.1, the time-weighted average recharge predicted by the hydrograph method is 24 mm/yr for the Eneabba Line bores and between 33 and 50 mm/yr for the Arrowsmith River bores since 1970. Although the impact of clearing on soil water fluxes is evident, recharge predicted with the chloride tracer method for a limited number of cleared sites is toward the low end of the range predicted by the hydrograph method (less than 27 mm/yr).

Groundwater sampled from private bores in 2001 provides little insight into post-clearing recharge: the well screens are generally much deeper than the rising water table, and groundwater ages predate clearing by more than 1000 years. Nevertheless, pre-clearing recharge estimates between 6.6 to 33.9 mm/yr were obtained based on the private bore data and groundwater chloride measured in 1965 and 1974, before water levels responded to clearing. The wide range of pre-clearing recharge estimates from the chloride data might be related to aquifer heterogeneity and localized recharge. These results have been the impetus to drill new piezometers and modify sampling techniques for acquiring recent recharge rates and to better constrain the rate of increased groundwater recharge due to clearing.

INTRODUCTION

The Early Cretaceous Parmelia Formation is the principal aquifer that outcrops approximately 200 to 300 km north of Perth on the northern portion of the Dandaragan Plateau. Groundwater resources in this region are of crucial importance to landholders, townships, and the mining industry; however, while pressure to allocate more water has increased, relatively few studies have been conducted to quantify groundwater recharge to the northern Perth Basin. Development plans for adjacent areas have increased interest in the groundwater recharge characteristics of the Parmelia aquifer. The outcomes of this study should guide extrapolation of recharge estimates to surrounding regions and should help identify efficient, low cost methods for estimating recharge that can be applied elsewhere.

A variety of physical and chemical methods for estimating groundwater recharge to an unconfined aquifer were evaluated for their practicality and applicability in this study. Groundwater-level fluctuation analysis coupled with physical measurements of specific yield, and the chloride mass balance or tracer method are relatively inexpensive and not particularly labor intensive compared to lysimetry, soil water mass balance, CFC tracer profiling or other commonly applied approaches to estimating recharge (Lerner et al., 1990; Cook and

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Herczeg, 1998; Bond, 1998; Walker, 1998). Groundwater recharge estimates are presented herein based on bore hydrograph analyses and chloride mass balance methods. Soil water fluxes were computed using chloride data from the unsaturated zone, which allow us to infer maximum drainage or recharge to the water table.

The first section of this report reviews the hydrology of the study area using long-term meteorological records from the Bureau of Meteorology and water level monitoring data from the Water and Rivers Commission. This information is supplemented with high frequency water level and meteorological data collected by CSIRO Land and Water since the end of March 2001. The weather station, monitoring bores and sampling locations are shown in Figure 1. Additional background on the Parmelia Formation is provided in the descriptions of the groundwater geochemistry and results from core and rotary air blast sample analyses. The crux of this report is the section on recharge estimation methods and comparison of results. The discussion section includes an evaluation of the reliability of recharge estimates obtained from different methods.

The outcomes from the first year of this investigation clearly indicate that further sampling is required to improve the accuracy of recharge estimates and confidence in their application for groundwater allocation purposes. A regional map showing the locations of proposed drill sites relative to geomorphic features is also provided for the next stage of the project.

HYDROLOGIC BACKGROUND

The study area has a semi-arid climate with 2.3 m of evaporation per year on average, well in excess of rainfall1. The average annual rainfall has decreased inland across the region from 500-570 mm at Eneabba and Badgingarra Research Station to <400 mm at Carnamah and Coorow to the east since 1960 (Figure 1)2.

Water levels in the Parmelia aquifer recorded after the mid-1960s increased remarkably and at an increasing rate on almost a decadal scale (Figures 2A, 2B). Water levels have been monitored several times per year from 1967 to 1994 for the Arrowsmith River bores and from 1974 to 1999 for the Eneabba Line bores3. The hydrographs for these bores indicate inflection points or changes in the water level gradient that may correspond to the drought events as suggested by the decrease in water level gradients in some of the Arrowsmith River bores.

1 Computed from 1960 to 2001 using evaporation data from the Shires of Three Springs, Carnamah and Eneabba; Source: Bureau of Meteorology, SILO data 2 Computed using rainfall data from the Shires of Three Springs, Carnamah, Coorow, Eneabba, Mingenew, Moora and Badgingarra Research Station; Source: Bureau of Meteorology, SILO data 3 Source: AQWABase from the Water and Rivers Commission

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Eneabba(523 mm)

Three Springs(389 mm)

Carnamah(376 mm)

Coorow(392 mm)

BadgingarraResearch Stn

(569 mm)

Mingenew (399 mm)

AR22AR21

AR17

EL1EL2EL3

Heitman 0

Tathra NP

Clausen 1-3

Alexander Morrison NP

9km to Moora (453 mm)

Parmelia Fm

Yarragadee Fm

UndifferentiatedCretaceous

Pre-Early JurassicFormations

Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Legend

0 10 20 30 40 km

Scale

Otorowiri Fm

W&RC monitoring bores with loggers

Sites drilled or cored July 2001- May 2002

CSIRO weather station

Major faults

Fractures

Distribution of unconfined Parmelia onthe Dandaragan plateau

Town centers and average annual rainfall

Eneabba(523 mm)

Three Springs(389 mm)

Carnamah(376 mm)

Coorow(392 mm)


(569 mm)

Mingenew (399 mm)

AR22AR21

AR17

EL1EL2EL3

Heitman 0

Tathra NP

Clausen 1-3

Alexander Morrison NP

9km to Moora (453 mm)

Parmelia Fm

Yarragadee Fm



Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Legend

0 10 20 30 40 km

Scale0 10 20 30 40 km

Scale

Otorowiri Fm

W&RC monitoring bores with loggers W&RC monitoring bores with loggers

Sites drilled or cored July 2001- May 2002Sites drilled or cored July 2001- May 2002

CSIRO weather stationCSIRO weather station

Major faultsMajor faults

Fractures


Town centers and average annual rainfall

Figure 1 Map of the study area indicating the geologic boundaries and outcrops relative to the Parmelia Formation on the Dandaragan Plateau. The locations of cored sites and the weather station are shown. Sediment samples were collected with rotary air blast from the Alexander

Morrison hole #3. The water table was reached in only the Clausen #3 hole. The average annual rainfall (1960-2001) is indicated for the town centers in the region

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20900

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08-Jan-65 01-Jul-70 22-Dec-75 13-Jun-81 04-Dec-86 26-May-92 16-Nov-97 09-May-03

Date

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evel

(cm

AH

D)

AR7AR14

AR15AR16AR17

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27-Mar-73 05-May-77 13-Jun-81 22-Jul-85 30-Aug-89 08-Oct-93 16-Nov-97 25-Dec-01 02-Feb-06

Date

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evel

(cm

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evel

(cm

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D)

AR7AR14

AR15AR16AR17

AR21AR22AR24

EL1C

25 cm/yr

55 cm/yr40 cm/yr

55 cm/yr

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Date

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evel

(cm

AH

D)

AR7AR14

AR15AR16AR17

AR21AR22AR24

EL1C

25 cm/yr

55 cm/yr40 cm/yr

55 cm/yr

21700

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22100

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27-Mar-73 05-May-77 13-Jun-81 22-Jul-85 30-Aug-89 08-Oct-93 16-Nov-97 25-Dec-01 02-Feb-06

Date

Wat

er L

evel

(cm

AH

D)

EL1C

EL2B

EL2C

EL3C

EL4A

40 cm/yr

30 cm/yr

10 cm/yr

A

B

A

B

Figure 2 (A) Groundwater hydrographs for the Eneabba Line bores, and (B) the Arrowsmith River

bores with EL1C included for comparison. Changes in the rate of water level rise for AR21 and AR22 are large compared with the other Arrowsmith River bores, which appear to have adjusted to

clearing before the Eneabba bores. Dashed line segments connect the last water level measurement from AQWABase and the data collected more recently by CSIRO

Regional differences exist in the pattern of water level rise over time, which may be related to the time and extent of clearing. The removal of native vegetation started in the 1950s in the Arrowsmith River region and was largely complete by the 1970s, whereas clearing was just getting underway to the south near the Eneabba Line bores after the mid-1960s. The lag between the time when clearing started in these two regions may explain why the rate of water level rise in the Eneabba Line bores was as much as three times greater after 1983 compared with the preceding decade, whereas water levels in the Arrowsmith River bores have increased generally at a uniform rate since monitoring started in 1967. Presumably, the response to clearing in the Arrowsmith River region preceded the start of water level monitoring by possibly 10 to 15 years. The Arrowsmith River region is also much more

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extensively cleared today than to the south where large areas of native vegetation have been preserved. This may explain why rates of water level rise are generally higher in the Arrowsmith River bores.

The long-term water level monitoring data are valuable for describing the groundwater response to the removal of native vegetation and for estimating recharge; however, more frequent water level and meteorological data were also required to calculate the aquifer barometric efficiency and to check for continuation of long-term water level trends. A weather station was installed near the Eneabba Line bores to concurrently measure barometric pressure, rainfall and evaporation every three hours (Figures 3, 4A). During the monitoring period March 2001 to July 2002, the cumulative excess rainfall over evaporation is nearly 350 mm. Detailed continuous monitoring of water levels is also being conducted at an hourly rate in four Eneabba Line bores and three Arrowsmith River bores in the Parmelia aquifer. One bore that is screened over the underlying Yarragadee aquifer is also being monitored at the same frequency as the other Eneabba Line bores.

0

50

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30-Mar-01 8-Jul-01 16-Oct-01 24-Jan-02 4-May-02 12-Aug-02Date

Cum

ulat

ive

Exce

ss R

ainf

all o

ver

Evap

orat

ion

(mm

)

Figure 3 The record of cumulative excess rainfall over evaporation as recorded at the CSIRO

weather station over a period of 485 days (March 2001 to July 2002). The total excess rainfall is nearly 350 mm.

A linear fit to the data over a one-year period beginning in March 2001 indicates that water levels in the Parmelia aquifer increased between 22 and 66 cm/yr, depending on the bore (Figures 4B, 4C). The water levels projected back to the final year of monitoring by the Water and Rivers Commission are consistent with the long-term trends. There does not appear to be a direct correlation between the thickness of the unsaturated zone and the rate of water level rise that could readily explain differences in the rates of water level rise between different bores. No seasonal patterns are apparent in the water level data most likely because the thick unsaturated zone creates diffuse wetting fronts. Barometric efficiencies were calculated over several pressure events and averaged for each bore. Overall, the Eneabba Line bores indicate a lower barometric efficiency (0.42) than the Arrowsmith River bores (0.73). These results indicate semi-confined conditions within the aquifer and suggest greater aquifer confinement in the Arrowsmith River region.

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950.00

955.00

960.00

965.00

970.00

975.00

980.00

985.00

990.00

25-Mar 14-May 3-Jul 22-Aug 11-Oct 30-Nov 19-Jan 10-Mar 29-Apr 18-Jun 7-Aug

Date

Bar

omet

ric P

ress

ure

(mb)

Barometric P

A

66 cm/yr

29 cm/yr

58 cm/yr

37 cm/yr

223.50

223.75

224.00

224.25

224.50

224.75

225.00

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225.50

225.75


Date

Gro

undw

ater

Ele

vatio

n (m

AH

D)

EL1C

EL2C

EL3C

EL2B

B

40 cm/yr

66 cm/yr

223.00

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Date

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undw

ater

Ele

vatio

n (m

AH

D)

AS17

AS21

AS22

22 cm/yr

C

950.00

955.00

960.00

965.00

970.00

975.00

980.00

985.00

990.00


Date

Bar

omet

ric P

ress

ure

(mb)

Barometric P

A

950.00

955.00

960.00

965.00

970.00

975.00

980.00

985.00

990.00


Date

Bar

omet

ric P

ress

ure

(mb)

Barometric P

A

66 cm/yr

29 cm/yr

58 cm/yr

37 cm/yr

223.50

223.75

224.00

224.25

224.50

224.75

225.00

225.25

225.50

225.75


Date

Gro

undw

ater

Ele

vatio

n (m

AH

D)

EL1C

EL2C

EL3C

EL2B

B

66 cm/yr

29 cm/yr

58 cm/yr

37 cm/yr

223.50

223.75

224.00

224.25

224.50

224.75

225.00

225.25

225.50

225.75


Date

Gro

undw

ater

Ele

vatio

n (m

AH

D)

EL1C

EL2C

EL3C

EL2B

B

40 cm/yr

66 cm/yr

223.00

224.00

225.00

226.00

227.00

228.00

229.00

230.00


Date

Gro

undw

ater

Ele

vatio

n (m

AH

D)

AS17

AS21

AS22

22 cm/yr

C

40 cm/yr

66 cm/yr

223.00

224.00

225.00

226.00

227.00

228.00

229.00

230.00


Date

Gro

undw

ater

Ele

vatio

n (m

AH

D)

AS17

AS21

AS22

22 cm/yr

C

Figure 4 (A) Barometric pressure recorded at the CSIRO weather station near the Eneabba Line

bore #2. Elevations of the water table measured in the Eneabba Line bores (B), and the Arrowsmith River bores (C) between March 2001 and July 2002. Barometric efficiencies of the Parmelia aquifer

were calculated using several of the largest changes in barometric pressure

– 6 –


Information about the water table in the Parmelia aquifer is fairly limited to where monitoring bores were installed in clusters or along transects; however, these groups of bores are widely spaced across the aquifer, which makes it difficult to interpolate water table contours and constrain groundwater flow directions (Figure 5). Given the limited coverage of data for 2002 water levels, a rough interpretation is that groundwater flow is toward the Arrowsmith River in the northern part of the study area and toward the Agaton and Moora Line bores to the south. The monitoring bore data indicate a groundwater divide near the Eneabba Line bores, but additional measurements are needed to confirm its presence. Water level data from monitoring bores and private bores in the early 1970s indicate a groundwater divide located south of the Eneabba Line (Commander, 1981). The depth to water is controlled by topography with the water table being shallower beneath drainage channels that dissect the study area. The depth to water varies from artesian near the Arrowsmith River to 78 m in EL3B.

Eneabba

Three Springs

Carnamah

Coorow


Mingenew

9km to Moora

Parmelia Fm

Yarragadee Fm



Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Legend

Otorowiri Fm

W&RC monitoring bores

Major faults

Fractures


0 10 20 30 40 km

Scale

228224

224

222

220

218

216

212

214

216

222

208

Contours of water table elevation (mAHD) in 2002.

214

Eneabba

Three Springs

Carnamah

Coorow


Mingenew

9km to Moora

Parmelia Fm

Yarragadee Fm



Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Arrowsmith River

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Yarra Yarra Lakes

Darling Fault

Urella Fault System

Legend

Otorowiri Fm

W&RC monitoring bores

Major faults

Fractures


0 10 20 30 40 km

Scale0 10 20 30 40 km

Scale

228224

224

222

220

218

216

212

214

216

222

208

Contours of water table elevation (mAHD) in 2002.

214

Figure 5 Contour map of the water table based partly on water level data from CSIRO-logged bores

in July 2002, and AQWABase data from 1994-1999 corrected to 2002 by extrapolating water level gradients

– 7 –


GROUNDWATER SAMPLING AND GEOCHEMICAL ANALYSES

Groundwater samples were collected from 18 actively used private bores and analyzed for major ions, oxygen-18 and deuterium (Tables 1A, 1B). A subset of nine of these bores was used to obtain groundwater ages by radiocarbon dating. Sampling groundwater from private bores with submersible pumps provided a low-cost option but bore construction details are sparse, making it difficult to determine the sampling depth. Sampling groundwater from the monitoring bores was considered, but the cost of purging several casing volumes was prohibitive for this investigation. The private bores actively supply water for domestic and farm use and did not require purging prior to sampling.

Table 1 (A) Groundwater chemistry data for 18 private bores sampled in October 2001 and (B) radiocarbon ages for a subset of the bores

(A)

Bore Ph Ec Na K Mg Ca Cl HCO3 SO4-S NO3-N del 18O del D ms/cm mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L per mil per milClausen 5.92 0.48 83.4 4.66 6.83 0.667 140 0.0 5 0.206 -4.8 -23.3Baker 7.01 0.48 88.8 11.1 7.17 2.55 105 47.7 19 0.005 -5.28 -25Nottle S.B 6.58 1.55 265 12.1 31.3 4.52 525 14.6 18 1.480 -4.55 -21.6Nottle D.B 6.45 1.56 265 12.7 31.6 4.49 525 14.7 17 1.310 -4.43 -19.9Culluton 6.6 1.62 301 16 17.5 7.11 595 0.0 1 0.009 -3.15 -15.6Nairn 4.45 1.79 295 3.09 49.1 4.87 665 0.0 8 4.160 -4.58 -22.1Fiegert 5.96 1.15 205 10.5 17.7 1.22 315 16.8 13 0.143 -4.67 -21.9Golden W.F 4.31 1.63 272 13.9 35.9 4.98 560 0.0 23 0.207 -4.84 -24.4Read 3.46 6.32 1167 51.6 153 17.8 2450 0.0 79 0.907 -4.74 -23.6Burley 5.96 0.87 149 11.4 15 2.03 210 12.6 8 0.026 -4.95 -23.4Cripps A. 6.05 0.87 159 8.32 11.6 1.67 210 14.3 10 0.324 -4.67 -21.8Cripps D. 6.13 1.13 202 9.9 18.4 2.35 315 16.3 13 0.297 -4.61 -21.5Mitchell 6 1.28 200 9.34 34.3 4.44 385 12.3 10 0.915 -4.53 -21.1Heitman 1 6.2 0.86 150 6.95 13 1.81 245 14.6 5 0.624 -4.57 -21.7Heitman 2 6.81 1.02 175 7.81 19 1.83 280 14.3 0.5 0.513 -4.73 -21Whittome 5.59 1.1 199 2.94 17.3 1.96 315 0.0 5 0.131 -4.75 -21.2Glover 5.38 1.4 244 8.84 26 2.13 525 0.0 13 0.218 -4.87 -22.9Watts 5.88 2.36 432 15.8 43.4 2.61 910 13.7 28 0.133 -4.88 -23.4*radiocarbon age dating conducted

(B)

Bore Radiocarbon Age Years Clausen 1809 Baker 35014 Nottle D.B 3553 Nairn modern; < 1000 years Fiegert 1903 Golden W.F 6301 Read 3636 Burley 5997 Cripps D. 5741

– 8 –


The total depth of the bore is generally the only construction data available for the private bores and it was used to provide a maximum estimate of the sampling depth. Ideally, the sampling depth should be determined from the difference in elevation between the water table and submersible pump (or well screen).

The age of groundwater appears to increase with depth for samples older than 5000 years, but there are too few data to establish a reliable predictive relationship (Figure 6). Essentially, the bores that were sampled indicate very old groundwater with ages between 1800 and 35000 years. There is only one “modern” aged groundwater sample from the Nairn farm bore. Modern in terms of radiocarbon dating for groundwater is probably less than 1000 years old, but difficult to determine with any certainty because of the potential for water-rock interactions and mixing of water with different flow paths

0

10

20

30

40

50

60

70

80

0 10000 20000 30000 40000

Radiocarbon Age (Years)

Dep

th o

f gro

undw

ater

sam

ple

belo

w th

e w

ater

tabl

e (m

)

Clausen, water table to pumpBaker, water table to screenBaker, water table to TDNottle D, water table to TDNairn, water table to TDGWF, water table to pumpGWF, water table to TDRead, water table to TDCripps D, water table to TD

Figure 6 Relationship between groundwater age from radiocarbon dating and the depth

of the sample below the water table. There are insufficient data to establish a reliable predictive relationship, but there is reasonable evidence to suggest groundwater age increases with depth for samples older than 5000 years. The sample depths are given

relative to the pump, screen and total depth of the bore, if available

The stable isotope and major ion chemistry of groundwater sampled from 18 private bores provides some insight into the source and type of water in the Parmelia aquifer. The oxygen-18 and deuterium concentrations were plotted relative to the Perth meteoric water line (Figure 7). The groundwater is derived mainly from a meteoric source. One outlier is the groundwater sample from an artesian bore on the Culluton property in the Arrowsmith River region. Since this is a groundwater discharge area and the water table is quite shallow, the source of groundwater is likely water that underwent evaporative concentration, thereby

– 9 –


depleting the concentration of heavier isotopes. This fractionation process is not evident in the samples from other parts of the aquifer, presumably because these samples were not sourced from water that evaporated substantially before recharging the aquifer.

-32.5

-30

-27.5

-25

-22.5

-20

-17.5

-15

-12.5

-10-5.4 -4.9 -4.4 -3.9 -3.4 -2.9

δ18O

!D

ClausenBakerNottle; Stock BoreNottle; Domestic BoreCullutonNairnFiegertGolden West FlowerReadBurleyCripps; AquacultureCripps; DomesticMitchellHeitman; Bore 1Heitman; Bore 2WhittomeGloverWattsdel D = 7.15del18O+10.6

Figure 7 Relationship between the δD and δ18O relative to the Perth meteoric water line. The

majority of the groundwater samples from private bores are derived from a meteoric source. The artesian groundwater sample from the Culluton bore shows evidence of isotopic fractionation, most

likely due to shallow circulation of water that has undergone evaporative concentration

The major ion chemistry reveals that the groundwater is a Na-Mg-Cl type water with low bicarbonate. Stiff diagrams for the groundwater samples reveal that the relative amounts of cations and anions are fairly uniform, but the total concentration of ions varies (Figure 8). The weathering of silicate minerals in the aquifer likely affects the groundwater composition. The groundwater pH is generally between 5 and 7, which is typical of water that has passed through the soil zone and reacted with CO2, but acidic water was sampled from three bores that are located close together (Read, Nairn, Golden West Farmer). The groundwater sample from Read has the lowest pH (3.46) and a relatively high total concentration of ions. It is likely that the very acidic water enhanced mineral dissolution, thereby increasing the amount of ions in solution. The cause of the acidic groundwater is not well understood, but these three samples show an increase in acidity with increasing sulfate concentration. It is interesting to note that the three acidic groundwater samples are from bores located near splays of the Urella Fault system. One potential source of sulfate is the Cattamarra Coal Member, which outcrops east of the Urella fault, and closest to the Read bore.

– 10 –


Heitman 1

Heitman 2Mitchell

Culluton

Cripps A. Cripps D.

Fiegert

Nottle S. Nottle D.

Clausen

Golden W.F.

WhittomeNairn Read

Burley

Baker

Glover

Yarra Yarra Lakes

Arrowsmith River

Darling Fault

Urella Fault System

#

#

##

#

#

#

#

#

#

##

#

#

#

#

##

Legend

Major faults

Fractures

0 10 20 30 40 km

Scale


Yarragadee Fm



Watts

meq/l AnionsCations

10203040506070 70605040302010

Na++K+ Cl-HCO3 - + CO3

-

SO42-

Ca2+

Mg2+

NO3-

Otorowiri Fm

Eneabba

Three Springs

Carnamah

Coorow

Mingenew

9km to Moora

Parmelia Fm

Heitman 1

Heitman 2Mitchell

Culluton

Cripps A. Cripps D.

Fiegert

Nottle S. Nottle D.

Clausen

Golden W.F.

WhittomeNairn Read

Burley

Baker

Glover

Yarra Yarra Lakes

Arrowsmith River

Darling Fault

Urella Fault System

#

#

##

#

#

#

#

#

#

##

#

#

#

#

##

Legend

Major faults

Fractures

0 10 20 30 40 km

Scale0 10 20 30 40 km

Scale


Yarragadee Fm



Watts

meq/l AnionsCations

10203040506070 70605040302010


-

SO42-

Ca2+

Mg2+

NO3-

meq/l AnionsCations

10203040506070 70605040302010meq/l AnionsCations

10203040506070 70605040302010


-

SO42-

Ca2+

Mg2+

NO3-

HCO3 - + CO3

-

SO42-

Ca2+

Mg2+

NO3-

Otorowiri Fm

Eneabba

Three Springs

Carnamah

Coorow

Mingenew

9km to Moora

Parmelia Fm

Figure 8 Stiff diagrams of groundwater in the Parmelia aquifer. Groundwater was sampled in

October 2001 from 18 privately owned domestic, stock and aquaculture bores. The ratios of cations to anions are generally similar, suggesting similar rock-water interactions have occurred throughout the aquifer. The Read sample is an extreme case with a total ion concentration of 3900 mg/L. The

high concentrations in the sample might be related to its proximity to a fault boundary and juxtaposition with other strata such as the Cattamarra Coal Member

On a Piper diagram for the groundwater samples, the anion composition in the Baker sample is an outlier with slightly more carbonate, bicarbonate and sulfate relative to chloride (Figure 9). This is the oldest and deepest groundwater sample with the highest pH (neutral) and yet one of the lowest concentrations of total dissolved solids.

– 11 –


C A T I O N S A N I O N S%meq/l

Na+K HCO +CO3 3 Cl

Mg SO 4

CaCalcium (Ca) Chloride (Cl)

Sulfa

te(S

O4)+

Chlor

ide(C

l) Calcium(Ca)+M

agnesium(M

g)

Carb

onat

e(CO

3)+B

icarb

onat

e(HC

O3)

Sodium(Na)+Potassium

(K)

Sulfate(SO4)M

agne

sium

(Mg)

80 60 40 20 20 40 60 80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

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60

40

20

ClausenBakerNottle S.BNottle D.BCullutonNairnFiegertGolden W.FReadBurleyCripps ACripps D.MitchellHeitman 1Heitman 2WhittomeGloverWatts

C A T I O N S A N I O N S%meq/l

Na+K HCO +CO3 3 Cl

Mg SO 4

CaCalcium (Ca) Chloride (Cl)

Sulfa

te(S

O4)+

Chlor

ide(C

l) Calcium(Ca)+M

agnesium(M

g)

Carb

onat

e(CO

3)+B

icarb

onat

e(HC

O3)

Sodium(Na)+Potassium

(K)

Sulfate(SO4)M

agne

sium

(Mg)

80 60 40 20 20 40 60 80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

20

40

60

80

20

40

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60

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20

ClausenBakerNottle S.BNottle D.BCullutonNairnFiegertGolden W.FReadBurleyCripps ACripps D.MitchellHeitman 1Heitman 2WhittomeGloverWatts

Figure 9 Piper plot showing that the Parmelia contains predominantly a Na-Mg-Cl type

groundwater with low bicarbonate. The groundwater sample from the Baker bore is an exception with slightly higher concentrations of bicarbonate, carbonate and sulfate relative to chloride

Core and Rotary Air Blast Sample Analyses

Additional background on recharge characteristics of the Parmelia aquifer was obtained from cores and rotary air blast samples collected from the unsaturated zone (Figure 10). The samples were collected from sites vegetated with low heathland shrubs and sites that were cleared of native vegetation. A hollow stemmed auger was used, except at one site (Alexander Morrison hole #3), where rotary air blast drilling was used. The sites include areas under native vegetation (Tathra and Alexander Morrison National Parks), and areas under cleared paddock (Heitman and Clausen farms). Maximum drill depths varied because of difficult drilling and several drilling attempts were made at some sites indicated by their sequential numbering (Table 2). The water table was reached only in the Clausen #3 hole.

– 12 –


Tathra 1

Figure 10 Descriptions of sediment samples collected by CSIRO with a hollow stemmed auger and rotary air blast (Alexander Morrison #3 site). Sediment color is mainly influenced by iron content

and the extent of weathering

– 13 –


Table 2 Record of sites drilled and cored for the study and summary details

Drill Site Method Maximum

Depth (mBGL)

Water table

reached? Location/Shire Land Use

Heitman 0 Hollow-stemmed auger 16.72 N Mingenew farm Clausen 1 Hollow-stemmed auger 9.75 N Eneabba farm Clausen 2 Hollow-stemmed auger 11.4 N Eneabba farm Clausen 3 Hollow-stemmed auger 20.25 Y Eneabba farm Tathra National Park 1 Hollow-stemmed auger 24.32 N Eneabba native veg. Alexander Morrison 1 Hollow-stemmed auger 19.5 N Coorow native veg. Alexander Morrison 2 Hollow-stemmed auger 19.5 N Coorow native veg. Alexander Morrison 3 Rotary air blast 40.5 N Coorow native veg.

The predominant litho-types in the cores are fine to very fine-grained sand, medium-grained sand and clay. There are isolated horizons of indurated ironstone and sand containing laterite nodules. Weathered, poorly sorted sands are quite common and generally grade into clay. Fine silt and clay size particles block the pores of poorly sorted sand, thereby reducing vertical infiltration rates.

Casing shoe samples of 10 cm in length were collected at intervals of approximately 0.75 m. These samples were used to obtain soil water chloride, moisture content, and bulk density from lab measurements. The soil water chloride and moisture content data were used to compute vertical fluxes in the unsaturated zone (Figures 11A, 11B).

Sub-samples of the sediment cores from Tathra National Park and the Heitman farm were used for determining specific yield. This involved sub-sampling above the casing shoe and repacking the sediment in soil rings. Porosity and specific retention were estimated from lab analyses and specific yield was obtained from the difference (Meinzer, 1932). The volumetric water content at field capacity of fine sand was obtained using an applied suction of 0.5 bars (Cassel and Nielsen, 1986). Specific retention was approximated using this value, assuming the volumetric water content at wilting point is negligible. Porosity was calculated from the bulk density determined in the lab and soil particle density, which was assumed a constant value of 2.65 g/cm3 for sandy soil (Blake and Hartge, 1986). Another technique for estimating specific yield involved particle size analyses, which were performed on five sub-samples of cores collected from the upper 4 m at the Tathra site. The percentages of sand, silt and clay were used to infer specific yield based on comparisons with published data from (Johnson, 1967; Fetter, 1994).

– 14 –


0.00

5.00

10.00

15.00

20.00

25.00

0 5000 10000 15000

Soil Water Chloride (mg/L)D

epth

(mB

GL)

Heitman 1Clausen 1Clausen 2Clausen 3

A0.00

5.00

10.00

15.00

20.00

25.00

0.00 0.10 0.20 0.30Gravimetric Water Content

Dep

th (m

BG

L)


0.00

5.00

10.00

15.00

20.00

25.00

0 10 20 30

Vertical Soil Water Flux (mm/yr)


0.00

5.00

10.00

15.00

20.00

25.00

0 5000 10000 15000

Soil Water Chloride (mg/L)D

epth

(mB

GL)


A0.00

5.00

10.00

15.00

20.00

25.00

0.00 0.10 0.20 0.30Gravimetric Water Content

Dep

th (m

BG

L)


0.00

5.00

10.00

15.00

20.00

25.00

0 10 20 30



0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

0 5000 10000 15000

Soil Water Chloride (mg/L)

Dep

th (m

BG

L)

Tathra National Park

Alexander Morrison 1

Alexander Morrison 2;only 1 sampleAlexander Morrison 3

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

0.00 0.10 0.20 0.30

Gravimetric Water Content

Dep

th (m

BG

L)




B0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

0.00 5.00 10.00 15.00


Dep

th (m

BG

L)




0.00

5.00

10.00

15.00

20.00

25.00

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35.00

40.00

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0 5000 10000 15000

Soil Water Chloride (mg/L)

Dep

th (m

BG

L)




0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

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Gravimetric Water Content

Dep

th (m

BG

L)




B0.00

5.00

10.00

15.00

20.00

25.00

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35.00

40.00

45.00

0.00 5.00 10.00 15.00


Dep

th (m

BG

L)




Figure 11 Vertical profiles of measured soil water chloride and gravimetric water content, and computed soil water fluxes for (A) cleared and (B) uncleared sites from analysis of casing shoe

samples. Soil water chloride is generally lower beneath the cleared sites, except Clausen #1, which contains fine-grained sediments that preserve the chloride bulge from former native vegetation.

Higher water contents generally correspond to samples with greater clay content

A wide range of specific yield values was obtained from the two analyses of cored sediments (Figures 12A, B; Table 3). All of the measurements were conducted on sediments from the unsaturated zone and may not be relevant to the saturated section. The specific yield values based on moisture retention experiments ranged from 1E-4 to 0.34. The lowest values generally correspond to cores of fine sand and clay. The average specific yield should be the harmonic mean, if flow is perpendicular to the strata; however, water may infiltrate along tortuous pathways, which make it difficult to define the mean. The mean specific yield obtained from the Tathra cores is 0.093, whereas the mean for the Heitman cores is much lower, 0.006 due to fine-grained sand between 11 and 12 m depth (Figure 12B). The specific yield estimates from the particle size analysis emphasize the dependence on lithology (Table 3). The lowest specific yield estimate < 0.01 predicted by correlating with published data corresponds to a sample with predominantly clay-size particles. The higher specific yield estimates of 0.3 to 0.4, corresponding to samples with a greater sand fraction, agree well with the maximum value obtained with the moisture retention experiments. A specific yield of 0.1 was used for computing recharge by the hydrograph method because it is a conservative value based on the mean value from the Tathra cores.

– 15 –


Table 3 Percentages of silt, sand and clay obtained from sieve analysis and estimates of specific yield based on the relationships from Johnson (1967). Five sub-samples of the

Tathra cores were analyzed

Core Sub-sampled

Depth (mBGL)

% Sand % Silt % Clay Esimate for Specific Yield

1 0.56 95.88 0.8 3.2 0.4 2 1.52 91.8 0 9.6 0.3 4 3.04 75.24 3.16 20.8 0.3 5 3.04 89.56 0.4 9.6 0.3 5 3.8 6.8 18.64 72 < .01

0

2

4

6

8

10

12

14

16

18

20

22

24

26

0.00 0.10 0.20 0.30 0.40 0.50Specific Yield

Dep

th (m

BG

L)

Cores from TathraNational Park, Eneabba

0

2

4

6

8

10

12

14

16

18

0.00 0.10 0.20 0.30 0.40 0.50Specific Yield

Dep

th (m

BG

L)

Cores from Heitmanpaddock, Mingenew

A B

Figure 12 Computed specific yield values based on the analysis of core data from holes drilled in (A)

Tathra National Park and (B) the Heitman paddock. Lower specific yield values generally correspond to very fine-grained, weathered sand and clay

DESCRIPTION OF METHODS FOR ESTIMATING GROUNDWATER RECHARGE

Hydrograph Technique Based on Specific Yield

For this technique, groundwater recharge is estimated from the product of specific yield and the average rate of water level change over time from bore hydrographs; however specific yield is extremely difficult to measure reliably (Sophocleous, 1985, 1991). Recharge rates were calculated using post-clearing rates of water level rise from different bores and a range of specific yield values. Since the rate of water level rise varies over time, different theories were considered. The first theory assumes the specific yield of the sediments is fairly uniform and a constant “effective” specific yield is applicable over the entire section through which

– 16 –


water is rising. In the first theory, changes in the rate of water level rise are attributed to dynamics in the aquifer from either a lag response to clearing and/or lateral flow from areas that have localized recharge. The second theory assumes that changes in the rate of water level rise are due to vertical changes in the specific yield as the water rises through different strata. This assumes the recharge rate is constant. Another theory could consider the combined effects of different recharge rates and variable specific yield.

Groundwater Recharge from Chloride Mass Balance

The chloride mass balance method for estimating groundwater recharge rates is well established (Allison and Hughes, 1978; Johnston, 1983, 1987; Stone, 1992). Since chloride is a conservative tracer, evaporation and plant uptake by transpiration concentrate rainwater-derived chloride in the soil. The profile of chloride in the soil varies with land use. In areas of native vegetation under arid conditions, the concentration of chloride typically forms a bulge in the root zone or near the surface. In cleared areas, the chloride bulge may be displaced downward in the soil profile or flushed away entirely (Stone, 1992).

Groundwater recharge estimated from the mass balance of chloride assumes steady-state conditions; therefore, it is most applicable to areas under native vegetation or cleared areas that have reached equilibrium. As a first approximation, advection was assumed dominant over diffusion, and groundwater recharge, R (in mm/yr) is estimated as:

gc/pPcR = ,

P is the mean annual precipitation [mm/yr]

pc is the chloride concentration of precipitation [mg/L]

gc is the groundwater chloride concentration at the water table [mg/L]

Groundwater chloride data were obtained from 8 initial water samples collected in 1974 for Eneabba Line bores and 1965-1966 for Arrowsmith River bores. Chloride measurements were also obtained for groundwater samples collected from 18 private bores in 2001. The groundwater samples obtained in 2001 however, do not reflect post-clearing recharge as indicated by older radiocarbon dates for a subset of the samples.

For this calculation of recharge, average values were used for the rainfall rate and concentration of rainwater chloride. The average rainfall for the nearest weather stations was used2. Rainwater collected at the CSIRO weather station in Eneabba and near the instrumented Arrowsmith River bores was analyzed for chloride. Six rainwater samples were taken between January and June 2002, and no significant difference in rainwater chloride can be determined between the two sites (Table 4). The average rainwater chloride is approximately 10.55 mg/L. The highest measured concentration of chloride (27.7 mg/L) occurs in January and rainwater chloride diminishes thereafter, as evaporation decreases.

– 17 –


Table 4 Concentrations of chloride in rainwater sampled near the weather station in Eneabba and the Arrowsmith River bores.

Rainwater Sample Date Location/ Shire

Chloride (mg/L)

30-Jan-02 Eneabba 27.70 16-Apr-02 Mingenew 15.00 18-Apr-02 Eneabba 4.70 16-May-02 Eneabba 6.65 05-Jun-02 Mingenew 7.07 05-Jun-02 Eneabba 2.16

Average: 10.55

Soil Water Flux from Chloride Mass Balance

Groundwater recharge rates were also compared with estimates of the time-averaged vertical flux of soil water , using the convective-diffusive transport equation from Peck et

al. (1981): wq

czcDpPc

wq

∂∂+

=θ

,

D is the dispersion/diffusion coefficient [mm2/yr]

θ is the volumetric water content [m3/m3]

c is the chloride concentration of the soil water [mg/l]

z is the vertical space coordinate (positive downward).

zc

∂∂ is the slope of the observed chloride concentration with respect to depth. Interstitial

water was extracted from unsaturated zone soil samples. The procedure for extracting soil water chloride is from Rayment and Higginson (1992). If groundwater velocities are sufficiently small, dispersion can be neglected. The diffusion coefficient was estimated for a sandy-clay aquifer based on the volumetric water content (Johnston, 1983). The method was applied using average rainfall data for the nearest weather stations2, and an average rainwater chloride of 10.55 mg/L.

RESULTS: ESTIMATION OF GROUNDWATER RECHARGE

Hydrograph Method

Recharge results are shown for both the Eneabba Line and Arrowsmith River bores. The response to clearing at the two locations is similar, given that a lag may exist due to earlier clearing in the Arrowsmith River region. The rates of water level rise in Eneabba may be increasing to the Arrowsmith levels. EL1C shows the typical response for the Eneabba Line bores with the following approximate rates of water level rise: 10 cm/yr (1974-1983), 30 cm/yr (1983-1995), and 40 cm/yr (1995-2002). For the Arrowsmith River bores, if we use AR21 as an example, the rates of water level rise are 55 cm/yr (1967-1978), 25 cm/yr (1978-

– 18 –


1984), and 55 cm/yr (1984-2002). However, this is at the high end because the other Arrowsmith River bores indicate a water level rise of approximately 40 cm/yr (1984-2002) and lower rates prior to 1984 that are between 20 and 40 cm/yr. The higher rates of water level rise in the Arrowsmith River bores compared to Eneabba might be due to lower specific yields in the region. This idea is consistent with sediment types that produce greater aquifer confinement in the Arrowsmith River region as interpreted from higher barometric efficiencies.

The results from applying the two main theories to explain the change in the rate of water level rise over time are shown. Based on a constant specific yield of 0.1 as per the first theory and the representative rates of water level rise for EL1C, the time-weighted average recharge rate weighted since 1974 is approximately 24 mm/yr for Eneabba Line bores. The time-weighted average recharge rate since 1970 in the Arrowsmith River region is between 33 and 50 mm/yr.

Application of the second theory predicts changes in specific yield within the vertical section through which the water level has risen over time. As an example, if we assume recharge is 35 mm/yr, the strata will have specific yield values ranging from 0.06 to 0.3, depending on the period of water level rise.

Groundwater Chloride Mass Balance

Groundwater recharge calculated based on chloride samples taken between 1966 and 1974 from the Eneabba Line and the Arrowsmith River bores is between 11 and 28 mm/yr (Table 5A). Similarly, groundwater recharge calculated using chloride data sampled in 2001 from private bores is between 6.6 and 33.9 mm/yr (Table 5B). The average recharge rate based on both data sets is 16 mm/yr. The water samples from the Baker, Culluton, Nairn and Read bores were omitted from this calculation for the following reasons. The groundwater sample from the Nairn bore has an uncertain groundwater age (<1000 years) and yields a recharge rate of 6.21 mm/yr. The Read sample has such an extraordinarily high ion concentration that suggests chloride may be influenced by saline groundwater from the Yarra Yarra saline lake; therefore the Read sample may not reflect direct rainfall recharge. The Baker sample is the oldest (>35000 years) and yet, remarkably, it has the lowest chloride concentration yielding a recharge rate of 57.2 mm/yr. The Culluton groundwater sample is from an artesian bore and it was not dated, thus we cannot be assured that it represents pre- or post-clearing recharge. The recharge rate for the Culluton sample is 7.1 mm/yr.

Some cleared areas of the Parmelia aquifer contain high groundwater chloride concentrations near the water table, which make it problematic to assume recharge is generally higher beneath cleared sites. Groundwater was sampled approximately 2 m below the water table from the Clausen #3 piezometer installed by CSIRO in 2002. The chloride concentration estimated from a measured electrical conductivity of 3230 µS is 930 mg/L. The estimate for recharge based on groundwater chloride is 5.1 mm/yr, which is too low to represent post-clearing conditions. The Clausen #3 site contains many layers of clay near the surface and may not be representative of where most of the recharge is occurring.

– 19 –


Soil Water Chloride Mass Balance

The soil water flux provides a measure of the proportion of chloride that has concentrated in the unsaturated zone, due to plant-water uptake, compared to the rate of chloride added from rainfall. The diffusion component of the soil water flux equation is extremely low compared to advection. Essentially, this means the soil water flux equation reduces to the equation for groundwater recharge based on chloride mass balance, but it requires the chloride concentration in soil water rather than groundwater. The flux of soil water should be quite different under cleared versus uncleared sites because native plants have a greater ability to use water. Some of the soil water flux profiles support this theory: the sites under native vegetation have soil water flux values less than 12 mm/yr, whereas soil water fluxes beneath the cleared paddocks are twice as large at some sites, up to 27 mm/yr (Figure 11). One exception is the Clausen #1 site where the soil water fluxes in the profile range from 0.4 to 1.7 mm/yr. The sediment cores from this paddock are predominantly fine to very fine, weathered sand containing high soil water chloride (approximately 3,000 to 12,000 mg/L). There is a chloride bulge in the profile, which is evidence that the signature of former native vegetation remains. The most likely explanation is that the recharge rate at this site is controlled more by lithology than the existence of cleared conditions. Another unusual profile was found beneath the Heitman paddock and again suggests control of fluxes by lithology. In this case however, the soil water flux and the water content increase to maximum values at nearly 16 m below the land surface. The soil water chloride in the profile is fairly uniform and low (< 1075 mg/L). One possibility is that vertical recharge has entirely removed a pre-existing chloride bulge, if one ever existed at this site. The simplest explanation for the maximum soil water flux at depth is related to the corresponding increase in soil water content. At this depth, the lithology changes to coarser, but highly weathered sand and interstitial clay with higher moisture content.

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Table 5 Estimates of recharge (pre-clearing) based on groundwater chloride mass balance using A) initial water chemistry data from W&RC bores collected in 1966 and 1974, and B) data collected from private bores (October 2001). The latter are data from bores with well screens well below the water table; therefore they likely represent pre-clearing

recharge. The average recharge rate based on both data sets is 16 mm/yr

(A) Bore Sampled for

Groundwater and Date Nearest Weather

Station Annual Rainfall

for Shire Groundwater

Cl Recharge*

mm/yr mg/L mm/yr

EL1C Carnamah+Eneabba 449.35 252 18.81 EL2B Carnamah+Eneabba 449.35 172 27.56 EL2C Carnamah+Eneabba 449.35 449 10.56 EL3C Carnamah+Eneabba 449.35 288 16.46 EL4A Carnamah+Eneabba 449.35 224 21.16 AR17 Mingenew 398.56 253 16.62 AR21 Mingenew 398.56 394 10.67 AR22 Mingenew 398.56 351 11.98

Average (mm/yr): 16.73 Minimum (mm/yr): 10.56 Maximum (mm/yr): 27.56

* Calculated using 10.55 mg/L for the rainwater chloride

(B)

Bore Sampled for Groundwater October 2001

Nearest Weather Station

Annual Rainfall for Shire

Recharge*

mm/yr mm/yr Clausen Carnamah+Eneabba 449.35 33.86 Nottle S.B Three Springs 389.00 7.82 Nottle D.B Three Springs 389.00 7.82 Fiegert Carnamah+Eneabba 449.35 15.05 Golden W.F Carnamah 375.68 7.08 Burley Coorow 391.65 19.68 Cripps A. Carnamah+Eneabba 449.35 22.57 Cripps D. Carnamah+Eneabba 449.35 15.05 Mitchell Mingenew 398.56 10.92 Heitman 1 Mingenew 398.56 17.16 Heitman 2 Mingenew 398.56 15.02 Whittome Coorow 391.65 13.12 Glover Badgingarra 569.25 11.44 Watts Badgingarra 569.25 6.60

Average (mm/yr)**: 14.51 Minimum (mm/yr)**: 6.60 Maximum (mm/yr)**: 33.86

* Calculated using groundwater chloride data in Table 1and 10.55 mg/L for the rainwater chloride

** Recharge estimates for Baker, Culluton, Nairn and Read samples were omitted (see text for explanation).

DISCUSSION

The semi-arid climate of the northern Perth Basin and heterogeneous clay-rich, weathered sands comprising the Parmelia aquifer offer little hope that significant groundwater recharge is occurring, despite the vast removal of native plants prior to 1970 and rising water levels. The impetus behind this project has been to determine current groundwater recharge rates to guide resource allocation; however, this information alone will not enable one to

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determine the maximum magnitude of sustainable development (Bredehoeft, 2002). The dynamic response of the aquifer to the removal of native plants and the resulting increase in recharge must be determined, if resource managers are to understand the size of sustainable groundwater development. To aid in this regard, both pre- and post-clearing estimates of groundwater recharge and their reliabilities are summarized below (Table 6).

Table 6 Summary of recharge estimates obtained using different methods

Recharge Estimation Method

Description Recharge rate mm/yr

Time Period

Groundwater Hydrograph EL bore hydrographs 24* post-clearing; since 1970 Method (Sy = 0.1) AR bore hydrographs 33 to 50* post-clearing; since 1970


W&RC and private bores samples

16** pre-clearing


Clausen #3 piezometer 5 possibly pre-clearing


Nairn farm bore 6 possibly pre-clearing


Culluton (1 artesian sample)

7 not certain

Soil Water Flux - Cl Maximum flux beneath cleared paddock

27 time-integrated; compare with post-clearing rates

Mass Balance Method Maximum flux beneath native vegetation

12 time-integrated; compare with pre-clearing rates

Soil Water Flux - Cl Deepest flux beneath cleared paddock

4 time-integrated; compare with post-clearing rates

Mass Balance Method Deepest flux beneath native vegetation

5 time-integrated; compare with pre-clearing rates

* time-weighted averages ** average recharge

In addition, one-dimensional models (WAVES) of vertical flux in the unsaturated zone were developed to compare average annual fluxes beneath cleared and uncleared land, using meteorologic and soil hydraulic data for the study area (Appendix). Simulated soil moisture content and fluxes within uniform and layered soil profiles were compared to illustrate the potential for lateral flow (Figures 13, 14). The models indicate that even under bare soil conditions, vertical recharge cannot exceed 33 mm/yr (Figure 15). This is contrary to infiltration rates obtained under plantation crops near Moora which gives unrealistically high rates of deep drainage (Lefroy et al., 2001 a;b). According to the authors a permanent fresh perched water table exists over a clay layer approximately 5-10 m below the surface. Realizing the fact that plantation roots will penetrate more than 10 m and that evapotranspiration will be more than the rainfall, it is not expected that the vertical infiltration will reach the aquifer as evident from the permanent perched aquifer. On the other hand the modelling results show that under Banksias for soils of similar properties the annual flux does not exceed 18 mm (Figure 15).

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0

5

10

15

20

25

30

35

40

45

Dep

th (m

)

Sp B

Sp B

Sp B

Sp B

0

5

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25

30

35

40

45

Dep

th (m

)

Sp B

Sp B

Sp B

Sp B

Figure 13 Layered soil profile consisting of Spearwood B, a loamy sand, interbedded with clay for

WAVES simulations.

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0

5

10

15

20

25

30

35

40

45

0 0.1 0.2 0.3 0.4 0.5Moisture content

Dep

th (m

)

Banksia + SpB and 3 clay layersBanksia + Uniform SpBBare soil + SpB and 3 clay layersBare soil + Uniform SpB

Figure 14 Comparison of moisture content computed with WAVES under Banksia and bare soil

from two soil profiles at the end of a simulation

0

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0 10 20 30 4Average annual flux (mm/yr)

Dep

th (m

)

0

Banksia + SpB and 3 clay layersBanksia + Uniform SpBBare soil + SpB and 3 clay layersBare soil + Uniform SpB

Figure 15 Comparison of average annual fluxes computed with WAVES under Banksia and bare soil

from two soil profiles for a 10-year simulation

– 24 –


One approach to estimating the increased recharge to groundwater from the removal of native plants is to compare soil water fluxes beneath cleared paddocks and native vegetation obtained using the chloride mass balance method. Among the six sites where the soil water chloride tracer method was applied, two sites with different vegetation conditions were appropriate for comparison. The maximum soil water flux at a depth of 4 m beneath the Clausen #2 site, a cleared paddock is 27 mm/yr or roughly twice the rate beneath native vegetation at the Alexander Morrison #3 site at the same depth and with a similar lithology profile. On this basis, capture of groundwater from the removal of native plants is nearly double the natural recharge rate. Variable amounts of highly weathered sands and interstitial clay also control soil water fluxes and ultimately recharge to the water table, thus one must be cautious about comparing soil water fluxes where the lithology in the unsaturated zone may be different.

The chloride tracer methods assume predominantly vertical flow; however, there could be some lateral flow, particular when water moving downward through the profile reaches an impasse and finds an adjacent pathway of higher conductivity at some lateral distance away. The implications are that the reliability of the chloride tracer method is very much dependent on heterogeneity within the aquifer.

The variability of specific yield throughout the Parmelia aquifer precludes accurate assessment of post-clearing recharge based on groundwater hydrograph analysis. If we assume that specific yield is uniform and as high as 0.1, the time-weighted average recharge rate near the Eneabba and Arrowsmith River bores is between 24 and 50 mm/yr since 1970. However, this range of recharge rates cannot be confirmed using results from the other methods. Specific yield clearly varies depending on the dominant lithology and degree of sediment sorting as indicated by estimates obtained from soil water retention experiments and particle size distributions. The presence of poorly sorted, fine-grained sand means that specific yield could be less than 0.01.

Although the groundwater chloride mass balance method provides an independent means for estimating recharge, the depths of the well screens beneath the water table in most of the bores have prevented sampling the most recent recharge under cleared conditions. If we assume that groundwater sampled near the water table (< 2 m) from the Clausen #3 piezometer represents post-clearing recharge, then the estimated recharge from the groundwater chloride method is 5.1 mm/yr. This estimate seems too low for post-clearing recharge and does not support the results from the hydrograph method. However, the recharge estimate from the groundwater chloride method does agree with the calculated soil water fluxes near the water table beneath the Clausen #3 site (Figure 11A). One should exercise great caution in assuming that groundwater near the water table is recent recharge.

The reliability of pre-clearing estimates of recharge can be tested by comparing chloride-based estimates of the soil water flux beneath native vegetation and recharge estimates using groundwater chloride samples from bores where either the groundwater age predates clearing and/or the well screens are much deeper than the current water table. The soil water fluxes beneath native vegetation are less than 12 mm/yr, whereas recharge estimates from groundwater chloride are between 6.6 and 33.9 mm/yr. The range of higher recharge values could be due to fractures or more permeable sediments.

– 25 –


CONCLUSIONS

Sustainable development of groundwater from the Parmelia aquifer will require a better understanding of increased recharge to groundwater due to the removal of native vegetation. Estimates of pre-clearing and post-clearing rates of groundwater recharge were obtained in this study using several different methods, but their reliability depends on acquiring more data on aquifer heterogeneity and the scale of measurements required by different methods. Soil water fluxes estimated using chloride mass balance should provide a reliable method for estimating increased recharge. The results from comparing soil water fluxes indicate increased recharge is approximately double the natural recharge rate under native vegetation.

The reliability of recharge estimates obtained from the groundwater chloride mass balance remains problematic because of uncertainty related to the amount of localized recharge. A wide range of estimates for pre-clearing recharge (6.6 to 33.9 mm/yr) was obtained using groundwater chloride data from bores where either the groundwater age predates clearing and/or the well screens are much deeper than the current water table. Future application of the groundwater chloride mass balance method should take into consideration the depth of the well screen beneath the water table and proportion of clay in the unsaturated zone. The observed water level rise in the aquifer suggest a large increase in recharge; however, the few estimates for post-clearing recharge acquired at this stage suggest the contribution of recent water to the groundwater in storage is relatively small. Groundwater sampling and age dating from carefully screened piezometers near the water table will be pursued in the next stage of the project.

Ideally, we should aim for independent confirmation of the recharge estimates predicted with the chloride tracer techniques, using a soil physical method such as the hydrograph method. Unfortunately, specific yield is not an easy parameter to constrain for a heterogeneous aquifer. The most realistic approach to determine recharge across the entire plateau will be to increase sampling and data collection for areas of suspected enhanced recharge and to use their proportion of the total plateau to upscale recharge estimates (Figure 16). The proportion of areas where enhanced recharge is less likely (i.e. broad uplands) should also be included in the total recharge calculation, especially if their combined area is quite extensive.

– 26 –


Color

3

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Elevation classificat ion

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Slope classificat ion

Color

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Elevation classificat ion

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Slope classificat ion

3 26A

1B

4

1A710

8

9A 9B

Legend

0 10 20 30 40 km

Scale0 10 20 30 40 km

Scale

Locations of proposed drill sites

3

2

1

Class

260-387

222-260

61-222

Elevation meters

1.97-25.18

0.79-1.97

0-0.79

Slopedegrees

3

2

1

Class

260-387

222-260

61-222

Elevation meters

1.97-25.18

0.79-1.97

0-0.79

Slopedegrees

Figure 16 Digital elevation map for the Parmelia aquifer and the

locations of 11 proposed drill sites

– 27 –


ACKNOWLEDGEMENTS

The authors wish to thank Chris Barber and Michael Martin for their useful discussions and comments. We also gratefully acknowledge Richard Silberstein for his assistance with WAVES, and Naoko Zwingmann for her assistance with interpreting the groundwater geochemistry. Fred Leaney provided valuable discussions on radiocarbon age dating and interpretations. This work was supported by the Centre for Groundwater Studies and the Centre partners (CSIRO Land and Water, Water and Rivers Commission, and Water Corporation).

REFERENCES

Allison GB, Hughes MW (1978) The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer. Australian Journal of Soil Research 16: 181-195.

Blake GR, Hartge KH (1986) Particle density, In: Klute A (ed) Methods of soil analysis, Part 1, 2nd edn., Physical and mineralogical methods, American Society of Agronomy and Soil Science of America, Agronomy Monograph 9:377-382.

Bond W (1998) Soil physical methods for estimating recharge. In: Studies in Catchment Hydrology, Basics of Recharge and Discharge Part 3. CSIRO Publishing, 17 pp.

Bredehoeft JD (2002) The water budget myth revisited: Why hydrogeologists model. Ground Water 40(4): 340-345.

Cassel, DK, Nielsen DR (1986) Field capacity and available water capacity, In: Klute A (ed) Methods of soil analysis, Part 1, 2nd edn., Physical and mineralogical methods, American Society of Agronomy and Soil Science of America, Agronomy Monograph 9:901-926.

Commander DP (1981) The hydrogeology of the Eneabba area, Western Australia. MSc University of Western Australia, Australia, 245 pp.

Cook PG, Herczeg AL (1998) Groundwater chemical methods for recharge studies. In: Studies in Catchment Hydrology, Basics of Recharge and Discharge Part 2. CSIRO Publishing, 17 pp.

Fetter CW (1994) Applied Hydrogeology, Third Edition. Maxwell Macmillan International, 691 pp.

Johnson AI (1967) Specific yield - Compilation of specific yields for various materials. U.S. Geological Survey Water-Supply Paper 1662-D.

Johnston CJ (1983) Estimation of groundwater recharge from the distribution of chloride in deeply weathered profiles from south-west Western Australia. In: Papers of the International Conference on Groundwater and Man. Australian Government Publishing Services, Aust. Water Resour. Counc. Conf. Series No. 8, 1:143-152.

Johnston CJ (1987) Distribution of chloride in relation to subsurface hydrology. Journal of Hydrology 94: 67-88.

Lefroy, E.C, Pate, J.S. and Stirzaker, R. J., 2001. Growth, water use efficiency and adaptive features of tagasaste (Chamaecytisus proliferus) at alley and plantation densities. Aust. J. Agric. Res. 52: 221-234.

Lefroy, E.C., Stirzaker, R.J. and Pate, J.S., 2001. The influence of tagasaste (Chamaecytisus proliferus) trees on the water balance of an alley cropping system. Aust. J. Agric. Res. 52: 235-246.

Lerner DN, Issar AS, Simmers I (1990) Groundwater recharge: a guide to understanding and estimating natural recharge, International Association of Hydrogeologists, v. 8, Heise, Hannover, 345 pp.

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Meinzer OE (1932) Outline of methods for estimating ground-water supplies. U.S. Geological Survey Water-Supply Paper 638-C:99-144.

Peck AJ, Johnston CD, Williamson DR (1981) Analyses of solute distributions in deeply weathered soils. Agricultural Water Management 4: 83-102.

Rayment GE, Higginson FR (1992) Australian laboratory handbook of soil and water chemical methods, v. 3, Inkata Press, Port Melbourne, 15-28.

Sophocleous M (1985) The role of specific yield in ground-water recharge estimations: a numerical study. Ground Water 23(1):52-58.

Sophocleous M (1991) Combining the soilwater balance and water-level fluctuation methods to estimate natural groundwater recharge: practical aspects. Journal of Hydrology 124: 229-241.

Stone WJ (1992) Paleohydrologic implications of some deep soilwater chloride profiles, Murray Basin, South Australia. Journal of Hydrology 132: 201-223.

Walker GR (1998) Using soil water tracers to estimate recharge. In: Studies in Catchment Hydrology, Basics of Recharge and Discharge Part 7. CSIRO Publishing, 27 pp.

Zhang L, Dawes WR (Eds.) (1998) WAVES: An integrated energy and water balance model, CSIRO Land and Water Technical Report No. 31/98.

Zhang L, Dawes, WR, Hatton, TJ (1996) Modelling hydrologic processes using a biophysically based model - application of WAVES to FIFE and HAPEX-MOBILHY. Journal of Hydrology 185: 147-169.

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– 30 –

APPENDIX

WAVES Modelling

Simulations of one-dimensional flow in the unsaturated zone were conducted using WAVES: Water, Atmosphere, Vegetation, Energy and Solutes Model (Zhang et al., 1996; Zhang and Dawes, 1998). Although the simulations are generic, conditions relevant to the study area were used. Inputs to the model include climate data from the Eneabba weather station since 1969 (SILO data), and soil hydraulic data for Spearwood sands since there were insufficient data for the Parmelia aquifer. Solute transport was not simulated. Vertical fluxes in the unsaturated zone were simulated and compared beneath Banksia and bare soil for two different soil profiles. Bare soil conditions represent an extreme case of cleared land. The soil water flux beneath crop or paddock planted with grasses would be slightly less than the flux beneath bare soil due to evapotranspiration.

Unsaturated flow simulations were conducted for two soil profiles, (i) a uniform profile consisting of loamy Spearwood sands, and (ii) a layered profile consisting of 2 m-thick clay bands interbedded with Spearwood soil (Figure 13). The soil properties are from Zhang and Dawes (1998; Table 7). The simulations involve a fixed water table boundary condition at 40 m depth in the profile. The models were initialized prior to conducting 10-year simulations by imposing an initial soil water potential of 2 m throughout the profile and then running the models until the change in storage over a 10-year period was zero. Banksia vegetation was simulated with a leaf carbon value of 0.1 kg/m2, equating to a leaf-area index of 0.6 (Silberstein, personal comm.). The Banksia rooting depth was initially set to 6 m and allowed to increase to a maximum of 10 m.

Table 7 Soil hydraulic parameters used for WAVES simulation

Soil Type Ks (m/d) θs θr λc (m) C Spearwood B 3.5 0.39 0.018 0.5 1.05

Clay 0.01 0.45 0.25 0.2 1.3

The presence of clay bands within a soil profile has a significant effect on the soil moisture content, regardless of the surface vegetation conditions (Figure 14). The high moisture in the clay can cause perching of water promoting lateral flow and discharge to lower parts of the landscape.

The average annual fluxes beneath bare soil are almost identical for the two different soil profiles since the water balance is nearly the same for both profiles (Figure 15). This is because there are no plant roots to extract water above the first clay layer. In contrast, the average annual fluxes beneath the Banksia are a maximum of 20% higher within the uniform soil profile than within the layered profile. The clay is much more effective at reducing the soil water flux if there is water loss from evapotranspiration. Although the wetting front pulses travel faster through the uniform soil profile, the average flux is constant beneath the rooting zone depth, because there is no further soil water extraction by roots (Silberstein, personal comm.). If the models were two-dimensional and lateral hydraulic gradients existed, then when water ponded above the clay, it would migrate laterally to discontinuities in the clay and become localized recharge. This might be nearly as large again as the flux through uniform sand (Silberstein, personal comm.).

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