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Looking aft er a l l our wat er needs
Ca p e l Ri v e r H y d r o l o g yS u m m a r y
Department of Water
Surface Water Hydrology Series
Report no. 24
July 2008
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Department of Water
168 St Georges Terrace
Perth Western Australia 6000
Telephone +61 8 6364 7600Facsimile +61 8 6364 7601
www.water.wa.gov.au
Government of Western Australia 2008
July 2008
This work is copyright. You may download, display, print and reproduce
this material in unaltered form only (retaining this notice) for your personal, non-
commercial use or use within your organisation. Apart from any use as permittedunder the Copyright Act 1968, all other rights are reserved. Requests and inquiries
concerning reproduction and rights should be addressed to the Department of
Water.
ISBN 978-1-921468-77-3 (print).
ISBN 978-1-921468-78-0 (online).
Acknowledgements
This publication was prepared by Jacqueline Durrant.
Subject of cover photograph:Capel River at Capel Railway Bridge (by RichardPickett)
For more information about this report, contact the Manager, Surface Water
Assessment Section of the Department of Water.
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Surface water hydrology Capel River Hydrology Summary
Department of Water iii
C o n t e n t sContents ..................................................................................................................... iii
1 Introduction..............................................................................................................1
2 Catchment description.............................................................................................2
3 Climate ....................................................................................................................5
4 Streamflow ..............................................................................................................8
5 Rainfall runoff modelling..........................................................................................9
5.1 Yates Bridge (610219) ..................................................................................................... 9
5.1.1 Annual streamflow ....................................................... .........................................................12
5.1.2 Floodflows.............................................................................................................................13
5.1.3 Monthly streamflow...............................................................................................................14
5.1.4 Daily streamflow .................................................. ............................................................ .....15
5.2 Capel Railway Bridge (610010) .....................................................................................19
5.2.1 Annual streamflow ....................................................... .........................................................22
5.2.2 Floodflows.............................................................................................................................23
5.2.3 Monthly streamflow...............................................................................................................24
5.2.4 Daily streamflow .................................................. ............................................................ .....24
6 Conclusion.............................................................................................................27
Appendix....................................................................................................................28
References ................................................................................................................31
F i g u r e s
Figure 1 Location of Capel River ..........................................................................2Figure 2 Capel River catchment, showing streamflow gauges, selected rainfall
gauges, extent of clearing, and location of the Darling and WhicherScarp......................................................................................................4
Figure 3 Annual rainfall at Capel Post Office (009516).........................................6
Figure 4 Mean monthly rainfall for Capel Post Office and Kirup for the period1975 to 2003 ..........................................................................................7
Figure 5 Streamflow gauging stations in the Capel River catchment....................8Figure 6 Observed versus modelled runoff at Yates Bridge, results for (a,b)
annual timescale and (c,d) monthly timescale......................................10Figure 7 Comparison of observed and modelled daily flows for a calibration year
(2003) normal scale (a) and log-scale y axis (b) and a verificationyear (1975) (c)......................................................................................11
Figure 8 Modelled and observed annual streamflow record for Capel River atYates Bridge (610219)..........................................................................12
Figure 9 Flood-frequency plot for Capel River at Yates Bridge (610219) ...........13
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iv Department of Water
Figure 10 Average and median monthly streamflow for Capel River at YatesBridge and average centroidal rainfall ..................................................14
Figure 11 Observed and modelled daily flow-duration curves categorised by yearfor Capel River at Yates Bridge (610219).............................................15
Figure 12 Observed daily flow-duration curves categorised by month for CapelRiver at Yates Bridge (610219) ............................................................16
Figure 13 Observed streamflow duration at Yates Bridge (610219).....................17Figure 14 Observed and modelled daily flow-duration curves categorised by
month for Capel River at Yates Bridge (610219)..................................18Figure 15 Observed versus modelled runoff at Capel Railway Bridge: results for
(a,b) annual timescale and (c,d) monthly timescale..............................19Figure 16 Comparison of observed and modelled daily flows for a calibration year
(2003) normal scale (a) and log-scale y axis (b) and a verificationyear (1994) (c)......................................................................................20
Figure 17 Comparison of observed, AWBM-modelled and modelled plus minimum
daily flow for a calibration year (2003) (log-scale y axis) ......................21Figure 18 Modelled and observed annual streamflow record for Capel River at
Capel Railway Bridge (610010)............................................................22Figure 19 Flood-frequency plot for Capel River at Capel Railway Bridge (610010)23Figure 20 Average and median monthly streamflow for Capel River at Capel
Railway Bridge and average centroidal rainfall.....................................24Figure 21 Observed and modelled daily flow-duration curves categorised by year
for Capel River at Capel Railway Bridge (610010) ...............................25Figure 22 Observed and modelled daily flow-duration curves categorised by
month for Capel River at Capel Railway Bridge (610010) ....................25Figure 23 Observed daily flow-duration curves categorised by month for Capel
River at Capel Railway Bridge (610010)...............................................26
T a b l e s
Table 1 Annual series flood-frequency data for Capel River at Yates Bridge(610219) (JDA 1998)............................................................................14
Table 2 Annual series flood-frequency data for Capel River at Capel RailwayBridge (610010) (JDA 1998).................................................................23
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Surface water hydrology Capel River Hydrology Summary
Department of Water 1
1 I n t r o d u c t i o n
The Capel River was proclaimed for surface water licensing in 1969 under the Rights
in Water and Irrigation Act 1914. The Department of Water is developing amanagement plan for the Capel River to guide water allocation and licensing.
This report discusses the hydrology of the Capel River and will be used to assist with
the development of ecological water requirements for the Capel River catchment.
Characteristics of surface-water hydrology were assessed at two gauging stations,
Yates Bridge (610219) and Capel Railway Bridge (610010).
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2 Department of Water
2 Ca t c h m e n t d e sc r i p t i o n
The Capel River catchment is located south of Bunbury and covers an area of
approximately 635 km2 (Figure 1). The river flows in a north-westerly direction,originating at the Darling Scarp (west of Kirup) and crossing the northern part of the
Blackwood Plateau, the Whicher Scarp and the Swan Coastal Plain before
discharging into the Indian Ocean (White & Comer 1999).
Figur e 1 Locat ion of Capel River
Historically, the Capel River discharged into the VasseWonnerup Estuary via the
Stirling Wetlands. In the late 1980s the river was diverted from the estuary to
discharge directly into Geographe Bay by way of an artificial river mouth cut through
the coastal sand dunes (Pen 1999). In order to prevent salt-water intrusion into the
Stirling Wetlands, levee banks were built along the Capel River. A sand bar
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Surface water hydrology Capel River Hydrology Summary
Department of Water 3
seasonally forms across the mouth of the river, and is broken under high-river-flow
conditions.
The lower reach of the Capel has been modified as part of an artificial drainage
system to drain the Swan Coastal Plain, enabling its use for dairy farming and otherforms of agriculture (Pen 1999). Land use within the catchment consists primarily of
agriculture, including dairy and beef production, orchards and viticulture. The mining
of mineral sands in the Capel region is also a primary land use. As well, there is an
increase in urban development across the catchment. The Water Corporation
operates the Kirup Dam, located in the upper catchment, north-east of the south
branch, to supply drinking water to the towns of Kirup and Mullalyup (Beckwith 2006).
A number of meteorological stations and streamflow gauging stations have operated
periodically on the Capel River (Figure 2). The gauging stations at Yates Bridge
(610219) and Capel Railway Bridge (610010) are currently in operation and areanalysed further in this study. The Capel River catchment is approximately 60 per
cent cleared (Figure 2). Upstream of the Yates Bridge gauging station, the level of
catchment clearing is approximately 35 per cent, predominantly on the north branch.
The upper portion of the catchment includes several areas of State forest and is
typical of the intermediate-rainfall-zone jarrah-forest catchments found in the south-
west of WA. To the west of the Darling Scarp the vegetation is primarily eucalypt
(tuart) and banksia woodlands (Beard 1990).
The Yarragadee Formation largely covers the extent of the Capel River, bounded onthe east by the Darling Fault. It is overlain by the Leederville Formation and areas of
Bunbury Basalt. The Yarragadee aquifer is high yielding and used for water supply at
Capel and for the mineral sands industry in the area. The Leederville aquifer is
overlain by the superficial formations on the Swan Coastal Plain. The part of the
aquifer between the Whicher Scarp and the Darling Fault is not overlain by the
superficial deposits (Figure 2) (Department of Water n.d.).
Groundwater salinity ranges from less than 200 milligrams per litre to about 400
milligrams per litre (Department of Water n.d.). Salinity levels in the Capel River vary
between the upper portion of the river and the townsite. The upper Capel isconsidered fresh (0500 mg/L), whereas downstream the water quality is classified
as marginal (5011100 mg/L) (Water and Rivers Commission 2003).
The north and south branches of the Capel River converge where the Darling Scarp
meets the Swan Coastal Plain and the terrain changes from rolling hills to flat plains.
In the upper catchment there is a large amount of surface-water storage, most of it in
the form of on-stream dams. As the topography changes to plains in the central
branch of the river, the ability to store water diminishes and direct pumping is the
primary method used to obtain water. In the lower catchment, there is a greater
reliance on abstraction from groundwater rather than surface water (CRLWUG n.d.;
Beckwith 2006).
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4 Department of Water
Figure 2 Capel River cat chment , showi ng st reamf low gauges, select ed rai nf al l
gauges, extent of clearing, and locat ion of the Darl ing and Whicher
Scarp
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Surface water hydrology Capel River Hydrology Summary
Department of Water 5
3 C l i m a t e
The Capel River catchment has a temperate climate (based on the Kppen
classification system) with a marked wet and cold winter and a distinctly dry andwarm summer (Bureau of Meteorology 2006). Winter rainfall is typically generated
from rain-bearing low-pressure systems (cold fronts) crossing the coast. High-
intensity summer storms may occur as a result of thunderstorms or ex-tropical
cyclones.
A number of Bureau of Meteorology rainfall gauges are located throughout the
catchment with varying periods of record. Long-term SILO derived/modelled rainfall
and evaporation data (from 1899) are available at the Capel Post Office (009516)
and Kirup (009714) gauges (Figure 2).
A centroidal daily rainfall series was developed for the upper part of the catchment by
using the Thiessen weighted polygon method to combine information from six of the
gauges selected for use in this study. Evaporation for the upper catchment was
described by the gauge at Kirup (009714).
The rainfall gauge at Capel Post Office (009516) operated from 1914 to 2003 and
has since been replaced by Capel North (009992). The Capel Post Office gauge was
selected to describe rainfall and evaporation in the lower catchment. The rainfall was
combined with the daily rainfall series generated for the upper catchment in order to
provide a single input for the modelling.
Consistent with the pattern formany other locations in the south-west of WA, the
mean annual rainfall in the catchment has shown a noticeable decrease since the
1970s from the long-term mean (Figure 3). For the period 1900 to 1974 the average
annual rainfall at Capel Post Office was 870 mm, while for 1975 to 2003 the average
was 735 mm, a decrease of 16 per cent.
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6 Department of Water
0
200
400
600
800
1000
1200
1400
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Annualrainfall(mm
-500
0
500
1000
1500
2000
2500
3000
3500
4000
Cumulativedeviationfromlong-te
rmmean
(mm)
Annual total Capel Post Office (009516)
Long-term mean rainfall 19002003 (830 mm)
19752003 mean rainfall (735 mm)
Cumulative deviation from long-term mean
Figure 3 Annual rai nf al l at Capel Post Of f ice (009516)
Mean annual rainfall varies throughout the catchment, increasing with distance from
the coast towards the scarp. The decreasing trend in annual rainfall is evident across
the whole catchment, with an average annual rainfall at Kirup for the period 1900 to
1974 of 1040 mm while for the period 1975 to 2003 the average was 955 mm.
Although the change is not as pronounced as near the coast, it is still a reduction of
8 per cent.
Rainfall is highly seasonal, with 79 per cent and 77 per cent of the annual rainfall at
Capel Post Office and Kirup respectively occurring between May and September for
the period 1975 to 2003 (Figure 4). The peak-rainfall month at Capel Post Office is
June, but in the upper Capel catchment at Kirup the peak month is July. The monthly
rainfall record for the period 1975 to 2003 at Kirup displays a small shift in
seasonality from the long-term record, with June being the peak-rainfall month over
the period 1900 to 1974. However, 77 per cent of the annual rainfall still occurred
between May and September.
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Surface water hydrology Capel River Hydrology Summary
Department of Water 7
0
20
40
60
80
100
120
140160
180
200
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Meanmonthlyrainfall(mm
)
Capel Post Office (009516)
Kirup (009714)
Figure 4 Mean mont hly rainf al l f or Capel Post Of f ice and Kir up f or t he per iod
1975 to 2003
Average annual evaporation for the period 1975 to 2003 was 1460 mm and 1320 mm
at Capel Post Office and Kirup respectively. There has been a very slight increase
(2 per cent) in evaporation over the long term (comparison of the period 1900 to 1974
and 1975 to 2003).
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8 Department of Water
4 S t r e a m f l o w
Five streamflow gauging stations have operated periodically on the Capel River
(Figure 2). Two historical stations operated on Gynudup Brook, which joins with thelower Capel before flowing into the Indian Ocean. The streamflow records produced
at these sites are not useful in this study due to the short periods of record. The
Capel River North and South branches have never been gauged. Two main gauges,
Yates Bridge (610219) and Capel Railway Bridge (610010), are currently in operation
and are analysed further in this study (Figure 5).
1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Capel River Yates Bridge
(610219)
Capel River Scott Road
(610129)
Capel River Above Capel
(610004)
Capel River Capel Railway
Bridge (610010)
Gynudup Brook Elgin Siding
(610130)
Gynudup Brook Capel-Elgin
Main Drain (610131)
Capel River River Mouth
(610024)
Figure 5 St reamf low gauging st at ions in t he Capel River cat chment
The Yates Bridge gauging station records flow from 315 km2 of the upper Capel
catchment (~50 per cent of entire catchment). The gauge was in operation from 1966
to 1976 and was reopened in 1996, resulting in an approximate 20-year period of
missing data. The flow record was complete from 1997 onwards, with no gaps. The
stage-discharge relationship has been developed from 94 discharge measurements
ranging from 0.01 m3/s to 39.5 m3/s. In the period of record since 1975, only one flow
was recorded above these measured flows (53.4 m3/s). The flows at Yates Bridge
are therefore considered relatively reliable.
The Capel Railway Bridge gauging station, located approximately 20 km downstream
of Yates Bridge, records flow from 395 km2 of the upper and middle Capel catchment
(~62 per cent of entire catchment) and has been in continuous operation since 1993.
The stage-discharge relationship is developed from 45 discharge measurements
ranging from 0.02 m3/s to 45.6 m3/s. One flow was recorded above these measured
flows (50.7 m3/s). A degree of uncertainty surrounds the rating curve, as there is no
stable control at the gauging station site. As a result, a large amount of the flow
record has had corrections or estimations applied.
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Surface water hydrology Capel River Hydrology Summary
Department of Water 9
5 Ra i n f a l l r u n of f m o d e l l i n g
There is a significant gap in flow data from the mid-1970s to the mid-1990s for the
Capel catchment, with no gauges recording flow during this time. To complete thehydrologic analysis, the records for Yates Bridge and Capel Railway Bridge needed
to be extended to cover the period 1975 to 2003. A lack of hydrologically similar
catchments with long gauged records meant that the data could not be extended
using correlation techniques.
The AWBM daily rainfall runoff model, within the CRC for Catchment Hydrology
Rainfall Runoff Library, was therefore run in order to extend the data sets. The
rainfall runoff model calculates surface runoff and baseflow from inputs of rainfall and
evaporation.
Calibration results were assessed using several methods, including the coefficient of
efficiency (E) (Nash & Sutcliffe 1970), the mean absolute error (MAE) and the root
mean square of the error (RMSE). The final parameter sets and statistical
assessments of fit can be found in the Appendix.
5 . 1 Ya t es Br i d ge (610219 )
For Yates Bridge gauging station (610219) the AWBM daily rainfall runoff model was
prepared and calibrated from 1997 to 2003 and verified for the years 1975 and 2004.
The centroidal rainfall derived using the Thiessen weighted polygon method was
used as the catchment rainfall series and the evaporation record at Kirup as thecatchment evaporation series.
The calibration produced a reasonable fit to the observed data. Monthly and annual
model efficiencies (E) for the calibration period were calculated at greater than 0.9
(Figure 6). At a daily timestep the model did not perform as well, with an efficiency of
0.71. Hydrographs for the calibration and verification periods further illustrate the fit to
the observed data (Figure 7).
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10 Department of Water
(a)
0
50
100
150
200
250
300
0 100 200 300
Observed annual runoff (mm)
Modelledannualrunoff(mm)
1:1 line
E = 0.93
(b)
0
50
100
150
200
250
300
350
1997 1998 1999 2000 2001 2002 2003
Runoff(mm)
Modelled runoff
Observed runoff
95% confidence limit
(c)
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
Observed monthly runoff (mm)
Modelledmonthlyrunoff(mm)
1:1 line
E = 0.96
(d)
0
10
20
30
40
50
60
70
80
Jan-97 May-98 Sep-99 Feb-01 Jun-02 Nov-03
Runoff(mm)
Modelled runoff
Observed runoff
Figure 6 Observed versus model led ru nof f at Yat es Bri dge, result s f or (a,b)
annual t im escale and (c,d) mont hly t imescale
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Surface water hydrology Capel River Hydrology Summary
Department of Water 11
0
1
2
3
4
5
6
7
8
1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
0.00001
0.0001
0.001
0.01
0.1
1
10
1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
0
1
2
3
4
5
6
7
8
1/01/1975 2/04/1975 2/07/1975 1/10/1975 31/12/1975
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
Figure 7 Compari son of observed and model led dail y f low s f or a cali br at ion
year (2003) nor mal scale (a) and l og-scale y axis (b) and averif icat ion year (1975) (c)
a
b
c
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12 Department of Water
On average, the annual flow estimates obtained from the modelling are within 5 per
cent of the observed annual flows at the Yates Bridge gauging station. The greatest
error (28 per cent) occurs in 2001, which was a very low flow year. The daily
hydrograph plotted on a log scale (Figure 7) shows that the modelled data is a poor
fit to the observed summer low flows, with the rainfall runoff modelling linearly
depleting the baseflow stores. For example, the average catchment runoff for the
months January to March is 0.006 mm, but the modelled flow recedes to effectively
zero (a large percentage error). A catchment runoff of 0.006 mm equates to
approximately 2 ML/day.
The calibrated model parameters were then applied to the longer period (1975 to
2003) of daily rainfall and evaporation data to obtain estimates of streamflow for the
extended period. Statistics were calculated for the flows using the extended period.
5.1.1 Annual st reamflow
The mean annual flow for the eight observed whole years of record is 46.1 GL with a
coefficient of variation of 0.53. The maximum recorded annual flow of 87.8 GL was
recorded in 1974. The minimum recorded annual flow of 6.66 GL was recorded in
2001 and is less than a sixth of the average flow.
The combined observed and modelled annual streamflow (1975 to 2003) has an
estimated mean annual flow of 39.9 GL and a coefficient of variation of 0.49
(Figure 8). The maximum and minimum recorded annual flows are 81.8 GL and 6.66
GL respectively. The flow in the Capel River at Yates Bridge is highly variable.
0
10
20
30
40
50
60
70
80
90
1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003
Streamflow
(GL)
Observed annual flow (GL)
Modelled annual flow (GL)
Average (19752003)
Figure 8 Modell ed and observed annual st reamf low record f or Capel River at
Yates Bridge (610219)
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Surface water hydrology Capel River Hydrology Summary
Department of Water 13
The mean annual runoff for the period 1975 to 2003 is 130 mm and the mean annual
rainfall at Kirup rainfall station (009714) is 955 mm. The annual runoff coefficient for
the upper Capel catchment is 13 per cent. The average annual baseflow index was
modelled at 0.55.
5.1.2 Floodflows
A flood-frequency curve for the Yates Bridge gauging station was developed using the
program AFAP. The curve was constructed using nine years of recorded data (1997 to
2005) and a Log Pearson Type III distribution was fitted to the data (Figure 9).
200
100
50
20
1052
1.
25
1.
11
1.
05
1.
02
1.
01
1.
005
1.
002
1.
001
500
0.1
1
10
100
1000
Annual exceedance (1 in y)
Peakflow
(m3/s)
Fitted LPIII distribution
610219 Yates Bridge
Figure 9 Flood-f requency plot for Capel River at Yat es Bri dge (610219)
The results were consistently lower than those produced by the flood-frequency
analysis undertaken for the Capel River as part of the Busselton Regional Flood
Study Review (JDA 1998, Table 1). The peak annual flow series was a combination
of FLIKE analysis and results of runoff routing modelling (RORB). The flows since1998 (i.e. those not included in the JDA study) have all been less than a 1 in 10 year
ARI.
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14 Department of Water
Table 1 Annual seri es f lood-f requency dat a f or Capel River at Yat es Bri dge
(610219) ( JDA 1998)
Average recurrenceinterval (1:year)
Annual exceedanceprobability (%)
Peak annual flow(m3/s)
Growth factor(Fx/F2)*
1:1.1 90.9 14
1:2 50 40
1:5 25 68 1.70
1:10 10 88 2.20
1:25 4 119 2.98
1:50 2 133 3.33
1:100 1 173 4.33
*Growth factor is calculated by dividing the X-year ARI peak annual flow by the 2-year ARI peakannual flow
5.1.3 Monthl y st reamflow
As with rainfall, the streamflow in the Capel River at Yates Bridge is highly seasonal,
with 95 per cent of the flow occurring from June to October (Figure 10). August is the
peak-flow month, with a mean flow of 12.1 GL. There is a lag between the peak-
rainfall month (July for the upper Capel) and the peak-flow month. This is generally
characteristic of catchments with large soil-storage capacities, but in this case may
be a response to the filling of a large surface storage in the upper catchment.
0
2000
4000
6000
8000
10000
12000
14000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Stream
flow
(ML)
0
50
100
150
200
250
Centroidalrainfall(mm)
Mean flow 19752003
Median flow 19752003
Average rainfall 19752003
Figure 10 Average and medi an mont hl y st reamf low f or Capel River at Yat es
Br i dge and average centr oidal r ainfal l
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Surface water hydrology Capel River Hydrology Summary
Department of Water 15
5.1.4 Daily st reamflow
Annual and monthly flow-duration curves were calculated using daily flows for each
year. The thick black line is the total flow-duration curve for the period 1975 to 2003,
the thin dashed blue lines are the observed yearly flow duration curves and the thingrey lines are the modelled yearly flow-duration curves (Figure 11). The flow-duration
analysis can be used to describe the flow regime, and it is important in the
development of environmental flows.
Based on observed and modelled data at Yates Bridge, the median daily flow is
12 ML, and flows greater than 0.01 ML are exceeded 97 per cent of the time. The
individual yearly flow-duration curves provide an indication of the variability in the
flow regime, with daily flows exceeding 0.01 ML between 87 per cent and 100 per
cent of days in each year. There is also variability on a vertical scale, with median
daily flows ranging from 3 ML to 42 ML.
0.01
0.1
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Probability of daily flows being exceeded
Dailystream
flow
(ML)
Total series
Annual observed series
Annual modelled series
Figure 11 Observed and modell ed dail y f low -dur at ion cur ves cat egori sed by year
for Capel River at Yates Bridge (610219)
The low flows (typically less than 1 ML/day) for the modelled years are not within the
range of natural variability due to the linear depletion of the baseflow stores
(Figure 11). This discrepancy was not reflected in the AWBM calibration statistics
due to the low flows being such a small component of the overall flow. It was,
however, evident in the daily hydrographs plotted on a log scale (Figure 7).
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16 Department of Water
There are three years (1975, 1976 and 1997) where the observed flow-duration
curves show a short period of zero flow each year. Monthly observed flow-duration
curves show these periods occur in the months of November to March (Figure 12). In
the latter years (1998 to 2003) the yearly curves are very flat at the low end, with
flows always greater than 0.1 ML/day (no zero flows). This constant flow typically
highlights the presence of groundwater contributions in the summer months. In this
instance, however, it is primarily due to the additional summer flow releases from a
large on-stream dam in the upper catchment to meet the needs of downstream water
users. If the discharge from the upstream dam was monitored, this could be more
accurately reflected in the modelled flow record.
Anecdotal evidence indicates that the upper reaches of the river were ephemeral
before these releases started (Capel River Local Water User Group pers. comm.).
This is confirmed by looking at the observed continuous-flow period (Figure 13). This
indicates that the upper reaches of the Capel River did cease to flow during thesummer months, but in recent times the daily flow has been continuous for most
years.
0.01
0.1
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Probability of daily flows being exceeded
Dailystreamflow(
ML)
January February March AprilMay June July AugustSeptember October November December
Figure 12 Observed dail y f low -dur at ion cur ves cat egori sed by mont h for Capel
River at Yat es Br id ge (610219)
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Surface water hydrology Capel River Hydrology Summary
Department of Water 17
Jan Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1966
1967
19681969
1970
19711972
197319741975
1976
19771978
1979
19801981
1982
19831984
1985
1986
19871988
1989
19901991
1992
19931994
1995
19961997
1998
19992000
2001
20022003
2004
2005
Observed flow data Gaps in flow record Zero flow day
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 13 Observed st reamf low d urat ion at Yat es Bri dge (610219)
An assessment of the monthly flow-duration curves is very important, as the inter-
annual variation of the curves (particularly low flow) over the period of record gives
an indication of the types of flow the river is adapted to. Its ability to adapt to different
flow regimes has implications for determining future ecological flow requirements, on
an environmental, social and economic scale, as recognised by Beckwith (2006).
Flow-duration curves were also calculated for each month using the observed and
modelled data (Figure 14). The months of April and May in particular exhibited
unusual results, with the modelled flows significantly lower than observed, particularly
for flows less than 1 ML/day. For most years, these months correspond with the
beginning of winter flows. As the modelled data excludes summer-flow releases from
the upstream storage and has a linear recession of the baseflow stores, there is an
apparent vertical jump in the streamflow record in order to produce the runoff
response required from the initial rainfall. As a result, the parameters chosen to
model this initial condition are likely to be misrepresented. Overall the flow-duration
curves were not consistent with those produced using observed data only
(Figure 12).
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18 Department of Water
0.01
0.1
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Probability of daily flows being exceeded
Dailystreamflow
(ML)
January February March AprilMay June July AugustSeptember October November December
Figure 14 Observed and model l ed dail y f low -dur at ion cur ves cat egori sed by
mont h f or Capel River at Yat es Bri dge (610219)
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Surface water hydrology Capel River Hydrology Summary
Department of Water 19
5 . 2 Ca p e l Ra i l w a y Br i d g e ( 6 1 0 0 1 0)
For Capel Railway Bridge (610010) the AWBM daily rainfall runoff model was
prepared and calibrated from 1995 to 2003 and verified for the years 1994 and 2004.
The centroidal rainfall derived using the Thiessen weighted polygon method was
used as the catchment rainfall series, as was the evaporation record at the Capel
Post Office.
The calibration produced a reasonable statistical fit to the observed data, although
not as good as for Yates Bridge. Monthly and annual model efficiencies (E) for the
calibration period were calculated at greater than 0.8 (Figure 15). At a daily timestep
the model did not perform as well, with an efficiency of 0.75. Hydrographs for the
calibration and verification periods further illustrate the fit to the observed data
(Figure 16).
(a)
0
50
100
150
200
250
300
0 100 200 300
Observed annual runoff (mm)
Modelledannualrunoff(mm)
1:1 line
E = 0.82
(b)
0
50
100
150
200
250
300
350
400
1995 1996 1997 1998 1999 2000 2001 2002 2003
Runoff(mm)
Modelled runoff
Observed runoff
95% confidence limit
(c)
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
Observed monthly runoff (mm)
Modelledmonthlyrunoff(mm)
1:1 line
E = 0.95
(d)
0
10
20
30
40
50
60
70
80
90
Jan-95 May-96 Sep-97 Feb-99 Jun-00 Nov-01 Mar-03
Runoff(mm)
Modelled runoff
Observed runoff
Figure 15 Observed versus model led ru nof f at Capel Rail way Bri dge: resul t s f or
(a,b) annual t im escale and (c,d) m onthl y t imescale
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20 Department of Water
0
1
2
3
4
5
6
1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
0.00001
0.0001
0.001
0.01
0.1
1
10
1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
0
0.5
1
1.5
2
2.5
3
3.5
1/01/1994 2/04/1994 2/07/1994 1/10/1994 31/12/1994
Runoff(mm)
Modelled runoff (mm)
Observed runoff (mm)
Figure 16 Compari son of observed and modell ed dail y f low s f or a cali brat ion year
(2003) nor mal scale (a) and l og-scale y axi s (b) and a veri f icat ion
year ( 1994) (c)
c
b
a
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Surface water hydrology Capel River Hydrology Summary
Department of Water 21
If the daily hydrographs are assessed using a log flow scale, it can be seen that the
calibration produces a reasonable fit to the observed winter months but does not
match well to the summer low flows (Figure 16). This is due to AWBM being a
conceptual model rather than a process-based model, which does not account for
continual baseflow contributions from stores. Instead, runoff is produced only in
response to rainfall and the baseflow stores are linearly depleted.
The low flows were investigated further in order to simulate the baseflow contribution
at the Capel Railway Bridge gauge. The minimum daily flow for the observed period
of record, from May 1993 to October 2006, was 3.3 ML. This value was added as a
constant yearly baseflow to all the modelled data from January 1975 to May 1993. As
an example of how this impacts the daily hydrograph, the minimum daily flow was
added to the modelled data for a calibration year (2003) (Figure 17). The impact is
negligible for the winter period and flattens out the summer low-flow period, rather
than linearly receding the baseflow stores.
0.00001
0.0001
0.001
0.01
0.1
1
10
1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003
Ru
noff(mm)
Modelled runoff (mm)
Observed runoff (mm)
Modelled runoff plus minimum baseflow (mm)
Figure 17 Compari son of observed, AWBM-modell ed and modell ed plus mi nim um
dail y f l ow f or a cali brat ion year (2003) ( log-scale y axis)
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22 Department of Water
5.2.1 Annual st reamflow
The mean annual flow for the observed whole years of record is 50.0 GL with a
coefficient of variation of 0.47. The maximum recorded annual flow of 94.6 GL was
recorded in 1999. The minimum recorded annual flow of 15.7 GL was recorded in2001 and is less than a third of the average flow. The minimum flow at Yates Bridge
also occurred in 2001, and was less than one-sixth of the average flow. The less
significant result at the Capel Railway Bridge indicates the influence of an increasing
baseflow contribution between the gauges.
For the period 1975 to 2003, the combined observed and modelled annual
streamflow at the Capel Railway bridge, with the low-flow correction applied, has an
estimated mean annual flow of 44.8 GL and a coefficient of variation of 0.53
(Figure 18). The maximum and minimum recorded annual flows were 96.3 and 11.5
GL respectively.
0
20
40
60
80
100
120
1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003
Stream
flow
(GL)
Observed annual flow (GL)
Modelled annual flow (GL)
Average (19752003)
Figure 18 Modelled and observed annual st reamf low r ecord f or Capel River atCapel Rail w ay Br id ge (610010)
The mean annual runoff for the period 1975 to 2003 is 113 mm, while the mean
annual rainfall for the Capel Post Office rainfall station (009516) is 735 mm. The
annual runoff coefficient is 15 per cent. The modelled average annual baseflow index
is 0.55, or 55 per cent, which is a significant proportion of the total streamflow.
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Surface water hydrology Capel River Hydrology Summary
Department of Water 23
5.2.2 Floodflows
A flood-frequency curve for the Yates Bridge gauging station was developed using
the program FLIKE. The curve was constructed using 12 years of recorded data
(1994 to 2005) and a GEV distribution was fitted to the data (Figure 19).
200
100
50
20
1052
1.
25
1.
11
1.
05
1.
02
1.
01
1.
005
1.
002
1.
001
500
0.1
1
10
100
1000
Annual exceedance (1 in y)
Peak
flow
(m3/s)
Fitted GEV distribution
610010 Capel Railway Bridge
Figure 19 Flood-f requ ency plot f or Capel River at Capel Rail way Bri dge (610010)
The results were consistently lower than those produced by the flood-frequency
analysis undertaken for the Capel River as part of the Busselton Regional Flood Study
Review (JDA 1998, Table 1). The peak annual flow series was a combination of FLIKE
analysis and results of runoff routing modelling (RORB) (Table 2). The flows since 1998
(i.e. not included in the JDA study) have all been less than a 1-in-10-year ARI.
Table 2 Annual seri es f lood-f requency dat a f or Capel River at Capel Rail way
Bridge (610010) (JDA 1998)
Average recurrenceinterval (1:year)
Annual exceedanceprobability (%)
Peak annual flow(m3s-1)
Growth factor(Fx/F2)*
1:1.1 90.9 12
1:2 50 31
1:5 25 56 1.81
1:10 10 79 2.55
1:25 4 103 3.32
1:50 2 121 3.90
1:100 1 157 5.06
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24 Department of Water
5.2.3 Monthl y st reamflow
As with rainfall, the streamflow in the Capel River at the Capel Railway Bridge is
highly seasonal, with 90 per cent of the flow occurring from June to October
(Figure 20). August is the peak-flow month, with a mean flow of 12.9 GL. The impactof the large soil-storage capacity is evident in the lag between the peak-rainfall month
and the peak-flow month.
0
2000
4000
6000
8000
10000
12000
14000
16000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Streamflow(
ML)
0
20
40
60
80
100
120
140
160
Centroidalrainfa
ll(mm)
Mean flow 19752003
Median flow 19752003
Average rainfall 19752003
Figure 20 Average and medi an mont hl y st reamf low f or Capel River at Capel
Railway Bridge and average centroidal rainfal l
5.2.4 Daily st reamflow
Annual and monthly flow-duration curves were calculated using daily flows for each
year. The thick black line is the total flow-duration curve for the period 1975 to 2003,
the thin dashed blue lines are the observed yearly flow-duration curves and the thin
grey lines are the modelled yearly flow-duration curves (Figure 21).
At Capel Railway Bridge (Figure 21) the median daily flow is 24 ML, and flows
greater than 3 ML are exceeded 100 per cent of the time. The curve is a gradual
decline to the low-flow end, suggesting large groundwater contributions to a steady
perennial river. The groundwater contribution, from the Leederville Aquifer, in the
summer months can be seen by the very similar monthly flow-duration curves for
January to March (Figure 22), indicating groundwater is the dominant source of flow.
In addition there is large surface storage (e.g. Capel Sands dams).
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Surface water hydrology Capel River Hydrology Summary
Department of Water 25
0.01
0.1
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Probability of daily flows being exceeded
Dailystreamflow
(ML)
Total series
Annual observed seriesAnnual modelled series
Figure 21 Observed and modell ed dail y f low -dur at ion cur ves cat egori sed by year
f or Capel River at Capel Rail way Bri dge (610010)
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Probability of daily flows being exceeded
Dailystreamflow
(ML)
January February March April
May June July August
September October November December
Figure 22 Observed and model l ed dail y f low -dur at ion cur ves cat egori sed by
mont h f or Capel River at Capel Rail way Bridge (610010)
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26 Department of Water
The monthly flow-duration curves for the combined modelled and observed data
(Figure 22) compare well to the observed monthly flow-duration curves (Figure 23)
with the exception of the summer months (January to May). Based on the slope of
the curves and the maximum and minimum daily streamflow, the observed monthly
flow-duration curves suggest that baseflow is a key component in these summer
months, corresponding to the lowest rainfall months. This suggests that the adjusted
modelled data do not fully account for the baseflow contribution.
1
10
100
1000
10000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Probability of daily flows being exceede d
Dailystreamflow(ML)
January February March April
May June July August
September October November December
Figure 23 Observed dail y f low -dur at ion cur ves cat egori sed by mont h for Capel
River at Capel Railway Bridge (610010)
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Surface water hydrology Capel River Hydrology Summary
Department of Water 27
6 C onc l us i on
Located in the south-west corner of Western Australia, the Capel River flows to
Geographe Bay. The catchment is 635 km
2
, of which approximately 60 per cent iscleared. Streamflow gauging stations at Yates Bridge (610219) and Capel Railway
Bridge (610010) are currently in operation and their recordingswere analysed further
to assist with the development of ecological water requirements for the Capel River.
The Capel River catchment has experienced a decline in annual rainfall since the
1970s, consistent with the trend in many locations in the south-west of WA. Over 75
per cent of average annual rainfall occurs between the months of May and
September.
Rainfall runoff modelling was undertaken to develop a time series for the period 1975
to 2003. The mean annual flow at Yates Bridge was 39.9 GL, with 95 per cent of the
streamflow occurring between the months of June and October. The annual runoff
coefficient for the upper Capel catchment was 13 per cent and the average annual
baseflow index was 55 per cent. Flow-duration analysis shows that the upper
reaches of the river were once ephemeral, but the flow record now shows a perennial
river due to summer flow releases from a large on-stream dam in the upper
catchment.
The mean annual flow at Capel Railway Bridge was 44.8 GL, with 90 per cent of the
streamflow occurring between the months of June and October. The annual runoff
coefficient for the upper Capel catchment was 15 per cent, while the average annualbaseflow index was 55 per cent. Flow-duration analysis suggests large groundwater
contributions to a steady perennial river.
To obtain a more accurate flow representation it would be necessary to run a
process-based model that explicitly models the surface water / groundwater
interaction in the Capel River system. As environmental flow studies assess low-flow
characteristics, it may not be practical to base the ecological water requirement study
on the baseflow adjusted modelled series. Instead it is recommended that the study
should be based on the observed record only (i.e. 1996 onwards for Yates Bridge
and 1993 onwards for Capel Railway Bridge).
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28 Department of Water
A p p e n d i x
Table A Final RRLAWBM parameter set for Capel River at Yates Bridge
(610219)
Parameter Value
Capacity C1 mm 32
Capacity C2 mm 230
Capacity C3 mm 720
Area A1 0
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Surface water hydrology Capel River Hydrology Summary
Department of Water 29
Table C Statistics comparing the observed and modelled results of the RRL
AWBM calibration for Capel River at Yates Bridge (610219)
Statistic (19761995) Observed Modelled
Annual
Average (mm) 119 126
Standard deviation (mm) 72 80
Mean absolute error (MAE) (mm) 5.8
Root mean square error (RMSE) (mm) 10.3
Coefficient of efficiency (E) 0.93
Monthly
Average (mm) 9.9 10.5
Standard deviation (mm) 15.9 17.0Mean absolute error (MAE) (mm) 0.62
Root mean square error (RMSE) (mm) 1.93
Coefficient of efficiency (E) 0.96
Daily
Average (mm) 0.33 0.34
Standard deviation (mm) 0.70 0.68
Mean absolute error (MAE) (mm) 0.04
Root mean square error (RMSE) (mm) 0.22
Coefficient of efficiency (E) 0.71
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30 Department of Water
Table D Statistics comparing the observed and simulated results of the RRL
AWBM calibration for Capel River at Capel Railway Bridge (610010)
Statistic (19761995) Observed Simulated
Annual
Average (mm) 133 129
Standard deviation (mm) 65 83
Mean absolute error (MAE) (mm) 10.5
Root mean square error (RMSE) (mm) 18.2
Coefficient of efficiency (E) 0.82
Monthly
Average (mm) 11.1 10.7
Standard deviation (mm) 17.6 18.4Mean absolute error (MAE) (mm) 1.11
Root mean square error (RMSE) (mm) 2.67
Coefficient of efficiency (E) 0.95
Daily
Average (mm) 0.36 0.35
Standard deviation (mm) 0.81 0.73
Mean absolute error (MAE) (mm) 0.07
Root mean square error (RMSE) (mm) 0.27
Coefficient of efficiency (E) 0.75
Where:
MAE is calculated asii
SON
1
and has a range of 0 to infinity and a perfect
score of 0
RMSE is calculated as ( )21
iiSO
N and has a range of 0 to infinity and a perfect
score of 0
and E is calculated as( )
( )22
1
OO
SO
i
ii
and has a range of negative infinity to 1 and a
perfect score of 1
and where O represents the observed data point, S the modelled data point and N
the total number of observations.
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Surface water hydrology Capel River Hydrology Summary
R e f e r e n c e s
Beard, JS 1990, Plant life of Western Australia, Kangaroo Press, Sydney.
Beckwith Environmental Planning 2006, Capel River surface water management
issue scoping report, Beckwith Environmental Planning Pty Ltd, Perth.
Bureau of Meteorology 2006, Australian climates, Bureau of Meteorology, Canberra,
viewed February 2007,
.
CRLWUG (Capel River Local Water User Group) n.d., Submission in response to the
draft blueprint for water reform in Western Australia, Capel River Local Water UserGroup, viewed January 2007, .
Department of Water, n.d., Hydrogeological atlas of WA, Department of Water,
Perth, viewed February 2007,
.
JDA 1998, Busselton regional flood study review, vol. 2, Appendices, JDA Consultant
Hydrologists, Perth.
Nash, JE & Sutcliffe, JV 1970, River flow forecasting through conceptual models, I, Adiscussion of principles, Journal of Hydrology, vol. 10, pp. 28290.
Pen, LJ 1999, Managing our rivers: a guide to the nature and management of the
streams of southwest Western Australia, Water and Rivers Commission, Perth.
Water and Rivers Commission 2003, Water quality and hydrology in the Capel River
19412001, South West inflo series, Water and Rivers Commission, Perth.
White, K & Comer, S 1999, Capel River action plan, prepared for the Geographe
Catchment Council, Water and Rivers Commission, Perth.