Capel River Hydrology

7/31/2019 Capel River Hydrology

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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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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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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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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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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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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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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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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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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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(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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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)



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)



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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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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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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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


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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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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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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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


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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0.01

0.1

1

10

100

1000

10000

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%


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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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


24/35


0

1

2

3

4

5

6

1/01/2003 2/04/2003 2/07/2003 1/10/2003 31/12/2003

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)



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)



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


25/35



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 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)


26/35


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.


27/35



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


28/35


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


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).


29/35



0.01

0.1

1

10

100

1000

10000

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%


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%


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)


30/35


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)


31/35



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).


32/35


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


33/35



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



Daily


Standard deviation (mm) 0.70 0.68





34/35


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




Monthly


Standard deviation (mm) 17.6 18.4Mean absolute error (MAE) (mm) 1.11



Daily


Standard deviation (mm) 0.81 0.73




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.


35/35


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.

Documents

Capel River Hydrology