1.1.5.3 Surface water flow


In the Sydney Basin bioregion, streamflow monitoring is undertaken by WaterNSW and DPI Water (formerly the NSW Office of Water). Figure 41 shows the location of DPI Water and WaterNSW streamflow gauges across the bioregion, including some discontinued sites.

Figure 41

Figure 41 Stream gauges in the Sydney Basin bioregion and along rivers that flow into the bioregion

Data: Bureau of Meteorology (Dataset 1, Dataset 3)

1.1.5.3.1 Monthly and annual flow characteristics

Sydney Basin streamflow data were obtained for 28 gauging stations from DPI Water and 110 gauging stations from WaterNSW for the Sydney Basin Bioregional Assessment. Sixty-eight gauging stations had more than ten years of data since 1981 (Figure 42).

Figure 43 summarises the mean monthly flow characteristics for the Sydney Basin bioregion based on the streamflow data from the 68 gauging stations. On average, February has the highest proportion (0.14) of annual streamflow of all the months, although there is considerable variability around this. February is also the month with the highest mean monthly rainfall (Figure 18 in Section 1.1.2). A significant proportion (0.125) of total annual streamflow occurs on average in August, which tends to be one of the lowest rainfall months in the bioregion. It is not clear from mean monthly climate drivers for the bioregion (Figure 18 in Section 1.1. 2) why August should have high streamflow, but it could relate to the characteristics of rainfall in August, including high rainfall variability, and catchment wetness meaning rapid runoff responses when rain does fall. It could also be driven up by the localised areas of higher rainfall in the area just south of Bowral and Wollongong in August, contributing to high flows in upper Nepean, Avon and Cordeaux catchments and the Kangaroo River. September through to January is typically a low-flow period (spring to summer), with relatively low variability. This is consistent with potential evapotranspiration (PET) being more than double precipitation (i.e. aridity index >2) during these months (Figure 1 8 in Section 1.1. 2). Streamflow across the bioregion tends to be higher during autumn and winter as mean monthly PET approaches and drops below mean monthly precipitation.

Figure 42

Figure 42 Streamflow gauges in the Sydney Basin bioregion and along rivers that flow into the bioregion that have at least ten years of daily flow data since 1981

Red rings identify stream gauges in the Hawkesbury-Nepean river basin that have annual streamflow data presented in Figure 44. Blue rings identify stream gauges in the other river basins that have annual streamflow data presented in Figure 45.

Data: Bureau of Meteorology (Dataset 1, Dataset 3)

Figure 43

Figure 43 Monthly streamflow ratio (monthly mean divided by annual mean) distribution, summarised for 68 streamflow gauges within the Sydney Basin bioregion

Red lines correspond to monthly medians; blue boxes summarise the interquartile range (25th to 75th percentiles); black whiskers identify full range

Data: Sydney Catchment Authority (Dataset 4); NSW Office of Water (Dataset 5)

Table 1 7 summarises mean annual flows for a selection of gauging sites in each of the major river basins in the Sydney Basin bioregion. Selection was based on gauges with the most number of days of data and the start and end dates vary across the sites. Data are collected at sites with very small contributing areas (a few square kilometres) through to sites with large contributing areas (thousands of square kilometres), hence there is considerable variability in the mean annual flows. Many of the gauges, particularly in the Hawkesbury and Shoalhaven river basins, are downstream of dams and their flow regimes will differ from pre-dam flows. The release of environmental flows to maintain healthy water ways is a key objective of the Metropolitan Water Plan (NSW Government, 2010) in the Hawkesbury-Nepean river basin and will be reflected in the flow record of some gauging stations.

Annual time series have been generated for a number of gauging stations to illustrate some of the different flow characteristics of the rivers in the Sydney Basin bioregion (Figure 44 and Figure 45). Only years with >90% of daily streamflow values are used. A mean daily flow is calculated from the available flow data and multiplied by 365 to obtain an annual total. Gaps in the record reflect insufficient data, rather than zero-flow.

Annual streamflow is highly variable, but patterns are generally consistent with the bioregion-averaged annual precipitation (Figure 1 9a in Section 1.1. 2). The size, location and physical characteristics of the catchments will contribute to local variations. The hydrographs tend to show relatively high flows through from mid-1980s to early 1990s, with 1990 having the highest flows at all sites for which data were available since 1980. However, where streamflow records are available from the 1970s, they indicate a period of higher streamflow at those sites than has been experienced in the 30 years since. However, the bioregion-averaged annual precipitation for this period is slightly lower than the mid-1980s to early 1990s period, which suggests that differences in the timing and magnitude of precipitation events varied in these periods. Annual streamflow volumes showed a decreasing trend from the early 1990s through to the mid- 2000s. The return to higher streamflow following the Millennium Drought has been variable across the bioregion, with some streams showing only moderate increases (e.g. at gauging stations 215220 and 216004) and others more significant increases (e.g. at gauging stations 212272, 213005 and 214003).

Table 17 Catchment areas and mean annual flow volumes for a subset of Sydney Basin bioregion stream gauging stations


River basin

Gauging station

Latitude

Longitude

Data length (days)

Catchment area

(km2)

Mean annual flow

(GL/y)

Hawkesbury

212008a

–33.425

150.0817

10980

199

20.30

212018

–33.1233

150.28

10479

1030

26.17

212048

–33.77

150.7617

9745

55

39.32

212201a

–33.7483

150.6833

11726

327

595.20

212202a

–33.865

150.6267

9571

386

190.39

212203a

–34.245

150.6667

9195

126

95.34

212208a

–34.205

150.63

8987

179

128.42

212209

–34.4783

150.5333

10912

67

44.14

212233a

–34.23

150.7433

11168

44

24.86

212241a

–33.8683

150.6017

8734

1413

424.22

212272a

–34.4967

150.3433

11900

60

59.84

212274

–34.5767

150.57

9707

4

4.50

212275a

–34.5367

150.4783

9356

35

29.71

212280

–34.1467

150.42

10181

360

34.73

212290

–33.42

150.725

10562

3305

376.14

212295

–33.6

150.93

9119

120

22.75

212320

–33.8783

150.7683

11699

90

8.76

Sydney Coast-Georges River

213005

–33.8

150.9817

10503

59

25.21

213006

–34.075

150.81

10482

3

0.72

213009

–33.85

150.945

9625

28

10.35

213011

–33.9033

150.9183

8993

68

13.95

213012

–33.9783

150.8867

8683

45

21.34

213013

–34.0267

150.8367

9967

37

7.52

Wollongong Coast

214003

–34.5767

150.705

11837

35

15.46

Shoalhaven

215216a

–34.845

150.4317

8768

840

827.61

215220

–34.73

150.52

11896

334

185.73

215233

–34.6067

150.5217

11794

8

3.96

215234a

–34.65

150.4833

9871

23

9.18

Clyde River-Jervis Bay

216004

–34.97

150.5983

11381

96

15.52

adownstream of a dam

Data: Sydney Catchment Authority (Dataset 4); NSW Office of Water (Dataset 5)

Figure 44

Figure 44 Annual streamflow volumes (1984 to 2013) at four gauging stations in the Hawkesbury river basin

212008 (Coxs River), 212272 (Wingecarribee River) and 212406 (Hawkesbury River) are located downstream of dams; 212406 is at Sackville, which is downstream of the tidal limit. 212320 (South Creek) records unimpaired flow. See Figure 42 for gauge locations.

Data: Sydney Catchment Authority (Dataset 4); NSW Office of Water (Dataset 5)

Figure 45

Figure 45 Annual streamflow volumes (1971 to 2013) for select gauges in four river basins

See Figure 42 for stream gauge locations.

Data: Sydney Catchment Authority (Dataset 4); NSW Office of Water (Dataset 5)

Last updated:
21 January 2019
Thumbnail of the Sydney Basin bioregion

Product Finalisation date

2018