9.4 Runoff to surface water

Supporting information

The total rainfall runoff to surface water for the 2012–13 year was 4,968,033 ML. The volume calculated represents the total runoff from the landscape into the rivers and storages within the Sydney region.

The following tables provide a summary and breakdown of rainfall runoff to surface water assets from the landscape in the Sydney region.

Runoff to surface water for the 2012–13 year: summary

Surface water asset



Storages managed by the Sydney Catchment Authority


Storages not managed by the Sydney Catchment Authority


River sections





evaporation loss in the river reaches upstream of reservoirs


runoff accounted in precipitation on river reaches upstream of reservoirs


runoff accounted in discharge from urban water system (see line item 9.9) 5,589




Volume of runoff to storages managed by the Sydney Catchment Authority for the 2012–13 year

Runoff to the storage (ML)

Avon 64,841
Blue Mountains1 4,939
Broughtons Pass Weir2 11,000
Cataract 68,596
Cordeaux 38,061
Fitzroy Falls 19,453
Lake Burragorang (Warragamba) 839,608
Lake Yarrunga (Tallowa) 839,045
Nepean 83,769
Pheasants Nest Weir3 32,400
Prospect 5,173
Wingecarribee 17,316
Woronora 29,727
Total 2,053,928

1 Blue Mountains storage reservoirs include Medlow Reservoir, Greaves Creek Reservoir, Upper Cascade Reservoir, Middle Cascade Reservoir, and Lower Cascade Reservoir.

2–3 Pheasants Nest and Broughton Pass weirs divert water to water treatment plants and Prospect Reservoir.

Volume of runoff to storages not managed by the Sydney Catchment Authority
Managing authority 


Runoff to the storage during 2012–13 (ML)

Shoalhaven City Council Danjera 31,633
Flat Rock Creek 1,981
Goulburn Mulwaree Council Pejar 16,902
Sooley 11,104
Lithgow City Council Farmers Creek No. 2 2,673
Wingecarribee Shire Council Bundanoon and Medway 32,379
Delta Electricity Lake Lyell 24,038
Lake Wallace 19,850
Thomsons Creek
Total 141,5221

1 There is no runoff volume provided for Bamarang as there is no real natural catchment for this storage.

The following table shows the total volume of runoff to rivers in the Sydney region. The volume was calculated by the Bureau of Meteorology.

Runoff to rivers


Runoff during 2012–13 (ML)

runoff to rivers


gauged flows




1 Volume stated for the gauged flows is the total observed data obtained from Sydney Catchment Authority. Some data were missing for the 2012–13 year. See line item 17.2 River outflow from the region for further details.

Quantification approach

Bureau of Meteorology

Data source

The Bureau: National Climate Centre daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric waterbody feature class; CSIRO: Australian Water Resources Assessment system Landscape model (AWRA-L) parameters.

Provided by

The Bureau; CSIRO.


Runoff to storages and rivers is estimated based on the Australian Water Resources Assessment system Landscape model (AWRA-L) version 3.0 (Van Dijk 2010) streamflow model outputs.

Using climate grid data for the Sydney region (including precipitation, temperature and solar radiation data), AWRA-L was used to estimate the runoff depth at each gridpoint within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major storages and rivers were excluded from the analysis.

The landscape was divided into the catchments of the major storages within the Sydney region for the purpose of determining inflows into individual storages. The Bureau of Meteorology Australian Hydrological Geospatial Fabric catchments were used to perform this break-up. Sections of the Shoalhaven, Coxs, Wollondilly, Capertee, and Macdonald river considered within the surface water store (1.2 Unregulated river) were removed from the Sydney region landscape.

The average runoff depth from the landscape into the surface water store was determined as the weighted mean of the relevant gridpoints within the region boundary. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. Mean runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages and major rivers).

Assumptions, limitations, caveats and approximations

  • The estimated runoff was compared against historical flows at unimpaired catchments within the Sydney region for the 2013 Account and provided a suitable representation of the runoff for this year.
  • The runoff estimates were subject to the assumptions of the AWRA-L model detailed in Van Dijk (2010).

Uncertainty information

The uncertainty estimate was not quantified.

Sydney Catchment Authority

Data source

Mass balance calculations provided in the Sydney Catchment Authority Water Balance 2012–13 for the Independent Pricing and Regulatory Tribunal (IPART). Note that inflow values in the Water Balance 2012–13 include both precipitation on storages (see line item 9.1 Precipitation on surface water) and inflow to storages (this line item); databases and calculated monthly catchment runoff (CMCR) and annual spreadsheet calculations; customer invoices.

Provided by

Sydney Catchment Authority.


The level sensors at the two gauging sites have been linked to Sydney Catchment Authority's radio telemetry system. Previously, developed rating tables have been used to convert water levels to a volume.

For all storages, mass balance calculation was used to estimate inflow to each storage. In the calculation, inflow, storage diversions, precipitation, evaporation, and other known losses, and beginning and end storage volumes for each storage were balanced for the 2012–13 year. Storage diversions, precipitation, evaporation, and beginning and end storage volumes were either measured data or calculated data (interpolations, application of rating tables) from measured data.

Assumptions, limitations, caveats and approximations


Uncertainty information

The volume is based on measured data. Estimated uncertainty based on meter accuracy, professional judgement on calculation methods and organisation practice is +/– 10%.