41.3 Runoff into storages

Supporting Information

Total volume of runoff into each storage during the 2011 Account year within the Sydney region was 2,032,808 ML. Details of runoff into storages is provided in the following tables.


Runoff volumes for storages managed by Sydney Catchment Authority (SCA)

Storage reservoir

Runoff (ML) – SCA











Lake Burragorang (Warragamba Dam)






Fitzroy Falls


Lake Yarrunga (Tallowa Dam)


Blue Mountains1


Pheasants Nest Weir2


Broughtons Pass Weir2




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

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

Runoff volumes for storages not managed by SCA

Storage reservoir

Runoff to the storage reservoir during the 2010–11 year (ML)



Flat Rock Creek






Thomsons Creek, Lyell and Wallace1


Farmers Creek No. 22


Bundanoon and Medway3




1The value represents retained inflow because outflow from the most downstream reservoir, Lake Lyell, is counted as runoff to Lake Burragorang (Warragamba Dam). Volume provided by Delta Electricity.

2The values represent net inflows to the reservoirs after accounting for precipitation, evaporation and other reservoir losses.  Precipitation and evaporation for these reservoirs are not separately presented. They have been included in relevant landscape items. The value for Farmers Creek No. 2 also includes 492 ML received from the Clarence Water Transfer Scheme.

3There is no runoff volume provided for Bamarang as there is no real natural catchment for this reservoir.

Quantification Approach

Bureau of Meteorology

Data Source

Bureau of Meteorology, National Climate Centre (NCC): daily climate grids (rainfall, temperature and solar radiation); Australian Hydrological Geospatial Fabric (AHGF), waterbody feature class. Commonwealth Scientific and Industrial Research Organisation (CSIRO): AWRA-L model parameters; monthly climatological average radiation grid data.

Provided by

Bureau of Meteorology (the Bureau).


Rainfall runoff to surface water was estimated based on the AWRA-L version 1.0.0 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 grid-point within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major storages were excluded from the analysis.

The landscape was divided into the catchments of the major reservoirs within the Sydney region for the purpose of determining inflows into individual reservoirs. The Bureau's Australian Hydrological Geospatial Fabric catchments were used to perform this break up.

The average runoff depth from the landscape into the surface water store was determined as the weighted mean of the relevant grid-points 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. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages).

Assumptions, Limitations, Caveats and Approximations

  • The estimated runoff was compared against historical flows at unimpaired catchments within the Sydney region for the 2010–11 water year 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.

Delta Electricity

Data Source

Data spreadsheets.

Provided by

Delta Electricity provided runoff to Lake Lyell, Lake Wallace and Thomsons Creek Reservoir.


River water level was converted to a volume using a rating table in a spreadsheet. Gauging stations for the water level measurement are located at the Farmers Creek (Gauge No. 212042), the Coxs River upstream of Lake Lyell (Gauge No. 212058) and Wallerawang (Gauge No. 212054). Water levels have been measured at a weir in the Coxs River upstream of Lake Lyell. For the other two stations, water levels have been measured at a well-defined cross-section of the streams.

Assumptions, Limitations, Caveats and Approximations

The provided value is the combined retained inflow to Thomsons Creek Reservoir, Lake Wallace and Lake Lyell, minus the outflows from Lake Lyell (which is the most downstream reservoir). The retained inflow value includes all sources of water from the catchment excluding return flows (e.g. discharges from power stations), upstream reservoir diversions and any spilling.

Uncertainty Information

The volume is based on measured data. Estimated uncertainty based on meter accuracy is +/– 5%.

The NSW Office of Water conducts an annual verification review on Delta Electricity's water data.

Sydney Catchment Authority

Data Source

Mass balance calculations provided in the Sydney Catchment Authority (SCA) Water Balance 2010–11 for the Independent Pricing and Regulatory Tribunal (IPART). Note that inflow values in the SCA Water Balance 2010–11 include both precipitation on reservoirs (line item 9.1) and inflow to reservoirs (this line item); databases and calculated monthly catchment runoff (CMCR) and annual spreadsheet calculations; customer invoices.

Provided by



Runoff to the following storage reservoirs are managed by SCA:

  • Cataract Reservoir
  • Cordeaux Reservoir
  • Avon Reservoir
  • Nepean Reservoir
  • Woronora Reservoir
  • Lake Burragorang (Warragamba Dam)
  • Prospect Reservoir
  • Wingecarribee Reservoir
  • Fitzroy Falls Reservoir
  • Lake Yarrunga (Tallowa Dam)
  • Blue Mountains reservoirs

         – Medlow Reservoir

         – Greaves Creek Reservoir

         – Upper Cascade Reservoir

         – Middle Cascade Reservoir

         – Lower Cascade Reservoir

  • Pheasants Nest Weir
  • Broughtons Pass Weir.

Inflows to Fitzroy Falls and Wingecarribee storages were calculated as follows:

  • Fitzroy Falls Reservoir: inflow was estimated as 4.4 times the gauged flow at Wildes Meadow (Gauge No. 215233), based on the extent of catchment area at the reservoir and the gauge.
  • Wingecarribee Reservoir: inflow was estimated as 2.5 times the gauged flow at Maugers in Caalang Creek (Gauge No. 212274), based on the extent of catchment area at the reservoir and the gauge.

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

For all other 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 2010–11 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. SCA Water Balance 2010–11 provides the results of the mass balance calculation. Note that inflow values in the SCA Water Balance 2010–11 include both precipitation on storages (line item 9.1) and inflow to storages (this line item).

Assumptions, Limitations, Caveats and Approximations


Uncertainty Information

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