Sydney
9.4 Runoff to surface water

The volume of runoff to surface water for the 2010–11 year is 4,173,310 ML. 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 – summary

Surface water asset	Runoff to the asset during 2010–11 (ML)
Storages managed by the Sydney Catchment Authority	1,822,007
Storages not managed by the Sydney Catchment Authority	150,801
River sections	2,142,348
Subtotal	4,175,156
Add evaporation loss in the river reaches upstream of reservoirs	13,414
Less runoff accounted in precipitation on river reaches upstream of reservoirs	9,071
Less runoff accounted in discharge from urban water system (see line item 9.9)	6,189
Total	4,173,310

 

Breakdown of runoff to storages managed by the Sydney Catchment Authority

Storage	Runoff to the storage during 2010–11 (ML)
Cataract	75,217
Cordeaux	41,379
Avon	49,657
Nepean	54,778
Woronora	31,260
Lake Burragorang (Warragamba Dam)	781,603
Prospect	4,563
Wingecarribee	14,249
Fitzroy Falls	22,960
Lake Yarrunga (Tallowa Dam)	779,209
Blue Mountains1	4,802
Pheasants Nest Weir2	11,412
Broughtons Pass Weir2	10,918
Total	1,882,007

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

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

Breakdown of runoff to storages not managed by the Sydney Catchment Authority

Storage	Managing authority	Runoff value was provided by	Runoff to the storage during 2010–11 (ML)
Danjera	Shoalhaven City Council	Bureau of Meteorology	23,533
Flat Rock Creek	Shoalhaven City Council	Bureau of Meteorology	1,739
Pejar	Goulburn Mulwaree Council	Bureau of Meteorology	20,036
Sooley	Goulburn Mulwaree Council	Bureau of Meteorology	10,683
Thomsons Creek, Lyell and Wallace1	Delta Electricity	Delta Electricity	62,450
Farmers Creek No. 2	Lithgow City Council	Bureau of Meteorology	3,047
Bundanoon and Medway	Wingecarribee Shire Council	Bureau of Meteorology	29,313
Total			150,8012

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

^2. There is no runoff volume provided for Bamarang as there is no real natural catchment for this reservoir.

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

Description	Runoff during 2010–11 (ML)
Runoff to rivers	2,142,348

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.

Bureau of Meteorology (the Bureau).

Runoff to storages and rivers 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 gridpoint 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 AHGF catchments were used to perform this break up. Sections of the Shoalhaven, Coxs, Wollondilly, Capertee and Macdonald rivers considered within the surface water store (line item 1.2) were removed from the Sydney subaccount 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. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages).

• The estimated runoff was compared against historical flows at unimpaired catchments within the Sydney region for the 2011 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 was not quantified.

Data spreadsheets.

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.

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 storage diversions and any spilling.

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

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.

SCA.

Runoff to the following storage reservoirs was provided 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 reservoirs 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 reservoirs, mass balance calculation was used to estimate inflow to each reservoir. In the calculation, inflow, reservoir diversions, precipitation, evaporation and other known losses, and beginning and end storage volumes for each reservoir were balanced for 2010–11. Reservoir 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 reservoirs (line item 9.1) and inflow to reservoirs (this line item).

Nil.

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

The following table compares the value for the 2009–10 year reported in this account with the corresponding value reported in the 2010 Account.

 

Comparative information from the 2010 Account for line item 9.4

2011 Account line item	2010 Account line item	Value reported in the 2010 Account (ML)	Value reported in the 2011 Account (ML)	Additional information
9.4 Runoff	13.4.1 Runoff to connected surface water	1,663,861	1,658,962	Restatement: In the 2010 year, this line item did not correct the formula for discharge from upstream storages from the urban water system.