Sydney
9.4 Runoff to surface water

The line item (8,252,376 ML) represents the volume of runoff to surface water for the 2011–12 year. The volume calculated represents the total runoff from the landscape into the drains, 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 2011–12 year: summary

	Surface water asset	Runoff (ML)
	Storages managed by the Sydney Catchment Authority	3,592,570
	Storages not managed by the Sydney Catchment Authority	238,591
	River sections	4,430,258
	Subtotal	8,261,419
add	Evaporation loss in the river reaches upstream of reservoirs	12,202
less	Runoff accounted in precipitation on river reaches upstream of reservoirs	12,069
less	Runoff accounted in discharge from urban water system (see line item 9.9)	9,176
	Total	8,252,376

 

Volume of runoff to storages managed by the Sydney Catchment Authority during the 2011–12 year

Storage	Runoff to the storage (ML)
Cataract	109,499
Cordeaux	63,345
Avon	55,681
Nepean	100,441
Woronora	49,386
Lake Burragorang (Warragamba Dam)	1,451,815
Prospect	2,141
Wingecarribee	23,513
Fitzroy Falls	28,760
Lake Yarrunga (Tallowa Dam)	1,653,168
Blue Mountains1	5,751
Pheasants Nest Weir2	22,670
Broughtons Pass Weir2	26,400
Total	3,592,570

1Blue 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.

Volume of runoff to storages not managed by the Sydney Catchment Authority during the 2011–12 year

Storage	Managing authority	Runoff to the storage during 2011–12 (ML)
Danjera	Shoalhaven City Council	46,403
Flat Rock Creek	Shoalhaven City Council	2,821
Pejar	Goulburn Mulwaree Council	23,258
Sooley	Goulburn Mulwaree Council	14,173
Farmers Creek No. 2	Lithgow City Council	5,772
Bundanoon and Medway	Wingecarribee Shire Council	65,809
Lake Lyell	Delta Electricity	44,211
Lake Wallace	Delta Electricity	34,402
Thomsons Creek Dam	Delta Electricity	1,742
Total		238,5911

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.

Runoff to rivers

Description	Runoff during 2011–12 (ML)
Runoff to rivers	2,828,303
Gauged flows	1,601,9551
Total	4,430,258

1 Volume stated for the gauged flows is the total observed data obtained from Sydney Catchment Authority. Some data was missing for the 2011-12 year. See 17.2 River outflow from the region for further details.

Bureau of Meteorology, National Climate Centre (NCC): daily climate grids (rainfall, temperature and solar radiation); Australian Hydrological Geospatial Fabric (AHGF), waterbody feature class and stream network.

The Bureau.

Runoff to storages and rivers is estimated based on the Australian Water Resources Assessment system Landscape model (AWRA-L) version 2.0.0 (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's 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 (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 and major rivers).

• The estimated runoff was compared against historical flows at unimpaired catchments within the Sydney region for the 2012 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.

Mass balance calculations provided in the Sydney Catchment Authority Water Balance 2011–12 for the Independent Pricing and Regulatory Tribunal (IPART). Note that inflow values in the Water Balance 2011–12 include both precipitation on storages ( line item 9.1) and inflow to storages (this line item); databases and calculated monthly catchment runoff (CMCR) and annual spreadsheet calculations; customer invoices.

Sydney Catchment Authority.

Inflows to Fitzroy Falls and Wingecarribee reservoirs are calculated as follows:

• Fitzroy Falls Reservoir: inflow is 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 is 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 Sydney Catchment Authority'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 2011–12. 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.

Nil.

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