Sydney
9.4 Runoff to surface water

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	Runoff (ML)
	Storages managed by the Sydney Catchment Authority	2,053,928
	Storages not managed by the Sydney Catchment Authority	141,522
	River sections	2,774,031
	Subtotal	4,969,481
add	evaporation loss in the river reaches upstream of reservoirs	13,563
less	runoff accounted in precipitation on river reaches upstream of reservoirs	9,422
less	runoff accounted in discharge from urban water system (see line item 9.9)	5,589
	Total	4,968,033

 

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

Storage	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	Storage	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	962
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

Description	Runoff during 2012–13 (ML)
runoff to rivers	1,446,169
gauged flows	1,327,8621
Total	2,774,031

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.

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.

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).

• 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).

The uncertainty estimate was not quantified.

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.

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.

Nil.

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