Sydney
9.1 Precipitation on surface water

The total volume of precipitation on surface water in the 2011–12 year is 164,426 ML. The following tables provide a summary and breakdown of precipitation on the surface water assets in the Sydney region.

Precipitation on surface water during the 2011–12 year

Surface water asset	Precipitation (ML)
Storages managed by the Sydney Catchment Authority	141,934
Storages not managed by the Sydney Catchment Authority	10,423
River sections	12,069
Total	164,426

 

Breakdown of precipitation on storages managed by the Sydney Catchment Authority during the 2011–12 year

Storage1	Precipitation (ML)
Avon	10,210
Blue Mountains1	528
Cataract	11,516
Cordeaux	9,643
Fitzroy Falls	8,407
Lake Burragorang (Warrangamba Dam)	66,883
Lake Yarrunga (Tallowa Dam)	12,881
Nepean	4,130
Prospect	7,194
Wingecarribee	6,587
Woronora	3,955
Total	141,934

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

Breakdown of precipitation on storages not managed by Sydney Catchment Authority during the 2011–12 year

Storage1	Precipitation (ML)
Bamarang	799
Bundanoon	290
Danjera	1,048
Farmers Creek No. 2	103
Flat Rock Creek	299
Lake Lyell	1,915
Lake Wallace	1,363
Medway	202
Pejar	1,562
Sooley	1,110
Thomsons Creek Dam	1,732
Total	10,4232

1 Data for storages not listed in the above tables are not available. Precipitation for those reservoirs has been considered within line item 9.4 Runoff to surface water.

2 Static surface area is used.

Breakdown of precipitation on rivers

River1	From	To	Precipitation during 2011–12 (ML)
Capertee	Glen Davis	Upper Colo	1,230
Coxs	Lithgow	Kelpie Point	1,894
Macdonald	Howes Valley	St Albans	158
Shoalhaven	Kadoona	Fossickers Flat	3,892
Wollondilly	Pomeroy	Golden Valley	4,895
Total			12,0692

1 Only the river sections for which river volumes have been calculated are included. Precipitation for all the other river reaches has been considered with line item 9.4 Runoff to surface water.

2 Static surface area is used.

The Bureau: National Climate Centre (NCC) daily climate grids (rainfall), geographical information system (GIS) layers; Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; Australian Water Resources Information System; New South Wales Office of Water: river channel cross-section and flow data from Hydstra and Computer Aided Improvements to River Operations (CAIRO) databases; New South Wales Office of Water: GIS layers; New South Wales Office of Water: GIS database, river channel sections included in the Sydney Connected Surface Water Store; New South Wales Office of Water: New South Wales waterbody polygon ArcSDE feature class.

The Bureau.

The Bureau provided precipitation data for the storages not managed by the Sydney Catchment Authority and for precipitation on the river sections.

Monthly precipitation data were produced by the Bureau. These were based on daily data from approximately 6,500 rain gauge stations and interpolated to a 0.05 degree (approximately 5 km) national grid (Jones et al. 2007). The average monthly surface area was calculated from daily storage levels and capacity tables. Where this data was not available a static surface area was produced from the AHGF. The surface area of the rivers was estimated using cross-section and long term average river level data and remains static in these calculations.

The precipitation at each waterbody was estimated from the proportionally weighted average of grid-points that intersected each water feature. The volume was then estimated using the surface area of each waterbody. The surface area varied dynamically with changing storage level for storages where the relationship between storage level and surface area had been derived.

• The precipitation estimates were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
• The dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore do not capture changes that occur on a shorter temporal scale.
• The use of the static default AHGF surface area is an approximation only. It represents the storages at capacity and therefore likely results in an overestimation of precipitation on the storages.
  

The uncertainty estimate was not quantified.

Daily return systems (DRS) database. Some stations from which DRS database receives data are managed by the Bureau. Data is also held in the Hydstra system.

Sydney Catchment Authority.

Precipitation for the following storages are managed by Sydney Catchment Authority:

• 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.

Rainfall received over the entire catchment area was calculated by interpolating rainfall values from point gauges (primarily tipping-bucket rain gauges). Rainfall volumes over storages were calculated by multiplying relevant surface areas and interpolated rainfall values.

Catchment average rain assumed to fall on the surface of a lake.

The precipitation volume is based on measured data. Estimated uncertainty based on accuracy of rain gauges, limitations in the calculation method and organisational practice is +/– 10%.