Sydney
9.1 Precipitation on surface water

The total volume of precipitation on surface water in the 2010–11 year is 112,395 ML. The following tables provide a summary and breakdown of precipitation on the surface water assets in the Sydney region.

Precipitation on surface water – summary

Surface water asset	Precipitation during 2010–11 (ML)
Storages managed by the Sydney Catchment Authority	97,242
Storages not managed by the Sydney Catchment Authority	4,879
River sections	10,274
Total	112,395

 

Breakdown of precipitation on storages managed by the Sydney Catchment Authority

Storage1	Precipitation during 2010–11 (ML)
Cataract	7,918
Cordeaux	6,470
Avon	8,774
Nepean	3,763
Woronora	1,914
Lake Burragorang (Warragamba Dam)	43,733
Prospect	3,763
Wingecarribee	5,722
Fitzroy Falls	7,531
Lake Yarrunga (Tallowa Dam)	7,110
Blue Mountains1	544
Total	97,242

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 the Sydney Catchment Authority

Storage1	Precipitation during 2010–11 (ML)
Bamarang	701
Danjera	893
Flat Rock Creek	274
Pejar	1,477
Sooley	1,020
Farmers Creek No. 2	97
Bundanoon	171
Medway	246
Total	4,879

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

Breakdown of precipitation on rivers

River1	From	To	Precipitation during 2010–11 (ML)
Capertee	Glen Davis	Upper Colo	1,066
Coxs	Lithgow	Kelpie Point	1,683
Macdonald	Howes Valley	St Albans	138
Shoalhaven	Kadoona	Fossickers Flat	3,059
Wollondilly	Pomeroy	Golden Valley	4,328
Total			10,274

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.

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

Bureau of Meteorology.

The Bureau provided precipitation data for following storages not managed by the Sydney Catchment Authority (SCA) :

• Bamarang Reservoir
• Danjera Reservoir
• Flat Rock Creek Reservoir
• Pejar Reservoir
• Sooley Reservoir
• Farmers Creek No. 2.
  

Precipitation for the river sections in:

• Capertee River
• Coxs River
• Macdonald River
• Shoalhaven River
• Wollondilly River.

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 precipitation at each waterbody was estimated from the proportionally weighted average of gridpoints 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 stoarges 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 will not capture changes that occur on a shorter temporal scale.

The uncertainty estimate was not quantified.

The source for rainfall data is SCA's daily return systems (DRS) database. Some stations from which DRS database receives data are managed by the Bureau. Data are also held in the Hydstra system.

SCA.

Precipitation for the following storages 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.

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%.

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 line item 9.1

2011 Account line item	2010 Account line item	Value reported in the 2010 Account (ML)	Value reported in the 2011 Account (ML)	Additional information
9.1 Precipitation on surface water	13.1 Precipitation on connected surface water	83,203	83,203	Change in line item name