9.1 Precipitation on surface water

Supporting information

The volume presented in the water accounting statements (48,406 ML) represents the precipitation falling on the surfaces of water storages.

The following table presents the breakdown of precipitation on water storages in the Melbourne region during the 2011–12 year.

Precipitation on storages within the Melbourne region
Storage operator Storage Volume (ML)
Melbourne water Cardinia Reservoir 12,894
Greenvale Reservoir 1,067
Maroondah Reservoir 2,217
O'Shannassy Reservoir 416
Silvan Reservoir 4,531
Sugarloaf Reservoir 4,393
Tarago Reservoir 3,498
Upper Yarra Reservoir 9,433
Yan Yean Reservoir 4,636
Southern Rural Water Melton Reservoir 1,008
Merrimu Reservoir 1,574
Pykes Creek Reservoir 1,364
Rosslynne Reservoir 1,375
Total 48,406

Quantification approach

Data source

National Climate Centre (NCC) daily climate grids and (rainfall) monthly precipitation grids, Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class, Australian Water Resources Information System (AWRIS)—Water storages.

Provided by

Bureau of Meteorology (the Bureau).


Monthly precipitation data were produced by the Bureau. It was 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 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 reservoir storage level for reservoirs where the relationship between storage level and surface area had been derived. In the Melbourne region, the surface area of all water storages was calculated dynamically.

Assumptions, limitations, caveats and approximations

  • Precipitation estimates were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
  • Dynamic storage surface areas calculated from the levels and capacity tables represent the monthly average and did not capture changes that occurred on a shorter temporal scale.
  • Melbourne Water uses rain gauges at each of its reservoirs to measure rainfall. These are generally assumed to have an accuracy of within +/– 5%.
  • Melbourne Water's calculations of rainfall on water storages during the 2011–12 year were approximately 10.5% lower than modelled estimates by the Bureau. Melbourne Water gauged rainfall volumes on water storages they manage are described in the following table.
Comparison of precipitation estimates
Storage Precipitation volume (ML)
—the Bureau
Precipitation volume (ML)
—Melbourne Water
Cardinia Reservoir
12,894 12,598
Greenvale Reservoir 1,067 1,127
Maroondah Reservoir 2,217 2,609
O'Shannassy Reservoir 416 450
Silvan Reservoir 4,531 4,261
Sugarloaf Reservoir 4,393 3,938
Tarago Reservoir 3,498 2,552
Upper Yarra Reservoir 9,433 6,884
Yan Yean Reservoir 4,636 4,596
Total 43,085 39,015

Uncertainty information

Uncertainty estimates were not available for the modelling method.

Comparative year

An error was made when calculating the 2010–11 year volume published in the 2011 Account due to the use of an incorrect precipitation data set. This error has been corrected in the 2012 Account. Consequently, the volume has been restated from 35,113 ML (as published in the 2011 Account) to 53,802 ML.

Comparative year information for line item 9.1 Precipitation on surface water
Storage operator Storage 2012 Account volume
for the 2010–11 year (ML)
2011 Account volume
for the 2010–11 year (ML)
Melbourne Water Cardinia Reservoir
12,578 2,885
Greenvale Reservoir 1,313 1,831
Maroondah Reservoir 2,774 424
O'Shannassy Reservoir 545 7,830
Silvan Reservoir 5,359 4,177
Sugarloaf Reservoir 5,131 344
Tarago Reservoir 3,904 374
Upper Yarra Reservoir 10,776 3,232
Yan Yean Reservoir 5,257 966
Southern Rural Water Melton Reservoir 1,529 329
Merrimu Reservoir 1,827 335
Pykes Creek Reservoir 1,552 3,514
Rosslynne Reservoir 1,257 8,872
Total                          53,802 35,113