9.1 Precipitation on surface water

Supporting information

The volume presented in the water accounting statements (35,591 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 2012–13 year.

Precipitation on storages within the Melbourne region
Storage operator Storage Volume (ML)
Melbourne Water Cardinia Reservoir 9,665
Greenvale Reservoir 815
Maroondah Reservoir 1,506
O'Shannassy Reservoir 370
Silvan Reservoir 3,150
Sugarloaf Reservoir 2,770
Tarago Reservoir 2,879
Upper Yarra Reservoir 6,889
Yan Yean Reservoir 3,114
Southern Rural Water Melton Reservoir 1,060
Merrimu Reservoir 989
Pykes Creek Reservoir 921
Rosslynne Reservoir 1,463
Total 35,591

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

The Bureau of Meteorology.


Monthly precipitation data were produced by the Bureau of Meteorology. 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 2012–13 year were approximately 5.5% 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
Greenvale Reservoir 815
Maroondah Reservoir 1,506 1,766
O'Shannassy Reservoir 370 342
Silvan Reservoir 3,150 2,772
Sugarloaf Reservoir 2,770 2,590
Tarago Reservoir 2,879 2,290
Upper Yarra Reservoir 6,889 5,278
Yan Yean Reservoir 3,114
Total 31,158

Uncertainty information

Uncertainty estimates were not available for the modelling method.