9.1 Precipitation on surface water
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.
|Storage operator||Storage||Volume (ML)|
|Melbourne Water||Cardinia Reservoir||9,665
|Upper Yarra Reservoir||6,889
|Yan Yean Reservoir||3,114|
|Southern Rural Water||Melton Reservoir||1,060|
|Pykes Creek Reservoir||921|
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.
|Storage||Precipitation volume (ML)
|Precipitation volume (ML)
|Upper Yarra Reservoir||6,889||5,278
|Yan Yean Reservoir||3,114