9.1 Precipitation on surface water
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.
|Storage operator||Storage||Volume (ML)|
|Melbourne water||Cardinia Reservoir||12,894|
|Upper Yarra Reservoir||9,433|
|Yan Yean Reservoir||4,636|
|Southern Rural Water||Melton Reservoir||1,008|
|Pykes Creek Reservoir||1,364|
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.
|Storage||Precipitation volume (ML)
|Precipitation volume (ML)
|Upper Yarra Reservoir||9,433||6,884|
|Yan Yean Reservoir||4,636||4,596|
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.
|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
|Upper Yarra Reservoir||10,776||3,232|
|Yan Yean Reservoir||5,257||966|
|Southern Rural Water||Melton Reservoir||1,529||329|
|Pykes Creek Reservoir||1,552||3,514|