The volume recognised in the water accounting statements (31,557 ML) represents the precipitation falling on the surfaces of the reservoirs listed in the following table.
| Reservoir | Precipitation (ML) |
|---|---|
| Cardinia Reservoir |
7,138 |
| Greenvale Reservoir |
887 |
| Maroondah Reservoir |
1,965 |
| Melton Reservoir |
281 |
| Merrimu Reservoir |
295 |
| O'Shannassy Reservoir |
340 |
| Pykes Creek Reservoir |
311 |
| Rosslynne Reservoir |
323 |
| Silvan Reservoir |
3,476 |
| Sugarloaf Reservoir |
2,435 |
| Tarago Reservoir |
2,982 |
| Upper Yarra Reservoir |
8,094 |
| Yan Yean Reservoir |
3,030 |
| Total |
31,557 |
Bureau of Meteorology: Australian Water Availability Project (AWAP) monthly precipitation grids; Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; Australian Water Resources Information System (AWRIS) water storage.
Bureau of Meteorology.
Monthly precipitation data was 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 (5 km) national grid (Jones et al. 2007b).
The precipitation at each waterbody was estimated from the average of grid-cells that intersected each water feature. The volume was then estimated using the surface area of each waterbody.
In the Melbourne region, Precipitation on the connected surface water store included precipitation falling on major storages only. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables.
Derived from measured data. Uncertainty is ungraded.
The precipitation estimates were subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007a).
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.
Melbourne Water uses rain gauges at each of its reservoirs to measure rainfall. These are generally assumed to have an accuracy of within +/–5%. This direct measurement of precipitation onto reservoirs yields different results to that determined by the Bureau of Meteorology using the methodology described above.