Melbourne
9.1 Precipitation on surface water
Supporting Information
The volume presented in the water accounting statements (35,115 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 2010–11 year.
| Water storage | Precipitation (ML) |
| O'Shannassy | 424 |
| YanYean | 3,514 |
| Greenvale | 966 |
| Maroondah | 1,831 |
| Silvan | 4,177 |
| Cardinia | 7,830 |
| UpperYarra | 8,872 |
| Rosslynne | 374 |
| PykesCreek | 329 |
| Merrimu | 344 |
| Sugarloaf | 2,885 |
| Melton | 335 |
| Tarago | 3,232 |
| Total | 35,115 |
Quantification Approach
Data Source
Provided by
Method
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.
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 2010–11 year were approximately 28% higher than modelled estimates by the Bureau of Meteorology. Melbourne Water gauged rainfall volumes on water storages they manage were described in the following table.
| Water storage | Precipitation Bureau of Meteorology (ML) |
Precipitation Melbourne Water (ML) |
| O'Shannassy | 424 | 557 |
| Yan Yean | 3,514 | 4,872 |
| Greenvale | 966 | 1,268 |
| Maroondah | 1,831 | 2,875 |
| Silvan | 4,177 | 4,946 |
| Cardinia | 7,830 | 12,575 |
| Upper Yarra | 8,872 | 8,172 |
| Sugarloaf | 2,885 | 4,700 |
| Tarago | 3,232 | 3,089 |
Uncertainty Information
Comparative year
This item corresponds to line item 13.1 Precipitation on connected surface water reported in the 2010 Account.
In the 2011 Account, improved datasets became available to the Bureau of Meteorology to recalculate precipitation for the 2009–10 year. Including the newly available data, the 2009–10 year value was re-estimated at 33,002 ML. This did not represent a material change from the volume reported in the 2010 Account (31,557 ML); therefore, the value was not restated in the water accounting statements.
2010 Account line items |
Storage Name |
Volume at 30 June 2010 reported in 2010 Account (ML) |
Difference due to data availability (ML) |
Volume at 30 June 2010 reported in 2011 Account (ML) |
13.1 Precipitation on connected surface water
|
9.1 Precipitation on surface water
|
31,557 |
1445 |
33,002 |