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.

Precipitation on water storages
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

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.

Provided by

Bureau of Meteorology.

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.

 

Precipitation on water storages comparison
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

Uncertainty estimates were not available for the modelling method.

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.

Comparative year information

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