17.1 Evaporation from surface water

Supporting information

The volume presented in the water accounting statements (42,190 ML) represents evaporation from the surface of water storages.

The following table presents the breakdown of evaporation from water storages in the Melbourne region during the 2011–12 year.

Evaporation from surface water storages
Storage operator Storage Volume (ML)
Melbourne Water Cardinia Reservoir 10,245
Greenvale Reservoir 1,577
Maroondah Reservoir 1,906
O'Shannassy Reservoir 200
Silvan Reservoir 2,757
Sugarloaf Reservoir 4,307
Tarago Reservoir 2,489
Upper Yarra Reservoir 5,535
Yan Yean Reservoir 5,237
Southern Rural Water Melton Reservoir 1,935
Merrimu Reservoir 2,608
Pykes Creek Reservoir 1,864
Rosslynne Reservoir 1,530
Total 42,190

Quantification approach

Data source

National Climate Centre (NCC) daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class, Australian Water Resources Information System (AWRIS)—Water storages.

Provided by

Bureau of Meteorology (the Bureau).


The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model (AWRA-L) version 2.0.0 (Van Dijk 2010) was used to calculate evaporation from the surface water store. The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. Daily AWRA-L potential evaporation grids were produced based on daily gridded climate data that were available on a 0.05 degree (approximately 5 km) national grid.

Potential evaporation is an estimate of the evaporative demand of the environment. The daily gridded climate datasets used to produce this estimate are generated by the Bureau of Meteorology and include downward solar radiation, and maximum and minimum air temperature. The methods used to generate these gridded datasets are outlined in Jones et al. (2007).

The evaporation at each waterbody was estimated from the proportionally weighted average of grid-points cells that intersected each water feature. The volume was then estimated using the monthly average surface area of each waterbody. The surface area varied dynamically with changing water 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

  • The AWRA-L potential evaporation estimates are subject to approximations associated with interpolating the observation point input data to a national grid as described in Jones et al. (2007)
  • The dynamic storage surface areas calculated from the levels and storage rating tables represent a monthly average and therefore will not capture changes that occur on a shorter timescale.
  • The total surface area of the surface water store within the Melbourne region included only the water storages not river channels.
  • Melbourne Water measures evaporation using the factored pan method. Melbourne Water's calculations of evaporation from water storages during the 2011–12 year were approximately 17% lower than modelled estimates by the Bureau. Melbourne Water evaporation estimates from water storages they managed were shown in the following table:
Comparison of evaporation estimates for the 2011–12 year
Storage Evaporation volume (ML)
—the Bureau
Evaporation volume (ML)
—Melbourne Water
Cardinia Reservoir
10,245 7,934
Greenvale Reservoir 1,577 1,706
Maroondah Reservoir 1,906 1,636
O'Shannassy Reservoir 200 242
Silvan Reservoir 2,757 2,236
Sugarloaf Reservoir 4,307 3,895
Tarago Reservoir 2,489 1,372
Upper Yarra Reservoir 5,535 5,129
Yan Yean Reservoir 5,237 4,170
Total 34,254 28,320

Uncertainty information

Uncertainty estimates were not available for the modelling method.

Comparative year

An error was made when calculating the 2010–11 year volume published in the 2011 Account due to the use of an incorrect evaporation data set.

In addition, data available for the evaporation calculation have improved since the 2011 Account publication, due to a change in modelling methods. The AWRA-L potential evaporation was used instead of the Priestly and Taylor potential evaporation method.  

The error and updated modelling approach has been applied retrospectively to the 2010–11 year volume. Consequently, the volume has been restated from 35,364 ML (as published in the 2011 Account) to 36,087 ML, as shown in the following table.

Comparative year information for line item 17.1 Evaporation from surface water
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                                    7,898 7,532
Greenvale Reservoir                                    1,479 1,667
Maroondah Reservoir                                    1,681 2,116
O'Shannassy Reservoir                                     180 263
Silvan Reservoir                                    2,479 3,260
Sugarloaf Reservoir                                    3,816 3,764
Tarago Reservoir                                    2,305 3,004
Upper Yarra Reservoir                                    5,051 6,754
Yan Yean Reservoir                                    4,638 4,731
Southern Rural Water Melton Reservoir                                    1,857 666
Merrimu Reservoir                                    2,095 617
Pykes Creek Reservoir                                    1,570 519
Rosslynne Reservoir
                                   1,038 471
Total                                 36,087 35,364