Melbourne
17.1 Evaporation from surface water

Supporting information

The volume presented in the water accounting statements (47,133 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 2012–13 year.

Evaporation from surface water storages

Storage operator

Storage

Volume (ML)

Melbourne Water

Cardinia Reservoir

11,688

Greenvale Reservoir

1,339

Maroondah Reservoir

2,126

O'Shannassy Reservoir

345

Silvan Reservoir

3,351

Sugarloaf Reservoir

4,028

Tarago Reservoir

3,601

Upper Yarra Reservoir

7,799

Yan Yean Reservoir

4,820

Southern Rural Water

Melton Reservoir

2,220

Merrimu Reservoir

1,949

Pykes Creek Reservoir

1,630

Rosslynne Reservoir

2,237

Total

47,133

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

The Bureau of Meteorology.

Method

The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model (AWRA-L) Version 3.0 (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 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-point 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 2012–13 year were not available for the full reporting period. Melbourne Water evaporation estimates from water storages they managed were shown in the following table:
Comparison of evaporation estimates for the 2012–13 year

Storage

Evaporation volume (ML)

Evaporation volume (ML)

 

The Bureau

Melbourne Water

Cardinia Reservoir

12,514

*

Greenvale Reservoir

1,430

1,898

Maroondah Reservoir

2,279

1,748

O'Shannassy Reservoir

369

257

Silvan Reservoir

3,590

*

Sugarloaf Reservoir

4,323

3,985

Tarago Reservoir

3,847

1,756

Upper Yarra Reservoir

8,313

5,805

Yan Yean Reservoir

5,147

4,685

Total

41,812

*

2012–13 evaporation volumes for Cardinia and Silvan reservoirs are unavailable for the full reporting period.

Uncertainty information

Uncertainty estimates were not available for the modelling method.

Comparative year

The comparative year volume for line item 17.1 Evaporation from surface water was restated in the water accounting statements from the 2012 Account due to an error identified in the method used to calculate this volume. Please refer to Restatement of comparative year information in the 'Water accounting policies' for more information.