Melbourne
17.1 Evaporation from surface water

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

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.

The Bureau of Meteorology.

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.

• 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 estimates were not available for the modelling method.

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.