Melbourne
17.1 Evaporation from surface water

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

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.

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.

• 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 estimates were not available for the modelling method.

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