Melbourne
17.1 Evaporation from surface water

The volume presented in the water accounting statements (35,365 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 2010–11 year.

Evaporation from water storages

Water storage	Evaporation (ML)
O'Shannassy	263
Yan Yean	4,731
Greenvale	1,667
Maroondah	2,116
Silvan	3,260
Cardinia	7,532
Upper Yarra	6,754
Rosslynne	471
Pykes Creek	519
Merrimu	617
Sugarloaf	3,764
Melton	666
Tarago	3,004
Total	35,365

Bureau of Meteorology, National Climate Centre (NCC): version 3 daily rainfall grids; daily maximum temperature grids; daily minimum temperature grids; daily satellite observed solar radiation grids; Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; Australian Water Resources Information System (AWRIS) water storage.

Bureau of Meteorology.

The Priestly and Taylor method to estimate potential evaporation (as calculated by the WaterDyn model (Raupach, 2008) was used to estimate evaporation from the connected surface water store. Monthly, open water evaporation data produced by the Bureau of Meteorology were used, based on daily gridded climate data that are 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 Priestly and Taylor 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 2010–11 year were approximately 28% lower than modelled estimates by the Bureau of Meteorology. Melbourne Water evaporation estimates from water storages they managed were shown in the following table:

 

Evaporation from water storages

Water storage	Evaporation (Bureau) (ML)	Evaporation (Melbourne Water) (ML)
O'Shannassy	263	241
Yan Yean	4,731	3,363
Greenvale	1,667	1,443
Maroondah	2,116	1,517
Silvan	3,260	1,974
Cardinia	7,532	5,896
Upper Yarra	6,754	4,907
Sugarloaf	3,764	3,356
Tarago	3,004	1,231

Uncertainty estimates were not available for the modelling method.

This corresponds to line item 14.1 Evaporation from connected surface water reported in the 2010 Account. In the 2011 Account, the methodology used to quantify the line item was improved. In 2010, the Penman open water evaporation method was used to estimate evaporation. In 2011 the Priestly and Taylor potential evaporation method was used to estimate evaporation, which resulted in a decrease to the comparative year volume.

Restatement of the volume published in the 2010 Account was made as the difference from these changes was material and increased the accuracy of the information provided to the users of the National Water Account. The changes and their respective volumes are detailed in the following table. The restated comparative year volume is 34,995 ML

Restated comparative year information for line item 17.1 Evaporation from surface water

2010 Account line item name	Volume at 30 June 2010 reported in 2010 Account (ML)	Value difference to volume reported due to method change (ML)	Volume at 30 June 2010 reported in 2011 Account (ML)
14.1 Evaporation from connected surface water	44,289	9,294	34,995