17.1 Evaporation from surface water

Supporting Information

This line item refers to total evaporation from all the major surface water storages within the Perth region.

Total volume of evaporation from each storage during the 2010–11 year within the Perth region is provided in the following table.


Evaporation from each storage within the Perth region during the 2010–11 year


Evaporation (ML)



Churchman Brook


Drakes Brook




Logue Brook




North Dandalup


Samson Brook




Serpentine Pipehead


South Dandalup













Quantification Approach

Data Source

Bureau of Meteorology, National Climate Centre: daily climate grids (rainfall, temperature and solar radiation), Bureau of Meteorology: Australian Hydrological Geospatial Fabric – waterbody feature class, Australian Water Resources Information System – water storages.

Provided by

Bureau of Meteorology.


The Priestly and Taylor method to estimate potential evaporation (as calculated by the WaterDyn model) was used to estimate evaporation from the surface water store. Monthly, open water evaporation data produced by the Bureau were used based on daily gridded climate data that are available on a 0.05 degree (5 km) national grid.

As a potential evaporation dataset, it was an estimate of the evaporative demand of the environment. The daily gridded climate datasets are generated by the Bureau and include downward solar irradiance 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 weighted average of the grid points within a 5 km radius of each water feature. The volume was then estimated using the surface area of each waterbody. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables.

The surface area of all of the storages in the Perth region was calculated using this dynamic method.

Assumptions, Limitations, Caveats and Approximations

The Priestly and Taylor potential evaporation estimates are subject to approximations associated with interpolating the observation point data to a national grid detailed 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 temporal scale.

The total surface area of the surface water store within the Perth region included only the storages (and not the rivers).

Uncertainty Information

The uncertainty estimate was not quantified.

Comparative year

The following table presents substantive changes in the line item reported in this 2011 Account with the corresponding line item in the 2010 Account.

Comparative year information for line item 17.1

2011 Account line item

2010 Account line item

2011 Account volume for the 2009–10 year (ML)

2010 Account volume for the 2009–10 year (ML)

Change from the 2010 Account to the 2011 Account

17.1 Evaporation from surface water

14.1 Evaporation from connected surface water



Methodology change: new model used

Prior period error: solar radiation data were incorrect

Change in line item name and number


The evaporation reported in the 2010 Account was overestimated. The bias correction applied to the satellite-derived solar radiation dataset was incorrect, which resulted in an overestimation of incoming solar radiation.

Using this improved dataset to recalculate evaporation from storages for the 2009–10 year resulted in a 27% decrease in volume compared with that reported in the 2010 Account. It is considered that this change is material and is an important line item for reliable estimates of inflows into water storages for the region.