Canberra
17.1 Evaporation from surface water

The evaporation from each of the storages listed in line item 1.1 Storages and line item 1.4 Lakes and wetlands can be seen in the following table.

¹ Evaporation calculated using dynamic surface area

² Evaporation calculated using static surface area.

The Priestly and Taylor method to estimate potential evaporation (as calculated by the WaterDyn model [Raupach 2008]) was used to estimate evaporation from the surface water store. Monthly, open water evaporation data produced by the Bureau of Meteorology were used, based on daily gridded climate data that were available on a 0.05 degree (approximately 5 km) national grid.

Potential evaporation was an estimate of the evaporative demand of the environment. The daily gridded climate datasets used to produce this estimate were generated by the Bureau of Meteorology 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 proportionally weighted average of grid–points 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 reservoir storage level for reservoirs where the relationship between storage level and surface area had been derived. In the Canberra region, the surface area of the four reservoirs was calculated dynamically and the surface area of the three urban lakes was a static value produced from the Bureau of Meteorology's Australian Hydrological Geospatial Fabric.

• 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 Canberra region included only the reservoirs and urban lakes (not the river channels).

The uncertainty estimate was not quantified.

This line item corresponds to Line Item 14.1 Evaporation from connected surface water reported in the 2010 Account. The data available for evaporation calculation have improved since the 2010 Account publication.

The volume of evaporation from surface water estimated for the comparison year (20,332 ML) using the improved dataset is less than the volume reported for the 2010 Account (25,578 ML). This was due to a change in modelling methods. The difference between the prior estimate of evaporation and the estimate produced for the comparison year can be attributed to the choice of the Priestly and Taylor potential evaporation over the Penman open water evaporation.

The difference of 5,246 ML represents a change of approximately 21% of the originally published volume. This change was considered material and a restatement for the 2010 Account volume has been undertaken.

These changes are summarised in the table below.