17.1 Evaporation from surface water
The total evaporation volume from surface water during the 2011–12 year is recorded as 20,097 ML. 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.
|Water storage type||Water storage name||Evaporation (ML)|
|Lakes2||Lake Burley Griffin||6,330|
1 Evaporation calculated using dynamic surface area
2 Evaporation calculated using static surface area.
The Bureau of Meteorology (The Bureau): 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 storage.
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 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 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 from each waterbody was estimated from the proportionally weighted average of grid-cells that intersected each water feature. The volume was then estimated using the surface area of each waterbody. The surface area varied dynamically with changing storage level for storages where the relationship between storage level and surface area had been derived. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables. In the Canberra region, the surface area of the four storages was calculated dynamically and the surface area of the three urban lakes was a static value produced from the AHGF.
Assumptions, limitations, caveats and approximations
- 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 Canberra region included only the reservoirs and urban lakes (not the river channels).
- The use of the static default AHGF surface area is an approximation only. It represents the lakes at capacity and therefore likely results in an overestimation of precipitation on the lakes.
The uncertainty estimate was not quantified.
In the 2011 Account, the Bureau used the Priestly and Taylor method to estimate potential evaporation. In 2012, the Bureau applied AWRA-L version 2.0.0 (Van Dijk 2010) which uses a modified version of the Penman-Monteith method to produce the potential evaporation. The Bureau recalculated the 2011 volume with the new method. Consequently, the volume has been restated from 21,689 ML (as published in the 2011 Account) to 18,982 ML.