Canberra
31.1 Evaporation from off-channel water storages

This line item refers to the evaporation that occurred on off–channel water storages within the Canberra region. The evaporation volume from off-channel water storages was 6,229 ML for 2012–13 year. These water storages are private storages and include farm dams. For more information on these storages see the Off-channel storages water storages region note. 

Bureau of Meteorology (The Bureau), National Climate Centre daily climate grids (rainfall, temperature and solar radiation); Geoscience Australia, mapping of human-made waterbodies spatial dataset, built-up areas spatial dataset and 9 arc-second digital elevation model (DEM).

The potential evaporation estimate produced by the Australian Water Resources Assessment system Landscape model  (AWRA-L) version 3.0 (Van Dijk 2010) was used to calculate evaporation from the private  water store. The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. The farm dam algorithm written by the Bureau was used to determining the amount of water available for evaporation from individual private storages.

Using climate grid data for the Canberra region (including precipitation, temperature, and solar radiation data), monthly, open water evaporation data produced by Bureau of Meteorology were calculated, based on daily gridded climate data that are available on a 0.05 degree (approximately 5 km) national grid.

The Canberra region was divided up into five subregions for the purpose of estimating the water balance of the off-channel water store. The four of the subregions were the catchments of the major storages and the fifth was the remaining land.

The off–channel storages filled primarily by rainfall-runoff. These were determined from waterbody mapping provided by Geoscience Australia and were waterbodies that were greater than 50 m from built-up areas in the Canberra region.

The potential average evaporation depth across the Canberra subregions was determined as the weighted mean of potential evaporation occurring from the relevant grid points within the region boundary. Points were weighted upon the area they represented within the Canberra landscape to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. The average potential evaporation was used as an input into the farm dam algorithm written by the Bureau. It determines the water stored in each off-channel water store at each time point and determines the volumetric potential evaporation by multiplying potential evaporation by reservoir surface area. It assumes that actual evaporation will occur at the same rate as potential evaporation unless reservoir empties, at which time evaporation will cease.

• 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 estimated volume available in storage for evaporation is subject to the assumptions associated with the farm dam algorithm written by the Bureau and the parameters used.
• The spatial extent of waterbodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.

The uncertainty estimate was not quantified.