Murray–Darling Basin
31.1 Evaporation from off-channel water storages

The volumetric value for the line item for the 2012–13 year was 1,257,805 ML. The line item represents the volume of water that passes into the atmosphere across a water/air interface from off-channel water storages within the Murray–Darling Basin (MDB) region. The following table presents breakdown information for the line item on a surface water resource plan area basis.

(1) Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall, temperature and solar radiation) and (2) Geoscience Australia: MDB human-made waterbody feature class and nine arc-second digital elevation model (DEM).

The potential evaporation estimate produced by the Australian water resources assessment system landscape (AWRA-L) model version 3.0 (Van Dijk 2010) was used to calculate evaporation from off-channel water storages. The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. The water-balance based farm dam algorithm written by the bureau was used to determine the amount of water available for evaporation from individual off-channel water storages.

Using daily gridded climate data for the MDB region (including precipitation, temperature and solar radiation data), open water evaporation data on monthly basis were calculated. Daily gridded climate data were available on a 0.05 degree (approximately 5 km) national grid.

The MDB was divided into 105 regions for the purpose of modelling the off-channel water store. The off-channel water store consisted of storages filled primarily by local catchment runoff. These were determined from waterbody mapping conducted by Geoscience Australia as those which:

• are not named storages (assuming that any storage with a name is unlikely to be a off-channel water storage); and
• are above 600 m in elevation; and/or
• are below 600 m in elevation in areas that receive greater than 400 mm per annum in precipitation and are not within 50 m of a major or perennial stream.

The above rules attempt to divide storages into those that are likely to be filled primarily by local catchment runoff and those which are filled by abstraction from surface water, groundwater or floodplain harvesting. The catchment of each individual storage was determined via analysis of the 9 arc-second DEM.

The potential average evaporation depth across the MDB region 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 MDB landscape to remove edge effects (where the area represented is not wholly within the MDB 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. The algorithm determines the water stored in each off-channel water storage at each time step and determines the volumetric potential evaporation by multiplying potential evaporation by storage surface area. The algorithm assumes that actual evaporation will occur at the same rate as potential evaporation unless storage empties, at which time, evaporation will cease.

• 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 spatial extent of water bodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.

The uncertainty estimate was not quantified.

There was a prior period error in the line item volume reported in the 2012 Account. The prior period error was a result of using solar radiation data with bias correction errors. Solar radiation is an input for line item volume calculation. The line item volume for the 2011-12 year has been recalculated accommodating the prior period error correction. As a result, the volume for the 2011-12 year has been changed for the whole MDB region from 1,611,332 ML reported in the 2012 Account to 1,480,233 ML in the 2013 Account.