Murray–Darling Basin
31.3 Off-channel water abstraction

The volumetric value for the line item for the 2012–13 year was 858,308 ML. The line item includes volume of water abstracted 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 water-balance based farm dam algorithm written by the bureau was used to determine the volume of water abstracted from the off-channel water store. The algorithm was provided inputs: climate data from gridded climate datasets, runoff from the Australian water resources assessment system landscape (AWRA-L) model version 3.0 (Van Dijk 2010) and storage details derived from spatial data.

The MDB region was divided into 105 units 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; 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 farm dam algorithm determines the water stored in each off-channel water storage at each time step and determines the volume required for use from the storage. The algorithm assumes that water will be abstracted from the storage at the rate required unless storage empties, at which time, abstraction will cease.

• The gridded climate input data are subject to approximations associated with interpolating observation point data to a national grid detailed 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.