31.3 Off-channel water abstraction
The volumetric value for the line item for the 2011–12 year was 973,602 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.
|Water resource plan area||Sustainable diversion limit area||State/Territory||Volume (ML) for the 2011–12 year|
|SW11 Barwon–Darling Watercourse||SS19||Barwon–Darling Watercourse||NSW|
|SW12 NSW Intersecting Streams||SS17||NSW Intersecting Streams||NSW|
|SW16 Qld Border Rivers||SS24||Qld Border Rivers||Qld||68,639|
|SW15 NSW Border Rivers||SS23||NSW Border Rivers||NSW|
|SW8 Murrumbidgee||SS15||Murrumbidgee NSW||NSW||111,748|
|SW7 NSW Murray and Lower Darling||SS18||Lower Darling||NSW||68,347|
|SW2 Vic. Murray||SS3||Kiewa||Vic.|
|SW5 SA Murray||SS11||SA Murray||SA|
|SS10||SA Non-prescribed areas||SA|
|SW3 Northern Victoria||SS4||Ovens||Vic.||27,283|
|SW6 Eastern Mount Lofty Ranges||SS13||Eastern Mount Lofty Ranges||SA||8,665|
|Whole MDB region||973,602|
(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 9 arc-second digital elevation model (DEM).
A water balance tool based on a Fortran code was used to determine the volume of water abstracted from the off-channel water store. The tool was provided inputs: climate data from gridded climate datasets, runoff from the Australian water resources assessment system landscape (AWRA-L) model version 2.0.0 (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)
- are above 600 m in elevation
- 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 water balance tool 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 tool assumes that water will be abstracted from the storage at the rate required unless storage empties, at which time, abstraction will cease.
Assumptions, limitations, caveats and approximations
- 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.
A change made to the calculation method resulted in the restatement of the 2010–11 year volume. The method used to quantify the line item was improved and resulted in a material change in volume.
The respective volumes associated with the change are detailed in the following table.
|2011 Account line item||Segment||2012 Account volume for the 2010–11 year (ML)||2011 Account volume for the 2010–11 year (ML)||Difference due to calculation method change (ML)|
|Line item 31.3 Water use||Northern Basin||596,566||566,505||30,061|
The volume estimated for the comparison year for the 2012 Account (1,106,511 ML) is higher than the volume reported for the 2011 Account (1,043,431 ML).
The difference between the previously reported volume and the estimate produced for the comparison year can be attributed to the choice of the AWRA-L v2.0.0 model (instead of the previously used AWRA-L v1.0.0) to provide inputs to the water balance tool. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff.
The difference of 63,079 ML represents a change of 6% of the previously reported volume.