31.3 Water use
The volumetric value for the line item for the 2010–11 year was 1,043,431 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.
|Surface water resource plan area||
|SW 11–12 and SW17–19||Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse||Qld and NSW||254,896|
|SW 15–16||Qld and NSW Border Rivers||Qld and NSW||74,467|
|Sub-total Northern Basin||566,505|
|SW 1 and 8||Murrumbidgee NSW and ACT||NSW and ACT||121,340|
|SW 2, 4, 5 and 7||NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee||NSW, Vic and SA||79,472|
|SW6||Eastern Mount Lofty Ranges||SA||12,851|
|Sub-total Southern Basin||476,926|
|Total for the region||1,043,431|
The tool for estimating dam impacts (STEDI) model (Sinclair Knight Merz 2011) was used to determine the volume of water abstracted from the off-channel water store. The model was provided inputs in the form of climate from gridded climate datasets, runoff from the AWRA-L v1.0.0 (Van Dijk 2010) model and dam 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 that:
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 that 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 STEDI model determines the water stored in each off-channel water storage at each time step and determines the volume required for use from the storage. STEDI 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 estimated volume available in storage for evaporation is subject to the assumptions associated with the STEDI model and the parameters used.
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.
In the 2011 Account, the following changes were made that caused the 2009–10 year value to be restated:
The scope of the line item was changed.
The methodology used to quantify the line item was improved and resulted in a material change in volume.
The change in the values for 2009–10 was due to a combination of the exclusion of off-channel storages that are not filled by local catchment runoff and the use of AWRA-L model to estimate rainfall-runoff into local catchment storages. Modelling changes were made to provide more reliable estimates of runoff volumes. The difference of 830,663 ML represents a change of approximately 56% of the volume provided for the 2010 Account. The changes and their respective values are detailed in the following table.
Volume for the 2009–10 year reported in the 2010 Account (ML)
Difference due to change in the calculation method (ML)
Volume for the 2009–10 year reported in the 2011 Account (ML)