30.1 Precipitation on off-channel water store
The volumetric value for the line item for the 2010–11 year was 1,437,489 ML. The line item includes the volume of all forms (rain, sleet, snow, hail, or drizzle) of water falls on 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||
|SW11–12 and SW17–19||Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse||Qld and NSW||290,710|
|SW15–16||Qld and NSW Border Rivers||Qld and NSW||161,378|
|Sub-total Northern Basin||857,566|
|SW1 and 8||Murrumbidgee NSW and ACT||NSW and ACT||144,707|
|SW2, 4, 5 and 7||NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee||NSW, Vic and SA||103,897|
|SW6||Eastern Mount Lofty Ranges||SA||10,069|
|Sub-total Southern Basin||579,923|
|Total for the region||1,437,489|
Monthly precipitation data were based on daily data from approximately 6,500 rain gauge stations and interpolated to a 0.05 degree (approximately 5 km) national grid (Jones et al. 2007).
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 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 which are filled by abstraction from surface water, groundwater or floodplain harvesting.
The average precipitation depth across the MDB sub-regions was determined as the weighted mean of precipitation 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 precipitation depth was used as an input into the tool for estimating dam impacts (STEDI) model (Sinclair Knight Merz 2011) converted the depth of precipitation to a volume using the surface area of off-channel water storages within the region.
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.
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.
Improved data sets were made available after the publication of the 2010 Account. Restatement of the value published in the 2010 Account was made as the difference was material and increased the accuracy of the information provided to the users of the National Water Account.
The methodology used to quantify the line item was improved and resulted in a material change in volume.
The value reported for the 2009–10 year in the 2010 Account was different to the restated value in the 2011 Account due to changes in the definition of off-channel water storages and changes to data availability. Only off-channel water storages that are filled by local catchment runoff have been included for the quantification of this line for the 2011 Account, whereas storages filled via pumping from surface water or groundwater were included in quantification of this line for the 2010 Account. It is also likely that there was a change in the data sources for precipitation to off-channel storages. This is because the gridded precipitation surfaces that these estimates are derived from a large number of precipitation monitoring sites. Many of these sites are manually monitored, with observations recorded on paper and mailed to the Bureau. There can be a considerable delay between the date of observation and the data becoming part of the interpolated surfaces thus historical surfaces can change as more data become available. 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 scope and calculation method change, and availability of improved data sets (ML)
Volume for the 2009–10 year reported in the 2011 Account (ML)