30.1 Precipitation on off-channel water store
The volumetric value for the line item for the 2011–12 year was 1,284,785 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.
|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||144,122|
|SW15 NSW Border Rivers||SS23||NSW Border Rivers||NSW|
|SW8 Murrumbidgee||SS15||Murrumbidgee NSW||NSW||110,410|
|SW7 NSW Murray and Lower Darling||SS18||Lower Darling||NSW||70,049|
|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.||25,079|
|SW6 Eastern Mount Lofty Ranges||SS13||Eastern Mount Lofty Ranges||SA||8,102|
|Whole MDB region||1,284,785|
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 water balance tool based on a Fortran code designed for the calculations used the average precipitation depth as an input for estimating storage impacts. The tool 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.
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.
|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)|
The volume of precipitation estimated for the comparison year for the 2012 Account (1,425,350 ML) is less than the volume reported for the 2011 Account (1,437,489 ML).The difference between the previously reported volume and the estimate produced for the comparison year can be attributed to the choice of the Australian water resources assessment system landscape (AWRA-L) v2.0.0 model (instead of the previously used AWRA-L v1.0.0) to provide inputs to the Fortran code. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 12,139 ML represents a change less than 1% of the previously reported value.