27.1 Off-channel water storages
The volumetric value for the line item for the 2010–11 year was 1,037,087 ML. The line item represents the volume of water held in constructed storages that are not connected either seasonally or perennially to rivers, filled predominantly by local catchment runoff within the Murray–Darling Basin (MDB) region. The following table presents breakdown information for the volumetric value on a surface water resource plan (WRP) area basis.
|Surface water resource plan area||
Volume (ML) as at 30 June 2011
|SW11–12 and SW17–19||Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse||Qld and NSW||185,804|
|SW15–16||Qld and NSW Border Rivers||Qld and NSW||54,628|
|Sub-total Northern Basin||456,528|
|SW1 and 8||Murrumbidgee NSW and ACT||NSW and ACT||147,753|
|SW2, 4, 5 and 7||NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee||NSW, Vic and SA||94,972|
|SW6||Eastern Mount Lofty Ranges||SA||11,065|
|Sub-total Southern Basin||580,559|
|Total for the region||1,037,087|
The tool for estimating dam impacts (STEDI) model (Sinclair Knight Merz 2011) was used to determine the volume of water stored in off-channel water storages. The model was provided inputs in the form of climate from gridded climate datasets, runoff from the AWRA-L model version 1.0.0 (Van Dijk 2010) and dam details derived from spatial data.
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 catchment of each individual storage was determined via analysis of the 9 arc-second DEM.The STEDI model performs a water balance on each individual storage at each time step using runoff and precipitation as inflows and spills, evaporation and usage as outflows. The volume of water held in storage is an output of this water balance.
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 are subject to the assumptions and methods associated with the data provided by Geoscience Australia.
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 datasets 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, to the quantification method and changes to data availability. Only off-channel storages that are filled by local catchment runoff have been included for the quantification of this line item for the 2011 Account, whereas storages filled via pumping from surface water or groundwater were included in quantification of this line item for the 2010 Account. The method for estimating rainfall-runoff (the primary inflow to local catchment storages) was updated from the unweighted mean of AWRA-L version 0.5 and WaterDyn version 26 used for quantification in the 2010 Account to AWRA-L version 1.0.0.
It is likely that there was a change in the data sources for precipitation to and evaporation from off-channel storages. This is because the gridded climate surfaces (including precipitation, temperature vapour pressure) that these estimates are derived from a large number of 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 as at 30 June 2010 reported in the 2010 Account (ML)
Difference due to scope and calculation method change, and availability of improved data sets (ML)
Volume as at 30 June 2010 reported in the 2011 Account (ML)