27.1 Off-channel water storages
The volumetric value for the line item for the 2011–12 year was 1,152,737 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 area basis.
|Water resource plan area||Sustainable diversion limit area||State/Territory||Volume on 30 June 2012 (ML)||Volume on 30 June 2011 (ML)|
|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||57,253||56,857|
|SW15 NSW Border Rivers||SS23||NSW Border Rivers||NSW|
|SW8 Murrumbidgee||SS15||Murrumbidgee NSW||NSW||168,527||158,443|
|SW7 NSW Murray and Lower Darling||SS18||Lower Darling||NSW||87,496||100,354|
|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.||32,570||33,315|
|SW6 Eastern Mount Lofty Ranges||SS13||Eastern Mount Lofty Ranges||SA||11,879||11,304|
|Whole MDB region||1,152,737||1,131,938|
A Fortran code water balance routine was used to determine the volume of water stored in off-channel water storages. The calculation routine 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 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 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 calculation routine 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 Fortran code and the parameters used.
- The spatial extent of water bodies are subject to the assumptions and methods associated with the data provided by the Geoscience Australia.
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 at 30 June 2011 (ML)||2011 Account volume at 30 June 2011 (ML)||Difference due to calculation method change (ML)|
The volume estimated for the comparison year for the 2012 Account (1,131,938 ML) is higher than the volume reported for the 2011 Account (1,037,087 ML). This was due to a change in modelling methods. 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 calculation routine. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 94,851 ML represents a change of 9% of the previously reported value.