30.3 Runoff harvesting into off-channel water store
The volumetric value for the line item for the 2010–11 year was 1,506,485 ML. The line item includes collection of runoff into off-channel water storages including local catchment runoff into farm dams 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) for the 2010–11 year
|SW11–12 and SW17–19||Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse||Qld and NSW||325,665|
|SW15–16||Qld and NSW Border Rivers||Qld and NSW||111,363|
|Sub-total Northern Basin||778,663|
|SW1 and 8||Murrumbidgee NSW and ACT||NSW and ACT||196,411|
|SW2, 4, 5 and 7||NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee||NSW, Vic and SA||127,832|
|SW6||Eastern Mount Lofty Ranges||SA||23,120|
|Sub-total Southern Basin||727,822|
|Total for the region||1,506,485|
Rainfall runoff harvesting to off-channel water storages was estimated based on the AWRA-L version 1.0.0 streamflow and the tool for estimating dam impacts (STEDI) farm dam water balance model outputs (Sinclair Knight Merz 2011).
Using climate grid data for the MDB region (including precipitation, temperature and solar radiation data), AWRA-L (Van Dijk 2010) was used to estimate the runoff depth at each grid-point within the region.
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 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 average runoff depth across the MDB sub-regions was determined as the weighted mean of runoff occurring from the relevant grid points within the region boundary. Points were weighted by 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 runoff depth was converted to a volume by multiplying depth by the total area and was used as an input into the STEDI model.
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 of runoff harvested 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 use of a 9 arc-second DEM to determine catchment area may result in storages being assigned a catchment much larger or smaller than the true catchment. In some cases a storage may be assigned the catchment of a stream line hundreds of metres away.
The uncertainty estimate was not quantified.
In the 2011 Account, the methodology used to quantify the line item was improved and resulted in a material change in volume.
The change in value was primarily due to the use of the AWRA-L v1.0.0 model in the 2011 Account to estimate runoff instead of the average of AWRA-L v0.5 and WaterDyn v26. Some change in the volume could also be attributed to changes the AWAP rainfall datasets as data from manually read gauges entered the dataset. These changes were made to provide more reliable estimates of runoff volumes. The difference of 251,177 ML represents a change of approximately 15.3% 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)