Murray–Darling Basin

The volumetric value for the line item is 44,270,703 ML. The following table provides a breakdown of run-off to connected surface water assets in the Murray–Darling Basin (MDB) region.

Data source

Bureau of Meteorology: climate data and geographic information system (GIS) layers; Australian Hydrologic Geospatial Fabric (AHGF) waterbody feature class

Commonwealth Scientific and Industrial Research Organisation (CSIRO): raster spatial data, WaterDyn and AWRA-L model parameters and radiation data

Geoscience Australia: Southwest Western Australia human-made waterbody feature class.

Data provider

Bureau of Meteorology (the Bureau).

Method

Run-off was calculated as the average of ‘discharge’ from the WaterDyn model and ‘streamflow’ from the AWRA-L model. These two model derived estimates were averaged because studies indicate that an ensemble of these two model outputs generally provide a better estimate of run-off than that given by either model individually (Bacon et al. 2010, Viney 2010).

The WaterDyn and AWRA-L models were run for the Australian continent using meteorological inputs from the Bureau’s National Climate Centre (NCC) and parameters from CSIRO. Gridded climate data used as model inputs included precipitation, temperature and solar radiation. WaterDyn and AWRA-L models were used to estimate the run-off depth for each grid-square within the region. Only run-off from the landscape is considered; therefore, the surface areas of the major reservoirs and the local catchment reservoirs were excluded from the analysis.

Run-off from the landscape is divided into two components: (i) run-off into the connected surface water system (major reservoirs, rivers and drains) and (ii) run-off into private reservoirs (local catchment reservoirs and other off-channel storages). Only run-off into the connected surface water system contributes to the total in this line item.

The average run-off depth from the landscape into the connected surface water store was estimated as the unweighted arithmetic mean run-off in the relevant model grid squares within the region boundary. Mean run-off depth was converted to a run-off volume by multiplying run-off depth by the total area of the region (excluding reservoirs).

Uncertainty

Uncertainty is ungraded.

Approximations, assumptions, caveats/limitations

• Only the run-off estimates were modelled and were not verified by real-time analysis of streamflow records. The reported value was an estimate of water that was likely to have entered the connected water system from the landscape.
• The run-off estimates were subject to the assumptions and limitations of the WaterDyn model detailed in Raupach et al. (2008) and the AWRA-L model detailed in van Dijk (2010). The current model deficiencies will be addressed over time through further development of the AWRA system including national calibration of AWRA-L, assimilation of satellite derived evapotranspiration/soil moisture and gauged streamflow and development of AWRA-R (Australian Water Resource Assessment system, river component).
• Where the volume of water intercepted by private reservoirs (local catchment reservoirs and other off-channel storages) has been calculated, the run-off estimates inherit the approximations, assumptions and caveats of the local catchment reservoir model (i.e. STEDI) and the parameters used.
• The calculation of the landscape run-off assigned each grid-square an equal contribution despite some grid-squares having a portion within or outside the reporting region boundaries. This had a limited influence because the average is then multiplied by the catchment area, which is not calculated based on the spatial dimensions of the model grid-cells.
• It is likely that a large proportion of run-off was lost to evaporation from the river, evaporation from the flood plain or drainage to groundwater. This is particularly applicable for terminal and semi-terminal rivers. These ‘run-off loss’ processes are not explicitly modelled in the MDB region.