Murray–Darling Basin
18.3 Discharge to landscape

The volumetric value for the line item for the 2012–13 year was 2,733,597 ML. The line item represents discharge from aquifers to landscape that includes soil, unconnected lakes and wetlands. The following tables provide detailed volumetric information for the line Item.

Sustainable diversion limit (SDL) areas other than those included in the detail table exist and may be relevant to the line item; however, relevant information was not available for the 2013 Account.

 (1) NSW Office of Water: inputs to the New South Wales groundwater models draw from knowledge of the hydrogeology and surface water processes and outputs from same models for sustainable diversion limit (SDL) areas Lower Lachlan Alluvium, Lower, and Mid-Murrumbidgee Alluviums and Upper Namoi Alluvium; (2) for the other SDL areas: (a) Bureau of Meteorology (the Bureau): National Climate Centre (NCC) daily climate grids (rainfall, temperature, vapour pressure and solar radiation), (b) Australian soil resources information system (ASRIS): soil data, and (c) Australian Bureau of Agricultural and Resource Economics and Sciences: land-use data.

The Bureau method (For SDL areas other than Lower Lachlan Alluvium, Lower and Mid Murrumbidgee Alluviums and Upper Namoi Alluvium)

Groundwater discharge was estimated along with diffuse groundwater recharge using the water atmosphere vegetation energy and solutes (WAVES) model (Zhang and Dawes 1998; Dawes et al. 1998). In the recharge calculations, depth to watertable was considered for all regions where the depth to watertable was shallow. A shallow watertable was assumed to be where the depth to watertable was 4 m or less below the ground surface. The shallow water table was interpolated using kriging with an external drift and the 9" digital elevation model as a physical constraint  following the methodology presented in Peterson et al. (2011). Where the watertable was not shallow, free drainage conditions were assumed.

For SDL areas with a shallow watertable, the model may produce a net discharge from groundwater over the calculation period.

New South Wales groundwater models method (For SDL areas Lower Lachlan Alluvium, Lower and Mid- Murrumbidgee Alluviums and Upper Namoi Alluvium)

Discharge was calculated for selected SDL areas by applying New South Wales groundwater models where the MODFLOW (USGS 2013) evapotranspiration routines were activated to represent groundwater discharge.

The Bureau method:

• The assumptions made in developing the WAVES model as described in Dawes et al. (1998) were all applicable to the discharge estimations carried out for this line item.

• The national land use grid (Australian Bureau of Agricultural and Resource Economics and Sciences 2010) was reclassified to three vegetation classes that include annuals, perennials, and trees.  The major vegetation classes modelled were C3 annual pasture, C3 perennial pasture and eucalypt trees with a grass understorey for SDL areas to the south of 31^oS, and C4 annual pasture, C4 perennial pasture and eucalypt trees with a grass understorey for the SDL areas to the north of 31^oS.

• Annual discharge was estimated using a shallow watertable surface estimated by interpolating measured groundwater levels.

New South Wales groundwater models:

• Groundwater models make many assumptions and approximations to represent water balance (refer to the United States Geological Survey website for more details).

The uncertainty estimate was not quantified.

For the Bureau method, the uncertainty in the input parameters and the corresponding impacts on the modelled discharge values were not studied and the uncertainty of the estimated discharge resulting from different interpolation methods was not estimated.

For New South Wales groundwater models, uncertainty of a discharge estimate was not evaluated for the groundwater models.

A prior period error and a change made to the calculation method resulted in the restatement of the 2011–12 year volume. The respective volumes associated with the change are detailed in the following table.

The volume estimated for the comparison year for the 2013 Account is higher than the volume reported for the 2012 Account. This was due to the following reasons:

• There was a prior period error in the line item volume reported in the 2012 Account. The prior period error was a result of using solar radiation data with bias correction errors. Solar radiation is an input for line item volume calculation.
• The difference between the previously reported volume and the estimate produced for the comparison year can be partly attributed to the choice of different bore hole locations for measured groundwater levels (data from same set of bore holes used in the 2012 Account were not available for calculations).