18.3 Discharge to landscape
The volume presented in the water accounting statement (18,379 ML) represents groundwater discharge via evapotranspiration from shallow water tables during the 2012–13 year.
Most of the groundwater discharge occurs from areas with shallow water tables in the south, southeast and southwest of the region.
Groundwater discharge was estimated using the WAVES model (Zhang and Dawes 1998). WAVES is a one-dimensional soil–vegetation–atmosphere–transfer model that integrates water, carbon, and energy balances with a consistent level of process detail. The input datasets required for WAVES include climate, depth to water table, soil, and vegetation data. The clipped sedimentary area was selected to estimate net discharge. The climate data used at selected points include rainfall, rainfall duration, maximum and minimum temperatures, vapour pressure deficit, and solar radiation. The relevant vegetation parameters required for modelling were selected from the WAVES user manual (Dawes et al. 1998). WAVES uses the soil hydraulic model of Broadbridge and White (1998) with saturated hydraulic conductivity, saturated moisture content, residual moisture content, inverse capillary length scale, and an empirical constant as input parameters calculated from soil properties accessed in the ASRIS database (Johnston et al. 2003).
The WAVES model has been used by CSIRO in its sustainable yields projects (Crosbie et al. 2008) and the Bureau built on this methodology. WAVES was run for all combinations of soil, vegetation, and depth to water table at each climate point. A groundwater discharge value was estimated for each 1 km × 1 km pixel across the region using annual rainfall, dominant soil and vegetation, and depth to water table. This discharge value can be positive or negative, due to evapotranspiration. Discharge within the Melbourne region was determined by summing the pixels with a positive estimate (red areas).
WAVES model dicharge areas
Assumptions, limitations, caveats and approximations
- Assumptions made when developing the WAVES model (Dawes et al. 1998) are all applicable to the discharge estimations carried out for the Melbourne region.
- The Bureau of Rural Science's land use map of the Melbourne region was reclassified to three vegetation classes: annuals, perennials and trees. The major vegetation classes modelled are C3 annual pasture, C3 perennial pasture, and eucalypt trees with a grassy understorey.
- Discharge was estimated to be within the clipped sedimentary area, considering the effects of shallow water table interpolated using kriging with external drift and the 9" Digital Elevation Model as an external driver following the methodology presented in Peterson et al. (2011).
- Diffuse discharge to groundwater from irrigation applied to the landscape is not included in the estimate.
The uncertainty in the input parameters and the corresponding impacts on the modelled discharge values have not been studied.
The uncertainty of the estimated discharge resulting from different discharge interpolation methods was not estimated.