Melbourne
18.3 Discharge to landscape

Supporting information

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.

Quantification approach

Data source

The Bureau of Meteorology National Climate Centre (NCC) Version 3 daily rainfall grids, Version 3 annual rainfall grids, daily maximum temperature grids, daily minimum temperature grids, daily satellite observed solar radiation grids and daily vapour pressure deficit grids; CSIRO: Australian Soil Resources Information System (ASRIS) soil information; Bureau of Rural Sciences Water 2010 land use mapping; Victorian Department of Environment and Primary Industries: bore locations, groundwater level data and aquifer attribution.

Provided by

The Bureau of Meteorology.

Method

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
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.

 

Uncertainty information

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.

 

Comparative year

The comparative year volume for line item 18.3 Discharge to landscape was restated in the water accounting statements from the 2012 Account due to an error identified in the method used to calculate this volume. Please refer to Restatement of comparative year information in the 'Water accounting policies' for more information.