18.3 Discharge to landscape
The volume presented in the water accounting statement (16,516 ML) represents groundwater discharge via evapotranspiration from shallow water tables during the 2010–11 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 of Meteorology 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 discharge 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 Sciences' 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 tables.
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 is not estimated.
This item corresponds to line item 16.3.1 Groundwater discharge to landscape from landscape water reported in the 2010 Account. This line item was unquantified in the 2010 Account. In the 2011 Account, the methodology used to quantify this item was improved and a volume included.
In the 2010 Account, the method yielded values that were off-set, which is not permitted by the Australian Water Accounting Standards; therefore, the figure was recorded as unquantifiable. In the 2011 Account, the method was corrected so that it yielded values of recharge (line item 10.3) and discharge (line item 18.3) that are not offset.
The value published in the 2010 Account was not recalculated using this improved quantification approach as these data were unavailable. The comparative year volume was unquantified.