10.3 Recharge from landscape
The volume presented in the water accounting statements (877,331 ML) represents the volume of groundwater recharge from the landscape in the Melbourne region during the 2010–11 year.
Recharge to groundwater is strongly linked with rainfall in the region. Most of the groundwater recharge occurs in the fractured rock aquifers in the north, northwest and northeast of the region.
Groundwater recharge was estimated using the WAVES model (Zhang and Dawes 1998; Dawes et al. 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 recharge. 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 recharge 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 recharge value can be positive or negative, due to evapotranspiration. Recharge was determined by summing the pixels with a negative estimate (grey areas).
WAVES model recharge areas
Assumptions, Limitations, Caveats and Approximations
- Assumptions made when developing the WAVES model (Dawes et al.1998) are all applicable to the recharge 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.
- Recharge was estimated to be within the clipped sedimentary area, considering the effects of shallow water tables.
- Diffuse recharge 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 recharge values have not been studied.
- The uncertainty of the estimated recharge resulting from different recharge interpolation methods is not estimated.
This item corresponds to line item 15.2.1 Diffuse groundwater recharge from landscape water reported in the 2010 Account. In the 2011 Account, the methodology used to quantify this item was improved.
In the 2010 Account, the method yielded values that were off-set, not in accordance with the Exposure Draft of Australian Water Accounting Standard 1. 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 is 143,063 ML.