18.3 Discharge to landscape
The volume reported (99,677 ML) included discharge through evapotranspiration from shallow groundwater and is summarised by groundwater management area in the following table.
|Region||Groundwater discharge to landscape (ML)|
|McLaren Vale Prescribed Wells Area
|Western Mount Lofty Ranges (fractured rocks*)||15,606|
* Recharge from the fractured rocks of the Myponga River catchment and Fleurieu Peninsula was not included as described in the quantification approach section.
Groundwater discharge to the landscape was estimated using the Water Atmosphere Vegetation Energy and Solutes (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. Climate, depth to water table (only for the sedimentary areas), soil and vegetation data were used as inputs to the model. The climate data include rainfall, rainfall duration, maximum and minimum temperatures, vapour pressure deficit, and solar radiation.
The WAVES model has been used by the CSIRO in its Sustainable Yields projects (Crosbie et al. 2008) and the Bureau of Meteorology has built on this methodology. WAVES was run at selected points from across the Adelaide region for all combinations of soil type, vegetation type and depth to water table. The point estimates of the groundwater discharge (evapotranspiration from the water table) fraction for the 2010–11 year were interpolated to a 1-km grid based on soil type, vegetation type and depth to water table, and multiplied by a grid of annual rainfall for 2010–11.
The discharge to the landscape was determined by summing the spatially interpolated negative recharge estimates.
The following figure illustrates the net groundwater discharge (in red) and recharge (in grey) across the Adelaide region during the 2010–11 year using the WAVES model.
Net groundwater recharge and discharge in the Adelaide region during the 2010–11
Assumptions, Limitations, Caveats and Approximations
- The assumptions made in developing the WAVES model as described in Dawes et al. (1998) were all applicable to the recharge estimations for the Adelaide region.
- The national land-use grid (Australian Bureau of Agricultural and Resource Economics – Bureau of Rural 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.
- Annual net recharge and discharge was estimated for the whole of Adelaide region including both sedimentary and fractured rock areas. Recharge and discharge to the sedimentary areas was modelled given the effects of a shallow water table interpolated using topo-to-raster methodology. For fractured rocks the shallow water table was interpolated using kriging with an external drift 9" Digital Elevation Model following the methodology presented in Peterson et al. (2011).
- Only the fractured rocks area contributing to the flow in the confined sediments was considered (see fractured rock boundaries in figure) for calculation of the diffuse groundwater discharge to landscape. This was done to maintain the groundwater balance.
This line item corresponds to line item 16.3.1 Groundwater discharge to landscape reported in the National Water Account 2010. In 2010 this item was not quantified; however in 2011 a suitable quantification approach enabled this item to estimated. The restated comparative year volume for groundwater discharge to landscape was 41,775 ML.