18.1 Groundwater outflow to outside region
The volume reported (405 ML) represents regional groundwater outflow from the Adelaide region at the northern boundary, through the major sedimentary aquifers in the Adelaide Plains (T1 and T2 aquifers). This represents the only groundwater outflow from the region, except from that occurring at the coast (reported at line 18.2 Groundwater outflow to outside region at coast).
Regional groundwater flow also occurs between the fractured rock aquifers and the sedimentary aquifers of the Adelaide Plains and McLaren Vale prescribed wells area (PWA); however, this flow is within the Adelaide reporting region and therefore is not included in the volume reported here. The Adelaide region boundary along the Mount Lofty Ranges is considered a groundwater divide with no groundwater flow across this divide. In general, groundwater flows from the fractured rock aquifers of the Mount Lofty Ranges west towards the Adelaide Plains and St Vincent Basin (Ellis and Fox 2004).
|Groundwater management area||Aquifer||Groundwater outflow (ML)|
South Australian Department of Environment, Water and Natural Resources (DEWNR): Drillhole Enquiry System; Aquaterra 2011.
Regional groundwater flow out of the Adelaide region was calculated using a simple Geographic Information System approach based on Darcy's Law (Bureau of Meteorology 2010). Groundwater levels measured at monitoring bores were interpolated to a groundwater-level grid for each season during the 2011–12 year using the ArcGIS Topo-to-Raster tool. Seasonal groundwater-flow grids were then derived from these groundwater-level grids along with aquifer thickness and hydraulic conductivity data using a modification of the ArcGIS Darcy velocity tool. Groundwater outflow was subsequently calculated across selected regional groundwater boundaries in the Adelaide Plains using a simple GIS analysis. Seasonal inflow volumes were summed to determine the total volume for the 2011–12 year.
The following two maps are presented to aid interpretation of the methodology used to quantify this item. The first map illustrates the location and concentration of bores used in the quantification approach.
Map showing the location and concentration of bores used in the quantification approach
The second map identifies the lateral groundwater flow boundaries used in the quantification approach. The purple line at the north of the Adelaide region illustrates the area across which groundwater outflow to outside region was calculated.
Map of the lateral groundwater flow boundaries in the Adelaide region
Assumptions, limitations, caveats and approximations
- Flow across the remaining unit boundaries was assumed to be small on an annual basis, either due to limited flow (e.g. in fractured rock) or because limitations of the method prevented its application.
- Regional groundwater outflow was estimated for the confined and semi-confined aquifers only (T1, T2). These productive aquifers are considered to be the most hydraulically conductive units and flow in other units is assumed to be insignificant.