Adelaide
18.1 Groundwater outflow to outside region

Supporting information

The volume reported (510 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 Gulf St Vincent (Ellis and Fox 2004). 

Regional groundwater inflow to the Adelaide region, broken down by aquifer in the 2012–13 year

Groundwater management area

Aquifer

Groundwater outflow (ML)

Adelaide Plains

T1

157

T2

353

Total

510


Quantification approach

Data source

South Australian Department of Environment, Water and Natural Resources (DEWNR): Drillhole Enquiry System; Aquaterra 2011.

Provided by

Bureau of Meteorology.

Method

Regional groundwater flow out of the Adelaide region was calculated using a simple Geographic Information System (GIS) approach based on Darcy's Law (Bureau of Meteorology 2010). A set of bores with current data was selected, including any bores within 20 km of the Adelaide region boundary.  Where no bores were present close to the boundary, the interpolated groundwater level surface was extended to the boundary and used to estimate groundwater flow across the boundary (this flow is small in magnitude).

Groundwater levels measured at monitoring bores were interpolated to a groundwater-level grid for each season during the 2012–13 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 2012–13 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
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 lateral groundwater flow boundaries in the Adelaide region
Map of 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.

Uncertainty information

The uncertainty estimate was not quantified.