Adelaide
18.1 Groundwater outflow to outside region

Supporting Information

The volume reported (390 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 at that occurring at the coast (reported at line 18.2).

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).


Region groundwater outflow from the Adelaide region in the 2010–11 year: broken down by aquifer
Groundwater management area Aquifer Groundwater outflow (ML)
Adelaide Plains
T1 65
T2 325
Total 390

Quantification Approach

Data Source

South Australian Department for Water Drillhole Enquiry System; Aquaterra 2011.

Provided by

Bureau of Meteorology

Method

Regional groundwater flow out of the Adelaide region was calculated using a simple GIS 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 2010–11 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 2010–11 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 throughout the McLaren Vale Prescribed Wells Area and Adelaide Plains used in the quantification approach.

Map showing location and concentration of bores used in the quantification approach
Map showing 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 blue 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 semiconfined 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.

Comparative year

This line item corresponds in part to line item 16.1.1 Regional groundwater outflow reported in the 2010 Account. In the 2011 Account, the following changes were made that caused the 2009–10 year value to be restated:

  • The presentation of the line items has changed since the 2010 Account: Groundwater outflow was partitioned into groundwater outflow at the coast (reported at line item 18.2) and regional groundwater outflow for the rest of the Adelaide region.
  • The methodology used to quantify the line item was improved as new aquifer properties have been used in the quantification approach.
Restatement of the value published in the 2010 Account was made as the difference was material and increased the accuracy of the information provided to the users of the National Water Account. The changes and their respective values are detailed in the following table. The restated comparative year volume is 408 ML.

Restatement of comparative year information for line item 18.1 Groundwater outflow to outside region
2010 Account line item name Groundwater area Aquifer Volume at 30 June 2010 reported in 2010 Account (ML) Value difference to volume reported due to reclassification (ML) Value difference to volume reported due to change in methodology (new aquifer properties)  (ML) Volume at 30 June 2010 reported in 2011 Account (ML)
16.1.1 Regional groundwater outflow
McLaren Vale PWA Maslin Sands 9 (9)   0
Port Willunga Formation 239 (239)   0
Adelaide Plains
T1 2183 (1,725) (354) 94
T2 0   314 314
Total 2,431 (1,973) (13) 408