10.2 Groundwater inflow from outside region at coast
The volumetric value for the line item for the 2010–11 year was 62 ML. The value applies only to the Southern Basin of the Murray–Darling Basin (MDB) region.
The regional groundwater flow at the coastline into the MDB region is a sum of flow along the Murray mouth through the major sedimentary aquifers. The flow was considered to occur in the Murray Group Limestone Aquifer and the Renmark Group Aquifer where these aquifers cross the vertical cross-section at the coastline (see the map available in line item 10.1 Groundwater inflow from outside region). The inflow may be either fresh or saline groundwater.
Groundwater flow was calculated using a simple geographic information system (GIS) approach based on Darcy's Law. Groundwater levels were interpolated for seasons using the ArcGIS Topo-to-Raster tool from reduced groundwater levels measured at monitoring bores.
The Geofabric version 2.0 (Bureau of Meteorology 2011a) was used to estimate aquifer thickness. The hydraulic conductivity values were sourced from the Mallee Prescribed Wells Area (PWA) – Murrayville Water Supply Protection Area (WSPA) Groundwater Model, Department of Water, Land and Biodiversity Conservation, South Australia (Barnett and Osei-bonsu 2006). The transmissivity values were calculated by multiplying the aquifer thickness with the relevant hydraulic conductivity.Seasonal groundwater flow-grids were derived from groundwater level grids, aquifer thickness and hydraulic conductivity using a modification of the ArcGIS Darcy Velocity tool. Groundwater flow across selected flow boundaries was then calculated using a simple GIS analysis and seasonal values were aggregated for the 2010–11 year.
Assumptions, Limitations, Caveats and Approximations
The uncertainty estimate was not quantified.
The uncertainty in the field-measured data (e.g. groundwater levels, hydraulic conductivity) was not specified and unknown and hence, the impacts of such uncertainty on the calculated groundwater flow were not estimated.
The regional flow estimations were based on the interpolated groundwater level grids produced using ArcGIS Topo-to-Raster tool. Use of different interpolation methods may impact on the values of the groundwater level grids and hence the estimated regional flow. However, a comparison of this methodology was carried out using a simple groundwater flow model developed on MODFLOW model (United States Geological Survey 2011). The results from the two methodologies indicated a 6% to 7% difference.
Groundwater flow was estimated for a simplified boundary constructed from a series of line segments. Groundwater flow across this boundary was calculated using the method described above. The uncertainty surrounding this simplification was not analysed.
This line item corresponds to line item 15.1.2 Sea water intrusion into aquifers reported in the 2010 Account.
The following change in the 2011 Account occurred, which did not cause the 2009–10 year value to be restated:
Improved data sets were made available after the publication of the 2010 Account. The value published in the 2010 Account was not restated as the difference was not material: the slight increase in the accuracy of the information was not likely to influence the decisions made by users of the National Water Account.