Murray–Darling Basin
10.2 Groundwater inflow from outside region at coast

The volumetric value for the line item for the 2012–13 year was 50 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.

(1) South Australia's Department of Environment, Water and Natural Resources: bore locations and groundwater level data in South Australia from the Drillhole Enquiry System; (2) Victorian Department of Environment and Primary Industries: bore locations and groundwater level data in Victoria; (3) National Groundwater Information System: assigning aquifers for bores; and (4) Murray–Darling Basin Authority: boundaries of the sustainable diversion limit (SDL) areas.

Bureau of Meteorology (the Bureau).

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 – Murrayville water supply protection area 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 2012–13 year.

Regional flow estimations were provided for the Murray Group Limestone Aquifer, which was chosen to represent the unconfined aquifer and the Renmark Group Aquifer. These were considered to be the main coastal aquifer systems that cross the boundary of the MDB region.

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 a simple GIS analysis. 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 2013). The results from the two methodologies indicated a 6–7% 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.