18.2 Groundwater outflow to outside region at coast
The volumetric value for the line item for the 2011–12 year was 1,551 ML. The value applies only to the Southern Basin of the Murray–Darling Basin (MDB) region.
The regional groundwater flow at the coastline out of the MDB region is a sum of flow along the Murray mouth through the major sedimentary aquifers. The outflow 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.
(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 (DEPI): bore locations and groundwater level data in Victoria; (3) National Groundwater Information System (NGIS): assigning aquifers for bores; and (4) Murray–Darling Basin Authority: boundaries of the sustainable diversion limit (SDL) areas.
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 of South Australian Department of Water, Land and Biodiversity Conservation (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 2011–12 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 a simple GIS analysis. Use of different interpolation methods may have an 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 (USGS 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.
In the 2012 Account, the set of bores used (based on the data availability) for calculations was changed that caused the 2010–11 year value to be restated. The set of bores used for 2011–12 year calculation was used for 2010–11 year calculations to have consistent volumes for both years. Resulting volume decrease for the 2010–11 year was 38 ML. The change applies only to the Southern Basin.