18.1 Groundwater outflow to outside region
The volumetric value for the line item for the 2011–12 year was 19 ML. The value applies only to the Southern Basin of the Murray–Darling Basin (MDB) region.
The value reported, represents the lateral outflow from the MDB region from both Murray Limestone and Renmark Group aquifers along the region boundary near the Murray mouth. The value was estimated for sections of the MDB boundary that are not co-incident with the coastline.
Regional groundwater flow across the MDB region boundary was considered material:
- in aquifers underlying the MDB surface drainage outlet to the Southern Ocean
- to and from the Great Artesian Basin (GAB).
The Great Artesian Basin (GAB) aquifers underlying the north of the MDB region were not considered to be part of the region based on its definition. Therefore groundwater flow to the GAB was considered flow to outside the region. No methodology is currently available to assess this outflow. All the other boundaries were assumed no flow boundaries mostly representing a groundwater divide due to bedrock outcrop.
Groundwater flow from the Northern Basin to the Southern Basin was also considered negligible. This was due to the fact that the segment boundary mainly represents a groundwater divide and it is mainly composed of fractured rocks with a local flow system and therefore negligible regional flow.
(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.
Regional groundwater flow across the MDB regional boundary was only considered to be significant in the area near the mouth of the River Murray in South Australia. Inflow from the Northern Basin to the Southern Basin was considered minor and could not be quantified. Groundwater inflow was estimated for the unconfined aquifer (Murray Group Limestone and Parilla Sands) and confined aquifer (Renmark Group).
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 2011–12 year.
Assumptions, limitations, caveats and approximations
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 aquifer systems that cross the boundary of the MDB region.
It was assumed that no major groundwater flow occurs between the SDL areas of Northern Basin and the Southern Basin. This was due to the groundwater flow within the fractured rocks (Lachlan Fold Belt: Lachlan and Western, Kanmantoo Fold Belt, and Orange Basalt) being local, therefore regional flow was negligible. In addition, most of the central–eastern boundary represents a no flow boundary (groundwater divide). Regional groundwater inflow in the Western Porous Rocks is also small due to small area and possibly low gradients (resulting from low groundwater recharge). It was assumed that groundwater outflow from the Upper Darling Alluvium was small due to the small outflow area. Interaction with the river would be the main recharge/discharge process in the area, with regional lateral outflow being a secondary process. Inflows and outflows for the Southern Basin were assumed to occur at or near the coast line only; all the other boundaries were assumed no flow boundaries mostly representing a groundwater divide.
Groundwater levels in the unconfined aquifer were assumed to be 0 metres Australian height datum along the coastline.
Groundwater flow from the GAB to the MDB and groundwater abstraction from the GAB were not evaluated for the 2012 Account due to lack of data (although this vertical leakage is recognised to be important in some SDL areas).
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 a simple GIS analysis for the unconfined and confined aquifers, respectively. 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 (USGS 2013). 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.
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 increase for the 2010–11 year was 11 ML. The change applies only to the Southern Basin.