Murray–Darling Basin

The volumetric value for the line item is 2,578 ML. Regional groundwater flow across the Murray–Darling Basin (MDB) reporting region boundary is only considered important in aquifers underlying the MDB surface drainage outlet to the Southern Ocean. (Note that groundwater flows in the Great Artesian Basin aquifers underlying the north of the MDB are not considered to be part of the water balance for the MDB region.)

Data source

Bore locations and groundwater level data in South Australia were sourced from the Water Regulations 2008 (category 2A) information returns provided by Department of Water South Australia (Provider Code W0078) to the Bureau of Meteorology.

Bore locations and groundwater level data in Victoria were sourced from the Department of Sustainability and Environment (DSE) through a database developed by the University of Melbourne (it is possible that small differences between the University of Melbourne database and the DSE groundwater database occur).

The geographic information system (GIS) data relating to the boundaries of the aquifers and Groundwater Management Units (GMUs) were extracted from the Interim Groundwater Geodatabase developed for the Bureau of Meteorology by Sinclair Knight Merz (SKM 2010a).

Data provider

Bureau of Meteorology.

Method

Regional groundwater flow across the MDB regional boundary is only considered to be significant in the area near the mouth of the Murray River in South Australia. The boundary through which groundwater flow is estimated is highlighted in the figure below. Groundwater flow was estimated for Murray Group Limestone and Renmark Group Aquifers that underlie this boundary but not including the boundary lengths co-incident with the coastline (flow across this boundary length is estimated in 15.1.2).

Groundwater flow was calculated using a simple 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. 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 (see the following figure) was then calculated using a simple GIS analysis and seasonal values were aggregated to the 2009–10 reporting period.

The surface water outlet of the Murray–Darling Basin region including the Murray River mouth and upstream (features shown include the boundary across which groundwater through-flow is calculated and groundwater observation bores used to estimate this flow)

Uncertainty

The uncertainty is ungraded.

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.

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.

Approximations, assumptions, caveats/limitations

• Regional flow estimations are provided for the Murray Group Limestone Aquifer subsystem and Renmark Group Aquifer. These are considered to be the main aquifer systems that cross the boundary of the MDB account region.
  
  
• The MDB Groundwater Status Report 2000–2005 (MDBC 2008a) estimates that regional groundwater flow systems in alluvial aquifers in the area of the lower MDB typically have a transmissivity of 100 m² per day. As no other information is available, this value of transmissivity is assumed for both the Murray Group Limestone Aquifer and Renmark Group Aquifer.
  
  
• Groundwater levels are assumed to be 0 metres Australian Height Datum along the coastline.
  
  
• Groundwater flow from the Great Artesian Basin to the MDB and groundwater abstraction from the Great Artesian Basin (GAB) are not evaluated for the National Water Account 2010, except in the Lower Namoi groundwater model where leakage upwards from the GAB is estimated (see Line item 15.1.3 Other lumped groundwater inflow from outside region).
  
  
• It is possible that small differences occur between the University of Melbourne database and the DSE groundwater database (from which bore locations and groundwater level data in Victoria were sourced).