Murray-Darling Basin
18.2 Groundwater outflow to outside region at coast

The volumetric value for the line item for the 2010–11 year was 1,663 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) The Drillhole Enquiry System of South Australian Department for Water: Bore locations and groundwater level data in South Australia and (2) Murray–Daring Basin Authority: The 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 2010–11 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 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 ArcGIS Topo-to-Raster tool. 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 (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 16.1.2 Groundwater discharge into sea, reported in the 2010 Account. In the 2011 Account, the following change was made that caused the 2009–10 year value to be restated:

• The methodology used to quantify the line item was improved and resulted in a material change in volume.

The changes and their respective values are detailed in the following table.

 

Restatement of comparative year information made for the line item 18.2 Groundwater outflow to outside region at coast

2010 Account line items	Segment	Volume for the 2009–10 year reported in the 2010 Account (ML)	Difference due to change in the calculation method (ML)	Volume for the 2009–10 year reported in the 2011 Account (ML)
16.1.2 Groundwater discharge into sea	Northern Basin	0	0	0
	Southern Basin	2,117	–449	1,668
	Whole region	2,117	–449	1,668

 

Change in the calculation method was due to the following reason:

• Regional flow values were estimated based on the sets of bores and the transmissivity values based on the method described above for the 2011 Account. These values were not same from the corresponding values provided in the 2010 Account because the sets of bores and transmissivity values used for the two accounts are not same. The sets of bores were not same because data for some of the bores used in the 2010 Account was not available for the 2011 Account.