Murray-Darling Basin
18.1 Groundwater outflow to outside region

The volumetric value for the line item for the 2010–11 year was 3 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 at 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.

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 flux. All the other boundaries were assumed no-flow boundaries, mostly representing a groundwater divide.

(1) The Drillhole Enquiry System of South Australian Department for Water: Bore locations and groundwater level data in South Australia, (2) Department of Sustainability and Environment (DSE) and Victorian Department of Primary Industries (DPI) through a database developed by the University of Melbourne: Bore locations and groundwater level data in Victoria, and (3) Murray–Daring Basin Authority: The boundaries of the sustainable diversion limit (SDL) areas.

Bureau of Meteorology (the Bureau).

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 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.

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 2011 Account due to lack of data (although this vertical leakage is recognised to be important in some SDL areas).

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).

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 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 16.1.1 Regional groundwater outflow, 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 18.1 Groundwater outflow to outside region

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)
16.1.1 Regional groundwater outflow	Northern Basin	0	0	0
	Southern Basin	53	–52	1
	Whole region	53	–52	1

 

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.