Murray–Darling Basin
18.4 Discharge to surface water

Supporting information

The volumetric value for the line item for the 2011–12 year was 42,366 ML. The following table provides detailed volumetric information for the line item.

 

Groundwater discharge to surface water in the Murray–Darling Basin region in the 2011–12 year
Groundwater resource plan area

Groundwater sustainable diversion limit area  

State

 Volume (ML) for the 2011–12 year 

Code

Name

GW 14 Namoi Alluvium GS29 Lower Namoi Alluvium NSW

2,600

GS47 Upper Namoi Alluvium NSW

29,121

GW12 Macquarie–Castlereagh Alluvium GS26 Lower Macquarie Alluvium NSW

3,486

Sub-total Northern Basin

35,207

GW 10 Lachlan Alluvium GS25 Lower Lachlan Alluvium  NSW

300

GW 9 Murrumbidgee Alluvium GS28 Lower Murrumbidgee Alluvium (shallow; Shepparton formation and deep: Calivil formation and Renmark Group) NSW

572

GS31 Mid-Murrumbidgee Alluvium NSW

6,287

Sub-total Southern Basin

7,159

Total for the region

42,366

Groundwater discharge to surface water was only calculated for those New South Wales sustainable diversion limit (SDL) areas for which there was a groundwater model and groundwater discharge to a river was considered to be an important component of the groundwater balance. SDL areas other than included in the table exist and may be relevant to the line item; however, relevant information was not available for the 2012 Account.

None of the groundwater models used for estimations calculated volumes for fractured basement rock areas. In these areas, both surface runoff and the drainage of water below the soil layers normally included in the models were assumed to contribute to a total annual stream flow. This implicitly accounts for groundwater flow and river discharge in local groundwater flow systems that usually dominate these fractured rock areas. These discharge volumes are not represented in this line item, but contribute to the value in line item 9.4 Runoff to surface water.

Quantification approach

Data source

Data sources were inputs to the New South Wales groundwater models developed from the knowledge of the hydrogeology and surface water processes to initialise parameters for river bed conductance and thickness.

Provided by

New South Wales Office of Water.

Method

Groundwater discharge to surface water (mainly groundwater discharge to a river) can be represented in MODFLOW models in several ways. Options that have been used in the New South Wales groundwater models are the MODFLOW river package and the MODFLOW drain package (USGS 2013).

Groundwater flow into the river is modelled when groundwater levels are higher than river water levels and water flow is out of the river when river water levels are higher than groundwater levels.

MODFLOW also has a subroutine to represent drains. When this is activated and groundwater levels are above the base of the drain, water flow to the drain is estimated and this water volume is removed from the cell of the groundwater model.

Assumptions, limitations, caveats and approximations

Groundwater models make numerous assumptions and approximations to represent water balance (refer to the groundwater techniques, methods, and models in United States Geological Survey website for more details).

Estimates of water level in rivers that are input to groundwater models are usually taken to be monthly average levels, and the levels would usually have a high level of uncertainty unless a river gauge is located within the groundwater model cell.

Uncertainty information

The uncertainty estimate was not quantified. It is currently not feasible to estimate the uncertainty of modelled groundwater discharge to surface water from outputs of a MODFLOW groundwater model.

Comparative year

A scope change resulted in the restatement of the 2010–11 year volume. The scope change was due to inclusion of the volume for Lower Namoi Alluvium (not included in the 2011 Account) within the Northern Basin in the 2012 Account. Resulting volume increase was 2,517 ML for the 2010–11 year.