Murray-Darling Basin
18.4 Discharge to surface water

Supporting Information

The volumetric value for the line item for the 2010–11 year was 13,211 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 2010–11 year
Groundwater resource plan area

Groundwater sustainable diversion limit area  


 Volume (ML) 



GW15 Namoi Alluvium GS51 Upper Namoi Alluvium NSW


GW12 Macquarie–Castlereagh Alluvium GS31 Lower Macquarie Alluvium NSW


Sub-total Northern Basin


GW10 Lachlan Alluvium GS30 Lower Lachlan Alluvium NSW


GW9 Murrumbidgee Alluvium GS33 Lower Murrumbidgee Alluvium (shallow: Shepparton Formation and deep: Calivil Formation and Renmark Group) NSW


GS36 Mid-Murrumbidgee Alluvium NSW


Sub-total Southern Basin


Total for the region



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 2011 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

NSW Office of Water.


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 (United States Geological Survey 2011).

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 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

The value reported as comparative year corresponds to the line item 13.3 Groundwater discharge to connected surface water that was reported in the 2010 Account.