Murray-Darling Basin
18.4 Discharge to surface water

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		State	Volume (ML)
	Code	Name
GW15 Namoi Alluvium	GS51	Upper Namoi Alluvium	NSW	1,390
GW12 Macquarie–Castlereagh Alluvium	GS31	Lower Macquarie Alluvium	NSW	3,151
Sub-total Northern Basin				4,541
GW10 Lachlan Alluvium	GS30	Lower Lachlan Alluvium	NSW	660
GW9 Murrumbidgee Alluvium	GS33	Lower Murrumbidgee Alluvium (shallow: Shepparton Formation and deep: Calivil Formation and Renmark Group)	NSW	410
	GS36	Mid-Murrumbidgee Alluvium	NSW	7,600
Sub-total Southern Basin				8,670
Total for the region				13,211

 

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.

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.

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.

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.

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.

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.