18.4 Discharge to surface water
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 resource plan area||
Groundwater sustainable diversion limit area
Volume (ML) for the 2011–12 year
|GW 14 Namoi Alluvium||GS29||Lower Namoi Alluvium||NSW||
|GS47||Upper Namoi Alluvium||NSW||
|GW12 Macquarie–Castlereagh Alluvium||GS26||Lower Macquarie Alluvium||NSW||
|Sub-total Northern Basin||
|GW 10 Lachlan Alluvium||GS25||Lower Lachlan Alluvium||NSW||
|GW 9 Murrumbidgee Alluvium||GS28||Lower Murrumbidgee Alluvium (shallow; Shepparton formation and deep: Calivil formation and Renmark Group)||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 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.
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.
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.
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.