Murray–Darling Basin

16.3.1 Groundwater discharge to landscape

Supporting information

The volumetric value for the line item is 28,130 ML (see the following table for the breakdown of the volume).

Volumes of groundwater discharge to landscape in the Murray–Darling Basin (MDB) in 2009–10
No.	Groundwater management unit (GMU)	State	Volume 2009–10 (ML)	Quantification method
1	Lower Gwydir Alluvium	NSW	0	BoM method
2	Lower Lachlan Alluvium	NSW	7,620	NSW models
3	Upper Lachlan Alluvium	NSW	0	BoM method
4	Lower Macquarie Alluvium	NSW	0	NSW models
5	Lower Murray Alluvium	NSW	0	BoM method
Lower Murrumbidgee – sum composed of GMUs 6 & 7			13,587	NSW models
6	Lower Murrumbidgee Deep Groundwater source	NSW
7	Lower Murrumbidgee Shallow Groundwater source	NSW
8	Mid Murrumbidgee Alluvium	NSW	6,923	NSW models
9	Lower Namoi Alluvium	NSW	0	NSW models
10	Upper Namoi Alluvium	NSW	0	NSW models
Katunga–Campaspe – sum composed of GMUs 11–13			0	BoM method
11	Campaspe Deep Lead Water Supply Protection Area	Vic
12	Katunga Water Supply Protection Area	Vic
13	Shepparton Irrigation Water Supply Protection Area	Vic
14	Mid Loddon Water Supply Protection Area	Vic	0	BoM method
Lower Murray–Darling Basin GMUs – sum composed of GMUs 15–27			0	BoM method
15	Balrootan (Nhill) Groundwater Management Area	Vic
16	Goroke Groundwater Management Area	Vic
17	Kaniva TCSA Groundwater Management Area	Vic
18	Murrayville Water Supply Protection Area	Vic
19	Nhill Groundwater Management Area	Vic
20	Telopea Downs Water Supply Protection Area	Vic
21	Angas–Bremer Prescribed Wells Area	SA
22	Coorong	SA
23	Ferries–McDonald	SA
24	Mallee Prescribed Wells Area	SA
25	Murraylands	SA
26	Peake, Roby and Sherlock Prescribed Wells Area	SA
27	River Murray Prescribed Water Course	SA
	Total Basin		28,130

Quantification approach

Data source

Bureau of Meteorology method (see the above table entitled ‘Volumes of groundwater discharge to landscape in the Murray–Darling Basin for 2009–10’ for the list of GMUs concerned)

The method used in the MDB assumes that there is no diffuse groundwater discharge to the landscape. No inputs required.

New South Wales groundwater models (see the above table entitled ‘Volumes of groundwater discharge to landscape in the Murray–Darling Basin for 2009–10’ for the list of GMUs concerned)

Diffuse groundwater discharge to the landscape can be estimated in a groundwater model using the ‘evapotranspiration’ (ET) subroutine. Input parameters required for this subroutine include the maximum possible groundwater ET and the ‘extinction depth’; if the water table goes below this depth there is no groundwater discharge.

Data provider

NSW Office of Water.

Method

Diffuse groundwater discharge to the landscape can be estimated in a MODFLOW groundwater model using the ET subroutine. Input parameters required for this subroutine include the maximum possible groundwater ET and the ‘extinction depth’. If the water table drops below the extinction depth there is no groundwater discharge, and if it rises above this depth there is a linear relationship between the depth to water table and the discharge of groundwater.

Diffuse groundwater discharge is represented in some New South Wales groundwater models using this MODFLOW subroutine.

Uncertainty

Bureau of Meteorology method: Ungraded.
New South Wales groundwater models: Uncertainties are not evaluated by the groundwater models.

Approximations, assumptions, caveats/limitations

For Bureau of Meteorology method:

The groundwater recharge estimates were made assuming free drainage conditions at the base of the soil profile in the MDB. This approach assumes that diffuse groundwater discharge is zero.

For New South Wales groundwater models:

Groundwater models make numerous assumptions and approximations to represent water balance components (refer to the United States Geological Survey website for more details).
Some New South Wales groundwater models do not allow diffuse groundwater discharge to occur (the ET subroutine is not activated in the model).
The NSW groundwater models that do represent diffuse groundwater discharge use the standard MODFLOW model representation of the process summarised above. This representation is an approximate conceptual model of the discharge process, which is not well understood.