Murray–Darling Basin

13.3 Groundwater discharge to connected surface water

Supporting information

The volumetric value for the line item is 14,156 ML (see the following table for the breakdown of the volume).

Groundwater discharge to connected surface water in the Murray–Darling Basin (MDB) region for 2009–10
No.	Groundwater management unit (GMU)	State	Volume 2009–10 (ML)
1	Lower Gwydir Alluvium	NSW	–
2	Lower Lachlan Alluvium	NSW	700
3	Upper Lachlan Alluvium	NSW	–
4	Lower Macquarie Alluvium	NSW	4,300
5	Lower Murray Alluvium	NSW	–
Lower Murrumbidgee – sum composed of GMUs 6 & 7	2,216
6	Lower Murrumbidgee Deep Groundwater source	NSW
7	Lower Murrumbidgee Shallow Groundwater source	NSW
8	Mid Murrumbidgee Alluvium	NSW	3,230
9	Lower Namoi Alluvium	NSW	1,100
10	Upper Namoi Alluvium	NSW	2,610
Katunga–Campaspe – sum composed of GMUs 11–13	–
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	–
Lower Murray–Darling Basin GMUs – sum composed of GMUs 15–27	–
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	*14,156*

– = no data available

These discharge volumes have only been calculated (using New South Wales groundwater models) for those New South Wales GMUs where groundwater discharge to a river is considered to be an important component of groundwater balance. The table above indicates that groundwater discharge to rivers is only estimated for a select number of GMUs where the groundwater balance is represented in a groundwater model.

None of the groundwater models used here model fractured basement rock areas. In these areas, both surface run-off and the drainage of water below the modelled soil layers are 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 13.4.1 Rainfall run-off to connected surface water.

Quantification approach

Data source

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

Data provider

NSW Office of Water.

Method

Groundwater discharge to connected 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.

The MODFLOW river package is also used for groundwater recharge from connected surface water. 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.

Uncertainty

Uncertainty is ungraded. It is currently not feasible to estimate the uncertainty of modelled groundwater discharge to connected surface water from outputs of a MODFLOW groundwater model.

Approximations, assumptions, caveats/limitations

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.