Murray–Darling Basin
18.3 Discharge to landscape

The volumetric value for the line item for the 2011–12 year was 617,753 ML. The line item represents discharge from aquifers to landscape that includes soil, unconnected lakes and wetlands. The following tables provide detailed volumetric information for the line Item.

Details of groundwater discharge to landscape in the MDB region in the 2011–12 year

Groundwater resource plan area	Groundwater sustainable diversion limit area		State	Volume (ML) for the 2011–12 year
	Code	Name
GW 15 Gwydir Alluvium	GS24	Lower Gwydir Alluvium	NSW	0
GW 14 Namoi Alluvium	GS47	Upper Namoi Alluvium	NSW	7,104
Sub-total Northern Basin				7,104
GW 10 Lachlan Alluvium	GS25	Lower Lachlan Alluvium	NSW	6,220
GW 9 Murrumbidgee Alluvium	GS28	Lower Murrumbidgee Alluvium (shallow; Shepparton formation)	NSW	15,664
	GS31	Mid-Murrumbidgee Alluvium	NSW	13,593
GW8 Murray Alluvium	GS27	Lower Murray Alluvium, shallow; Shepparton formation	NSW	90,161
	GS46	Upper Murray Alluvium	NSW	55,234
GW2 Goulburn–Murray	GS8	Goulburn–Murray (Ovens–Kiewa sedimentary plain)	Vic	1,026
	GS8	Goulburn–Murray (Victorian Riverine sedimentary plain)	Vic	293,430
GW3 Wimmera–Mallee (groundwater)	GS9	Wimmera–Mallee (sedimentary plain)	Vic	538
GW5 Eastern Mount Lofty Ranges	GS01	Angas Bremer (Quaternary sediments and Murray Group limestone)	SA	6,419
GW4 SA Murray	GS03	Mallee (Pliocene Sands and Murray Group limestone)	SA	9,348
	GS05	Peake–Roby–Sherlock (unconfined and confined)	SA	119,016
Sub-total Southern Basin				610,649
Total for the region				617,753

Sustainable diversion limit (SDL) areas other than those included in the detail table exist and may be relevant to the line item; however, relevant information was not available for the 2012 Account.

(1) NSW Office of Water: inputs to the New South Wales groundwater models draw from knowledge of the hydrogeology and surface water processes and outputs from same models for SDL areas Lower Lachlan Alluvium, Lower and Mid Murrumbidgee Alluviums and Upper Namoi Alluvium; (2) For the other SDL areas (a) Bureau of Meteorology (the Bureau): National Climate Centre (NCC) daily climate grids (rainfall, temperature, vapour pressure and solar radiation), (b) Australian soil resources information system (ASRIS): soil data, and (c) Australian Bureau of Agricultural and Resource Economics and Sciences: land-use data.

The Bureau and NSW Office of Water.

The Bureau method (For SDL areas other than Lower Lachlan Alluvium, Lower and Mid Murrumbidgee Alluviums and Upper Namoi Alluvium)

Groundwater discharge was estimated along with diffuse groundwater recharge using the water atmosphere vegetation energy and solutes (WAVES) model (Zhang and Dawes 1998; Dawes et al. 1998). In the recharge calculations, depth to watertable was considered for all regions where the depth to watertable was shallow. A shallow watertable was assumed to be where the depth to watertable was 4 m or less below the ground surface. The shallow water table was interpolated using kriging with an external drift and the 9" Digital Elevation Model as a physical constraint  following the methodology presented in Peterson et al. (2011). Where the watertable was not shallow, free drainage conditions were assumed.

For SDL areas with a shallow watertable, the model may produce a net discharge from groundwater over the calculation period.

New South Wales groundwater models method (For SDL areas Lower Lachlan Alluvium, Lower and Mid Murrumbidgee Alluviums and Upper Namoi Alluvium)

Discharge was calculated for selected SDL areas by applying New South Wales groundwater models where the MODFLOW (USGS 2013) evapotranspiration routines were activated to represent groundwater discharge.

The Bureau method:

• The assumptions made in developing the WAVES model as described in Dawes et al. (1998) were all applicable to the discharge estimations carried out for this line item.

• The national land use grid (Australian Bureau of Agricultural and Resource Economics and Sciences 2010) was reclassified to three vegetation classes that include annuals, perennials, and trees.  The major vegetation classes modelled were C3 annual pasture, C3 perennial pasture and eucalypt trees with a grass understorey for SDL areas to the south of 31^oS and C4 annual pasture, C4 perennial pasture and eucalypt trees with a grass understorey for the SDL areas to the north of 31^oS.

• Annual discharge was estimated using a shallow watertable surface estimated by interpolating measured groundwater levels.

New South Wales groundwater models:

• Groundwater models make many assumptions and approximations to represent water balance (refer to the United States Geological Survey website for more details).

The uncertainty estimate was not quantified.

For the Bureau method, the uncertainty in the input parameters and the corresponding impacts on the modelled discharge values were not studied and the uncertainty of the estimated discharge resulting from different interpolation methods was not estimated.

For New South Wales groundwater models, uncertainty of a discharge estimate was not evaluated for the groundwater models.

The following changes resulted in the restatement of the 2010–11 year volume:

• scope change
• a change in the calculation method.

The changes and their respective values are detailed in the following table.

Restatement of comparative year information made for the line item18.3 Discharge to landscape

Segment	2012 Account volume for the 2010–11 year (ML)	2011 Account volume for the 2010–11 year (ML)	Difference due to changes in scope and calculation method (ML)
Northern Basin	36,992	36,992	0
Southern Basin	200,575	271,136	-70,561
Whole region	237,567	308,128	-70,561

 

The scope change was due to the following reasons:

• Southern Basin: inclusion of Upper Murray Alluvium (29,274 ML), Angas Bremer (4,692 ML), Malle (5,778 ML) and Peake–Roby–Sherlock (80,141 ML).
• Southern Basin: extension of Wimmera–Mallee sedimentary plains (1,280 ML).

 

The change of methodology was due to the following reason:

• The depth to watertable was interpolated using the methodology presented in Peterson et al. (2011).
• New set of bores was used based on the data availability.