Murray–Darling Basin
18.3 Discharge to landscape

Supporting information

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.

Quantification approach

Data source

(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.

Provided by

The Bureau and NSW Office of Water.

Method

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.

Assumptions, limitations, caveats and approximations

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 31oS and C4 annual pasture, C4 perennial pasture and eucalypt trees with a grass understorey for the SDL areas to the north of 31oS.

  • 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).

Uncertainty information

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.

Comparative year

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.