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.
|Groundwater resource plan area||
Groundwater sustainable diversion limit area
Volume (ML) for the 2011–12 year
|GW 15 Gwydir Alluvium||GS24||Lower Gwydir Alluvium||NSW||
|GW 14 Namoi Alluvium||GS47||Upper Namoi 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)||NSW||
|GW8 Murray Alluvium||GS27||Lower Murray Alluvium, shallow; Shepparton formation||NSW||
|GS46||Upper Murray Alluvium||NSW||
|GW2 Goulburn–Murray||GS8||Goulburn–Murray (Ovens–Kiewa sedimentary plain)||Vic||
|GS8||Goulburn–Murray (Victorian Riverine sedimentary plain)||Vic||
|GW3 Wimmera–Mallee (groundwater)||GS9||Wimmera–Mallee (sedimentary plain)||Vic||
|GW5 Eastern Mount Lofty Ranges||GS01||Angas Bremer (Quaternary sediments and Murray Group limestone)||SA||
|GW4 SA Murray||GS03||Mallee (Pliocene Sands and Murray Group limestone)||SA||
|GS05||Peake–Roby–Sherlock (unconfined and confined)||SA||
|Sub-total Southern Basin||
|Total for the region||
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.
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).
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.
|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)|
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.