10.3 Recharge from landscape
The volumetric value for the line item for the 2011–12 year was 3,133,191 ML. The following table provides detailed volumetric information for the line item.
|Groundwater resource plan area||
Groundwater sustainable diversion limit area
Type of recharge
Volume (ML) for the 2011–12 year
|GW15 Gwydir Alluvium||GS24||Lower Gwydir Alluvium||NSW||Rainfall recharge||
|GW14 Namoi Alluvium||GS29||Lower Namoi Alluvium||NSW||Rainfall recharge||
|GS47||Upper Namoi Alluvium||NSW||Rainfall recharge||
|GW12 Macquarie–Castlereagh Alluvium||GS26||Lower Macquarie Alluvium||NSW||Rainfall recharge and irrigation channel||
|Sub-total Northern Basin||
|GW 10 Lachlan Alluvium||GS25||Lower Lachlan Alluvium||NSW||Rainfall recharge||
|GW 9 Murrumbidgee Alluvium||GS28||Lower Murrumbidgee Alluvium (shallow; Shepparton formation)||NSW||Rainfall recharge||
|GS31||Mid-Murrumbidgee Alluvium||NSW||Rainfall recharge||
|GW8 Murray Alluvium||GS27||Lower Murray Alluvium, shallow; Shepparton formation||NSW||Rainfall recharge||
|GS46||Upper Murray Alluvium||NSW||Rainfall recharge||
|GW2 Goulburn–Murray||GS8||Goulburn-Murray (Ovens–Kiewa sedimentary plain)||Vic||Rainfall recharge||
|GS8||Goulburn-Murray (Victorian Riverine sedimentary plain)||Vic||Rainfall recharge||
|GW3 Wimmera–Mallee (groundwater)||GS9||Wimmera–Mallee (sedimentary plain)||Vic||Rainfall recharge||
|GW5 Eastern Mount Lofty Ranges||GS01||Angas Bremer (Quaternary sediments and Murray Group limestone)||SA||Rainfall recharge||
|GW4 SA Murray||GS03||Mallee Murray Group limestone||SA||Rainfall recharge||
|GS05||Peake–Roby–Sherlock (unconfined)||SA||Rainfall recharge||
|Sub-total Southern Basin||
|Total for the region||
1 Bureau = Bureau of Meteorology, NSW models = New South Wales models
Sustainable diversion limit (SDL) areas other than those included in the table exist and may be relevant to the line item; however, relevant information was not available for the 2012 Account.
For the SDL areas for which 'Data source' is shown as the Bureau in the table
The Bureau estimated the diffuse groundwater recharge volumes using the water atmosphere vegetation energy and solutes (WAVES) model (Zhang and Dawes 1998; Dawes et al. 1998). In the recharge calculations, depth to water table was considered for all regions where the depth to watertable was shallow. A shallow water table was assumed to be where the depth to the watertable was 4 m or less below the ground surface. The shallow water table was interpolated using kriging with 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. The recharge within the MDB region was determined by summing the spatially interpolated positive recharge estimates.
For the SDL areas for which 'Data source' is shown as New South Wales models in the table
Recharge volumes were calculated for selected SDL areas applying New South Wales groundwater models based on MODFLOW (United States Geological Survey 2013) modelling process.
Groundwater recharge is both an input to and, an output from, a groundwater model. There is no single method for estimating recharge used in the New South Wales groundwater models; however, several models estimate recharge as a percentage of rainfall. The magnitude of recharge (as a percentage of rainfall) can be adjusted during the calibration of a groundwater model so that the observed groundwater levels are reproduced in model outputs as accurately as possible, typically for a period of around 20 years if data are available.
Assumptions, limitations, caveats and approximations
The assumptions made in developing the WAVES model as described in Dawes et al. (1998) were all applicable to the recharge estimations carried out for the MDB region.
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 recharge was estimated using a shallow water table surface estimated by interpolating measured groundwater levels.
New South Wales groundwater models:
Groundwater models make many assumptions and approximations to represent water balance (United States Geological Survey 2013).
Several of the New South Wales groundwater models assume estimation of recharge volume as a percentage of rainfall.
The uncertainty estimate was not quantified.
For the Bureau method:
The uncertainty in the input parameters and the corresponding impacts on the modelled recharge values were not studied.
The uncertainty of the estimated recharge resulting from different recharge interpolation methods was not estimated.
For New South Wales groundwater models:
Uncertainty of a recharge estimate is not evaluated for the groundwater models.
The following changes resulted in the restatement of the 2010–11 year volume:
- changes made to the calculation method
- scope change.
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 (45,435 ML) and Angas Bremer (12,821 ML)
- Southern Basin: inclusion of rainfall recharge volume for Lower Murrumbidgee Alluvium (211,820 ML)
- Southern Basin: extension of the area used in the 2011 Account for Wimmera Mallee sedimentary plains.
The change of methodology was due to the following reason:
- The depth to water table was interpolated using the methodology presented in Peterson et al. (2011).
- New set of bores was used based on the data availability.
- Northern Basin: Application of New South Wales groundwater models instead of the Bureau method to estimate the recharge volume for Lower Namoi Alluvium.