Modelling limitations

The report for the Queensland Water Commission (GHD, 2012) states that the 2012 Office of Groundwater Impact Assessment (OGIA) model was developed

‘to provide a suitable tool for assessing the impacts of CSG activities on water levels in the aquifers present within the Surat CMA. The modelling work was therefore undertaken with this single purpose in mind. Hence, according to the definitions presented in the MDBC modelling guideline (Middlemis et al., 2000), the regional scale model developed is considered to be an ‘impact assessment model’. The model is therefore not considered to be an ‘aquifer simulator’ and is therefore on its own not necessarily suitable for:

  • Predicting responses to arbitrary changes in hydrological conditions;
  • Developing sustainable water resource management policies;
  • Assessing impacts on groundwater dependent ecosystems; or
  • Quantifying surface water – groundwater interactions.

In particular the regional scale of the model and hence the relatively coarse model grid is likely to represent a significant limitation to use of the model for these purposes’ (GHD, 2012, p. 50).

The first generation OGIA model built in 2012 has the best available representation of CSG development in the Surat cumulative management area (CMA) for cumulative groundwater impact assessment purposes. It is considered a fit-for-purpose regional-scale groundwater model for the bioregional assessment of the Maranoa-Balonne-Condamine subregion, with the exception of criteria related to integration with surface water numerical modelling and receptor impact modelling that are related to the representation of water fluxes in surficial aquifers, as discussed in Section .

The representation of surface water – groundwater interactions, mine pit dewatering, CSG activities and horizontal and vertical discretisation are identified as having the greatest potential effect on model predictions in the qualitative uncertainty analysis. Drain cell conductance, which represents mine pit dewatering, has a major effect on modelled drawdown and water balance. Higher drain cell conductance values will increase the slope of modelled groundwater drawdown above the drain cell elevation in the surrounding model cells. However, it is not considered in the formal uncertainty analysis. The MODFLOW EVT package is considered a superior approach for representing CSG depressurisation in MODFLOW models compared to using the well or drain packages. However, while alternative modelling approaches using dual-phase model code would improve the accuracy of predicted drawdowns they have greater data, resources and technical or computational requirements. Horizontal discretisation affects model predictions, particularly where large hydraulic gradients are to be expected, such as near coal mines where hydraulic gradients are steepest. Local-scale modelling is the only way to resolve this issue. The integrated approach that OGIA used to model groundwater levels in the Condamine Alluvium is an example of using a more detailed local-scale model to make predictions at an appropriate scale. While an increased vertical discretisation allows for a more accurate representation of hydraulic properties, especially storage values, there are insufficient data to justify parameterising and constraining a higher vertical resolution at the regional scale.

The quantitative uncertainty analysis considered hydraulic conductivity, recharge and storage values, but did not consider the parameters used to specify boundary conditions such as drain and river boundary conductance and the lateral head dependent boundary conditions. Model conceptualisation is not considered as part of the formal uncertainty analysis. The calibration-constrained uncertainty analysis is used to quantify the effect of horizontal hydraulic conductivity, vertical anisotropy, modelled watertable recharge and specific storage/specific yield on model predictions. These parameters are not re-calibrated after the boundary conditions that represent the open-cut coal mines are added into the model. However, this is not likely to affect modelled predictions as the pre 1995 groundwater levels used for model calibration represent a period before significant CSG extraction and open-cut coal mine development affected the regional groundwater levels. Model calibration with additional groundwater level or flux observations and a finer resolution model grid in the vicinity of the coal mines would improve model predictions, but is not feasible in the existing regional-scale groundwater model.

Last updated:
17 October 2018
Thumbnail of the Maranoa-Baloone-Condamine subregion

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