The Namoi subregion groundwater model was developed with MODFLOW-USG (Panday et al., 2013) to probabilistically assess the drawdown due to additional coal resource development, and year of maximum change (tmax), as well as provide the change in surface water – groundwater flux as a boundary condition for the surface water modelling reported in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018).
For the majority of the model domain, the median value of the simulated drawdown is less than 0.2 m. The probability of exceeding this threshold is 100% within the immediate vicinity of the mine footprint area and decreases rapidly with increasing distance from the development. One outcome of the groundwater modelling that is carried forward to the impact and risk analysis (companion product 3-4 for the Namoi subregion) is the zone of potential hydrological change; this is defined as the the contour of 5% probability of exceeding 0.2 m drawdown. The zone of potential hydrological change is generally within 10 km of the development footprint boundary. This also means that the cumulative drawdown of multiple developments can overlap when they are within 20 km of each other; this occurs in several areas of the Namoi subregion. In most cases the drawdown is attenuated at the alluvium boundary due to the high transmissivity and so the largest magnitude drawdowns occur in the consolidated rock rather than the alluvium.
Generally, the magnitude of drawdown due to additional coal resource development, which is the difference in drawdown between CRDP and baseline, is most sensitive to the hydraulic properties of the interburden. The drawdown is not sensitive to parameters such as the recharge scalers, depth of incision of the streambed or the alluvium hydraulic properties.
The tmax varies between 2012 and 2102 and thus spans the entire simulation period. It indicates that while maximum difference in drawdown (dmax) can be achieved during mining operations, it is very likely that dmax is attained in the decades after mining ceases. The tmax increases with increasing distance from mine tenements. The largest drawdowns due to additional coal resource development occur in close vicinity of the mines, within or shortly after the peak mining period and within the simulation period. Further away from the mines, the drawdown due to additional coal resource development takes longer to reach a maximum, potentially beyond the simulation period. However, as the drawdowns are not likely to be significant and are increasingly uncertain, there is little to be gained through extending the simulation period to provide a more precise estimate of tmax.
The simulated changes in surface water – groundwater flux are integrated into the surface water modelling, reported in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018). Additional coal resource development may increase baseflow for some parameter combinations. The possibility of baseflow increases is consistent with the understanding of the dynamics of the groundwater system during and after mining as well as with the conceptualisation of the groundwater model, although observations of mine-induced baseflow increases have not been reported in the literature.
The probabilistic estimates of dmax are constrained by a distance-based weighting of groundwater level observations, estimates of total streamflow and by estimates of mine water makes. The groundwater level and streamflow observations mostly constrained the depth of incision of the streambed assigned to river nodes in the model and to a lesser extent the scaler on diffuse recharge. The predictions of dmax were not sensitive to the depth of incision of the streambed or the scaler on diffuse recharge, giving limited value to these observations. The simulation of mine water makes were sensitive to the hydraulic properties of the coal-bearing formations and the interburden. Data on mine water makes are useful for constraining predictions of dmax.
The probabilistic hydrological changes presented in this product will form the basis of the further receptor impact modelling reported in companion product 2.7 and impact and risk analysis reported in companion product 3-4 for the Namoi subregion.
Input data, model files (including pre- and post-processing scripts and executables) and results are available at www.bioregionalassessments.gov.au.
Product Finalisation date
- 184.108.40.206 Methods
- 220.127.116.11 Review of existing models
- 18.104.22.168 Model development
- 22.214.171.124 Boundary and initial conditions
- 126.96.36.199 Implementation of the coal resource development pathway
- 188.8.131.52 Parameterisation
- 184.108.40.206 Observations and predictions
- 220.127.116.11 Uncertainty analysis
- 18.104.22.168 Limitations
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product