2.6.2.8 Uncertainty analysis

Summary

The formal uncertainty analysis generates predictive distributions of drawdown (dmax) – the maximum difference in drawdown between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development – and year of maximum change (tmax) at the model nodes of the regional groundwater analytic element model (GW AEM). For the model nodes in the alluvium, the predicted dmax generally is less than 5 m and occurs in the second half of the simulation period. Beyond 20 km from the mine footprints, the probability of exceeding a drawdown of 0.2 m is small.

None of the model nodes associated with the Clematis Group have median drawdown in excess of 1 m. These drawdowns are linked to propagation of drawdown through the Cenozoic cover rather than propagation of drawdown through the Rewan Group.

Although drawdowns predicted in the upper Permian coal measures decrease rapidly with increasing distance to the mine footprints, dmax is generally in excess of 5 m throughout the model domain.

The discussion of model assumptions and their effect on predictions highlighted that the assumptions with the highest potential to affect predictions are the implementation of the CRDP, the representation of the Cenozoic and alluvial sediments and the conceptualisation of the surface water – groundwater interaction. The predictions of drawdown are considered conservative; more local hydrogeological information is required to make more precise predictions.