2.6.2.3.1 Objectives

The primary objective of bioregional assessment (BA) groundwater modelling is to quantify the hydrological changes on regional groundwater due to additional coal resource development, which is based on the difference in results between the baseline and coal resource development pathway (CRDP) simulations. The significance of the hydrological changes due to the additional coal resource development can only be understood when considered against the results of the hydrological changes from the baseline simulation. In order to represent uncertainty, a probabilistic approach is used, which requires the groundwater model to be run thousands of times with different parameter combinations. This can have high computational overheads if the model domain is large and finely resolved. For the Hunter subregion, the modelling domain must be at least 17,000 km² and 2 to 3 km deep. Given this large domain and the requirement to do thousands of simulations, the groundwater model must be computationally efficient, represent just the key processes for a regional-scale assessment and have a spatial resolution appropriate for representing local to regional effects of coal resource development.

The model needs to represent the main causal pathway groups that link mine hazards to groundwater responses on and off the mine sites: ‘Subsurface depressurisation and dewatering’, which involves subsurface depressurisation and dewatering from excavation of coal seams and mine water pumping; 'Subsurface physical pathways’, which involves changes in subsurface physical pathways due to hydraulic conductivity changes resulting from rock deformation due to mining; and ‘Surface water drainage’, which involves changes to surface water drainage through its interaction with groundwater (see companion product 2.3 for the Hunter subregion (Dawes et al., 2018).

Key outputs from the model are groundwater drawdowns and changes in baseflow, which are summarised as changes in key groundwater and surface water hydrological response variables at model nodes across the modelling domain (Section 2.6.2.1.2.1).

Drawdowns due to the additional coal resource development are reported as probability distributions of the differences in drawdown between the coal resource development pathway (CRDP) and the baseline simulations. The drawdowns are reported by the groundwater model at each model node in the model domain, but since most water-dependent assets are near the ground surface in the alluvium, these are the model nodes of greatest interest.

The changes in baseflow from the CRDP and baseline simulations are used in the Hunter river model, where they are incorporated into the hydrological response variables generated as part of the surface water modelling (see companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018)).

Since the main objective of the BA numerical modelling is to quantify the difference between two modelled futures, the emphasis on producing a well-calibrated model is lower than if the objective were to predict the state of groundwater in the future under baseline and coal resource development pathway (Section 2.6.2.1.2).