2.6.2.1 Methods

Summary

This section summarises the groundwater modelling approach with reference to the Namoi subregion conceptual model and the objectives of the bioregional assessment.

The groundwater numerical modelling is designed to provide probabilistic estimates of groundwater drawdown and changes in the surface water – groundwater flux due to additional coal resource development in the Namoi subregion. Results can be expressed in terms of contour maps of the percent chance of exceeding a specified drawdown and/or percentiles of drawdown. The approach in the Namoi subregion is consistent with the Bioregional Assessment Programme’s companion submethodology M07 (as listed in Table 1) for groundwater modelling (Crosbie et al., 2016), and companion submethodology M09 (as listed in Table 1) for propagating uncertainty through models (Peeters et al., 2016).

The model chain comprises a subregion-wide groundwater model and surface water model (see companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018)). This model chain is evaluated many times to characterise the prediction uncertainty for two hydrological response variables:

• dmax, the maximum difference in drawdown
• tmax, the year of maximum change.

The modelled changes in surface water – groundwater flux inform the potential changes in total streamflow, modelled using the Namoi subregion surface water model.