2.6.2.9.1 Data gaps and opportunities to reduce predictive uncertainty

An important outcome from the Assessment is identifying the main sources of uncertainty and the opportunities for improving regional-scale groundwater modelling in the Namoi subregion. The qualitative uncertainty analysis in Section 2.6.2.8 highlighted several model choices and assumptions that have a high potential impact on the predictions. The implementation of the coal resource development pathway (CRDP) has the highest potential to impact the predictions, particularly in situations where the CRDP becomes out of date if identified developments do not proceed or new developments are proposed. This has occurred with one of the developments in the Namoi subregion CRDP not proceeding (Caroona Coal Project). In line with companion submethodology M04 on developing a coal resource development pathway (Lewis, 2014), once the CRDP has been accepted in a bioregional asssessment it is not revisited.

The knowledge base of hydraulic properties in the deeper geological layers of the Gunnedah Basin is limited in the Namoi subregion. The sensitivity analysis of model predictions highlighted that especially the hydraulic conductivity of the interburden has a high influence on the model predictions. Any information that can constrain the prior distributions of these parameters, including on processes that can affect interburden integrity such as faults, will increase the predictability of the drawdown by the groundwater model.

While estimates of recharge and discharge are essential for groundwater management in the subregion, they are of lesser importance when assessing the drawdown caused by coal resource development. Additional information would undoubtedly make for a better conceptual model and a groundwater model that can reproduce historical observations more accurately, but would have very limited potential to reduce the predictive uncertainty of drawdowns and fluxes.

Related to this is the quality of the current groundwater level observations. Analysis of the metadata of the observations highlighted that the horizontal and vertical accuracy of many observation locations is insufficient to be used in formal model evaluation. While this can be addressed by additional quality control in the database in combination with field verification of observation locations, the reduction in predictive uncertainty is limited as the groundwater level observations in the alluvial aquifers cannot constrain the parameters relevant to drawdown predictions.

The depth of incision of the streambed below topography is a very influential parameter for the simulation of groundwater level in the regional watertable aquifer and the surface water – groundwater flux. It is feasible to conduct a longitudinal survey of streambed elevation to provide a measured value for this parameter in the model; being able to constrain this parameter might enable the groundwater level observations and surface water – groundwater flux estimates to be more useful in constraining the hydraulic properties within the model.