3.2 Methods

Summary

The impact and risk analysis for the Gloucester subregion followed the overarching methodology described in companion submethodology M10 (as listed in Table 1) for analysing impacts and risks (Henderson et al., 2018). The impact analysis quantified the magnitude and extent of the potential hydrological or ecosystem changes due to additional coal resource development, including direct, indirect and cumulative impacts. The risk analysis considered not only the magnitude and extent of the potential impact, but also the likelihood of the impact.

Impacts to water-dependent landscapes and assets can be caused by changes in surface water and changes in groundwater in the regional watertable, from which most ecological assets source water. The impact and risk analysis used the conceptual model of causal pathways and probabilistic estimates of hydrological change to identify where potential impacts to landscapes and assets might occur. In some of these locations, receptor impact models were used to translate potential hydrological changes to potential ecosystem changes.

For bioregional assessment (BA) purposes, the regional watertable is the upper groundwater level within the unconfined, near-surface aquifer (not perched), where pore water pressure is equal to atmospheric pressure. It is constructed by combining the watertable from all the near-surface geological units (or layers) in which it occurs. Within the Gloucester subregion, the regional watertable exists in the alluvia of the Gloucester and Karuah river systems and in the weathered and fractured zone outside these alluvia. The change in drawdown at the regional watertable was obtained from the analytic element groundwater model for the weathered and fractured zone and combined with the change in drawdown in the alluvia from the MODFLOW groundwater model.

Surface water modelling was undertaken using the Australian Water Resources Assessment landscape model (AWRA-L). Results for eight hydrological response variables were reported for 34 model nodes across the subregion and extrapolated to stream links to better represent changes in surface water across the assessment extent.

The results from the groundwater and surface water modelling were used to define the zone of potential hydrological change due to additional coal resource development. Potential impacts to landscapes and assets were then assessed by overlaying their location on the zone of potential hydrological change. Outside this zone, landscapes and assets were ruled out from potential impacts and not analysed further. Inside this zone, potential impacts were summarised for each landscape class or asset using indicators of hydrological change (hydrological response variables) and ecosystem change (receptor impact variables).

The databases, tools and geoprocessing that supported the impact and risk analysis are summarised in this section.