2.7.6.2 Limitations of the receptor impact modelling

Section 2.7.1 and companion submethodology M08 (as listed in Table 1) for receptor impact modelling (Hosack et al., 2018) detail the strengths and limitations of the expert elicitation process used in BAs for building qualitative ecosystem models and quantitative receptor impact models. There is no need to revisit these here, except to acknowledge that the qualitative models and receptor impact models that were developed to represent the landscape classes in the zone of potential hydrological change for the Hunter subregion reflect the subjectivity and bias inherent in the knowledge base of the assembled experts – for example, in defining the scope of the model; its components and connections; ecologically important hydrological variables; representative receptor impact variables; and magnitude and uncertainty of responses to change. Thus, each model represents ‘a view’ of a landscape class or ecosystem; a view that might brook argument about some of the specifics, but would generally be accepted as an adequate high-level conceptualisation of the important components of the ecosystem(s) it represents.

However, some knowledge gaps and limitations were identified at the expert elicitation workshops, which limit the assessment of potential impacts from hydrological changes due to additional coal resource development for some landscape classes or components of landscape classes within the zone of potential hydrological change. In other words, they limit this BA and must be flagged as areas requiring further investigation.

While some models include salinity and/or nutrient components, the expert elicitations to define the results space for the receptor impact models are premised on changes in the timing, magnitude, or level of surface water and groundwater. Changes in water quality parameters that could occur with a shift in the relative contributions of surface runoff and groundwater to streamflow or due to enhanced connectivity between aquifers of differing water quality, for example, are not represented. Thus, the potential ecological impacts due to additional coal resource development reported in the impact and risk analysis for the Hunter subregion (companion product 3-4 (Herron et al., 2018)) reflect the risk from hydrological changes only; they could differ if changes in key water quality parameters had been included in the model formulation.

No specific limitations were identified in the receptor impact models for perennial streams and intermittent streams. The riffle-breeding frog, Hydropsychidae larvae and hyporheic invertebrate taxa receptor impact variables for the perennial streams and intermittent streams were selected as indicators of instream ecosystems that are sensitive to changes in hydrology and can represent the response of other components of the ecosystem to changes in hydrology, and components that depend on those components. The extent to which they are suitable indicators of ecosystem response for all instream ecosystems across the Hunter subregion is not known. The interpretation of results of the receptor impact models presented in companion product 3-4 for the Hunter subregion (Herron et al., 2018) is couched in terms of risk to instream habitat, rather than risks to the receptor impact variables themselves.

Ephemeral streams were assumed to not be connected to groundwater, hence unlikely to be impacted by drawdown of the regional watertable and changes in baseflow. While there is potential for additional coal resource development to affect catchment runoff to ephemeral streams, the changes were considered unlikely to impact communities, which are assumed to be highly adapted to variable runoff or opportunistic users of creek flows when they occur.

No specific limitations were identified in relation to the wet and dry sclerophyll forests receptor impact model. The density of foliage cover was generally considered a good indicator of water availability.

The forested wetland receptor impact model was specifically developed for riparian vegetation along unregulated rivers in the Hunter river basin and is not considered suitable for evaluating the impact of hydrological changes due to additional coal resource development on the regulated Hunter River, nor for the ‘Coastal swamp forests’ and ‘Coastal floodplain wetlands’ vegetation classes found in the Macquarie-Tuggerah lakes basin. The receptor impact modelling results for the forested wetlands presented in Section 3.4 of companion product 3-4 for the Hunter subregion (Herron et al., 2018) represent less than 50% of the mapped extent of forested wetlands identified as intersecting the zone of potential hydrological change.

The rainforest qualitative model assumes rainforests occur in sheltered gullies and slopes in hilly-to-steep terrain of the coast and escarpment of the Macquarie-Tuggerah lakes hinterland. About 10 km² of the mapped rainforest groundwater-dependent ecosystems (GDEs) in the zone of potential hydrological change occur on the alluvium of larger perennial rivers, for which the rainforest qualitative model is not considered appropriate. The potential for adverse impacts to these riparian rainforests along the Wyong River and Jilliby Jilliby Creek has not been assessed as part of this BA. Given they occupy a similar landscape position to forested wetlands, the forested wetland receptor impact model might be appropriate for a first pass assessment of potential impacts from the proposed underground mines in this area. This idea was not tested with the invited experts and the ecological water requirements of these rainforests is a knowledge gap of this assessment.

Experts at the receptor impact modelling workshop lacked the expertise to develop a quantitative model for the ‘Freshwater wetlands’ landscape class. Uncertainty about the connectivity of freshwater wetlands to regional groundwater was also identified during the qualitative modelling workshop. The available literature suggests they are probably rain-fed, local aquifers, with poor connection to deeper groundwater. Better groundwater information is needed for each of the subregion’s freshwater wetlands to assess potential impacts from hydrological changes due to additional coal resource development.

Experts were uncertain about the potential impacts of changes in salinity and groundwater drawdowns on saline wetlands (including saltmarsh and mangroves), represented in the qualitative model for inter-tidal wetlands. This stemmed, in part, from uncertainty about the connection of Hunter subregion saline wetlands to regional groundwater. Seagrasses, represented in both the intertidal wetlands and subtidal benthos qualitative models, were considered particularly sensitive to changes in light attenuation, which could result from base-level lowering caused by mine subsidence below seagrass beds, but not from underground mines away from the lakes. Experts generally considered there was little threat from changes in surface water inflows and groundwater drawdown.

A summary of the assumptions and limitations of the qualitative and receptor impact models that emerged during the expert elicitation workshops is provided in Table 30. Knowledge gaps and research opportunities are identified for some models. A more comprehensive listing of the gaps and opportunities that have emerged during the BA for the Hunter subregion is provided in Section 3.7 of companion product 3-4 for the Hunter subregion (Herron et al., 2018).

Table 30 Assumptions and limitations of qualitative (QM) and receptor impact models (RIM) developed for Hunter subregion landscape classes that intersect the zone of potential hydrological change