Breadcrumb

2.7 Receptor impact modelling for the Gloucester subregion

Executive summary

View of the Gloucester valley NSW with the Barrington River and associated riparian vegetation in the foreground and the township Gloucester in the distance looking south from the Kia Ora Lookout, 2013 Credit: Heinz Buettikofer, CSIRO

This product details the development of qualitative mathematical models and receptor impact models for the Gloucester subregion. Receptor impact models enable the Bioregional Assessment Programme (the Programme) to quantify the potential impacts and risks that coal resource developments pose to water-dependent landscape classes and ecological assets. Using receptor impact models to investigate landscapes provides a better understanding of how changes in hydrology may result in changes in ecosystems.

A receptor impact model describes a relationship between:

Coal resource developments

Bioregional assessments consider two potential coal resource development futures in the Gloucester subregion:

  • baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are commercially producing as at December 2012. The two baseline open-cut coal mines are Duralie Coal Mine in the south and Stratford Mining Complex in the north
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as the additional coal resource development (those that are expected to begin commercial production after December 2012). In this assessment the potential hydrological impacts of the expansion of the two baseline open-cut coal mines, Duralie and Stratford Mining Complex developments, and a new open-cut coal mine at Rocky Hill in the north of the Gloucester Basin, are modelled. AGL’s proposed CSG development, the Gloucester Gas Project, is included because these futures were finalised in October 2015 before AGL withdrew from this project in February 2016. As per companion submethodology M04 for developing a coal resource development pathway, the CRDP was not revisited.

The difference in results between CRDP and baseline is the change that is primarily reported in a bioregional assessment. This change is due to additional coal resource development. Potential hydrological changes have been presented in companion product 2.6.1 (surface water) and companion product 2.6.2 (groundwater); the process of developing qualitative mathematical models and receptor impact models is summarised.

Methods

Receptor impact model development is both qualitative and quantitative due to the complexity and uncertainty associated with describing relationships between hydrological change and ecological components of the system. The absence of direct relevant theory and relevant ecological response data means the expert judgement, obtained through structured elicitation approaches, has a key role in the receptor impact modelling. It is involved in mapping ecological processes and key components (as signed digraphs), constructing qualitative models that record – as an increase, decrease or no change – landscape class response to sustained hydrological change, selecting ecological indicators (receptor impact variables) from the ecological components or processes and the hydrological regimes (hydrological response variables) that support them, and quantifying the potential response of those indicators to specific hydrological elicitation scenarios. The resulting statistical model quantifies how changes in hydrological response variables due to coal resource development may potentially impact the receptor impact variables in a short-term (2013 to 2042) and long-term (2073 to 2102) period within a landscape class.

Ecosystems

Landscape classes are used to categorise ecosystems into groups that are expected to respond similarly to changes in groundwater and surface water due to coal resource development for the receptor impact models in the Gloucester subregion. In the Gloucester subregion there are 25 landscape classes aggregated further into five landscape groups: (i) ‘Riverine’, (ii) ‘Groundwater-dependent ecosystem (GDE)’, (iii) ‘Non-GDE’, (iv) ‘Estuarine’, and (v) ‘Economic land use’.

A zone of potential hydrological change was defined to ‘rule out’ potential impacts. In the Gloucester subregion this zone is 250 km2. Water-dependent landscapes and ecological assets outside of this zone are very unlikely (less than 5% chance) to experience hydrological change due to additional coal resource development. Within the zone, potential impacts are considered further using qualitative mathematical models and receptor impact models.

Riverine’ landscape group

Stream reaches in the ‘Riverine’ landscape group within the zone of potential hydrological change are almost entirely represented by two closely related landscape classes which are the focus of receptor impact modelling, ‘Perennial – gravel/cobble streams’ and ‘Intermittent – gravel/cobble streams’.

The qualitative mathematical model for the ‘Perennial – gravel/cobble streams’ landscape class indicates that potential changes in surface water and groundwater regimes due to coal resource development lead to negative effects across almost all processes and components of the system. Based on this knowledge, three receptor impact models were developed that show how a change in surface water or groundwater may cause a response in an ecological indicator. Examples of the output from the receptor impact models include:

  • Percent canopy cover of woody riparian vegetation as an ecological indicator responding to a 6 m potential reduction in groundwater levels (from average conditions between 1983 and 2012), may lead to an approximate 20% decrease in percent canopy cover of woody riparian vegetation in both the short: 2013 to 2042 and long: 2073 to 2102 assessment years.
  • Average density of net-spinning caddisfly larvae as an ecological indicator responding to a change in surface water flow (>200 zero-flow days per year) may drop to values <1 per m2 of riffle habitat in both the short: 2013 to 2042 and long: 2073 to 2102 assessment years. Noting that the qualitative model suggests that larvae density can vary substantially across the landscape class (<100 to 1000 per m2) under conditions of constant flow.
  • Average density of the eel-tailed catfish as an ecological indicator responding to changes in surface water flow may see a decline from 5 individuals per 600 m2 transect under continuous flow, to less than 1 individual in two transects as flow becomes more intermittent.

The qualitative mathematical model for the ‘Intermittent – gravel/cobble streams’ landscape class reports a decrease or zero (no change) response in most of the model variables under three scenarios of hydrological change due to coal resource development. The receptor impact model built for this landscape class reports the response of the mean richness of hyporheic invertebrate taxa to changes in zero-flow days. Hyporheic taxa are organisms found where surface water and groundwater mix below the bed of a stream and is an ecological indicator because it can persist in intermittent rivers and streams but is sensitive to the length and frequency of zero-flow spells. Mean richness of hyporheic taxa may drop from 10 to 20 per sampling unit under conditions of constant flow to values between 1 and 8 under very intermittent flow conditions (>300 zero-flow days per year).

‘Groundwater-dependent ecosystem (GDE)’ landscape group

In the ‘Groundwater-dependent ecosystem (GDE)’ landscape group, qualitative mathematical models were developed for three landscape classes: (i) ‘Forested wetlands’, (ii) ‘Wet sclerophyll forests’, and (iii) ‘Dry sclerophyll forests’. Receptor impact models, however, were not able to be developed for any of these landscape classes within the constraints of the workshop and due to availability of suitable experts.

The qualitative mathematical model for the ‘Forested wetlands’ landscape class focused on the role that forest canopies play as a food source and habitat and their response to a simultaneous decrease in shallow and deep groundwater. This model was also used as a basis for qualitative mathematical modelling of the ‘Wet sclerophyll forests’ and ‘Dry sclerophyll forests’ landscape classes. Outputs for all three landscape classes indicate an ambiguous or negative response for all the biological variables in the model with the exception of the zero (no change) response of ground-layer and mid-storey vegetation in the ‘Dry sclerophyll forests’ landscape class. Ambiguous responses arise from positive effects associated with the potential release from predation or competitive dominance, potentially being countered by negative effects resulting from reduced nectar production.

Ecosystems not modelled

No qualitative mathematical models or receptor impact models were developed for the ‘Estuarine’ landscape group, or the ‘Freshwater wetlands’ landscape class, located along the Karuah River estuary because they lay entirely outside the zone of potential hydrological change. No qualitative mathematical models or receptor impact models were developed for the landscape classes from the ‘Non-GDE’ landscape group because they lack a dependence on water other than rainfall and the ecological and economic assets within this group are not anticipated to be impacted by coal resource development through groundwater or surface water mediated pathways.

Bioregional assessments also consider risk to, and impacts on, economic and sociocultural water-dependent assets, however, receptor impact models are not constructed for these assets. Potential impacts on water-dependent economic assets are assessed through availability of groundwater or surface water and against legislated make good provisions and cease-to-pump days. Potential impacts on sociocultural assets are limited to characterising the hydrological changes that may be experienced by those assets in the impact and risk analysis (product 3-4).

Future work

The receptor impact modelling described in this product guides how companion product 3-4 (impact and risk analysis) is framed. Companion product 3-4 will describe impacts on, and risks to, water-dependent assets in the Gloucester subregion.

 

Last updated:
4 June 2018