The impact and risk analysis (Herron et al., 2018c) investigated how hydrological changes due to additional coal resource development may affect ecosystems. These ecosystems were classified into 26 landscape classes and 5 landscape groups (Box 6, Table 1).
The impact and risk analysis (Box 7) focused on landscape classes that intersect the zone of potential hydrological change (Box 4). Any ecosystem or asset wholly outside of this zone is considered very unlikely to be impacted due to additional coal resource development.
For potentially impacted ecosystems within the zone, receptor impact models (Box 8) were used to translate predicted changes in hydrology into a distribution of ecological outcomes that may arise from those changes. These models used indicators of the health of the ecosystem, such as taxa richness or canopy cover of vegetation, to infer the potential ecological impacts of hydrological changes.
The natural and human-modified ecosystems in the subregion were classified into 26 landscape classes (Table 1 and Section 2.3.3 in Dawes et al. (2018)) to enable a systematic and comprehensive analysis of potential impacts on, and risks to, the water-dependent assets nominated by the community. These landscape classes were aggregated into five landscape groups, based on their likely shared response to hydrological changes. The landscape classification was based on the subregion’s geology, geomorphology, hydrogeology, land use and ecology. Definitions for landscape classes and landscape groups for the Hunter subregion are available online at environment.data.gov.au/def/ba/landscape-classification/hunter-subregion.Box 6 Understanding the landscape classification
Box 7 Analysing impact and risk
Potential impacts to water-dependent ecosystems and assets were assessed by overlaying their location on the zone of potential hydrological change (Box 4) to identify the hydrological changes that a particular asset or ecosystem might experience.
- Outside this zone, ecosystems and assets are very unlikely to be impacted by hydrological changes due to additional coal resource development.
- Inside this zone, ecosystems and assets are potentially impacted.
Within the zone, not all water-dependent ecosystems or assets will be affected by hydrological changes, as this depends on their reliance on groundwater or surface water. Hydrological changes due to additional coal resource development may be large, but within the range of natural seasonal and climatic variability, and so may not affect water-dependent ecosystems or assets. Alternatively, small changes may affect sensitive ecosystems that have a strong reliance on groundwater or surface water.
For ecological assets, the assessment considered the potential impact to the habitat of species, not potential impacts to the species themselves.
Ecosystems that fall within the mine pit exclusion zone are likely to be directly impacted, but as estimates of drawdown are unreliable, the degree of impact is not possible to quantify. Similarly, the surface water modelling close to mine pits cannot quantify the degree of impact on some streams.
Box 8 Receptor impact models
Receptor impact models translate predicted changes in hydrology into ecological outcomes that may arise from those changes. Applying receptor impact models across ecosystems assists in identifying where changes in hydrology may result in ecosystem changes, and consequently where additional local-scale information and further investigation may be warranted.
To assess potential ecological outcomes, experts identified receptor impact variables, characteristics that serve as indicators of the ecological condition of an ecosystem, and which are also likely to respond to hydrological changes as well as being within the expertise of the available experts. These variables were specifically chosen to be representative of a landscape class in the Hunter subregion.
One or more hydrological response variable(s) that affect each indicator were identified with ecological experts. Receptor impact models (statistical models) were then developed to represent the relationship between each indicator and its important hydrological response variables. Details are found in Hosack et al. (2018a) and Dataset 13.
For riverine forests in forested wetlandsin the Hunter subregion, one ecological indicator (bolded) was chosen to predict changes that are sensitive to the following hydrological response variables:
- projected foliage cover: overbench flow, overbank flow, groundwater drawdown.
For perennial streams, two indicators (bolded) were chosen to predict changes that are sensitive to the following hydrological response variables:
- abundance of riffle-dwelling Hydropsychidae (caddisfly) larvae: zero-flow days (averaged over 30 years) and the mean maximum spell duration of zero-flow days
- the probability of presence of riffle-breeding frogs: zero-flow days (averaged over 30 years) and the mean maximum spell duration of zero-flow days.
For intermittent streams, two indicators (bolded) were chosen to predict changes that are sensitive to the following hydrological response variables:
- hyporheic invertebrate taxa richness: zero-flow days (averaged over 30 years) and the mean maximum spell duration of zero-flow days
- the probability of presence of riffle-breeding frogs: zero-flow days (averaged over 30 years) and the mean maximum spell duration of zero-flow days.
Hydrological models were used to quantify changes in the hydrological response variable(s). Predictions of an ecological indicator at a specific location were made by applying the receptor impact model for that indicator to the predicted hydrological response variable(s) at that location.
Receptor impact models were used to predict changes in the indicator for a landscape class that result from the changes in hydrological response variables. The changes in the indicator reflect the magnitude of potential ecological impacts for that ecosystem. The indicators provide a measure of the risk to the ecosystem, rather than a prediction about (for example) caddisfly and frog populations per se.
Importantly, receptor impact models were not used in isolation but were applied along with other lines of available evidence, including expert advice, hydrological modelling results and other existing data and knowledge, to assess potential ecological impacts.
Table 1 Area (km2) or length (km) of all landscape classes in the assessment extent and the zone of potential hydrological change
aThe ‘Lowly to moderately intermittent’ and ‘Moderately to highly intermittent’ landscape classes were collapsed into a single ‘Intermittent’ landscape class for analysis (Hosack et al., 2018a).
GDE = groundwater-dependent ecosystem, na = not applicable
Data: Bioregional Assessment Programme (Dataset 12)
Ecosystems
Which ecosystems are very unlikely to be impacted?
Ecosystems outside the zone of potential hydrological change are very unlikely to be impacted, including 1232 km of perennial streams, 1450 km of intermittent streams and 8840 km of ephemeral streams.
Within the zone, most (3012 km2) of the ecosystems are classified as non-groundwater-dependent vegetation (51%) and economic land uses (43%), such as irrigated and dryland agriculture, production forestry, mining and industrial uses. These are ruled out as they are predominantly rainfall dependent and therefore not the focus of bioregional assessments.
1347 km of ephemeral streams in the zone are unlikely to be impacted because, by definition, ephemeral streams are not connected to groundwater, and none of these 1347 km of streams are disrupted by changes in surface water drainage.
Seagrasses, which occur in coastal lakes within the zone of potential hydrological change, are particularly sensitive to changes in water levels which can be caused by subsidence above underground mines. However, in NSW, mines are required to prepare subsidence management plans that detail how they will minimise impacts from subsidence as a condition of approval. In the Macquarie and Tuggerah coastal lake systems, subsidence exclusion zones have been delineated, in which there are restrictions on underground mining practices. As a result, the risk from subsidence was considered to be low (for more details see Section 3.4 in Herron et al. (2018c)).
Which ecosystems are potentially impacted?
Ecosystems that intersect the zone of potential hydrological change (Box 4) are potentially at risk of impact due to additional coal resource development (Table 1, Figure 13).
Of the 14,659 km of rivers and streams in the Hunter subregion, 3137 km (21%) are in the zone of potential hydrological change. This includes 634 km of perennial streams, 518 km of intermittent streams, and 1985 km of ephemeral streams (Table 1). Of the 1985 km of ephemeral streams, 638 km are potentially disrupted by changes in surface water drainage. The remaining 1347 km of ephemeral streams are unlikely to be impacted.
There are 102 km2 of ecosystems identified as potentially groundwater dependent in the zone of potential hydrological change, including rainforests (60% of the total in the assessment extent), forested wetlands (38%), and wet and dry sclerophyll forests (18%) (Table 1 and Figure 13).Key finding 5
Small to no changes in the number of zero-flow days (averaged over 30 years) modelled for most perennial and intermittent streams suggest generally low risk to instream habitat. Exceptions are the perennial Wyong River and the intermittent Saddlers and Loders creeks, where potentially large flow regime changes are predicted to affect instream ecosystems. Impacts on instream habitat cannot be ruled out in a number of intermittent streams that were not modelled, but which flow close to additional coal resource developments.
Groundwater-dependent ecosystems in the zone of potential hydrological change represent 28% of the total area of groundwater-dependent ecosystems within the assessment extent (Table 1).
Figure 13 Landscape classes within the zone of potential hydrological change
Landscape classes are shown for the ‘GDE’ landscape group and the ‘Riverine’ landscape group. Groundwater-dependent ecosystems (GDEs) are exaggerated (not to scale) for clarity. Riverine hydrological changes are either ‘unlikely’ (outside the zone of potential hydrological change) or ‘potential’ inside the zone. Mines in the coal resource development pathway (CRDP) includes baseline and additional coal resource developments (ACRD).
Data: Bioregional Assessment Programme (Dataset 1)
Forested wetlands
Modelled changes in ecologically important flows indicate a higher risk to the condition of riverine forested wetlands along the Goulburn River compared to other riverine forested wetlands in the subregion.Key finding 6
A receptor impact model (Hosack et al., 2018a) was used to predict whether changes in groundwater drawdown and frequency of overbench and overbank flows result in changes in projected foliage cover of riverine forests (in the ‘Forested wetland’ landscape class) along unregulated rivers of the Hunter river. The model was not appropriate for quantifying impacts on coastal forested wetlands, nor riverine forests along the regulated river.
Most of the riverine forests on unregulated streams in the Hunter river basin are very unlikely to experience groundwater drawdown of more than 0.2 m, and it is very unlikely that more than 0.1 km2 will experience drawdown exceeding 2 m. Changes in overbench and overbank flows are mainly predicted along the Goulburn and Hunter rivers. Due to flow regulation, the modelled changes in overbench and overbank flows along the Hunter River, and hence the risk to Hunter River forested wetlands, are difficult to determine.
The median result suggests little likelihood of changes in projected foliage cover in most of the riverine forested wetlands. Details can be found in Section 3.4.4.3.2 of Herron et al. (2018c). To better resolve the risk to riverine forests along the Goulburn River, local information is needed to constrain the predictions of hydrological change and put these potential changes in the context of other factors influencing ecosystem condition.
Impacts to coastal forested wetlands in the modelled drawdown zone from the proposed Wallarah 2 mine and Mandalong expansion were not modelled and cannot be ruled out.
Perennial streams
Modelled flow regime changes in the perennial Wyong River and the intermittent Saddlers and Loders creeks indicate a higher risk to instream habitat compared to other modelled streams in the subregion.Key finding 7
Receptor impact models (Hosack et al., 2018a) were developed to predict the risk to instream habitats of perennial streams due to additional coal resource development. Experts identified perennial stream ecosystems as sensitive to increases in the long-term average number of zero-flow days (see Box 8). Potentially large changes in the number of zero-flow days were modelled in the Wyong River.
The Wyong River is the only modelled perennial stream where increases in the mean annual number of zero-flow days (averaged over 30 years) due to additional coal resource development were modelled to exceed 3 days (95% chance of exceeding), with median estimates suggesting increases of between 20 and 80 days per year. There is a 5% chance of an increase of more than 200 zero-flow days per year (averaged over 30 years), although this impact becomes much less when local-scale hydraulic data is incorporated into the model (see Surface water).
Two receptor impact models were used to quantify the risk from these hydrological changes in terms of numbers of caddisfly larvae and the probability of presence of riffle-breeding frogs (see Box 8), two components of the ecosystem that rely on permanent flow for their persistence. Details can be found in Section 3.4.3.3.1 of the impact and risk analysis (Herron et al., 2018c). Local information is needed to better resolve the level of risk, including better definition of the magnitude and likelihood of the hydrological changes and the implications of these potential changes given local considerations.
Surface water modelling was not undertaken for the perennial Dora Creek. Similar hydrological changes to those modelled for the Wyong River might be expected given its similar geography and magnitude of modelled groundwater drawdown. This suggests a risk of impacts on instream habitats in Dora Creek also.
No significant changes in zero-flow days (averaged over 30 years) are predicted in perennial streams of the Hunter river basin, hence instream ecosystems that are adapted to current flow regimes are very unlikely to be impacted in this basin.
Intermittent streams
Two ecological indicators of instream habitat were chosen (Box 8) which potentially change in response to changes in the number of zero-flow days (averaged over 30 years). Results of regional-scale hydrological modelling indicate a 50% chance of changes in zero-flow days (averaged over 30 years) in Saddlers Creek exceeding 20 days per year, and exceeding 3 days per year in Loders Creek and in an unnamed creek in the Bayswater Creek catchment. There is a 5% chance that increases in zero-flow days (averaged over 30 years) in Saddlers and Loders creeks exceed 80 days per year. Conversely, there is also a 5% chance of no significant change in zero-flow days (averaged over 30 years) in these creeks. There are other intermittent streams near mines where hydrological changes were not quantified, but where impacts may be expected.
Saddlers Creek (at least a 50% chance) and Loders Creek (at least a 5% chance) have the potential to experience changes in the two chosen ecological indicators (Box 8) as a result of the reductions in zero-flow days (averaged over 30 years) due to additional coal resource development. Hence, it is possible that the instream habitat of these streams may be impacted. Details can be found in Section 3.4.3.3.2 of Herron et al. (2018c).
Again, these results identify areas where further investigation is warranted, rather than being site-specific predictions about ecological changes per se. Local factors, such as presence of faults or aquitards, geomorphic condition and quality of the water, will influence the extent to which risks identified from regional-scale modelling warrant further attention.
Changes in the two chosen ecological indicators (Box 8) due to additional coal resource development are very unlikely in the upper Goulburn River, Wollar Creek and Saltwater Creek. Hence, it is unlikely that the instream habitat of these streams would be impacted.
The lack of groundwater modelling of the Wilpinjong additional coal resource development means that streamflow changes in Wollar Creek and the Goulburn River may be underpredicted, with potentially larger impacts than those presented here.
Explore potential impacts on ecosystems in more detail on the BA Explorer, available at www.bioregionalassessments.gov.au/explorer/HUN/landscapes. Conceptual modelling, product 2.3 (Dawes et al., 2018) Receptor impact modelling, product 2.7 (Hosack et al., 2018a) Impact and risk analysis, product 3-4 (Herron et al., 2018c) Receptor impact modelling, submethodology M08 (Hosack et al., 2018b) Impacts and risks, submethodology M10 (Henderson et al., 2018) Impact and risk analysis database (Dataset 1) Landscape classification (Dataset 9) Receptor impact models (Dataset 13) Results from applying receptor impact models (Dataset 14)FIND MORE INFORMATION
