The zone of potential hydrological change is the area within the subregion where changes in hydrology due to additional coal resource development exceed defined thresholds for groundwater and surface water. The impact and risk analysis presented in Section 3.4 and Section 3.5 focuses on landscape classes and assets that intersect this zone. Any landscape class or asset wholly outside of the zone of potential hydrological change is considered very unlikely (less than 5% chance) to be impacted due to additional coal resource development.
The zone of potential hydrological change is defined as the union of the groundwater zone of potential hydrological change (Section 3.3.1.1) and the surface water zone of potential hydrological change (Section 3.3.1.2). It is presented in Section 3.3.1.3.
3.3.1.1 Groundwater
The groundwater zone of potential hydrological change is defined with a greater than 5% chance of exceeding 0.2 m of drawdown in the regional watertable due to additional coal resource development. Groundwater impacts of coal mines and coal seam gas (CSG) projects are regulated under state legislation and state regulatory and management frameworks. This 5% chance is determined based on the uncertainty analysis, described in Section 2.6.2.8 of companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b). It means that, for each individual assessment unit in the groundwater zone, 95% of groundwater model runs exceeded this level of drawdown. The 0.2 m drawdown threshold adopted in bioregional assessments (BAs) is consistent with the most conservative minimal impact threshold in the NSW Aquifer Interference Policy (DPI Water, 2012) and Queensland’s Water Act 2000.
Figure 14 shows the areas with at least a 5% chance of drawdown exceeding 0.2 m under the baseline and the CRDP. The extent of drawdown under the baseline is 4307 km2 (25% of the assessment extent). This increases to 5129 km2 (30% of assessment extent) under the CRDP, which represents the combined extent of drawdown under baseline and due to additional coal resource development. It is the area where the drawdown due to the additional coal resource development has at least a 5% chance of exceeding 0.2 m that forms the basis of the groundwater zone of potential hydrological change.
The groundwater zone of potential hydrological change (Figure 15) covers an area of 2441 km2, or 14% of the assessment extent. It represents the area with at least a 5% chance of exceeding 0.2 m of drawdown due to additional coal resource development. The locations of the additional coal resource developments that were not modelled are shown to identify where, had they been included in the modelling, the groundwater zone may be expected to differ from what is shown.
The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).
Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3)
Figure 15 Groundwater zone of potential hydrological change for the Hunter subregion
Additional coal resource developments (ACRDs) that were not modelled in the groundwater model are shown to identify where, had they been included, the groundwater zone might differ.
Data: Bioregional Assessment Programme (Dataset 2, Dataset 3, Dataset 4, Dataset 5)
3.3.1.2 Surface water
The threshold hydrological change adopted for each hydrological response variable for defining the zone of potential hydrological change is given in Table 6. Together the hydrological response variables represent potential changes across the full flow regime, from low flows (P01, ZFD, LFD, LFS, LLFS) to high flows (P99 and FD), including two to represent changes in flow volume (AF) and variability (IQR) (see companion submethodology M06 (as listed in Table 1) for surface water modelling (Viney, 2016)).
Table 6 Surface water hydrological response variables and the thresholds used in defining the zone of potential hydrological change
A location on the river is deemed to be in the zone if the change in at least one of the nine variables exceeds its threshold. Probability estimates are derived from the predictions of 300 model replicates, each of which uses a unique set of model parameter values. A 5% threshold implies that at least 15 of the 300 replicates have modelled changes that exceed the relevant change threshold. If fewer than 15 replicates have modelled changes that exceed the threshold at a particular location, then the change in that hydrological response variable at that location is considered very unlikely to impact water-dependent landscape classes and assets. Table 11 and Figure 28 in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018) identify the model nodes and links in the river-modelling network where the modelled hydrological change exceeds at least one of the hydrological response variable thresholds.
The surface water zone of potential hydrological change includes reaches that make up the AWRA‑R link-node network (see Figure 34 in companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018a)), but also needs to include reaches that were not modelled, but which could potentially be impacted due to additional coal resource development. They include:
- Perennial and intermittent streams within the groundwater zone of potential hydrological change. It is assumed that within the groundwater zone of potential hydrological change, streams connected to regional groundwater could potentially be affected by additional coal resource development. Streams tagged as ‘perennial’, ‘lowly to moderately intermittent’ or ‘moderately to highly intermittent’ in the modelled flow regime spatial layer for the Hunter subregion (Bioregional Assessment Programme, Dataset 6) are assumed to be connected to groundwater.
- Ephemeral streams within areas of disruption to surface water drainage. By definition, ephemeral streams flow only in response to precipitation and have no baseflow component. In other words, they are not connected to regional groundwater, and are unlikely to be affected by groundwater drawdown due to additional coal resource development. However, they can potentially be affected by disruption to surface water drainage on coal mining sites. The ‘highly intermittent to ephemeral’ stream reaches in the modelled flow regime spatial layer for the Hunter subregion (Bioregional Assessment Programme, Dataset 6) that intersect the surface water maximum footprint areas for open-cut mines for additional coal resource development (Bioregional Assessment Programme, Dataset 2) are potentially impacted.
Of the perennial, intermittent and ephemeral streams identified above, some have been or will be materially altered by mine site excavations under baseline developments, and are unlikely to be affected further due to additional coal resource development. A visual inspection was undertaken of the selected streams, comparing remotely sensed imagery and the surface water maximum footprint areas for open-cut mines under the baseline (Bioregional Assessment Programme, Dataset 2); these reaches were manually removed. The remaining reaches were clipped upstream of the groundwater zone of potential hydrological change (since there can by definition be no changes in this area). They were also extended downstream of the groundwater zone of potential hydrological change to where they join a reach already in the surface water zone of potential hydrological change or a lake, and added to the network of potentially impacted streams. In all, about 1228 km of streams were identified as potentially impacted. These 1228 km of potentially impacted streams were used to select the 1 km x 1 km assessment units (Bioregional Assessment Programme, Dataset 7) that intersect the stream network (Bureau of Meteorology, Dataset 8) or contain riparian groundwater-dependent ecosystems (GDEs) (Bioregional Assessment Programme, Dataset 6), to define the surface water zone of potential hydrological change. GDEs within 200 m of the stream network were selected automatically, and this selection was inspected and manually adjusted to ensure that riparian vegetation that could potentially be impacted by changes in surface water hydrology is included in the zone. The surface water zone of potential hydrological change is shown in Figure 16. It shows the mine footprints that were included in the surface water modelling.
Figure 16 Surface water zone of potential hydrological change for the Hunter subregion
Additional coal resource developments (ACRDs) that were not included in the surface water modelling are shown to identify where, had they been included, the surface water zone might differ.
Data: Bioregional Assessment Programme (Dataset 2, Dataset 3, Dataset 4, Dataset 5), Bureau of Meteorology (Dataset 8)
The surface water zone of potential hydrological change covers an area of 1426 km2 (about 8% of assessment extent). Given the wide distribution of mines across the subregion, it includes most of the Goulburn River, most of the Hunter Regulated River, Wollombi Brook, and many smaller tributaries of the Goulburn and Hunter rivers. The Wyong River, Jilliby Jilliby Creek, parts of Ourimbah Creek that flow into the Tuggerah Lakes, and a number of smaller streams that drain to Lake Macquarie are part of the surface water zone of potential hydrological change – predominantly due to changes in groundwater.
3.3.1.3 Zone of potential hydrological change
Hydrological changes assessed as part of a BA are summarised for the zone of potential hydrological change. This is derived from the union of the groundwater zone of potential hydrological change (Figure 15) and the surface water zone of potential hydrological change (Figure 16) clipped to the assessment extent, and is shown in Figure 17. The Hunter zone of potential hydrological change covers an area of 3213 km2 (19% of assessment extent). A graphical summary of the areas (km2) of the zone and its surface and groundwater components is provided in Figure 18. The zone contains approximately 3136 km of stream (based on the Geofabric stream network (Bureau of Meteorology, Dataset 8)), of which 1228 km are potentially impacted (Section 3.3.1.2) and 1908 km are predominantly low-order, ephemeral streams that can be ruled out as unlikely to be affected by hydrological change. Five reporting areas (Table 7) are defined for reporting purposes to provide greater detail around key coal resource development areas within the subregion.
The zone of potential hydrological change is the first filter applied to landscape classes and water-dependent assets in the Hunter subregion as part of a ‘rule-out’ process for the impact and risk analysis. Landscape classes and assets that are completely outside the zone are very unlikely (less than 5% chance) to be impacted due to additional coal resource development and do not have qualitative landscape models or receptor impact models. Landscape classes that intersect the zone have qualitative models and/or receptor impact models, which are used to assess the potential impact of the modelled hydrological changes on the ecosystems represented by the qualitative model or receptor impact models. Details of the qualitative models and receptor impact models are provided in companion product 2.7 for the Hunter subregion (Hosack et al., 2018). Results from the receptor impact modelling are presented in Section 3.4.
3.3.1.3.1 Mine pit exclusion zone
Figure 17 also shows the mine pit exclusion zone defined for the Hunter subregion. It is based on open-cut mine footprints under the CRDP within the zone of potential hydrological change. The mine pit exclusion zone identifies areas within the zone of potential hydrological change that are within, or in close proximity to, open-cut mine pits, and where:
- modelled drawdowns are highly uncertain due to the very steep hydraulic gradients at the mine pit interface
- changes in the drawdown are inevitable where the mine pit intersects the regional watertable and will be at least to the depth of the mine pit
- other factors, such as physical removal of a wetland or creek, may have a larger impact on a landscape class than the predicted decrease in groundwater level
- impacts are predominantly site-scale, and assumed to be adequately addressed through existing development approval processes, and hence not the primary focus of BAs.
The modelled estimates of drawdown, while large, are considered unreliable for use in the receptor impact modelling. Local-scale groundwater models are expected to give better estimates of drawdown around mine pits than is possible using a regional-scale model. The mine pit exclusion zone within the zone of potential hydrological change covers an area of 435 km2 (Figure 18).
In the impacts on landscape classes and assets sections (Section 3.4 and Section 3.5, respectively), the initial rule-out assessment determines what is in the zone of potential hydrological change and, within that, what is in the mine pit exclusion zone. Features that have a groundwater dependency and occur in the mine pit exclusion zone do not have receptor impact modelling results generated for them; they are assumed to be ‘potentially impacted but not quantified’.
Changes in surface water were analysed on an individual stream link basis. Stream links, where it was determined that the change in hydrology for that stream link could not be interpolated from a nearby model node are labelled as ‘potential hydrological change’ in the maps presented in this section, and are reported in results tables under the header ‘potentially impacted but not quantified’.
3.3.1.3.2 Reporting areas
The zone of potential hydrological change has five discrete drawdown areas that correspond to the main areas potentially impacted due to additional coal resource development. Five reporting areas, which encompass the drawdown and potentially impacted surface water network associated with each mining area, have been defined to summarise results (Figure 17). Table 7 identifies the additional coal resource developments within each reporting area. In the Hunter river basin, four drawdown areas are connected by the surface water zone of potential hydrological change, which means that results reported for the Lower Hunter include changes from the Central Hunter, which include changes from the Lower Goulburn and the Upper Goulburn. The Macquarie-Tuggerah reporting area is almost entirely contained within the Macquarie-Tuggerah lakes basin and not hydrologically connected to the Hunter river basin reporting areas.
Table 7 Reporting areas and modelled additional coal resource developments
Figure 17 Zone of potential hydrological change for the Hunter subregion
Additional coal resource developments (ACRDs) that were not modelled in one or both of the hydrological models are shown to identify where, had they been included, the zone of potential hydrological change might differ. The reporting areas show where results are summarised as part of the impact and risk analysis.
Data: Bioregional Assessment Programme (Dataset 2, Dataset 3, Dataset 4, Dataset 5)
