3.3 Potential hydrological changes


Potential hydrological changes were derived for the two futures considered in bioregional assessments (BAs): the baseline and the coal resource development pathway (CRDP).

The groundwater zone of potential hydrological change is defined as the area with at least a 5% chance of drawdown exceeding 0.2 m in the regional watertable due to additional coal resource development (as predicted by numerical groundwater modelling). In the Hunter subregion, it spans an area of 2441 km2 and comprises five discrete drawdown areas in the Upper Goulburn, Lower Goulburn, Central Hunter, Lower Hunter and Macquarie-Tuggerah lakes basins.

The surface water zone of potential hydrological change corresponds to the area along the modelled stream network where the change in at least one of nine surface water hydrological response variables exceeds its threshold due to the additional coal resource development. The thresholds can be generally described as at least a 5% chance of a 1% (or 3 day) or greater change in a flow volume or frequency. It also includes non-modelled perennial and intermittent streams in the groundwater zone of potential hydrological change and ephemeral streams that are directly affected by open-cut mining and site facilities. A total stream length of 1228 km was used to select the assessment units that define the surface water zone.

The combined groundwater and surface water zone of potential hydrological change in the Hunter assessment extent covers an area of 3213 km2.

It is very likely (greater than 95% chance) that an area of at least 528 km2 could experience 0.2 m of drawdown due to additional coal resource development (additional drawdown); it is very unlikely (less than 5% chance) that more than 2441 km2 exceeds 0.2 m of additional drawdown. It is very unlikely that more than 927 km2 exceeds 2 m of additional drawdown, and very unlikely that more than 524 km2 exceeds 5 m of additional drawdown. When drawdown predictions are constrained using local information, the range of predicted drawdown extent can be reduced. For example, drawdown extents predicted around Wallarah 2 based on local hydrogeological information are predicted to be smaller than extents based on the regional parameter set.

The potential impacts due to additional coal resource development on surface water are assessed using hydrological response variables: low-flow days, high-flow days and annual flow. Results of regional-scale modelling suggest large changes in flow regime are very likely in Loders Creek, Dry Creek and two unnamed creeks near the Mount Pleasant and Mount Thorley–Warkworth coal mines. Dry Creek and the unnamed creeks are small and the hydrological changes are localised. The Hunter Regulated River, into which these creeks flow, is not very sensitive to changes in inflows from them. Wollar Creek, Saddlers Creek and the Wyong River are modelled to have relatively large hydrological changes at the 50th percentile. Wollar Creek, Saddlers and Loders Creek have a hydrological effect on the Goulburn and Hunter rivers into which they flow. However, changes in baseflow to the Goulburn and Hunter rivers due to groundwater drawdown could be more significant than changes in tributary inflows on Goulburn River and Hunter River flows.

Results for the Hunter Regulated River show that decreases in mean annual flow of between 1% and 2% are very likely. These changes need to be interpreted with caution, since the Australian Water Resources Assessment river model (AWRA-R) has not been constructed to specifically represent operational management of releases from Glenbawn and Glennies Creek storages.

The potentially large changes in hydrology predicted in the Wyong River reduce considerably when the regional results are constrained using local information. The small chance of at least 200 more low-flow days per year based on the regional analysis becomes a small chance of at least 7 additional low-flow days when the baseflows from the groundwater model are based on local hydrogeological data.

Generally, the modelled changes are small relative to the interannual variability due to climate, especially for annual flow and high-flow days. There is a chance that changes in low-flow days could significantly impact the values associated with streams near all the mining areas, with smaller intermittent and perennial streams close to Central Hunter and Lower Hunter additional coal resource developments particularly at risk. Areas identified as at risk of large hydrological changes require further investigation using local-scale information.

Any change in hydrology could result in changes in stream water quality; however, this was not modelled. A range of regulatory requirements are in place in NSW, which are intended to minimise potential salinity impacts from coal resource development. In the Hunter Regulated River, a salinity trading scheme is managing mine and industry discharges to the river in order to keep salinity to acceptable levels. Discharges to unregulated streams are managed through licences, with conditions attached governing the volume, quality and timing of discharges. Groundwater is typically more saline than surface runoff, which suggests that the predicted reductions in baseflow are more likely to lead to decreases in stream salinity. However, the actual effects depend very much on local conditions, and increases in stream salinity cannot be ruled out.

Users can visualise more detailed results for hydrological changes using a map-based interface on the BA Explorer, available at www.bioregionalassessments.gov.au/explorer/HUN/hydrologicalchanges.

Potential hydrological changes due to additional coal resource development are summarised using hydrological response variables based on results from regional-scale surface water and groundwater modelling, reported in companion product 2.6.1 (Zhang et al., 2018) and companion product 2.6.2 (Herron et al., 2018b) for the Hunter subregion. These hydrological response variables have been defined to represent the maximum difference between the CRDP and baseline for groundwater drawdown and a range of streamflow characteristics. They have also been used to define the zone of potential hydrological change – the focal extent for the impact and risk analysis (Section 3.3.1).

Potential changes in groundwater and surface water within the zone of potential hydrological change are presented in Section 3.3.2 and Section 3.3.3, respectively. Areas are identified that are more at risk of hydrological changes, and hence potentially adverse impacts, due to additional coal resource development. Local scale information is needed to refine the assessment of risk and determine the appropriate management response in these areas. While changes in water quality were not part of the hydrological modelling, the potential for changes in water quality due to additional coal resource development in the Hunter subregion is considered in Section 3.3.4.

Additional hydrological response variables have been defined for input into the landscape class qualitative models and receptor impact models (companion product 2.7 for the Hunter subregion (Hosack et al., 2018)), and for quantifying potential impacts on economic assets. They represent key water dependencies in these systems and are based on average differences over 30-year and 90-year periods. Changes in these variables are presented as part of the impact and risk analysis in Section 3.4 and Section 3.5.

Last updated:
15 March 2019
Thumbnail of the Hunter subregion

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