- Bioregional Assessment Program
- Namoi subregion
- 3-4 Impact and risk analysis for the Namoi subregion
- 3.3 Potential hydrological changes
The baseline and the coal resource development pathway (CRDP) futures provide the basis for assessing the hydrological changes attributable to additional coal resource development in the Namoi subregion.
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 modelled additional coal resource development. It spans an area of 2299 km2.
The surface water zone of potential hydrological change is defined as the area in which the change in any one of nine surface water hydrological response variables exceeds the specified thresholds. In addition to the modelled reaches, the surface water zone also includes non‑modelled streams in the groundwater zone of potential hydrological change. A total stream length of 5521 km is included in the surface water zone.
The combined groundwater and surface water zone of potential hydrological change covers an area of 7014 km2.
It is very likely that an area of at least 156 km2 will experience 0.2 m of drawdown due to modelled additional coal resource development (additional drawdown); it is very unlikely that more than 2299 km2 exceeds 0.2 m of additional drawdown. It is very unlikely that more than 853 km2 exceeds 2 m of additional drawdown, and very unlikely that more than 520 km2 exceeds 5 m of additional drawdown. These numbers include the 116 km2 in the mine pit exclusion zone, where the modelled drawdowns are considered unreliable due to steep hydraulic gradients at the pit face.
The potential hydrological changes due to modelled additional coal resource development on surface water are assessed using the hydrological response variables: zero-flow days, high-flow days and annual flow. Large changes in flow regime are very likely in the Back Creek, Merrygowen Creek, Bollol Creek, Maules Creek, Driggle Draggle Creek and two unnamed creeks near Lake Goran. Most of the creeks have catchment areas much less than 100 km2 and their effects are localised. The Namoi Regulated River, into which these creeks flow, is not very sensitive to changes in inflows from these creeks. Only the Namoi River and all of its potentially affected tributaries were modelled since the effects of small intermittent streams cannot be ascertained without a detailed finer resolution modelling. The modelled streams make up about 35% of the total stream length in the zone of potential hydrological change.
Results for the Namoi Regulated River show that widespread decreases in mean annual flow of greater than 5% is unlikely.
Generally, the predicted changes are small relative to the rainfall-related interannual variability, especially for annual flow and high-flow days. There is a chance that changes in zero-flow days could significantly change the zero-flow regime in streams near all the mining areas, with smaller intermittent streams close to Boggabri, Maules Creek, Tarrawonga and Watermark additional coal resource developments particularly at risk.
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 to minimise potential water quality impacts from coal resource development. 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 could occur.
Increases in baseflow, potentially leading to increases in alluvial aquifer and stream salinity, cannot be ruled out. However, the magnitude and extent of water quality changes cannot be determined without specifically representing water quality parameters in the modelling. This remains a knowledge gap.
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/NAM/hydrologicalchanges.
Potential hydrological changes due to additional coal resource development are based on hydrological response variables resulting from the surface water and groundwater modelling, reported in companion product 2.6.1 (Aryal et al., 2018) and companion product 2.6.2 (Janardhanan et al., 2018) for the Namoi subregion. These hydrological response variables represent the maximum difference in results between the coal resource development pathway (CRDP) and baseline for groundwater drawdown for a range of streamflow characteristics. They are also the basis for defining the zone of potential hydrological change – the focal extent for the impact and risk analysis (Section 3.3.1).
Potential impacts on groundwater and surface water within the zone of potential hydrological change are presented in Section 3.3.2 and Section 3.3.3, respectively. 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 Namoi subregion is considered in Section 3.3.4. All analyses are carried out on ‘assessment units’ which, in the Namoi subregion, consist of 1 km2 (1 km x 1 km) grid cells that cover the entire assessment extent.
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 Namoi subregion (Ickowicz 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 hydrological response variables are presented as part of the impact and risk analysis in Section 3.4 and Section 3.5.
Product Finalisation date
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.4.1 Overview
- 3.4.2 Landscape classes that are unlikely to be impacted
- 3.4.3 'Floodplain or lowland riverine' (non-Pilliga) landscape group
- 3.4.4 'Non-floodplain or upland riverine' (non-Pilliga) landscape group
- 3.4.5 Pilliga riverine (upland and lowland)
- 3.4.6 Potentially impacted landscape classes lacking quantitative ecological modelling
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that are not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product