Summary and discussion

The prediction results show that the additional coal resource development in the Gloucester subregion has more noticeable impacts on hydrological response variables in northern receptors than in the southern receptors. They are particularly apparent in streamflows along the Avon River, a tributary of the Gloucester River, and where two of the three coal mines and most of the proposed CSG field are located. Despite there being one coal mine with an additional coal resource development footprint in the Karuah river basin, there is comparatively little hydrological impact on any response variables in the southern part of the subregion.

The comparison among the 30 receptors shows that the relative hydrological changes are larger for the receptors where the maximum additional coal resource development percentage is larger. For instance, the receptors with the two largest additional coal resource development footprints are receptors 14 and 11, where the percentage increases in footprint are 18% and 6%, respectively. The resulting median pmax values for the three high-flow flux-based variables (AF, P99 and IQR) are in the range between –15% and –17% for node 14 and in the range –4% to –6% for node 11.

For every hydrological response variable, the biggest impacts (in terms of pmax for the flux-based variables and in terms of amax for the frequency-based variables) are predicted to occur at node 14. This node is located downstream of the expansions to the Stratford Mining Complex and within the proposed Gloucester Gas Project stage 1 field. There are bigger predicted changes in amax at nodes further downstream, but the proportional impacts of these changes are diluted by relatively unaffected inflows from the Upper Gloucester (node 8) and Barrington (node 3) rivers.

The impacts due to additional coal resource development on the low-streamflow hydrological response variables (daily streamflow at the first percentile, low-flow days, number of low-flow spells and the longest low-flow spell) do not appear to be more noticeable than those on the high-streamflow hydrological response variables (annual flow, daily streamflow at the 99th percentile and flood days). The flux-based variables (AF, IQR, P99 and P01) have similar median pmax values at the most heavily impacted nodes. Similarly, the two frequency-based variables that are most directly comparable – FD and LFD – have roughly commensurate changes in median amax values. However, the uncertainty in predicted pmax (for the flux-based variables) and amax (for the frequency-based variables), and in predicted tmax is greater for the low-flow variables.

For high-streamflow hydrological response variables, the tmax at receptors with noticeable changes occurs approximately when the maximum additional coal resource development occurs. This indicates that the instantaneous streamflow reduction caused by the additional mine footprint dominates amax and pmax in these hydrological response variables while the changes from the cumulative impact on baseflow over time caused by watertable drawdown are negligible.

For low-streamflow hydrological response variables, the tmax at receptors with noticeable changes does not occur consistently with the time when the maximum additional coal resource development footprint occurs. Furthermore, at the most heavily impacted node (node 14), the predicted median tmax values tend to be a little later for two of the low-flow hydrological response variables, P01 and LLFS. This indicates that the causes of the impacts on the low-flow variables are controlled by a combination of the instantaneous impact from the additional mine footprints and the cumulative impact on baseflow over time caused by watertable drawdown. Therefore, it is expected that uncertainty in predicting the changes on low-streamflow hydrological response variables is much larger than that on high-streamflow response variables.

Last updated:
31 October 2018
Thumbnail of the Gloucester subregion

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