Hydrological changes due to coal mining in the Hunter subregion are summarised in the form of water balances. Separate summaries are provided for groundwater and surface water; however, the streamflow term in the surface water balances includes a contribution from the groundwater model.
The groundwater balance covers the entire groundwater modelling domain. The key water balance term for the mining impact is the mine pumping volume. The reported values are based on mean annual extraction rates over each 30-year period and were inputs to the simulations. Between 2013 and 2042, the increase in the median 30-year mean annual pumping rate due to the additional coal resource development was 8.5 GL/year, a 46% increase on the baseline rate of 18.6 GL/year. Between 2043 and 2072, all baseline developments had ceased, but there was a comparatively small change in the rate of pumping (i.e. 30-year mean of 1.1 GL/year) for the additional coal resource development. The modelled surface water – groundwater flux response to mine water pumping and hydraulic enhancement is a decreasing trend over the three 30-year periods. The additional coal resource development results in reductions of the median groundwater contribution to streamflow of 0.1 GL/year in the 2013 to 2042 period, 0.9 GL/year difference between 2043 and 2072 and 1.2 GL/year between 2073 and 2102. These are equivalent to reductions of 0.2%, 1.8% and 2.4% relative to the baseline fluxes. The trend reflects the lagged response of the surface water – groundwater flux to mine pumping.
The changes due to the additional coal resource development upstream of node 41 on the Goulburn River flows are reductions of 0.5 GL/year between 2013 and 2042 and 0.2 GL/year for the 2043 to 2072 and 2073 to 2102 periods. At node 6 on the Hunter River, the median reductions in mean annual streamflow due to the additional coal resource development are 10.3 GL/year (2013–2042), 7.3 GL/year (2043–2072) and 4.5 GL/year (2073–2102). Surface water changes are correlated with mine footprint area, which reaches a maximum between 2013 and 2042. Following the cessation of mining, the lagged groundwater response to mining and the gradual rehabilitation of open-cut mines and enduring impacts on surface runoff from final voids and above longwall mines, mean the impacts on streamflow of coal resource development are sustained beyond the life of the mines. In the Goulburn River, the changes to streamflow due to the additional coal resource development are 1.0% to 0.4% of the baseline streamflow; in the Hunter River at Singleton, it varies from 2.4% to 1.1% of baseline streamflow over the reporting periods.
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- 2.5.1 Methods
- 2.5.2 Water balances
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