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2.5 Water balance assessment for the Gloucester subregion

Executive summary

View of the Gloucester valley NSW with the Barrington River and associated riparian vegetation in the foreground and the township Gloucester in the distance looking south from the Kia Ora Lookout, 2013 Credit: Heinz Buettikofer, CSIRO

This product uses results from surface water and groundwater numerical modelling to assess how water balances might be different as a result of coal resource development in the Gloucester subregion for three time periods: 2013 to 2042, 2043 to 2072, and 2073 to 2102.

 Water balances are reported for the two futures considered in a bioregional assessment:

  • baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas fields that were commercially producing as at December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and coal seam gas fields that are in the baseline as well as those that were expected to begin commercial production after December 2012.

The difference in results between these potential futures is due to the additional coal resource development – all coal mines and coal seam gas fields, including expansions of baseline operations, that were expected to begin commercial production after December 2012.

For this water balance assessment, there were four additional coal resource developments: a new open-cut coal mine at Rocky Hill, expansions of the Duralie mine and Stratford mining complex, and the AGL Gloucester Gas Project Stage 1 coal seam gas development. AGL announced in February 2016 it would not proceed with this development. However, this development was already included in the hydrological modelling for the CRDP, which was finalised in October 2015.  

Water balances for the Gloucester subregion

The product reports separate groundwater and surface water balances.

Potential surface water impacts due to coal resource development accumulate along the north-flowing Avon River and south-flowing Karuah River. These rivers are not hydrologically connected (i.e. water does not flow between them). Water balances are reported at two nodes: one on the Avon River and one on the Mammy Johnsons River within the Karuah river basin.

In the Avon river basin, modelling shows that additional coal resource development at the open-cut Stratford and Rocky Hill coal mines and the AGL coal seam gas development reduces mean annual surface water outflow by 1105 megalitres per year between 2013 and 2042. Smaller reductions are predicted for the subsequent two 30-year periods to 2102 (717 megalitres per year for 2043 to 2072 and 712 megalitres per year for 2073 to 2102). This is primarily due to the retention of surface water runoff on the mine sites rather than a decrease in baseflow due to groundwater extraction. This change in outflow is less than 2% of the baseline coal resource development (baseline) flow and is within the uncertainty of the predicted surface water outflow.

In the Mammy Johnsons river basin, expansion of the Duralie Coal Mine leads to modelled reductions of surface water outflows of 163 megalitres per year between 2013 and 2042, rising to 186 megalitres per year in 2043 to 2072, a level that persists through the subsequent 30-year period. These reductions are dominated by the retention of surface water runoff on the mine sites rather than the baseline responses due to groundwater extraction. This change in outflow represents less than 0.3% of the baseline flow and is within the uncertainty of the predicted surface water outflow.

It was not possible to provide a reliable estimate of the groundwater balance because the magnitude of upward flow of groundwater from deeper layers into the alluvium is not adequately constrained.

This analysis is part of a series of products, which describe how water balance is potentially impacted by coal resource development into the future. This product informs impact and risk analysis (Component 3 and 4) products, which examine the risk to, and potential impacts on, water resources and water-dependent assets due to coal seam gas or coal mining developments.

Last updated:
15 June 2018