2.5 Water balance assessment for the Namoi subregion

Executive summary

Gulligal Lagoon, which is located about halfway between Gunnedah and Boggabri on the western side of the Namoi River, NSW, 2005 Credit: Courtesy of Neal Foster

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 Namoi 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.

The Namoi subregion had six operational mines in the baseline and ten additional coal resource developments. The NSW Government bought back BHP’s Caroona coal exploration licences in August 2016. This occurred after the coal resource development pathway was finalised, so the Caroona Coal Mine remained part of the modelling for this assessment.

Water balances for the Namoi

The groundwater results for the Namoi subregion are reported for an area larger than the subregion (59,000 square kilometres) that includes the Gunnedah and Surat geological basins and the alluvium along the major water courses. The conceptual groundwater balance, 30 years into the future, shows the increase in the median 30-year mean annual pumping rate due to additional coal resource development is 9.1GL/year which is more than an order of magnitude greater than the baseline rate of 0.2 GL/year (but still very small (<3%) compared to the total licensed extractions of 316 GL/year).

The surface water balance was modelled at a specific location on the Namoi River downstream of the junction with Bohena Creek, as this area shows the largest combined changes in streamflow as a result of all coal mines in the Namoi subregion and the Narrabri Gas Project. The model predicts a less than 1% change to annual surface water outflow due to the additional coal resource developments.

The surface water – groundwater flux is the net volume of water exchanged between the groundwater and the river, calculated as the difference between groundwater flow to the river (i.e. baseflow) and leakage from the river to groundwater. A positive number indicates a stream leakage to groundwater. The modelled surface water – groundwater flux response to mine water pumping and hydraulic enhancement is a decreasing trend in river losses to groundwater over the three 30-year periods. The additional coal resource development results in losses of streamflow of 3.5 GL/year in the 2013 to 2042 period, 2.1 GL/year between 2043 and 2072, and 0.6 GL/year between 2073 and 2102. These additional losses are small compared to the trend in increasing stream losses through time in the baseline due to licensed extractions (40.7 GL/y for 2013 to 2042 up to 62 GL/y for 2073 to 2102).

Relationship to other products

This analysis is part of a series of products, which describe how surface water and groundwater is potentially impacted by coal resource development into the future. The 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 resource developments.

Last updated:
6 December 2018