2.5.2.1 Groundwater balance for the groundwater modelling domain

Groundwater balances for the Namoi subregion modelling domain (Figure 4) are provided in Table 4 for 2013 to 2042, Table 5 for 2043 to 2072 and Table 6 for 2073 to 2102. Water balance terms represent the mean annual volume in GL/year for each 30-year period across the entire groundwater modelling domain, an area of 59,000 km². The median and the 10th and 90th percentile values for each variable of the baseline and CRDP set of simulations are reported, whereas the difference reflects the change attributable to additional coal resource development, obtained from subtracting the median baseline value from the median CRDP value. As the values in the table represent percentiles of many model runs rather than the output of single model runs, the water balance components do not necessarily sum to zero (single model run water balance components sum to zero).

Table 4 Groundwater balance for Namoi subregion groundwater model domain for 2013 to 2042

The first number is the median, and the 10th and 90th percentile numbers follow in brackets. The difference is between the two median values. Numbers are rounded to one decimal place. Positive values represent water entering the model domain; negative values represent water leaving the model domain.

Data: Bioregional Assessment Programme (Dataset 1)

Table 5 Groundwater balance for Namoi subregion groundwater model domain for 2043 to 2072

The first number is the median, and the 10th and 90th percentile numbers follow in brackets. The difference is between the two median values. Numbers are rounded to one decimal place. Positive values represent water entering the model domain; negative values represent water leaving the model domain.

Data: Bioregional Assessment Programme (Dataset 1)

Table 6 Groundwater balance for Namoi subregion groundwater model domain for 2073 to 2102

The first number is the median, and the 10th and 90th percentile numbers follow in brackets. The difference is between the two median values. Numbers are rounded to one decimal place. Positive values represent water entering the model domain; negative values represent water leaving the model domain.

Data: Bioregional Assessment Programme (Dataset 1)

The dominant water balance terms are recharge and surface water – groundwater flux on the input side and licensed extractions and evapotranspiration on the output side. The recharge term in the baseline and CRDP simulations varies across the three periods due to changes in rainfall and evapotranspiration from the increases in temperature assumed in the modelling to reflect global warming (see companion submethodology M06 (as listed in Table 1) for surface water modelling (Viney, 2016)). However, the difference between the baseline and CRDP recharge in each 30-year period is predicted to be zero because the groundwater model does not account for changes in recharge that might arise as a result of mine subsidence or excavation of open-cut mines.

Licensed extractions (under NSW’s Water Management Act 2000) for non-mining uses, such as irrigation, stock and domestic, town water supply and industrial uses, were modelled in the groundwater model. It was assumed that extractions were the same under the baseline and under the CRDP (i.e. changes in mine water use under the CRDP do not affect water use by other licence holders). It was assumed that rates of extraction were at the maximum entitlement under each licence and that there was no increase in licensed entitlements into the future. Thus the non-mining groundwater extractions do not differ between baseline and CRDP in any of the 30-year periods.

The primary differences in the water balance between the baseline and CRDP are due to the extractions for coal resource development; however, groundwater extraction for mining and CSG purposes is small (~3%) compared to the licensed extractions for non-mining uses. The greatest difference between the baseline and CRDP is seen in the first 30-year period (2013 to 2042) and by the last 30-year period all CRDP developments have completed and there are no extractions in either the baseline or CRDP.

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. In all three 30-year periods, the median of the 30-year mean annual surface water – groundwater flux under the CRDP is greater than under baseline, indicating an increase in the river losses to groundwater with more intensive coal resource development.

Evapotranspiration from groundwater is a major component of the water balance in each 30-year period, this is consistent with remotely sensed actual evapotranspiration (AET) being greater than rainfall in large parts of the model domain (Guerschman et al., 2009). Evapotranspiration from groundwater is influenced by not just atmospheric conditions, but also the position of the watertable relative to the evapotranspiration extinction depth (a parameter in the model which approximates the maximum rooting depth below which vegetation and atmospheric processes cannot extract water). Since the hydrological change due to additional coal resource development is through pumping-induced drawdown of the watertable around the mining operations, there is a difference between the CRDP and baseline simulations in each of the 30-year periods, which reflects changes in the area of watertable above the evapotranspiration extinction depth.

The flow across the model boundaries is small and does not change between time periods or between the baseline and CRDP. The change in storage is always a positive value indicating a reduction in the stored volume through each time period. Under the baseline the change in storage is decreasing through the three time periods indicating that the model has not come to a dynamic equilibrium; this is caused by the licensed extractions and the reduction in recharge due to the global warming signal applied.

The changes in the groundwater balance due to additional coal resource development are small compared to the changes due to licenced extractions (not coal related) and reductions in recharge through time.