- Bioregional Assessment Program
- Hunter subregion
- 2.5 Water balance assessment for the Hunter subregion
- 2.5.2 Water balances
- 220.127.116.11 Groundwater balance for the groundwater modelling domain
Groundwater balances for the Hunter subregion modelling domain (Figure 5) are provided in Table 3 for 2013 to 2042, 2043 to 2072 and 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 34,000 km2. The median, 10th and 90th percentile values for each variable of the baseline and coal resource development pathway (CRDP) set of simulations are reported, whereas the difference reflects the change attributable to the additional coal resource development, obtained from subtracting the median baseline value from the median CRDP value. Figure 5 summarises the difference in the median values of each groundwater balance variable for the three 30-year periods.
The dominant water balance terms are recharge and evapotranspiration. 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 submethodology M06 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 pits.
Evapotranspiration, on the other hand, 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 the 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. While, intuitively, one might expect increasing drawdown to increase the area of watertable below the evapotranspiration extinction depth, model results indicate small increases in evapotranspiration under the CRDP. This is because the modelled redistribution of the watertable reflects the combined effects of hydraulic enhancement and landscape position and, as illustrated in Section 18.104.22.168 of companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b), can lead to localised areas of higher watertable. It must be noted that these increases in the median 30-year mean annual evapotranspiration range from 0.5 to 2.1 GL/year, less than a 1% increase in all periods and much smaller than the difference in the 10th and 90th percentiles for the baseline for all periods.
Table 3 Groundwater balance for Hunter subregion groundwater model domain for 2013 to 2042, 2043 to 2072 and 2073 to 2102
Groundwater balance equation: Re = ET + ExL + ExM +Qbf + B + ΔS
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.
Data: Bioregional Assessment Programme (Dataset 1)
The key water balance term for the mining impact is the mine pumping volume. The reported values are based on the 30-year annual mean extraction rates, which were inputs to the simulations. Between 2013 and 2042, the difference in the median 30-year mean annual pumping rates under the baseline and CRDP 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, thus the comparatively small change in the rate of pumping (1.1 GL/year) was due to the additional coal resource development. In the third period, no mining operations were occurring in the model simulations and there is no difference between the inputs to the baseline and CRDP simulations. The range of values, as represented by the 10th and 90th percentiles, reflects the modelled range using multipliers between 0.5 and 1.5 of the pumping estimates from mine environmental assessment reports (see Section 2.1.6 of companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018a)).
Figure 5 Change in the median value of each groundwater balance term due to the additional coal resource development
ET = evapotranspiration
Data: Bioregional Assessment Programme (Dataset 1)
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 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 level permitted 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.
Changes in other groundwater balance terms, caused by mine pumping and hydraulic enhancement, are reflected predominantly in the surface water – groundwater flux. This is the net volume of water exchanged between the regional aquifer and the river, calculated as the difference between groundwater flow to the river (i.e. baseflow) and leakage from the river to groundwater. In all three 30-year periods, the median of the 30-year mean annual surface water – groundwater flux under the CRDP is smaller than under baseline, indicating a reduction in baseflow in the river with more intensive coal resource development. Across the range (10th to 90th percentile) of modelled runs, this flux varies from about 30% to 160% of the median, which reflects the variation in modelled pumping rate, as well as the influence of varying hydraulic parameters and depth of the river in the model. The difference in medians attributable to the additional coal resource development varies from a decrease of 0.1 GL/year in the 30-year mean annual surface water – groundwater flux for the 2013 to 2042 period to a 0.9 GL/year decrease for the 2043 to 2072 period to a 1.2 GL/year decrease in the 2073 to 2102 period. This trend of an increasing impact over the three 30-year periods reflects a lagged response to mine pumping, which is at its maximum in the first 30-year period. In percentage terms, the reduction in groundwater flow to the river due to the additional coal resource development represents 0.2%, 1.8% and 2.4% of the groundwater flow to the river under the baseline for the three periods.
The boundary flow represents the net flux at the general head boundaries of the modelling domain. These boundaries were selected so that they have no impact on the predictions. The differences in boundary flows between the baseline and CRDP are much smaller than the difference in the 10th and 90th percentiles of the baseline indicating that the assumptions about these boundaries are valid. These boundary fluxes might be of concern if the volumes were large, but the volumes are small and therefore largely inconsequential in terms of quantifying the changes on the water balance due to the additional coal resource development.
The change in storage term is typically a balancing term in a water balance, and for a system in equilibrium should be on average zero. In the 2013 to 2042 period, both the baseline and CRDP show decreases in stored water due to pumping of mine water. The difference in stored water between the baseline and CRDP is significant, with almost 10 GL/year (67%) less stored water under the CRDP relative to the baseline. This largely reflects the greater losses from mine pumping under the CRDP. Once groundwater pumping ceases, the changes in storage under the CRDP and baseline are smaller, as is the difference between them.
Product Finalisation date
- 2.5.1 Methods
- 2.5.2 Water balances
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product