balances are provided for two subdomains of the Australian Water Resources Assessment river (AWRA-R) modelling area (Figure 4).
Rainfall was not varied as part of the analysis, so only a single rainfall volume is provided for the water balance for each 30-year period. The decreasing mean annual rainfall input over the three 30-year periods reflects the global warming changes assumed in the modelling (see submethodology M06 surface water modelling ()). In the , this assumes three seasons with rainfall reductions, with rainfall increases in the summer months (December to February) only.
The streamflow term in the surface water balances includes the surface water – flux component from the groundwater model. As stated in Section 2.5.1, water balances are reported separately for surface water and groundwater modelling, as they apply to different areas.
These water balances do not account for water that groundwater but is not discharged to the stream. The residual is a balancing term that comprises evapotranspiration, change in storage and leakage from the river.
balances at node 41 on the Goulburn River (corresponding to stream gauge 210006 at Kerrabee) are provided in Table 4 for 2013 to 2042, 2043 to 2072 and 2073 to 2102. The contributing area to node 41 is just over 3400 km2 and includes the : Bylong open-cut and underground, Moolarben open-cut and underground, Ulan West underground and Wilpinjong open-cut. The median, 10th and 90th percentile values for each variable of the and set of simulations are reported. The difference reflects the change attributable to the additional coal resource development, obtained from subtracting the median baseline value from the median CRDP value.
Mine and from the river for economic use were not modelled in the Goulburn River. Extractions from the Goulburn River are low compared to the Hunter Regulated River (see Section 126.96.36.199.2) and would have been modelled the same for the baseline and CRDP, so were not actively pursued. While mine discharges would be expected to change under the CRDP, suitable data were not obtained from which to develop rules for representing discharge behaviour in the model. Furthermore, mining reports indicated that:
- anticipated increases in mine discharge from Ulan West would be to the Talbragar River outside the subregion ()
- Moolarben mine would experience a water deficit for most of its Stage 2 development and not be discharging water to streams; and that it would need to source water from Ulan and Wilpinjong mines to meet demand, thereby reducing discharges to streams from those mines also ()
- the proposed Bylong mine aims to be a nil discharge site ().
Thus mine discharges to the Goulburn River system from additional coal resource development were considered likely to be small.
Table 4 Surface water balance for 2013 to 2042, 2043 to 2072 and 2073 to 2102 at node 41 (Goulburn River)
Surface water balance equation: P + M = Q + Ex + Res
For some (but not all) terms, three numbers are provided. The first number is the median, and the 10th and 90th percentile numbers follow in brackets. Numbers are rounded to one decimal place. NM = data not modelled
In all three 30-year periods, the additional coal resource development results in reductions in median streamflow relative to baseline. The biggest is registered in the 2013 to 2042 period with a 0.5 GL/year reduction averaged over the 30 years, equivalent to a 1.0% decrease from the median baseline streamflow. This is the main period of mining, during which the maximum mining footprint occurs, relatively limited rehabilitation of open-cut sites is assumed to have occurred and reductions in surface are greatest. By 2072, rehabilitation of open-cut mine sites to just a final void is assumed to have been completed. The difference in the median streamflow reduces to 0.2 GL/year (~0.4% of baseline) in the 2043 to 2072 and 2073 to 2102 periods, reflecting the cessation of mining and gradual rehabilitation of much of the open-cut mine areas.
The persistence of small differences in the median streamflow in the 2073 to 2102 period likely reflects the impacts on the surface water – flux from still propagating through the system from the mine sites, and the fact that in the surface water modelling the reduction in runoff above longwall mines is assumed to be enduring and that open-cut mines generally leave final voids. Thus surface runoff response does not return to a pre-disturbance condition.
Even though coal extraction and pumping of mine water are assumed to have finished by 2044, the impacts of groundwater drawdown on surface water – groundwater fluxes continue to work their way through the system over the subsequent two 30-year periods. Table 3 show the reductions in the median 30-year mean surface water – groundwater flux due to the additional coal resource development across the entire groundwater modelling domain increase from 0.1 GL/year in the 2013 to 2042 period to 0.9 GL/year in the 2043 to 2072 period to 1.2 GL/year in the 2073 to 2102 period.
balances at node 6 on the Hunter River (corresponding to stream gauge 210001 at Singleton) are provided in Table 5 for 2013 to 2042, 2043 to 2072 and 2073 to 2102. The contributing area to node 6 is about 16,485 km2 and includes, in addition to the in the contributing area to node 41 (Section 188.8.131.52.1), the open-cut mines at Ashton, Bengalla, Bulga, Drayton South, Liddell, Mount Arthur, Mount Owen, Mount Pleasant and Mount Thorley-Warkworth; and underground mines at Mount Arthur and Wambo. The median, 10th and 90th percentile values for each variable of the and set of simulations are reported. The difference reflects the change attributable to the additional coal resource development, obtained from subtracting the baseline value from the CRDP value.
Table 5 Surface water balance for 2013 to 2042, 2043 to 2072 and 2073 to 2102 at node 6 (Hunter River)
Surface water balance equation: P + M = Q + Ex + Res
For some (but not all) terms, three numbers are provided. The first number is the median, and the 10th and 90th percentile numbers follow in brackets. Numbers are rounded to one decimal place.
Mine s and from the river for economic use were modelled in the Hunter Regulated River. The regulated river is the main source of water to meet the needs of electricity generators, irrigators, mining operations and town water supply along these reaches. Annual discharges by the mines can vary in the model based on flow regime, but the model results suggest the additional coal resource development is not causing a significant change in mine discharges. Extractions from the river also do not vary between the baseline and CRDP because the same management rules are assumed for both. However, the model predicts a decrease in extractions over the 90 years in response to a drying climate, corresponding to reductions in the average annual allocation over time.
The additional coal resource developments are predicted to cause reductions in streamflow at node 6 in all three periods relative to the baseline streamflow. The biggest reduction of 10.3 GL/year occurs in the 2013 to 2042 period and represents a reduction of 2.4% of the median baseline streamflow. The difference between CRDP and baseline decreases to 7.3 GL/year (a 1.7% reduction relative to the baseline) in the 2043 to 2072 period and to 4.5 GL/year (a 1.1% reduction relative to the baseline) between 2073 and 2102.
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