The formal uncertainty analysis considered hydraulic conductivity, recharge and storage values, but not model conceptualisation or the parameters used to specify drain and river boundary conditions. However, model predictions of groundwater drawdown are unlikely to be sensitive to the relatively high Drain and River cell conductance values. Posterior distributions of calibration-constrained hydraulic conductivity parameters (Kh and Kv) vary over several orders of magnitude in the OGIA model layers. Modelled recharge values have a skewed distribution, with most values being less than 10 mm/year (Figure 15).
Hydrological changes arising from coal resource development for two possible futures – the baseline and the coal resource development pathway (CRDP) – are assessed using a probabilistic approach. Thresholds used to describe hydrological changes in the Maranoa-Balonne-Condamine subregion are consistent with those described in the Surat Underground Water Impact Report (UWIR). Maximum baseline groundwater drawdown associated with coal seam gas (CSG) production (in excess of 700 m) is predicted near the towns of Chinchilla and Roma. Hydrological changes in excess of 0.2 m baseline groundwater drawdown in the vicinity of the five baseline coal mines are generally within 5 to 10 km (maximum 15 to 20 km) of the modelled pits.
Additional groundwater drawdown in the vicinity of proposed coal mines in excess of 0.2 m (probability, p=0.05) is generally within 20 to 40 km (maximum 50 to 60 km) of the modelled pits. Hydrological changes in excess of 5 m (p=0.05) additional drawdown are predicted in 153 of the 9213 bores in New Acland Coal Mine water balance area and 7 of the 411 bores in The Range water balance area. Overall, 86 of the 19,000 economic bores are predicted to experience more than 5 m additional groundwater drawdown in the Maranoa-Balonne-Condamine subregion.
The qualitative uncertainty analysis describes the rationale for and scores the effect of nine assumptions or model choices on model predictions. The representation of surface water – groundwater interactions, mine pit dewatering, CSG activities and horizontal and vertical discretisation in the model and the quantitative uncertainty analysis are judged to have the greatest potential effect on model predictions.
This section describes the quantitative and qualitative uncertainty analyses used to give probabilistic estimates of the hydrological changes associated with coal resource development in the Maranoa-Balonne-Condamine subregion. The quantitative uncertainty analysis numerically evaluates the degree to which each of the model inputs (parameters) affects the model predictions. Model predictions of groundwater drawdown associated with coal resource development are presented in two ways. Firstly, the 95th percentile of groundwater drawdown under the baseline future and the probability of exceeding additional groundwater drawdown thresholds under the CRDP future are presented spatially for each model layer. Secondly, the maximum additional drawdown and time to maximum drawdown at the economic bores within the two water balance areas is presented. The qualitative uncertainty analysis describes the rationale for and scores the effect of each assumption on model predictions.