What are the potential hydrological changes?

The predicted drawdown (Box 4) is used to define a zone to ‘rule-in’ or ‘rule-out’ potential hydrological change. The zone is the area with at least a 5% chance of greater than 0.2 m drawdown due to additional coal resource development (Figure 5). This threshold is consistent with the most conservative minimal impact thresholds in Queensland or NSW state regulations. Because impact and risk analysis was not undertaken for this bioregion, only groundwater hydrological changes were used to define the zone. The zone is defined by changes in the near-surface aquifer from which most ecological assets source water. Water-dependent landscapes and ecological assets outside of this zone are very unlikely to experience any hydrological change due to additional coal resource development. Within the zone, potential impacts may need to be considered further in an impact and risk analysis and smaller-scale analyses that take into account local conditions.

The zone of potential hydrological change can also be defined in deeper geological layers. Impact and risk analysis was not carried out for the Clarence-Moreton bioregion and hence impacts at these deeper layers were not assessed. Figure 42 of Cui et al. (2016b) shows the probability of exceeding 0.2 m drawdown at model nodes in deeper layers.

The models used in the assessment produced a large number of predictions of groundwater drawdown and streamflow characteristics rather than a single number. This results in a range or distribution of predictions, which are typically reported as probabilities – the percent chance of something occurring (Figure 6). This approach allows an assessment of the likelihood of exceeding a given magnitude of change, and underpins the assessment of the risk.

Hydrological models require information about physical properties such as the thickness of geological layers and how porous aquifers are. It is unknown how these properties vary across the entire assessment extent (both at surface and at depth), and therefore the hydrological models were run thousands of times using different sets of values from credible ranges of those physical properties each time. The model runs were optimised to reproduce historical observations, such as groundwater level and changes in water movement and volume.

A narrow range of predictions indicates more agreement between the model runs about the result, which enables decision makers to anticipate potential impacts more precisely, and a wider range indicates less agreement and hence more uncertainty in the outcome.

The distributions created from these model runs are expressed as probabilities that hydrological variables (such as drawdown) exceed relevant thresholds, as there is no single ‘best’ estimate of change. In this assessment, the estimates of drawdown are shown as a 5%, 50% or 95% chance of exceeding thresholds. Throughout this synthesis, the term ‘very likely’ is used to describe where there is a greater than 95% chance that the model results exceed thresholds, and ‘very unlikely’ is used where there is a less than 5% chance. While the model was based on the best available information, if the range of parameters used was not realistic, or if the modelled system does not reflect reality sufficiently, these modelled probabilities might vary from the actual probability of exceeding thresholds.

Figure 7 Key areas for reporting probabilistic results

The assessment extent was divided into smaller square assessment units and the probability distribution (Figure 6) was calculated for each. In this synthesis, results are reported with respect to the following key areas (Figure 7):

1. outside the zone of potential hydrological change, where hydrological changes (and hence impacts) are very unlikely (defined by maps showing the 5% chance)
2. inside the zone of potential hydrological change, comprising the assessment units with at least a 5% chance of exceeding the threshold (defined by maps showing the 5% chance). Further work is required to determine whether the hydrological changes in the zone translate into impacts for water-dependent assets and landscapes
3. with at least a 50% chance of exceeding the threshold (i.e. the assessment units where the median is greater than the threshold; defined by maps showing the 50% chance)
4. with at least a 95% chance of exceeding the threshold (i.e. the assessment units where hydrological changes are very likely; defined by maps showing the 95% chance).