The Gloucester and models were designed to quantify potential changes in hydrology caused by undertaken at multiple coal resource developments. This enabled an assessment of the of coal resource development at a regional scale. This analysis focused on the , outside of which potential from are (less than 5% chance). See Section 3.3.1 for more details.
A deeper was coupled to a shallow numerical alluvial model to estimate spatially explicit probabilities of hydrological changes due to coal resource development. Potential hydrological changes were calculated for two futures considered in : the and the (CRDP). The difference in results between and baseline is the change that is primarily reported in a BA. This change is due to the , which in the comprises three open-cut coal mines and one CSG field (Section 188.8.131.52).
The deeper analytic element model was developed using TTim () to predict the response in the weathered zone to coal resource development. The bottom of the weathered zone is the lower boundary of the alluvium (where present) and the land surface layer outside of the alluvium. The analytic element model stochastically simulated the representation of faults and hydraulic properties, thus incorporating both conceptual and parameter in the predictions. The numerical model of the alluvium was developed in MODFLOW (). It propagated the from the weathered zone below to the and also predicted the change in – flux to the stream network. Results were generated at across the modelling domain. representing maximum drawdown and the year of maximum drawdown were defined for summarising model results. The details of the groundwater modelling are reported in companion product 2.6.2 for the Gloucester subregion ().
Groundwater modelling results are reported for the , from which most ecological source water. For BA purposes, the regional watertable is the upper groundwater level within the unconfined, near-surface (not perched), where pore water pressure is equal to atmospheric pressure. It was constructed by combining the watertable from all the near-surface geological units (or layers) in which it occurs. Within the Gloucester subregion, the regional watertable exists in the alluvia of the Gloucester and Karuah river systems and in the weathered and fractured zone outside these alluvia. The change in drawdown in the regional watertable was obtained from the analytic element model for the weathered and fractured zone and combined with the change in drawdown in the alluvia from the MODFLOW models.
Drawdown from the groundwater model nodes was spatially interpolated to obtain valid posterior distributions at all across the modelling domain. A Delaunay Triangulation was generated in the R package ‘tripack’ (). For each assessment unit, the of maximum drawdown at the nodes of the triangle enclosing the assessment unit were linearly interpolated to the new location. A forward-backward cubed-root transform was applied during the interpolation to improve performance over potentially non-linear surfaces.
Section 184.108.40.206 describes how the groundwater modelling results were used to define the .
modelling for the was undertaken using the Australian Water Resources Assessment landscape model (AWRA-L). Details of the application of this model to the Gloucester subregion are reported in companion product 2.6.1 (). No river modelling was carried out because the rivers in the subregion are unregulated and their catchments are relatively small. Instead, streamflow was predicted by accumulating output from a spatially explicit streamflow model (AWRA-L). Coal resource development affects surface water hydrology directly through disruption of surface water drainage and some aspects of operational water management, and indirectly through changes in surface water – fluxes in response to from mine and CSG .
Results for eight were reported for 34 across the Gloucester subregion. The locations of these model nodes are shown in Figure 7. In order to carry out the and analysis, results from these model nodes needed to be extrapolated to stream links. Extrapolating these changes is important in order to get some sense of the changes in surface water across the entire .
The process for extrapolating hydrological response variable values from model nodes to stream links is shown schematically in Figure 8. The schematic includes a number of stream links with no model nodes (dashed lines) for which model results were not generated, but were important for doing the extrapolations. The junctions of these non-modelled streams with the modelled network correspond to significant changes in streamflow and hence represent limits to extrapolation from the nearest upstream or downstream model node. Extrapolations were also not undertaken for stream links potentially affected by changes in from open-cut coal mine operations (e.g. Avondale Creek, the reach between nodes 14 and 16 on Dog Trap Creek, and the reach between nodes 12 and 13 on Waukivory Creek). Because the impact of a mine on streamflow diminishes with increasing distance downstream of the mine, it is difficult to know how far along the reach it is reasonable to extrapolate from the nearest model node before the hydrological changes at that node are no longer representative of the hydrological changes at that point in the reach. Thus, they were classified as potentially impacted and included in the .
Section 220.127.116.11 describes how the surface water modelling results were used to define the .
18.104.22.168 Representing predictive uncertainty
The models used in the assessment produced a large number of predictions of 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 9). This approach allowed an assessment of the of exceeding a given magnitude of change, and underpinned the assessment of the .
Groundwater models require information about physical properties such as the thickness of geological layers, how porous are, and whether faults are present. As the exact values of these properties are not always known, modellers used a credible range of values, which are based on various sources of data (commonly point-scale) combined with expert knowledge. The groundwater model was run thousands of times using a different set of plausible values for those physical properties each time. Historical observations, such as groundwater level and changes in water movement and volume, were used to constrain and validate the model runs.
The complete set of model runs produced a range or distribution of predictions (Figure 9) that are consistent with available observations and the understanding of the modelled system. The range conveys the confidence in model results, with a wide range indicating that the expected outcome is less certain, while a narrow range provides a stronger evidence base for decision making. The distributions created from these model runs are expressed as probabilities that drawdown or a change in streamflow will exceed relevant thresholds, as there is no single ‘best’ estimate of change.
In this assessment, the estimates of drawdown or streamflow change are shown as 95th, 50th or 5th results, corresponding to a 5%, 50% or 95% chance of exceeding thresholds. Figure 10 illustrates this predictive within a spatial .
Throughout this product, the term ‘very likely’ is used to describe where there is a greater than 95% chance of something occurring, and ‘very unlikely’ is used where there is a less than 5% chance.
The chart on the left shows the distribution of results for drawdown, obtained from an ensemble of thousands of model runs that use many sets of parameters. These generic results are for illustrative purposes only and are not actual results from the Gloucester subregion.
The assessment extent was divided into smaller square assessment units (see Section 22.214.171.124) and the probability distribution (Figure 9) was calculated for each. In this product results are reported with respect to the following key areas:
A. outside the zone of potential hydrological change, where hydrological changes (and hence impacts) are very unlikely (defined by maps showing the 95th percentile)
B. 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 95th percentile). Further work is required to determine whether the hydrological changes in the zone translate into impacts for water-dependent assets and landscapes
C. 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 50th percentile)
D. 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 5th percentile).
Product Finalisation date
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that were not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product