The hydrological analysis encompasses theand modelling reported in companion product 2.6.1 ( ) and companion product 2.6.2 ( ), respectively, for the Namoi subregion. The Namoi surface water and groundwater models quantify potential changes in hydrology from multiple coal resource developments and enable an assessment of the of coal resource development at a regional scale. For streamflow, nine , which incorporate changes in groundwater fluxes from the groundwater model, allow an assessment of the on low-, average- and high-flow characteristics of the time-series data. For groundwater, a single hydrological response variable quantifies the maximum attributable to the modelled .
Companion submethodology M06 for surface water modelling () and companion submethodology M07 for groundwater modelling ( ) (as listed in Table 1) define thresholds for each hydrological response variable. A modelled change at or above the threshold identifies the level of hydrological change due to additional coal resource development that needs to be considered further. Preliminary zones of potential hydrological change for surface water (companion product 2.6.1 ( )) and for groundwater (companion product 2.6.2 ( )) identify the areas beyond which is considered .
For surface water, ais included in the if it registered a change above the threshold in at least one of the nine hydrological response variables. The Namoi link-node mapping shows where results from surface water model nodes define upstream and downstream links in the stream network (see companion product 2.6.1 ( )), and it identifies which stream reaches are within the (see Figure 37 in ).
The zones of potential hydrological change for surface water and groundwater modelling, identified in companion product 2.6.1 (3.2.5 for details). This resolution recognises the input data resolutions in the hydrological modelling and allows for an assessment at the bioregional level. Section 3.3 outlines the process of this zone development for the . The additional processing steps needed to incorporate stream reaches that are not explicit in the AWRA-R link-node network are also available in Section 3.3. This includes those that have an inherent surface water dependency and intersect with these stream reaches. This overcomes the spatial limitation of representing streams as line features, which do not include and floodplain areas. The zone of potential hydrological change underpins the ‘rule-out’ overlay analysis of landscape classes and (Section 3.2.4).) and companion product 2.6.2 ( ), form the basis for a combined zone of potential hydrological change that is spatially explicit and at a 1 km pixel resolution, and this resolution is aligned with the shape of . Assessment units are the basis of the subsequent assessment (see Section
18.104.22.168 Representing predictive uncertainty
The models used in the assessment produce a large number of predictions of Figure 7). This approach allows an assessment of the of exceeding a given magnitude of change, and underpins the assessment 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 (
Groundwater models, for example, require information about physical properties such as the thickness of geological layers, how porousare, and whether faults are present. As the exact values of these properties are not always known, the modellers used a credible range of values, which are based on various sources of data (commonly point-scale) combined with expert knowledge. Incorporation of this credible range included running the model 3500 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 from across the subregion, help to constrain and validate the model runs subsequently.
The complete set of model runs produces a range or distribution of predictions (Figure 7) that is consistent with the available regional 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 5th, 50th or 95th Figure 8 illustrates this predictive spatially.results, corresponding to a 95%, 50% or 5% chance of exceeding thresholds.
Throughout this product, the term ‘very likely’ describes 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.
The assessment extent was divided into smaller square assessment units (see Section 3.2.5) and the probability distribution (Figure 7) 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).
Percentile estimates of drawdown enable the reader to choose their own drawdown thresholds. For example, an ecologist may be interested in potential hydrological changes in an area of floodplain remnant vegetation where their conceptual ecological model indicates that herbaceous species are affected by 1 to 2 m of drawdown and floodplain trees are affected by 10 to 20 m of drawdown. The ecologist can use the 5th, 50th and 95th percentile estimates of drawdown for the relevantor to assess the likelihood and extent of potential on and risks to that due to coal resource development.
In contrast, the percent chance of exceeding important threshold values enables the reader to choose their level of certainty. A regulator may be interested in the likelihood of a groundwaterexceeding defined regulatory thresholds. The regulator can then determine the number of bores where there is a 20% chance of exceeding 5 m drawdown.
Product Finalisation date
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.4.1 Overview
- 3.4.2 Landscape classes that are unlikely to be impacted
- 3.4.3 'Floodplain or lowland riverine' (non-Pilliga) landscape group
- 3.4.4 'Non-floodplain or upland riverine' (non-Pilliga) landscape group
- 3.4.5 Pilliga riverine (upland and lowland)
- 3.4.6 Potentially impacted landscape classes lacking quantitative ecological modelling
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that are not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product