- Bioregional Assessment Program
- Namoi subregion
- 2.6.1 Surface water numerical modelling for the Namoi subregion
- 188.8.131.52 Prediction
- 184.108.40.206.4 Defining thresholds for hydrological change
The consequences of the changes to streamflow characteristics on landscape classes and water-dependent assets are considered in companion product 3-4 (impact and risk analysis) for the Namoi subregion (as listed in Table 2). In order to rule out surface water dependent landscape classes and assets that are unlikely to be impacted by changes in surface water hydrology, it is necessary to define a threshold, above which changes in hydrology will be considered further, and which reaches of the stream network are and are not showing this hydrological change. Specified thresholds of hydrological change are provided for each of the nine hydrological response variables in Section 8.1.4 of companion submethodology M06 (as listed in Table 1) for surface water modelling (Viney, 2016):
- the flux-based hydrological response variables – AF, P99 and IQR – is a 5% or greater chance of a 1% or greater change in the variable (i.e. if at least 5% of model replicates show a maximum difference between the CRDP and baseline of at least 1% of the baseline value)
- the flux-based hydrological response variable – P01 – is a 5% or greater chance of a 1% or greater change in the variable and the change in runoff depth is greater than 0.0002 mm. Note that the addition of a runoff depth threshold is a departure from Viney (2016) and is designed to exclude reaches where the absolute change in runoff is minimal
- the frequency-based metrics – FD, LFD, LLFS and ZFD – is a greater than 5% chance of there being a change in the variable of at least 3 days in any year
- the frequency-based metric – LFS – is a greater than 5% chance of there being a change in the variable of at least 2 spells in any year.
Table 13 summarises the hydrological changes due to the additional coal resource development for each of the surface water modelling nodes in the Namoi subregion for each hydrological response variable that exceed the specified thresholds.
At 18 nodes (node 6, node 17 and nodes 38 to 53, all of which are upstream of the proposed developments) there are no hydrological changes in any hydrological response variable that exceed the specified threshold. This includes Baradine Creek, the uppermost nodes of Bohena Creek and Mooki River, and all reaches on the Namoi River and its tributaries upstream of the junction with the Mooki River. At nodes 21, 22, 25, 27, 31, 34 and 54, there are changes in all nine hydrological response variables that exceed the specified thresholds. At all other nodes, there are changes in some hydrological response variables where the specified thresholds are exceeded, but not in others.
The last row in Table 13 gives the number of nodes for which there is a change for each hydrological response variable that exceeds the specified thresholds. The low-streamflow hydrological response variables (LLFS, LFD, P01, ZFD and LFS), each of which has change that exceeds the specified thresholds in at least 30 nodes, are markedly more affected than the high-streamflow hydrological response variables (AF, FD and P99).
Figure 27 shows reaches predicted to experience a change in at least one hydrological response variable that exceeds the specified threshold due to additional coal resource development.
Table 13 Hydrological changes due to additional coal resource development at each model node (row) for each hydrological response variable (column)
E = exceeds specified threshold; – indicates that it does not exceed specified threshold
Shortened forms of hydrological response variables are defined in Section 220.127.116.11.1. Specified thresholds for each hydrological response variable are provided in Section 18.104.22.168.4.
Data: Bioregional Assessment Programme (Dataset 1)
Figure 27 Model nodes and links with changes in at least one hydrological response variable that exceeds specified thresholds due to additional coal resource development in the Namoi subregion
ACRD = additional coal resource development; AWRA-R = Australian Water Resources Assessment river model
Data: Bioregional Assessment Programme (Dataset 1); Bureau of Meteorology (Dataset 2)
For some reaches (e.g. those below nodes 38 and 39 but above node 37), hydrological change can change from exceeding specified thresholds to not exceeding these thresholds between the two nodes. These reaches are shown as dashed pink lines and other information is needed to delineate the change on these reaches. Similarly, upstream of the headwater nodes exceeding the threshold change in hydrology, there will be some length of stream that is also potentially affected by coal resource development.
To define the zone of potential hydrological change for the impact and risk analysis – that is, the area outside of which it is very unlikely that landscape classes and assets will be impacted – we need to determine the upstream extents of the stream network likely to experience hydrological changes that exceed the threshold. This final step is reported in companion product 3-4 (impact and risk analysis) for the Namoi subregion (as listed in Table 2), where drawdown results from the groundwater modelling and mine footprint data are used to identify stream reaches that are not explicit in the surface water node-link network and where hydrological changes from additional coal resource development are likely to exceed specified thresholds.
The implications of potential changes in hydrology from additional coal resource development for the Namoi subregion landscape classes and assets are described in companion products 2.7 (receptor impact modelling) and 3-4 (impact and risk analysis) for the Namoi subregion (listed in Table 2).
Product Finalisation date
- 22.214.171.124 Methods
- 126.96.36.199 Review of existing models
- 188.8.131.52 Model development
- 184.108.40.206 Calibration
- 220.127.116.11 Uncertainty
- 18.104.22.168 Prediction
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product