3.4.5.1 Description
As discussed in Section 3.4.1, the unique ecosystems within the Pilliga and Pilliga Outwash IBRA subregions were treated separately when developing ecological models. Given the relatively low relief and subtle changes between those streams classified as ‘lowland’ and ‘upland’ across much of the Pilliga region, the experts recommended evaluating impacts on these riverine landscape classes together for the purposes of eliciting models of their ecological changes under altered surface water and groundwater regimes. The stream network across most of the Pilliga region is predominantly infilled Quaternary sediments derived from the sandstone and deposited by dendritic streams draining north and west (Norriss, 1996). The sediments become finer towards the Namoi River valley where extensive clearing for agriculture has occurred. The eastern margins of the Pilliga region include the recharge beds of the GAB and palaeochannels of overlying alluvium, which can be incised into aquifer units such as the Pilliga Sandstone (see companion product 2.3 for the Namoi subregion (Herr et al., 2018)).
The riverine landscape classes within the Pilliga region all tend to be intermittent and/or ephemeral with only a small portion of the stream network classified as ‘Permanent lowland stream’ (14.3 km in the zone of potential hydrological change). Lowland streams (‘Temporary lowland stream’, ‘Temporary lowland stream GDE’ and a small portion of ‘Permanent lowland stream’) are mainly confined to the Pilliga Outwash portion of the Pilliga where the streams flow north onto the broad floodplains of the Castlereagh-Barwon Plains IBRA region (SEWPaC, 2012). Bohena Creek is one of the major streams flowing through the Pilliga, and flows north-east towards Narrabri and is classified predominately as ‘Temporary lowland stream GDE’ and ‘Temporary lowland stream’ (Figure 25). The ‘Temporary lowland stream’ (624.6 km in the zone) and ‘Temporary upland stream’ (530.4 km in the zone) landscape classes are by far the largest landscape classes within the zone of potential hydrological change of the Pilliga region (Table 28). The ‘Temporary upland stream GDE’ landscape class occupies only a small fraction of the stream network (11.5 km in the zone) (Table 28). Further description of the Pilliga landscape and associated landscape classes can be found in Section 2.7.3 and Section 2.7.4 of companion product 2.7 (Ickowicz et al., 2018)).
Table 28 Pilliga riverine landscape classes within the Pilliga region of the zone of potential hydrological change and their corresponding lengths and percentage contributions
Data: Bioregional Assessments (Dataset 2)
3.4.5.2 Potential hydrological impacts
The Pilliga riverine landscape classes (upland and lowland) of the Pilliga region were assigned three hydrological response variables based on their importance to the corresponding qualitative model for the Pilliga riverine landscape, which includes streams classified as both lowland and upland (Table 29). The relevant hydrological response variables are expressed as changes relative to a baseline reference period (1983 to 2012); mean increase in number of zero-flow days, mean increase in annual maximum no-flow spells and maximum decrease in groundwater drawdown (Table 29).
3.4.5.2.1 Groundwater
Most of the riverine streams in the Pilliga region of zone of potential hydrological change are ‘Temporary lowland stream’ (624.6 km) and ‘Temporary upland stream’ (530.4 km). Additional coal resource development is expected to have a larger impact on groundwater drawdown on the ‘Temporary upland stream’ landscape class with 14.8 km exposed to a 50% chance of greater than 2 m drawdown compared with 11.5 km of the stream network under a baseline future (Table 30 and Figure 39). For the lowland riverine landscape classes, there is a total of 33.5 km exposed to a 5% chance of greater than 2 m groundwater drawdown due to additional coal resource development compared to 0.4 km under the baseline future (Table 30 and Figure 39).
Table 29 Summary of the hydrological response variables and corresponding receptor impact variables used in the receptor impact models for the Pilligaa riverine landscape classes, together with the corresponding qualitative model (signed digraph) that describes the ecosystem linkages among different components
The proportion of landscape classes with surface water modelling is also provided.
a‘Pilliga’ as used here refers to those parts of the zone of potential hydrological change that fall within the ‘Pilliga region’.
See Section 2.7.3 and Section 2.7.4 in companion product 2.7 for the Namoi subregion (Ickowicz et al., 2018) for further details.
Additional drawdown is the maximum difference in drawdown between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development. The extent of the coal resource developments in the CRDP is the union of the extents in the baseline and the additional coal resource development (ACRD).
Data: Bioregional Assessment Programme (Dataset 1)
Table 30 Length (km) of landscape classes in the Pilliga riverine landscape classes potentially exposed to varying levels of baseline drawdown in the zone of potential hydrological change
The area potentially exposed to ≥0.2, ≥2 and ≥5 m baseline drawdown and additional drawdown is shown for the 5th, 50th and 95th percentiles. Baseline drawdown is the maximum difference in drawdown under the baseline relative to no coal resource development. Additional drawdown is the maximum difference in drawdown (dmax) due to additional coal resource development relative to the baseline. Areas within mine pit exclusion zones are excluded from further analysis.
Data: Bioregional Assessment Programme (Dataset 2).
3.4.5.2.2 Surface water
Only 6% of the lowland and upland riverine landscape classes in the Pilliga region had surface water modelling associated with them (Table 29). The surface water modelling for the Pilliga region shows a small chance (5%) of increase in zero-flow days (Table 31) for the zone of potential hydrological change. Only 0.3 km of ‘Temporary upland stream’ landscape class is exposed to a 5% chance of an increase in zero-flow days of greater than 20 days, while both upland (6.5 km) and lowland (85.6 km) landscape classes are exposed to a 5% chance of an increase in zero-flow spells of greater than 3 days for the 2013 to 2042 simulation period (Table 31).
For increases in maximum zero-flow spells, only 20 km of lowland and 0.3 km of upland riverine landscape classes are exposed to a 5% chance of an increase in length of zero-flow spells of greater than 3 days for the 2073 to 2102 simulation period (Table 32). No changes in maximum zero-flow spells were predicted for the 2013 to 2042 simulation period (Table 32).
Table 31 Length (km) of landscape classes in the Pilliga riverine landscape classes (upland and lowland) potentially exposed to an increase in zero-flow days for two different simulation periods: 2042 and 2102, in the zone of potential hydrological change
The length potentially exposed to ≥3, ≥20 and ≥80 days increase in zero-flow days from the baseline period (1983 to 2012) is shown for the 5th, 50th and 95th percentiles.
Data: Bioregional Assessment Programme (Dataset 2)
Table 32 Length (km) of landscape classes in the Pilliga riverine landscape classes (upland and lowland) potentially exposed to an increase in annual maximum zero-flow spells for two different simulation periods: 2042 and 2102, in the zone of potential hydrological change
The length potentially exposed to ≥3, ≥10 and ≥40 days increase in the length of the maximum zero-flow spell during the 30-year simulation period compared to the baseline period (1983 to 2012) is shown for the 5th, 50th and 95th percentiles.
Data: Bioregional Assessment Programme (Dataset 2)
3.4.5.3 Potential ecosystem impacts
The potential for ecosystem impacts for relevant landscape classes within the Pilliga region in the zone of potential hydrological change was estimated using two separate receptor impact models applied to the riverine landscape classes (upland and lowland) (see Table 29). The receptor impact variables, average number of families of aquatic macroinvertebrates and projected foliage cover of riparian vegetation were modelled under the baseline and CRDP futures. These two models were combined across the relevant assessment units to define the aggregated risk where model inputs were available. The risk thresholds used for defining risk and their associated terminology are identical to that applied to the receptor impact variables assigned to the landscape classes in the ‘Floodplain or lowland riverine’ landscape group (see Section 3.4.3.3).
The Pilliga region has relatively few model nodes along the stream network, with only two nodes along Bohena Creek (classified as a ‘Temporary lowland stream’). There is a large proportion (94%) of the entire length of the Pilliga riverine stream network that is not quantified under the surface water modelling approach used here (Figure 40 and Table 29). The risk composite across all modelled assessment units shows areas exposed to ‘at some risk’ along Bohena Creek and some adjacent assessment units intersecting tributaries (Figure 40). The subsequent sections describe the specific results of each model that contribute to the observed location and magnitude of risks described here.
The level of risk: ‘at minimal risk of ecological and hydrological changes’ (‘at minimal risk’), ‘at some risk of ecological and hydrological changes’ (‘at some risk’) and ‘more at risk of ecological and hydrological changes’ (‘more at risk’) is presented for different assessment units where the receptor impacts are modelled for the different landscape classes. Remaining assessment units for the relevant classes in the Pilliga riverine without receptor impact modelling and surface water modelling are also shown (green). Extent captures areas with ‘at some risk’ or ‘more at risk’ assessment units.
Data: Bioregional Assessment Programme (Dataset 5)
3.4.5.3.1 Pilliga riverine
There were no detectable differences in predicted mean changes in either projected foliage cover or average number of families of aquatic macroinvertebrates across the Pilliga riverine landscape classes between the baseline and CRDP futures across the different simulation periods (2042 and 2102) (Figure 41a and c). This is reflected in the very small amount of stream segments impacted by changes in surface water regime (Table 31 and Table 32) and the limited impacts from additional groundwater drawdown (Figure 39).
However, an assessment of the modelled changes in the number of families of aquatic macroinvertebrates at a given assessment unit identified locations across the extent of the lowland riverine landscape classes that are at risk due to coal resource development (Figure 41b and d). At the 5th percentile declines in projected foliage cover were greater than 0.2 in two assessment units and less than 0.1 in four assessment units. Changes in the average number of families of aquatic macroinvertebrates due to additional coal resource development were similar between simulation periods and were as low as –19 families at the 5th percentile and as low as approximately –12 families at the 50th percentile (Figure 41d). An increase in projected foliage cover and the average number of families of aquatic macroinvertebrates was observed at the 95th percentile (Figure 41b and d).
Box and whisker plots of modelled (a) projected foliage cover and (c) average number of families of aquatic macroinvertebrates in Pilliga riverine landscape classes in 2042 and 2102 under both baseline and coal resource development pathway (CRDP) futures. Differences in (b) projected foliage cover and d) average number of families of aquatic macroinvertebrates between CRDP and baseline futures for each assessment unit containing Pilliga riverine landscape classes. The relevant thresholds used to delineate changes in the receptor impact variable associated with ‘at some risk of ecological and hydrological changes’ and ‘more at risk of ecological and hydrological changes’ are indicated by the orange and red dashed horizontal lines.
Data: Bioregional Assessment Programme (Dataset 5)
