2.7.5 Pilliga riverine landscape classes

Summary

The Pilliga and Pilliga Outwash Interim Biogeographic Regionalisation for Australia (IBRA) regions represent a unique set of ecological systems within the Namoi subregion. In consultation with experts in this region, it was considered appropriate to develop a set of separate ecological models for the relevant riverine and terrestrial landscape classes in this region to improve the assessment of potential ecological impacts.

The zone of potential hydrological change identified for the Pilliga region extends across a large portion of the Pilliga and Pilliga Outwash IBRA regions within the assessment extent. Both lowland and upland riverine landscape classes are represented in the zone of potential hydrological change. The lowland riverine classes (‘Temporary lowland stream’, ‘Temporary lowland stream groundwater-dependent ecosystem (GDE)’ and a small portion of ‘Permanent lowland stream’ landscape classes) cover approximately 13.1% of streams in the zone of potential hydrological change and are mainly confined to the Pilliga Outwash portion where the streams flow north onto the broad floodplains of the Castlereagh-Barwon IBRA region. Bohena Creek is one of the major streams in the Pilliga region and has highly intermittent flow, flowing less than 20% of days on average.

Among the upland riverine landscape classes, the ‘Temporary upland stream’ landscape class (530.4 km or 9.6% of the zone of potential hydrological change) is the most widespread, whereas the ‘Temporary upland stream GDE’ landscape class occupies only a small fraction of the stream network (11.5 km or 0.2% of the zone). The ‘Grassy woodland GDE’ landscape class occupies a very large area of the terrestrial environment in the Pilliga region (561.7 km² or 8.0% of the zone). Non-floodplain wetlands are also found within the zone of potential hydrological change and tend to be located in the northern part of the Pilliga region where the Pilliga Outwash adjoins the Castlereagh-Barwon IBRA region. These wetlands include the ‘Non-floodplain wetland’ (2.5 km² or <0.1% of the zone) and ‘Non-floodplain wetland GDE’ (1.8 km² or <0.1% of the zone) landscape classes.

A qualitative model for all Pilliga riverine landscape classes was developed that included the aquatic habitat as well as the fringing riparian vegetation. From this model, key hydrological response variables were identified: groundwater drawdown, change in annual zero-flow days (averaged over 30 years) and maximum zero-flow spells. The receptor impact modelling workshop used two different receptor impact variables to evaluate potential ecological impacts in this system: projected foliage cover of riparian trees and number of families of aquatic macroinvertebrates in instream pool habitats. A qualitative model for the ‘Grassy woodland GDE’ landscape class was also formulated, but no quantitative modelling was developed due to limitations in resources and local expertise.

Receptor impact modelling indicates that foliage cover during the reference period is an important predictor of foliage cover in the future. It also indicates that the experts’ opinion provides some evidence for the maximum additional drawdown having a negative effect, and strong evidence for the number of zero-flow days having a negative effect on average percent projected foliage cover. Receptor impact modelling also supports the hypothesis that an increase in the mean number of zero-flow days and/or the mean maximum length of zero-flow spells will have a slightly negative effect on the number of families of macroinvertebrates despite the experts being somewhat uncertain about its average value. There is also strong evidence for additional drawdown having negative effect on macroinvertebrates, and that the number of families of macroinvertebrates will decrease as groundwater drawdown increases due to coal resource development. There is, however, considerable uncertainty in these predictions.