The Pilliga and Pilliga Outwash IBRA regions represent a unique set of ecological systems within the Namoi subregion. By comparison to the other landscapes across the Namoi subregion, the Pilliga and Pilliga Outwash is unique in many attributes of its ecology, geomorphology, hydrogeology, soils and ecohydrology. In consultation with ecological experts in this region it was deemed necessary to develop a specific set of receptor impact models to define potential ecological impacts for the Pilliga and Pilliga Outwash IBRA regions. Here, the Pilliga and Pilliga Outwash IBRA regions under consideration are referred to as simply the ‘Pilliga region’. Figure 23 provides a conceptual model of a typical landscape in the Pilliga Nature Reserve and examples of the hydrologic connectivity and water movement found in it.
GDE = groundwater-dependent ecosystem, ‘veg.’ = vegetation
Source: Symbols courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/)
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. Lowland streams (‘Temporary lowland stream’, ‘Temporary lowland stream GDE’ and a small portion of ‘Permanent lowland stream’ landscape classes, Table 31) are mainly confined to the Pilliga Outwash portion of the Pilliga where the streams flow north onto the broad floodplains of the Castlereagh-Barwon IBRA region (SEWPaC, 2012). Bohena Creek is one of the major streams flowing through the Pilliga, and flows north-east towards Narrabri. It is classified predominately as ‘Temporary lowland stream GDE’ and ‘Temporary lowland stream’. This is a highly intermittent stream – flowing less than 20% of days on average (Table 32). The flow regime is highly seasonal with lowest monthly volumes occurring in May and higher monthly flow volumes occurring between July and November.
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) (Table 31). 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) (Table 31). 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 (Figure 24). 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 (Table 31).
Table 31 Areas and/or lengths of the ‘Floodplain or lowland riverine’ and ‘Non-floodplain or upland riverine’ landscape groups within the entire assessment extent and the Pilliga region of the zone of potential hydrological change
na = not applicable
Data: Bioregional Assessment Programme (Dataset 2)
Figure 24 Location of the 'Floodplain or lowland riverine' landscape group within the Pilliga region
IBRA = Interim Biogeographic Regionalisation for Australia
Data: SEWPaC (Dataset 1); Bioregional Assessment Programme (Dataset 2); Bureau of Meteorology (Dataset 3)
Table 32 Maximum, median, 10th, 25th, 75th and 90th percentile flows (ML/day) for the Pilliga region
|
Gauge # |
Gauge name |
Landscape class |
Maximum |
10th |
25th |
Median |
75th |
90th |
|---|---|---|---|---|---|---|---|---|
|
419905 |
Bohena Creek |
Temporary lowland stream GDE |
22,826 |
0 |
0 |
0 |
0 |
334 |
GDE = groundwater-dependent ecosystem
Data: Bioregional Assessment Programme (Dataset 2)
The stream network across most of the Pilliga region is predominantly infilled Quaternary sediments derived from sandstone and deposited by dendritic streams draining north and west (Norris, 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 Great Artesian Basin and paleochannels of overlying alluvium can be incised into aquifer units such as the Pilliga Sandstone (see companion product 2.3 for the Namoi subregion (Herr et al., 2018)). Under these conditions upward leakage of groundwater can occur where the paleochannel has eroded the underlying low-permeability saprolite. Such connectivity is poorly understood, but is considered to be highly variable across the region (Ransley et al., 2012). It is likely that water in the Pilliga Sandstone aquifer is providing baseflow to the Namoi River near the confluence of the Narrabri River and Bohena Creek (companion product 2.3 for the Namoi subregion (Herr et al., 2018)).
The ephemeral nature of streamflow across most of the lowland riverine stream network means that riffle habitat is uncommon and there are few permanent, disconnected pools within the stream channel. In some reaches, there may be longitudinal subsurface flow of water from the underlying watertable. Most fauna in these riverine systems are adapted to desiccation and wet–dry phases. This means they are transitory and take advantage of available pools or damp patches along the streambed.
Riparian vegetation along lowland streams in the Pilliga region is often dominated by Eucalyptus spp. including E. chloroclada, E. blakelyi, E. fibrosa, E. melliodora, E. conica and E. crebra. Angophora floribunda and Callitris columellaris are also found in association with these eucalypts. Sedges are also found interspersed along the riparian areas and streambed (Figure 25).
Credit: Bioregional Assessment Programme, Patrick Mitchell (CSIRO), January 2016
It was decided during the qualitative modelling workshop by experts of this region that the ecological processes and associated biota were similar across much of the upland and lowland riverine streams in the Pilliga region (excluding some areas in the north where the Pilliga Outwash adjoins the broader floodplains of the Castlereagh-Barwon IBRA subregion). Thus, there is an assumption here that modelled hydrological changes across all riverine landscape classes in the Pilliga region have similar impacts on associated ecological receptor impact variables. This means that the ecological modelling presented for riverine landscape classes in the Pilliga region include both upland and lowland stream landscape classes.
The ‘Grassy woodland GDE’ landscape class covers a range of vegetation communities across the Pilliga region and tends to be concentrated around the drainage lines and stream network. This is probably attributable to both groundwater discharge from surrounding sandstone aquifers particularly in the lowland riverine systems within the Pilliga Outwash area (northward portion) and localised subsurface runoff (Figure 23). Some plant communities such as the broombush plain (dominated by Melaleuca uncinata) in the Pilliga are known to receive subsurface run-on from upslope sites where soils are shallow and can become waterlogged (Norris, 1996). The level of groundwater dependency of these plant communities was inferred from multiple criteria (including remote sensing of canopy greenness and density) that were not necessarily verified by field assessment (Kuginis et al., 2016). Thus, some caution needs to be exercised when discussing the nature of water dependency in this landscape class (as noted in Section 2.3.3.3 of companion product 2.3 for the Namoi subregion (Herr et al., 2018)).
Some of the non-floodplain wetlands located within the zone of potential hydrological change (particularly in the outwash part of the Pilliga region) are ‘tank gilgai’ type wetlands described in Bell et al. (2012). They are commonly fringed by buloke (Allocasuarina luehmannii) and various sedge, rush and other herbaceous plant communities (Bell et al., 2012) and tend to occur within a mosaic of woodlands and shrublands largely dominated by buloke, E. chloroclada, E. pilligaensis, E. sideroxylon and Melaleuca densispicata (Benson et al., 2010). The nature of wet and dry phases within these wetlands is determined by localised runoff from rainfall, which means that their dependency on flow systems at larger scales is likely to be negligible.
