2.7.2.1 Potentially impacted landscape classes

The landscape classification for the assessment extent of the Namoi subregion identified 29 landscape classes that were aggregated into 6 broad landscape groups (see Section 2.3.3 of companion product 2.3 for the Namoi subregion (Herr et al., 2018b)). Once the zone of potential hydrological change was developed for the Namoi subregion (as outlined in companion product 3‑4 for the Namoi subregion (Herr et al., 2018a)) it was used to: (i) identify ecological landscape classes that intersect it and are potentially impacted by the modelled hydrological changes due to additional coal resource development, and (ii) rule out landscape classes that do not intersect the zone and are therefore considered very unlikely (less than 5% chance) to be impacted by changes in hydrology. Qualitative and/or receptor impact models are only needed for those ecological landscape classes that are potentially impacted.

There are two landscape groups that are automatically ruled out of this component of BA regardless of their extent within the zone of potential hydrological change. Firstly, the ‘Dryland remnant vegetation’ landscape group is ruled out from potential impacts because it comprises vegetation communities that are deemed to be reliant on incident rainfall and local runoff and do not include features in the landscape that have potential hydrological connectivity to surface water or groundwater features (for further information, see Section 2.3.3 of companion product 2.3 for the Namoi subregion (Herr et al., 2018b)). Secondly, the ‘Human-modified’ landscape group (comprising six landscape classes) is excluded from this analysis because it primarily comprises agricultural and urban landscapes that are highly modified by human activity, and contains a set of ecohydrological attributes distinct from the other landscape groups (for further information, see Section 2.3.3 of companion product 2.3 for the Namoi subregion (Herr et al., 2018b)). Attributes of the water-dependency of some aspects of these landscapes are considered elsewhere (see Section 3.5 of companion product 3‑4 for the Namoi subregion (Herr et al., 2018a)), that is, the potential impact of coal resource development on economic assets such as groundwater bores.

None of the 15 springs of the ‘Non-GAB springs’ landscape class found in the assessment extent are located within the zone of potential hydrological change. Therefore, this landscape class can be ruled out as it is very unlikely to be impacted due to additional coal resource development.

The remaining 21 landscape classes, comprising 4 landscape groups that intersect the 7014 km² zone of potential hydrological change, are considered dependent on groundwater or surface water regimes. These landscape groups, therefore, are potentially impacted due to additional coal resource development and are considered further in this product and through the remainder of the impact and risk analysis of the BA for the Namoi subregion (i.e. as presented in companion product 3‑4 (Herr et al., 2018a)). The landscape groups are ‘Floodplain or lowland riverine’, ‘Non‑floodplain or upland riverine’, ‘Springs’ and ‘Rainforest’.

When developing ecological models for the Namoi subregion it was deemed necessary to develop a separate model for defining potential ecological impacts for the Pilliga and Pilliga Outwash IBRA subregions (SEWPaC, 2012). The Pilliga and Pilliga Outwash IBRA subregions, or simply termed the ‘Pilliga region’ here, represent a unique set of landscapes within the Namoi subregion. By comparison to the other landscapes across the Namoi subregion, the Pilliga region has many unique attributes in terms of its ecology, geomorphology, underlying hydrogeology, soils and ecohydrology. Consistent with the structuring of the ecological models, outlined further in this product (see Section 2.7.5), the potential hydrological changes and ecosystem impacts are presented for both a separate Pilliga region of the zone of potential hydrological change and the remaining ‘non-Pilliga’ areas or reporting regions for the relevant landscape classes (see Section 3.3.2.2 of companion product 3‑4 for the Namoi subregion (Herr et al., 2018a) for details on reporting regions).

2.7.2.1.1 ‘Floodplain or lowland riverine’ landscape group

The zone of potential hydrological change contains all four riverine landscape classes within the ‘Floodplain or lowland riverine’ landscape group and includes 61% of the entire extent of this group within the assessment extent (Table 5). Among these four riverine classes, the largest contribution is from the ‘Temporary lowland stream’ (2062.2 km) and ‘Permanent lowland stream’ (979.6 km) landscape classes (Table 5). All six of the non-riverine classes occur within the zone of potential hydrological change (Table 5). The largest non-riverine class by area is the ‘Floodplain grassy woodland GDE’ (421.7 km²) and ‘Floodplain grassy woodland’ (121.3 km²) (Table 5). Almost half of all the ‘Floodplain riparian forest GDE’ in the assessment extent (148.7 km²) is included in the zone of potential hydrological change (Table 5). Most of the areas classified as ‘Floodplain wetland’ and ‘Floodplain wetland GDE’ in the assessment extent (30.1 km²and 151.8 km² respectively) form part of the zone of potential hydrological change (Table 5).

A single signed digraph model was created for the ‘Floodplain or lowland riverine’ landscape group that captured most of the key linkages within and between the riverine and floodplain habitats. From this model, key hydrological response variables were selected for a subset of the landscape classes. Given the expertise and resources available at the qualitative modelling workshop for the Namoi subregion, it was decided that receptor impact models be developed for a subset of landscape classes of this group (Table 5). There are two landscape classes in the ‘Floodplain or lowland riverine’ landscape group (non-Pilliga region) where groundwater drawdown was assigned as a hydrological response variable: ‘Floodplain riparian forest’ and ‘Floodplain riparian forest GDE’. The corresponding receptor impact variable for riparian forests was identified as change in projected foliage cover. The frequency of overbank flows was identified as being an important driver of the riparian ecosystem (‘Floodplain riparian forest’ and ‘Floodplain riparian forest GDE’ landscape classes) as well as the off-channel water bodies or floodplain wetlands (‘Floodplain wetland’ and ‘Floodplain wetland GDE’ landscape classes). The experts at the qualitative modelling workshop considered the presence of tadpoles from the Limnodynastes genus as the key receptor impact variable for floodplain wetlands. The cease-to-flow attributes of the surface water regime were considered as critical hydrological response variables for the riverine landscape classes and were assigned: annual number of zero-flow days and annual maximum zero-flow spells. Assemblages of macroinvertebrates in the edge habitat were deemed to be appropriate receptor impact variables for gauging impacts on these cease-to-flow attributes of the flow regime.

Receptor impact models were not constructed for the ‘Floodplain grassy woodland’ and ‘Floodplain grassy woodland GDE’ landscape classes. While these classes occupy a large proportion of this landscape group within the zone of potential hydrological change, they are considered less sensitive to hydrological change given their reduced reliance on groundwater and surface water (see Section 2.7.3 for more details).