- Bioregional Assessment Program
- Namoi subregion
- 3-4 Impact and risk analysis for the Namoi subregion
- 3.4 Impacts on and risks to landscape classes
The diverse range of natural and human-modified ecosystems in the Namoi subregion were classified into 29 landscape classes, and then aggregated into six landscape groups based on the classification criteria and water dependency: ‘Floodplain or lowland riverine’, ‘Non‑floodplain or upland riverine’, ‘Rainforest’, ‘Springs’, ‘Dryland remnant vegetation’ and ‘Human-modified’.
Landscape classes that are unlikely to be impacted
Two landscape groups were ‘ruled out’ of the ecological modelling component of the assessment based on the criteria used in the initial landscape classification. Firstly, the ‘Dryland remnant vegetation’ landscape group can be ruled out because it comprises vegetation communities that are deemed to not be dependent on surface water or groundwater. Secondly, the ‘Human-modified’ landscape group (comprising six landscape classes) is excluded because it primarily comprises agricultural and urban landscapes that are highly modified by human activity. However some aspects of this group are considered elsewhere in the bioregional assessments (BAs) (i.e. economic assets).
Additionally, the ‘Non-GAB springs’ landscape class was excluded because none of the 15 springs found in the assessment extent are located within the zone of potential hydrological change.
Within the riverine lowland landscape classes, ‘Permanent lowland stream GDE’, ‘Temporary lowland stream GDE’ and ‘Floodplain riparian forest’, are very unlikely to be impacted, given no change in their relevant hydrological response variables for any of the simulation periods.
‘Floodplain or lowland riverine’ landscape group
The lowland riverine landscape classes in this group include the Namoi River and its major tributaries. The impacts on surface water were defined using increases in the cease-to-flow periods – defined using the variables mean annual zero-flow days (averaged over 30 years) (ZQD, subsequently referred to in this Section as ‘zero-flow days’) and mean annual maximum zero-flow spells. Approximately 16.9 km of the ‘Permanent lowland stream’ landscape class is exposed to a 50% chance of an increase of greater than 20 days in zero-flow days and an increase of greater than 10 days in maximum zero-flow spells over the 2013 to 2042 simulation period. Only approximately 10 km of ‘Temporary lowland stream’ is exposed to a similar magnitude of changes to 2042.
Hydrological responses of the riparian vegetation in these lowland riverine systems, classified almost exclusively as ‘Floodplain riparian forest GDE’, were defined by decreases in the frequency of overbank flows (based on a pre-defined threshold of a 1 in 3 year flood event in the reference period) and maximum groundwater drawdown. Of the 72 km2 of ‘Floodplain riparian forest GDE’ within the zone of potential hydrological change, only 0.5 km2 has a 50% chance of a decrease of at least one overbank event every 20 years during the 2013 to 2042 simulation period. There is also a 50% chance of greater than 2 m groundwater drawdown in 0.6 km2 due to additional coal resource development.
Hydrological responses in floodplain wetlands are also defined by decreases in the frequency of overbank flows. Any change in overbank flow events across the extent of floodplain wetlands is very unlikely, with 7.7 km2 of ‘Floodplain wetland’ and 19.7 km2 of ‘Floodplain wetland GDE’ having a 5% chance of experiencing one less event every 50 years during the 2013 to 2042 simulation period. However, no declines in the frequency of overbank flows for floodplain wetlands were detected for the 2073 to 2102 simulation period.
Potential ecosystem impacts estimated from the receptor impact modelling showed changes in one or more of the receptor impact variables at a confined set of locations across the distribution of the associated landscape class. For example, largest predicted declines in average number of families of aquatic macroinvertebrates due to additional coal resource development (ranging from approximately 16 to 17 families at the 5th percentile and approximately 4 to 3 families at the 50th percentile) were observed around Maules Creek and Bollol Creek. The receptor impact model results were combined to provide a combined assessment of potential ecosystem impacts based on thresholds indicative of a relative measure of risk across a given landscape class. The greatest concentration of ‘more at risk of ecological and hydrological changes’ and ‘at some risk of ecological and hydrological changes’ assessment units (see Section 220.127.116.11 for the definitions of these risk categories) is located along the Namoi River and its tributaries, Maules Creek, Back Creek and Bollol Creek. Of the 1425 assessment units included in one or more of the receptor impact models, 51 were predicted to have ‘at minimal risk’ and 29 ‘more at risk’, with most of these risk categories being determined by potential impacts on lowland riverine landscape classes and floodplain wetland landscape classes. A more detailed and local consideration of risk needs to consider the specific values at the location that the community are seeking to protect (e.g. particular assets), and bring in other lines of evidence that include the magnitude of the hydrological change and the qualitative mathematical models.
‘Non-floodplain or upland riverine’ landscape group
Both the instream and riparian habitats were modelled in the ‘Non-floodplain or upland riverine’ landscape group. Impacts on the riverine system were defined using the same cease‑to-flow variables as the lowland riverine classes. Only two upland riverine classes, ‘Temporary upland stream’ and ‘Temporary upland stream GDE’, are exposed to a 50% chance of increases in zero-flow days greater than 20 days (2.2 and 2.6 km, respectively) for both the 2013 to 2042 and 2073 to 2102 simulation periods. Patterns in the increase in maximum zero‑flow spells (50% chance of increases in maximum zero-flow spells of greater than 10 days) affect the same landscape classes and stream lengths as increases in zero-flow spells for both simulation periods.
It is very unlikely that the riparian vegetation represented by the ‘Upland riparian forest GDE’ landscape class will be impacted for either hydrological response variables, maximum groundwater drawdown and frequency of overbank flow events. Areas predicted to have a 5% chance of drawdown of greater than 0.2 m intersect with only 0.1 km2 of this landscape class. Only a small area (0.3 km2) is at risk of a 5% chance of a change in one less overbank flow event every 50 years.
Pilliga riverine classes (upland and lowland)
The Pilliga region, which encompasses both the Pilliga and Pilliga Outwash Interim Biogeographic Regionalisation for Australia (IBRA) subregions, was considered as a separate entity for the purposes of the ecological modelling due to its distinctive biophysical attributes. Ecological impacts on the Pilliga riverine landscape classes (including predominantly temporary upland and lowland landscape classes) were defined based on changes in maximum groundwater drawdown, increases in zero-flow days and increases in maximum zero-flow spells. Groundwater drawdown due to additional coal resource development across the Pilliga riverine landscape classes is predicted to have the largest impact on the ‘Temporary upland stream’ landscape class with 14.8 km exposed to a 50% chance of greater than 2 m drawdown.
The surface water modelling for the Pilliga region shows that chances of increases in zero‑flow days and maximum zero-flow spells are small (5%) and of a small extent. Only 0.3 km of ‘Temporary upland stream’ landscape class was 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) classes were exposed to a 5% chance of an increase in zero-flow days of greater than 3 days for the 2013 to 2042 simulation period. For increases in maximum zero-flow spells, only 20 km of lowland and 0.3 km of upland riverine classes were exposed to a 5% chance of an increase in zero-flow days of greater than 3 days for the 2073 to 2102 simulation period. No changes in maximum zero‑flow spells were predicted for the 2013 to 2042 simulation period. Only 6% of the total length of the Pilliga riverine landscape classes had surface water modelling data, thus hydrological impacts across most of the stream network in the Pilliga were not determined.
The potential ecosystem impacts on the Pilliga riverine landscape classes were assessed using the number of aquatic macroinvertebrate families and projected foliage cover as receptor impact variables. Declines in both of these variables were confined to assessment units along Bohena Creek and were equivalent to ‘at some risk’ level of risk across the Pilliga riverine landscape class based on the risk thresholds defined for other receptor impact models.
Potentially impacted landscape classes lacking quantitative ecological modelling
There were parts of some landscape classes for which there was limited or no surface water modelling. This is due to both the distribution of potential model nodes across the assessment extent and the limitations on interpolating model outputs across the stream network. It followed that receptor impacts that depended on surface water were also not available at the same locations; these locations have hydrological or ecological changes that cannot be quantified.
There were also several landscape classes where receptor impact modelling was not performed and where ecosystem impacts cannot be explicitly defined. These include the two ‘GAB springs’ that intersect the Pilliga region. It is unclear whether these springs source their water from the regional watertable used to define the zone, so it is not known whether they are potentially impacted. The classification as GAB springs is based on their association with underlying sandstone formations; their connection to the GAB requires further investigation.
The ‘Grassy woodland GDE’ landscape class is widespread in the Pilliga and given its likely dependence on groundwater at least in some locations may be impacted by additional groundwater drawdown. However, more detailed studies are required to identify the nature of this groundwater reliance.
Product Finalisation date
- 3.1 Overview
- 3.2 Methods
- 3.3 Potential hydrological changes
- 3.4 Impacts on and risks to landscape classes
- 3.4.1 Overview
- 3.4.2 Landscape classes that are unlikely to be impacted
- 3.4.3 'Floodplain or lowland riverine' (non-Pilliga) landscape group
- 3.4.4 'Non-floodplain or upland riverine' (non-Pilliga) landscape group
- 3.4.5 Pilliga riverine (upland and lowland)
- 3.4.6 Potentially impacted landscape classes lacking quantitative ecological modelling
- 3.5 Impacts on and risks to water-dependent assets
- 3.6 Commentary for coal resource developments that are not modelled
- 3.7 Conclusion
- Contributors to the Technical Programme
- About this technical product