2.7.4 'Groundwater-dependent ecosystem' landscape group

Summary

Eight of the nine groundwater-dependent ecosystem (GDE) landscape classes in the Hunter subregion occur within the zone of potential hydrological change. For five of these landscape classes, four qualitative models were developed: ‘Forested wetland’, ‘Wet and dry sclerophyll forest’, ‘Rainforest’, and ‘Freshwater wetlands’.

The qualitative mathematical model for the ‘Forested wetland’ landscape class was based upon the biological processes and environmental factors that regulate competition of casuarina with other tree species, shrubs and herbs. The model recognises the possibility for coal resource development to impact the groundwater regimes that support these forest communities. Groundwater level, overbench flow and overbank flow were identified as critical hydrological determinants of the condition of the forested wetlands ecosystem. Qualitative analysis of the signed digraph model generally indicates a negative predicted response of casuarina trees, seeds and seedlings to each of the cumulative impact scenarios, with a corresponding decline in shade, habitat structure, bank stability and orchids and fungi. Most of the other variables have a zero or ambiguous response prediction.

Relationships identified in the qualitative modelling workshop were formalised into a forested wetlands receptor impact model that predicts the response of annual mean projected foliage cover of woody riparian vegetation (predominately Eucalyptus tereticornis, Casuarina cunninghamiana and E. camaldulensis) in a 0.25-ha transect extending from the channel to the top of the bank (including floodplain overbank), to changes in the timing and magnitude of drawdown and the occurrence of overbench and overbank flows. The model reflects the experts’ view that:

  • initial foliage cover in the reference period has a positive effect on future foliage cover
  • groundwater extraction has a negative effect on average projected foliage cover
  • increased frequency of overbench flows has a positive effect on projected foliage cover
  • increased frequency of overbank flows has a positive effect on projected foliage.

The qualitative mathematical model for ‘Wet sclerophyll forest’ and ‘Dry sclerophyll forest’ landscape classes is based on the model structure for forested wetlands. Unlike casuarina trees, the sclerophyll forests were not expected to have a requirement for two-to-three-yearly permanent inundation. The qualitative modelling workshop identified one variable, groundwater, as the key hydrological factor governing the quality of the wet and dry sclerophyll forest ecosystem. Qualitative analysis of the signed digraph model generally indicates a negative predicted response for trees, seedlings and shrubs to groundwater depletion, with a corresponding decline in shade, habitat structure, nectar, nectar consumers, predators, sap- and leaf-eating insects, gliders and koalas. Insects were predicted to increase as a consequence of a release from predation pressure, while no change was predicted for saline soils and herbs.

Relationships identified in the qualitative modelling workshop were formalised into a receptor impact model that described the response of annual mean projected foliage cover of sclerophyll forest (predominately Angophora costata, Corymbia gummifera, Eucalyptus capitellata, Banksia spinulosa) in a 0.25-ha plot to changes in the timing and magnitude of groundwater. The model reflects the experts’ opinion that:

  • initial foliage cover in the reference period has a positive effect on future foliage cover
  • groundwater extraction has a negative effect on average projected foliage cover.

A model for rainforest communities was developed based on a modified version of the model for wet and dry sclerophyll forests. In this system, epiphytic vegetation in the form of vine thickets are an important component of habitat structure, and trees, shrubs and herbaceous vegetation provide a key resource to consumers of fleshy fruits. Owing to the typical location of these rainforests in sheltered gullies and slopes in hilly-to-steep terrain, experts thought it likely that this vegetation would probably use local groundwater (e.g. perched watertables, springs) opportunistically, and this groundwater would not be connected to regional watertables. Thus, no cumulative impact scenarios were developed for qualitative analyses of the model system, and no quantitative models were developed for the ‘Rainforest’ landscape class.

The ‘Freshwater wetland’ landscape class was described as a complex of marsh (littoral) and pond (open water) habitats, where the relative area of each habitat type was determined by the average level and degree of fluctuation in the water surface. Given that the best available information is that these wetlands are likely to be dependent on local, perched groundwater systems no cumulative impact scenarios were developed for qualitative analyses of the model system, and no quantitative models were developed for this landscape class.

Last updated:
18 January 2019
Thumbnail of the Hunter subregion

Product Finalisation date

2018

ASSESSMENT