3.4.7 'Non-floodplain, terrestrial GDE' landscape group

3.4.7.1 Description

The ‘Non-floodplain, terrestrial GDE’ landscape group includes ecosystems that rely on the subsurface presence of groundwater and are not associated with floodplains or palustrine, lacustrine or riparian wetlands. Non-floodplain landforms include clay plains, loamy and sandy plains, inland dunefields, or areas of fine-grained and coarse-grained sedimentary rocks. The ‘Non‑floodplain, terrestrial GDE’ landscape group includes two landscape classes: ‘Non-floodplain, terrestrial GDE’ that covers 268 km² (or less than 0.1% of the assessment extent) and ‘Non-floodplain, terrestrial GDE, remnant vegetation’ that covers 20,532 km² (or 3% of the assessment extent) (Table 19).

A high-level conceptual model for unconfined, permeable rock aquifers is relevant to this landscape group (Figure 63). Near-surface discharge may occur where there is a change in geology or topography (e.g. at a geological boundary between sandstone and an underlying shale) or at the break of slope at the base of hills or ranges. Here, terrestrial vegetation can access relatively shallow unconfined aquifers via the capillary zone.

Terrestrial vegetation may source groundwater up-gradient of the contact between the plains and elevated areas such as mesas and ironstone jump-ups (Figure 63). Terrestrial GDEs in these areas are likely to support regional ecosystems (REs) dominated by Corymbia spp. (DSITI, 2015). Terrestrial vegetation also may source groundwater where it discharges at the break of slope of sandstone ranges. A less common type of permeable rock aquifer occurs in basalts, which stores and transmits groundwater through vesicles, fractures and weathered zones. Terrestrial vegetation may source groundwater at the edge of basalt plains and hills. Connectivity of groundwater in the zone of potential hydrological change is complicated as the Galilee subregion contains a series of stacked groundwater systems, as described in detail in companion product 2.1-2.2 (Evans et al., 2018a) and Section 2.3.2 of companion product 2.3 (Evans et al., 2018b) for the Galilee subregion. The Galilee Basin groundwater system, which includes the Rewan Group and Clematis Group, is the most likely source of groundwater for the ‘Non-floodplain, terrestrial GDE’ landscape group in the zone of potential hydrological change. In particular:

GDEs that access regional groundwater systems within the zone of potential hydrological change may be impacted by changes in groundwater (for example, from drawdown or recharge) across a broad area. However, some proportion of these GDEs will not be connected to the regional watertable and will be independent of broader-scale changes due to the additional coal resource development.

The water requirements and the degree of groundwater dependency of the vegetation in the ‘Non-floodplain, terrestrial GDE’ landscape group will depend on a number of factors, including:

Vegetation within the ‘Non-floodplain, terrestrial GDE’ landscape group can typically use deep roots to access groundwater in the capillary zone above the watertable via capillary action or hydraulic lift. Previous research has revealed links between groundwater depth and tree condition, but critical thresholds that lead to rapid and potentially irreversible change have been difficult to quantify. For example, maximum rooting depth in a global study was 5.2±0.8 m for sclerophyllous shrubland and forest, 7.0±1.2 m for trees and 9.5±2.4 m for desert vegetation (Canadell et al., 1996). Further, the mean maximum rooting depth of 11 species of sclerophyllous trees was 12.6±3.4 m (Canadell et al., 1996). This includes tree species such as Eucalyptus marginata, where roots have been reported at depths of around 40 m (Dell et al., 1983).

Tree water uptake of groundwater from deeper watertable levels is generally less than where the watertable is shallower (e.g. Zencich et al., 2002; O’Grady et al., 2006a, 2006b). Standing water levels for 98 bores associated with the ‘Non-floodplain, terrestrial GDE’ landscape group averaged 37.3 m and ranged between 157 m and 0.3 m. The rate of drawdown can also be critical to vegetation survival. Plant roots can only remain in contact with a declining watertable if the rate of decline does not exceed potential root growth rate; 3 to 15 mm/day for arid shrub and grass species (Naumberg et al., 2005). However, there is a critical knowledge gap in the ecohydrology of groundwater-dependent vegetation, particularly relating to the sensitivity of vegetation to changes in the rate of groundwater drawdown across different watertable depths (and whether this response is linear).

The ‘Non-floodplain, terrestrial GDE’ landscape group supports 90 REs in the assessment extent. There is considerable uncertainty related to the water regime required to support many of these REs. However, the nature of the dependency on groundwater is likely to vary among and within vegetation communities as a function of groundwater availability, depth and quality (companion product 2.1-2.2 for the Galilee subregion (Evans et al., 2018a)).

The majority of REs have Eucalyptus and/or Corymbia species as dominant/co-dominant in the upper storey. A smaller number of REs have Acacia and/or Melaleuca species as dominant/co-dominant.