3.4.4 'Groundwater-dependent ecosystem (GDE)' landscape group


3.4.4.1 Description

Groundwater-dependent ecosystems (GDEs) are those that rely on the surface or subsurface expression of groundwater to meet all or some of their life-cycle requirements (Eamus et al., 2006). The dependence of GDEs on groundwater varies both spatially and temporally (Eamus et al., 2006). Ecosystems may be obligate GDEs, with a continuous or entire dependence on groundwater, or facultative GDEs, with an infrequent or partial dependence on groundwater (Zencich et. al., 2002). Plants that depend solely on moisture held within the soil profile are known as vadophytes and are not groundwater dependent (Sommer and Froend, 2010). In the Gloucester subregion, as in much of Australia, there is considerable uncertainty as to the nature of groundwater dependency for much terrestrial vegetation. The hydroclimatic environment of the Gloucester subregion is subtropical. Annual rainfall ranges from about 960 to 1400 mm/year, and is highest in summer when potential evaporation is also highest (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014). Nonetheless, the region is still classified as being water limited inasmuch as potential evaporation (1400 to 1700 mm/year) exceeds rainfall in most months of the year. Rainfall is also highest along the margins of the subregion because this area is associated with the higher elevation regions, whereas the deficit of rainfall, relative to potential evaporation, is greater throughout much of the lowland areas of the subregion. The Gloucester Basin underlies the Gloucester subregion and is characterised as a closed hydrogeological system. Thus, water entering the system must leave as either surface water or groundwater discharge (companion product 2.3 for the Gloucester subregion (Dawes et al., 2018)). Groundwater recharge is estimated as up to 17% of rainfall under steady-state conditions and up to 28% of rainfall under transient conditions, with high values associated with alluvial aquifers (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)). This combination of rainfall deficit and surface water and groundwater recharge creates the potential for GDEs to exist within the Gloucester subregion.

The subregion has three main hydrogeological units (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)) relevant to sustaining GDE structure and function, which provide a useful conceptual framework for examining landscape classes dependent on groundwater:

  • alluvial aquifers along major creek lines
  • relatively shallow weathered/fractured rock aquifers
  • impermeable Alum Mountain Volcanics that underlie these hydrogeological units.

The geomorphology of the Gloucester subregion has been described in detail elsewhere (companion product 1.1 (McVicar et al., 2014); companion product 2.3 (Dawes et al., 2018) for the Gloucester subregion), and only a brief summary is presented here as context (Figure 48). The Quaternary alluvial aquifers are developed in proximity to the rivers. Soils in these alluvial deposits are dominated by Tenosols and are composed of clay layers and highly permeable sediments with high hydraulic conductivities (up to 500 m/day). The thickness of the alluvia varies from 9 to 15 m and the watertable is shallow and responsive to rainfall and flood events close to the river.

The Permian fractured rock and weathered zone is up to 150 m thick. It underlies the alluvial system and extends to the edges of the subregion. These shallow-rock hydrogeological units are composed of interbedded sandstone, silt and claystone. Generally, hydraulic conductivities of these aquifers are low with a sluggish response to rainfall. However, these hydraulic conductivities are highly variable as a result of fracturing and fault zones within the formation. Soils of the fractured rock and weathered zone tend to be dominated by Kurosols. Typically, these soils have a sharp, abrupt boundary between the upper coarser-textured ‘A horizon’ and the finer-textured ‘B horizon’, which may provide a pathway for subsurface lateral flows of water.

The outcropping Alum Mountain Volcanics formations are generally considered to be impermeable but localised fractures may provide pathways for localised groundwater flow paths (companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)). These may be expressed as springs along the margins of the basin, driven by localised circulation of meteoric water.

Figure 48

Figure 48 Conceptual model of the major groundwater processes in the Gloucester Basin

GDE = groundwater-dependent ecosystem

The water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use within the Gloucester subregion. In general, transpiration of groundwater is expected to decline as the depth to groundwater increases, but there is very limited evidence to support this assumption within Australia. O’Grady et al. (2010) reviewed estimates of groundwater discharge in Australia and concluded that there is considerable variation in the relationship between transpiration of groundwater and depth to groundwater. Factors such as the rooting depth of a particular species (which is usually not known), hydroclimatic environment and groundwater salinity all impact on groundwater use by vegetation. Zolfaghar et al. (2014) examined the structure and productivity of eucalypt forest across a depth-to-watertable gradient in the upper Nepean catchment in NSW. They found that where groundwater was shallow, vegetation had significantly higher biomass and productivity than sites where groundwater was deeper than approximately 10 m. The relationships between depth to groundwater and the structural and functional attributes of the vegetation communities were highly non-linear, with steep declines in leaf area index and biomass over a range of 5 to 10 m depth to groundwater. However, it is important to note that the study was largely correlative in nature and did not quantify the groundwater requirements of the vegetation. Specific studies of GDEs within the Gloucester subregion are limited. Existing mapping of GDEs is based on a multiple-lines-of-evidence approach that incorporated existing vegetation mapping, modelled groundwater levels and remote sensing (Kuginis et al., 2012). Modelled depths to groundwater (Summerell and Mitchell, 2011) for the subregion are generally shallow (within 16 m of the ground surface). However, there is likely to be uncertainty in the mapping and this is indicated by relatively shallow groundwater modelled within the bordering Alum Mountain Volcanics.

Of the five GDE landscape classes that were identified as likely to be groundwater dependent, four were present in the zone of potential hydrological change: ‘Forested wetlands’, ‘Wet sclerophyll forests’, ‘Rainforests’ and ‘Dry sclerophyll forests’. GDEs occur within each of the three hydrogeological units described previously but they are predominantly associated with the weathered/fractured rock zone and alluvial aquifers (Table 21). Few GDEs are present above the Alum Mountain Volcanics.

Table 21 Area of groundwater-dependent ecosystem landscape classes within each of three hydrogeological units across the assessment extent


Landscape class

Alluvium

(ha)

Weathered/fractured rock zone

(ha)

Alum Mountain Volcanics

(ha)

Dry sclerophyll forests

0.8

19

0.1

Forested wetlands

60

138

6.9

Rainforests

80

61

3.3

Wet sclerophyll forests

0

12

5.8

The wet sclerophyll forests of NSW occur on moderately fertile soils in high rainfall areas, and are characterised by a tall, open, sclerophyllous tree canopy and a luxuriant understorey of soft-leaved, mesophyllous, shrubs, ferns and herbs. Many understorey plants are rainforest species or have close rainforest relatives. Rainforests may be embedded within a matrix of wet sclerophyll forest and the two often blend together as intermediate forms. More than 30% crown cover of emergent, non-rainforest species (including eucalypts, brushbox and turpentine) results in a classification of wet sclerophyll forest rather than rainforest (DECC, 2007). As discussed in Section 2.7.2 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b), a single conceptual model was developed for both the ‘Wet sclerophyll forests’ and ‘Rainforests’ landscape classes in the Gloucester subregion. The main vegetation communities are described in Table 22.

Table 22 Main vegetation communities within the ‘Wet sclerophyll forests’, ‘Rainforests’ and ‘Dry sclerophyll forests’ landscape classes


Vegetation community

Source

North coast wet sclerophyll forests have a subdominant stratum of mesophyllous small trees or tall shrubs up to 15 m tall and a second understorey layer of mesophyllous shrubs above a continuous ground stratum of ferns and herbs. Vines are also present on shrubs and smaller trees. They occur both in coastal ranges and foothills, and on alluvium in sheltered creek flats. They grade into both northern hinterland forests (with decreasing shelter or moisture) and subtropical rainforests (with increasing shelter, moisture or fertility). Dominant canopy species include Eucalyptus acmenioides (white mahogany), E. microcorys (tallowwood), E. pilularis (blackbutt), E. saligna (Sydney blue gum), Lophostemon confertus (brush box) and Syncarpia glomulifera (turpentine) which occur in various combinations.

NSW Office of Environment and Heritage (n.d. a)

Northern warm temperate rainforests consist of closed forest up to 30 m tall, generally lacking emergents. The canopy comprises 4–15 species but is dominated by Acmena smithii (lilly pilly), Ceratopetalum apetalum (coachwood) and Doryphora sassafras (sassafras). It occurs in sheltered gullies and slopes in the hilly-to-steep terrain of the coast and escarpment on moderately fertile soils in high rainfall areas, extending above 1000 m in elevation, on granites, rhyolites, syenites or sedimentary substrates that yield acid soils with moderate levels of nutrients. Occasional lianas and epiphytes, open shrub/sapling stratum and variable fern/herb groundcover amongst copious leaf litter. Mosses, liverworts and lichens may be conspicuous on tree trunks or the forest floor.

NSW Office of Environment and Heritage (n.d. b)

Hunter-Macleay dry sclerophyll forests are dry open eucalypt forests to 30 m tall that are associated with the major coastal river valleys along the NSW coast. They have a mixed sclerophyll and mesophyll shrub stratum, and grassy ground layer. They occur below 400 m elevation in foothills and undulating terrain in rain-shadow valleys, on well-drained loams derived from shales. Main overstorey species include Corymbia maculata (spotted gum), Eucalyptus crebra (narrow-leaved ironbark), E. moluccana (grey box), E. propinqua (grey gum), E. siderophloia (grey ironbark) and Syncarpia glomulifera (turpentine).

NSW Office of Environment and Heritage (n.d. c)

Qualitative mathematical models for the ‘Dry sclerophyll forests’ and ‘Forested wetlands’ and ‘Wet sclerophyll forests’ landscape classes are presented in Section 2.7.4 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b)).

3.4.4.2 Potential hydrological change

The area of GDE landscape classes within the zone of potential hydrological change is 3.2 km2 of which 0.4 km2 is in the mine pit exclusion zone (Table 23). Of the remaining 2.8 km2 of GDEs, 1.2 km2 is potentially subject to groundwater drawdown of more than 0.2 m under the baseline future based on the 95th percentile, compared to 0.6 km2 based on the 50th percentile and 0.4 km2 based on the 5th percentile (Table 23). The majority of the GDE area potentially subjected to groundwater drawdown under the baseline future is forested wetland. The majority of the GDE area potentially subjected to groundwater drawdown under the baseline future is subject to a drawdown of less than 2 m; 0.3 km2of forested wetland is potentially subject to a drawdown of 2 to 5 m under the baseline future.

Of GDEs, 1.1 km2 is potentially subject to additional groundwater drawdown as a result of additional coal resource development based on the 95th percentile, compared to 0.5 km2 based on the 50th percentile and 0.1 km2 based on the 5th percentile (Table 24). The majority of the GDE area potentially subjected to additional groundwater drawdown is in the ‘Forested wetlands’ landscape class. No rainforest GDE is potentially subject to additional groundwater drawdown. All of the GDE area potentially subjected to additional groundwater drawdown is subject to a drawdown of less than 2 m.

3.4.4.3 Potential ecosystem impacts

Based on the modelling of the 95th percentile presented in Table 23 and Table 24, it was concluded that approximately an additional 1.1 km2 of GDEs would be subjected to a groundwater drawdown of greater than 0.2 m but less than 2 m, and that most of the impact would be in the ‘Forested wetlands’ landscape class. As noted in Section 3.4.4.1, the water requirements of GDEs are poorly understood and there is large uncertainty as to the frequency, timing and duration of groundwater use within the Gloucester subregion. In general, transpiration of groundwater is expected to decline as the depth to groundwater increases, but the relationships between depth to groundwater and the structural and functional attributes of the vegetation communities are highly non-linear, with steep declines in leaf area index and biomass over a range of 5 to 10 m depth to groundwater.

Hence, for the ‘GDE’ landscape group in the Gloucester subregion, qualitative mathematical models of the ‘Forested wetlands’, ‘Wet sclerophyll forest’ and ‘Dry sclerophyll forest’ landscape classes were developed to make qualitative predictions about the impact of coal resource development on GDEs (see Sections 2.7.4.2, 2.7.4.3 and 2.7.4.4 of companion product 2.7 for the Gloucester subregion (Hosack et al., 2018b) for details of the models). GDE vegetation may form habitat for several threatened plant species included in the Gloucester subregion asset register (see companion product 1.3 for the Gloucester subregion (McVicar et al., 2015); Bioregional Assessment Programme, 2017; Bioregional Assessment Programme, Dataset 11), including the Charmhaven apple (Angophora inopina), white-flowered wax plant (Cynanchum elegans), leafless tongue orchid (Cryptostylis hunteriana), slaty redgum (Eucalyptus glaucina) and trailing woodruff (Asperula asthenes). In addition, GDE vegetation can form habitat for a range of vertebrate and invertebrate fauna. Examples of vertebrate fauna from the Gloucester subregion asset register listed under the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) that might use GDE vegetation, either for habitat or feeding, include the grey-headed flying-fox (Pteropus poliocephalus), red goshawk (Erythrotriorchis radiatus), regent honeyeater (Anthochaera phrygia), swift parrot (Lathamus discolor), giant barred frog (Mixophyes iteratus), stuttering frog (Mixophyes balbus), Hastings River mouse (Pseudomys oralis) and the koala (Phascolarctos cinereus). Riparian vegetation, in particular, provides migration corridors for aquatic and terrestrial fauna and habitat for a range of threatened species, such as the spot-tailed quoll (Dasyurus maculatus ssp. maculatus).

Table 23 Area (km2) of groundwater-dependent ecosystem (GDE) landscape classes potentially exposed to varying levels of baseline drawdown in the zone of potential hydrological change


Landscape class

Area in zone of potential hydrological change (km2)

Area in mine pit exclusion zone (km2)

Area with baseline drawdown ≥0.2 m (km2)

Area with baseline drawdown ≥2 m (km2)

Area with baseline drawdown ≥5 m (km2)

5th

50th

95th

5th

50th

95th

5th

50th

95th

Dry sclerophyll forests

0.2

0.0

0.0

0.1

0.2

0.0

0.0

0.0

0.0

0.0

0.0

Forested wetlands

2.0

0.4

0.3

0.4

0.8

0.0

0.0

0.3

0.0

0.0

0.0

Rainforests

0.9

0.0

0.0

0.1

0.2

0.0

0.0

0.0

0.0

0.0

0.0

Wet sclerophyll forests

0.2

0.0

0.0

0.1

0.1

0.0

0.0

0.0

0.0

0.0

0.0

Total

3.2

0.4

0.4

0.6

1.2

0.0

0.0

0.3

0.0

0.0

0.0

The area potentially exposed to ≥0.2, ≥2 and ≥5 m baseline drawdown is shown for the 5th, 50th and 95th percentiles. Baseline drawdown is the maximum difference in drawdown (dmax) under the baseline relative to no coal resource development. Areas within mine pit exclusion zones are excluded from further analysis.

Due to rounding, some totals may not correspond with the sum of the separate numbers.

Data: Bioregional Assessment Programme (Dataset 1)

Table 24 Area (km2) of groundwater-dependent ecosystem (GDE) landscape classes potentially exposed to varying levels of drawdown due to additional coal resource development


Landscape class

Area in zone of potential hydrological change (km2)

Area in mine pit exclusion zone (km2)

Area with additional drawdown 0.2 m (km2)

Area with additional drawdown 2 m (km2)

Area with additional drawdown 5 m (km2)

5th

50th

95th

5th

50th

5th

50th

95th

95th

Dry sclerophyll forests

0.2

0.0

0.1

0.1

0.2

0.0

0.0

0.0

0.0

0.0

0.0

Forested wetlands

2.0

0.4

0.0

0.3

0.8

0.0

0.0

0.0

0.0

0.0

0.0

Rainforests

0.9

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Wet sclerophyll forests

0.2

0.0

0.0

0.0

0.1

0.0

0.0

0.0

0.0

0.0

0.0

Total

3.2

0.4

0.1

0.5

1.1

0.0

0.0

0.0

0.0

0.0

0.0

The area potentially exposed to ≥0.2, ≥2 and ≥5 m baseline drawdown is shown for the 5th, 50th and 95th percentiles. Additional drawdown is the maximum difference in drawdown (dmax) due to additional coal resource development relative to the baseline. Areas within mine pit exclusion zones are excluded from further analysis.

Data: Bioregional Assessment Programme (Dataset 1)

The models (summarised in Table 25) predicted decreases in all vegetation-related variables, including overstorey and understorey (ground layer) cover, and recruitment. Forest habitats and nectar production were also expected to be negatively impacted, while fragmentation was predicted to increase. This was predicted to have negative impacts on populations of arboreal mammals, including koalas but not flying foxes, as well as nocturnal raptors and aggressive native honeyeaters. The impact of hydrological changes on flying foxes, regent honeyeaters, swift parrots and diurnal raptors was uncertain. Although the conclusions from this modelling are qualitative, and therefore highly uncertain, it is reasonable to assume that any ecosystem impacts will be greatest where the groundwater drawdown is greatest in the immediate vicinity of resource development. The impacts on assets associated with these landscape classes are further assessed in Section 3.5.2.

Table 25 Predicted response of the signed digraph variables in the ‘GDE’ landscape group to cumulative changes in hydrological response variables


Signed digraph variable

Forested wetlands

Wet sclerophyll forest

Wet sclerophyll forest

Dry sclerophyll forest

Dry sclerophyll forest

Summary of all models

Full name

Short form

C1

C1 model 1

C1 model 2

C1 model 1

C1 model 2

Diurnal raptor

DR

?

?

?

?

?

?

Nocturnal raptor

NR

Aggressive native honeyeaters

ANHE

Swift parrot

SP

?

?

?

?

?

?

Regent honeyeater

RHE

?

?

?

?

?

?

Grey-headed flying fox

GHFF

?

?

?

?

?

?

Forest habitats

FH

Forest fragmentation

FF

+

+

+

+

+

+

Flowers and nectar

FN

Precipitation

Ppt

0

0

0

0

0

0

Koala

Koa

Arboreal mammals

AM

Recruitment

Rec

Micro-climate

MC

Overstorey (vegetation)

FWOS, WSOS, DSOS

Ground layer vegetation

HWV, WSGL, DSGL

0

Mid-storey vegetation

WSMS, DSMS

na

0

Wetland community

WC

na

na

na

na

Predictions with a low probability (less than 0.80) of sign determinacy are denoted by a question mark. Zero denotes completely determined predictions of no change. na = not applicable

Last updated:
9 November 2018
Thumbnail of the Gloucester subregion

Product Finalisation date

2018