3.4.6 'Floodplain, terrestrial GDE' landscape group


3.4.6.1 Description

The ‘Floodplain, terrestrial GDE’ landscape group includes ecosystems that rely on the subsurface presence of groundwater on a permanent or intermittent basis to maintain growth or to avoid water stress and adverse impacts on condition (Eamus et al., 2006). These landscape classes are broadly defined as groundwater-dependent woodlands or shrublands that occur on floodplains but are not associated with palustrine, lacustrine or riparian wetlands and are classified as Type 3 GDEs (Richardson et al., 2011). The ‘Floodplain, terrestrial GDE’ landscape group includes two landscape classes: ‘Terrestrial GDE’, which covers 750 km2 (or 0.1% of the assessment extent) and ‘Terrestrial GDE, remnant vegetation’, which covers 78,479 km2 (or 13% of the assessment extent) (Table 19).

The alluvial aquifers that support these GDEs are formed from particles such as sand, silt and/or clay deposited within channels or on floodplains as a result of highly intermittent flooding processes. Floodplains in the lower parts of catchments tend to be significantly wider and deeper than alluvial floodplains that occur in higher parts of the catchments. These floodplains are commonly underlain by sediments deposited in fluvial (riverine) environments.

The ‘Floodplain, terrestrial GDE’ landscape group typically occurs in areas that are water limited, with low annual rainfall and high evaporation (companion product 1.1 for the Galilee subregion (Evans et al., 2014)). Low and sporadic rainfall, coupled with high evaporative demand, means that groundwater may be a more reliable source of water for groundwater-dependent vegetation than surface water (Eamus et al., 2006). Several processes acting either individually or in combination control recharge into the alluvial aquifers of this landscape group, including direct infiltration of rainfall, inundation by floodwaters or discharge from surrounding water-bearing geological units.

Shallow groundwater systems in alluvial sediments that underlie the floodplains include perched aquifer systems isolated from the regional watertable and groundwater systems that are connected with deeper aquifers underlying the alluvial sediments (Figure 57). The degree of connection with alluvial sediments is governed by a number of factors, including hydraulic head pressures in the different aquifer systems and whether sedimentary layers in alluvial deposits impede upward groundwater movement from underlying aquifers. If there is sufficient hydraulic head (pressure) in underlying aquifers and a connective pathway, then groundwater may discharge from underlying aquifers into overlying aquifers in alluvial sediments. This may occur if there is not a sufficiently competent aquitard (e.g. thick clay-rich layers) to impede upwards groundwater flow. Deeper aquifers that may discharge to overlying alluvial sediments in the Burdekin river basin include the Clematis Group, Dunda beds (a part of the Rewan Group), upper Permian coal measures and the Joe Joe Group.

Potentiometric mapping of water levels for various aquifers outlined in companion product 2.1-2.2 for the Galilee subregion (Evans et al., 2018a) suggests that there is potential for discharge from deeper aquifers to overlying alluvium where the:

  • Clematis Group and Dunda beds aquifers occur near the surface under shallow cover of the Moolayember Formation or where these units directly underlie alluvium in the Carmichael River valley (in the vicinity and downstream of the Doongmabulla Springs complex)
  • Belyando River floodplain is underlain by upper Permian coal measures and Joe Joe Group, in particular, in the vicinity of the confluence of the Carmichael River and Belyando River (including Mellaluka Springs complex and Albro Springs).

Surface water – groundwater interactions in floodplain environments vary depending on local geological conditions (e.g. distribution of sand-rich sediments in the floodplain), hydrogeological conditions and climate. For example, unconfined groundwater levels can rise after heavy rainfall, resulting in temporary discharge to streams. During drier periods, groundwater levels can fall, resulting in surface water recharging the shallow groundwater system.

Regardless of aquifer configuration and connectivity, vegetation can extract groundwater if the watertable, or capillary zone that forms above the watertable, comes within reach of the plant root system (generally at depths of less than 20 m). Most vegetation in the ‘Floodplain, terrestrial GDE’ landscape group is likely to source groundwater from alluvium and Cenozoic sediments. As a result, GDEs connected to the regional groundwater systems within the zone could be impacted by changes in groundwater (for example, from drawdown or recharge) across a broad area.

The ‘Floodplain, terrestrial GDE’ landscape group supports over 85 regional ecosystems (REs) in the assessment extent. However, there is considerable uncertainty related to the water regime required to support many of these REs. 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. Additionally, groundwater dependency may be influenced by the age of the vegetation within the REs, for example, younger trees commonly have shorter roots than older trees and thus their groundwater requirements can be met from relatively shallower groundwater systems. The nature of this groundwater dependency may have implications for vegetation recruitment and community persistence, as well as stand structure.

Eucalyptus, Corymbia or Acacia species are common dominant/co-dominant overstorey species in these REs. Specifically, eucalypt woodlands dominate alluvial river and creek flats of the floodplains. Eucalyptus brownii or E. coolabah woodlands and open woodlands dominate the alluvial plains. Acacia woodlands, including A. argyrodendron, A. cambagei and A. harpophylla are also common on the alluvial plains. Smaller areas of Corymbia woodlands are associated with alluvial plains or river terraces. The waxy cabbage palm (Livistona lanuginosa) is part of the riparian vegetation along river channels and on floodplains on alluvial duplex soils in a small area of the Burdekin river basin that includes Doongmabulla Springs complex on the Carmichael River. The waxy cabbage palm relies on subsurface availability of groundwater (Pettit and Dowe, 2004; Department of Environment, 2015).

`Figure 57

Figure 57 Pictorial conceptual model of the potential interactions between ecosystems and groundwater within alluvial aquifers, such as those of the 'Floodplain, terrestrial GDE' landscape group

GDE = groundwater-dependent ecosystem

Source: adapted from Queensland Department of Science, Information Technology and Innovation (Dataset 4) © The State of Queensland (Department of Science, Information Technology and Innovation) 2015

3.4.6.2 Potential hydrological impacts

The ‘Floodplain, terrestrial GDE’ receptor impact model focused on the influence that surface water and groundwater hydrology had on trees that support a woodland community, and create local conditions and a microclimate (i.e. shade, leaf litter and soil moisture) that favours mesic vegetation and suppresses xeric vegetation (companion product 2.7 for the Galilee subregion (Ickowicz et al., 2018)).

For the ‘Floodplain, terrestrial GDE’ receptor impact model, the relevant hydrological response variables are:

  • maximum difference in drawdown under the baseline future or under the coal resource development pathway future relative to the reference period (1983 to 2012) (dmaxRef)
  • mean annual number of events with a peak daily flow exceeding the threshold (the peak daily flow in flood events with a return period of 2.0 years as defined from modelled baseline flow in the reference period (1983 to 2012)). This metric is designed to be approximately representative of the number of overbank flow events in future 30-year periods (EventsR2.0).

3.4.6.2.1 Groundwater

Vegetation classified as ‘Floodplain, terrestrial GDE’ in the zone of potential hydrological change is located along the western edge of the zone, upstream of the proposed Hyde Park and China Stone coal mines in the north and upstream of the proposed Kevin’s Corner, Alpha and South Galilee coal mines in the south (Figure 58). Vegetation in the Carmichael River valley and Belyando River floodplain in the zone of potential hydrological change are predominantly classified as ‘Floodplain, terrestrial GDE’.

Most groundwater-dependent vegetation in the zone of potential hydrological change is classified as ‘Floodplain, terrestrial GDE’ (2433 km2 of 3776 km2, or 64% of groundwater-dependent vegetation in the zone). It is very unlikely that additional drawdown in excess of 0.2 m in the uppermost aquifer (i.e. Quaternary alluvium and Cenozoic sediment layer) will affect more than 1967 km2 of vegetation classified as ‘Floodplain, terrestrial GDE’ (Table 27 and Figure 59).

The median (50th percentile) estimate of greater than 2 m drawdown due to additional coal resource development is less extensive, potentially affecting 319 km2 of vegetation classified as ‘Floodplain, terrestrial GDE’, or 8% of groundwater-dependent vegetation in the zone (Table 27). Additional drawdown in excess of 5 m is very unlikely to affect more than 296 km2 of vegetation classified as ‘Floodplain, terrestrial GDE’.

Figure 58

Figure 58 'Floodplain, terrestrial GDE' and 'Non-floodplain, terrestrial GDE' landscape groups: location of groundwater-dependent vegetation in the zone of potential hydrological in the Galilee subregion

GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 1, Dataset 9)

Figure 59

Figure 59 'Floodplain, terrestrial GDE' landscape group: area (km2) of groundwater-dependent vegetation potentially exposed to varying levels of additional drawdown and changes to recurrence of overbank flows per year (EventsR2.0) in 2042 and 2102 in the zone of potential hydrological change

GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 1)

Table 27 'Floodplain, terrestrial GDE' landscape group: area (km2) of groundwater-dependent vegetation potentially exposed to varying levels of additional drawdown in the zone of potential hydrological change


Landscape class

Area in assessment extent

Area in zone of potential hydrological change

Area in mine exclusion zone

Area with additional drawdown ≥0.2 m

Area with additional drawdown ≥2 m

Area with additional drawdown ≥5 m

5th

50th

95th

5th

50th

95th

5th

50th

95th

Terrestrial GDE

750

75.2

7.4

12.0

20.5

57.2

4.6

11.3

16.9

1.4

5.5

10.8

Terrestrial GDE, remnant vegetation

78,479

2358

189

356

734

1910

86

308

588

23.2

113

286

Subtotal

79,229

2433

196

368

754

1967

90.6

319

605

24.6

118

296

Some totals reported here have been rounded.

GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 1)

3.4.6.2.2 Surface water

Over half of the groundwater-dependent vegetation on floodplains (716 km2) in the surface water zone of potential hydrological change is potentially impacted but are located on temporary streams that are not modelled. This includes parts of Bimbah, Cattle, Dyllingo and North creeks and the Carmichael and Belyando rivers in the northern zone, and Alpha, Lagoon, Sandy and Tallarenha creeks in the southern zone (Figure 60).

By 2042, it is very unlikely that modelled overbank flows will decrease by more than 0.1 events per year, potentially affecting more than 68 km2 of groundwater-dependent vegetation in the zone of potential hydrological change (Figure 59 and Table 28). This includes vegetation along parts of Alpha, Bully, Sandy and Tallarenha creeks and the Belyando River. A reduction of 0.1 events per year means one fewer overbank flow events every 10 years. Based on the median estimate, the number of modelled overbank flows per year in the 30-year period preceding 2042 is predicted to decrease by more than 0.1 in an area of 3 km2, 0.05 in an area of 47 km2 and 0.02 in an area of 85 km2 of groundwater-dependent vegetation. Predictions in the 30-year period preceding 2102 are less extensive; median estimates of the number of overbank flows per year are predicted to decrease by more than 0.02 in an area of less than 1 km2 of groundwater-dependent vegetation in the zone of potential hydrological change (Figure 59).

Figure 60

Figure 60 'Floodplain, terrestrial GDE' landscape group: modelled decrease in recurrence of overbank flows per year (EventsR2.0) due to additional coal resource development in 2042 and 2102 in the zone of potential hydrological change

ACRD = additional coal resource development; GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 9)

Table 28 ‘Floodplain, terrestrial GDE’ landscape group: area (km2) of groundwater-dependent vegetation potentially exposed to changes in recurrence of overbank flows per year (EventsR2.0) due to additional coal resource development in 2042 and 2102 in the zone of potential hydrological change


Landscape class

Area in zone of potential hydrological change

Area potentially impacted but not quantified

Area with 0.02 decrease of overbank flows (events per year)

Area with 0.05 decrease of overbank flows (events per year)

Area with 0.1 decrease of overbank flows (events per year)

5th

50th

95th

5th

50th

95th

5th

50th

95th

2013–2042

Terrestrial GDE

75.2

21.9

14.1

3.1

0

3.5

1.7

0

2.7

0.3

0

Terrestrial GDE, remnant vegetation

2358

694

341

81.6

0

86.8

45.0

0

65.7

2.4

0

Subtotal

2433

716

355

84.7

0

90.4

46.7

0

68.4

2.7

0

2073–2102

Terrestrial GDE

75.2

21.9

6.4

0.1

0

1.4

0

0

0

0

0

Terrestrial GDE, remnant vegetation

2358

694

185

0.4

0

10.0

0

0

0

0

0

Subtotal

2433

716

191

0.5

0

11.4

0

0

0

0

0

Some totals reported here have been rounded.

A reduction of 0.02 events per year means one fewer overbank flow event every 50 years, 0.05 is one fewer overbank flow event every 20 years and 0.1 is one fewer overbank flow event every 10 years.

GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 1)

3.4.6.3 Potential ecosystem impacts

The key hydrological determinants of ecosystem function identified by experts for the ‘Floodplain, terrestrial GDE’ landscape group are related to overbank flooding and access to relatively shallow alluvial groundwater sources. Tree foliage cover is related to both the rate of groundwater drawdown and its maximum depth, such that tree roots maintain contact with groundwater. Seasonal floods are important contributors to groundwater recharge on floodplains, but could potentially contribute to saturated, anoxic soil conditions that may suppress deep-rooted vegetation.

For the ‘Floodplain, terrestrial GDE’ receptor impact model, the receptor impact variable is the percent foliage cover of floodplain trees, such as Eucalyptus, Corymbia or Acacia species that dominate the alluvial river and creek flats in the landscape group. Percent foliage cover is the mean annual value measured in a 1 ha plot. The experts’ opinion provides strong evidence that:

  • antecedent foliage cover has a strong effect on future foliage cover, which reflects the lag in the response of canopy cover to changes in hydrological response variables that would be expected of mature trees with long life spans
  • mean percent foliage cover would decrease from approximately 10% by approximately 3% if groundwater depth increases by 6 m and all other model variables are held at their median values
  • mean percent foliage cover would increase by less than 1% as the number of flood events with peak daily flows exceeding the 1983 to 2012 2-year return period (EventsR2.0) increases to a maximum value of 1.6 and all other model variables are held at their median values.

There is considerable uncertainty in these predictions, particularly related to the effect of flow regime on percent foliage cover, which means that the large uncertainty in the receptor impact model does not preclude the small possibility of EventsR2.0 having a negligible effect on percent foliage cover.

Median estimates of the difference in percent foliage cover due to additional coal resource development in the 30-year periods preceding 2042 and 2102 indicate no change from that under the baseline (Figure 61). The large uncertainty in the elicited model is reflected by the relatively wide range of model predictions. Results indicate there is a 5% chance that percent foliage cover in some assessment units may decrease by up to 20% in 2042 and up to 11% in 2102, and a 95% chance that it may increase by up to 15% in 2042 and up to 6% in 2102, due to additional coal resource development.

Risk thresholds for the ‘Floodplain, terrestrial GDE’ receptor impact model are:

  • ‘at some risk of ecological and hydrological changes’ decreases of greater than 5% foliage cover
  • ‘more at risk of ecological and hydrological changes’ decreases of greater than 10% foliage cover.

Groundwater-dependent vegetation where receptor impact modelling indicated greater than ‘at minimal risk’ level occurs along floodplains associated with Alpha, North, Sandy and Tallarenha creeks and the Belyando and Carmichael rivers (Figure 62). Receptor impact variables were not calculated for 5503 (59%) assessment units for this landscape group in the zone of potential hydrological change (where hydrological changes were not quantified). Of the 1119 assessment units where receptor impact variables were calculated, 105 (or 9%) are considered to be ‘at some risk’ and 141 (or 13%) are considered to be ‘more at risk’. Thus, there is some level of risk to 22% of assessment units with receptor impact modelling, and 3% of the total number of assessment units when both the quantified and unquantified changes are considered for this landscape group. The groundwater-dependent ecosystems on floodplains where there is some level of risk occur along parts of Alpha, North, Sandy and Tallarenha creeks and the Belyando and Carmichael rivers. A more detailed and local consideration of risk needs to consider the specific values at the locations that the community are seeking to protect (e.g. particular assets) because that will help to identify meaningful analysis thresholds. It is also necessary to incorporate other lines of evidence that include the magnitude of the hydrological change and the information from the qualitative mathematical models.

Figure 62

Figure 62 'Floodplain, terrestrial GDE' landscape group: composite risk to groundwater-dependent vegetation due to additional coal resource development

ACRD = additional coal resource development; GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 11)

Last updated:
4 January 2019
Thumbnail of the Galilee subregion

Product Finalisation date

2018
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