2.7.5.2 Qualitative mathematical model

Page 29 of 48

As part of this bioregional assessment (BA), a qualitative mathematical model was developed for the 'Floodplain, terrestrial GDE' landscape group. This model focused on the influence that surface water and groundwater hydrology had on trees that support a woodland community, creating local conditions and a microclimate (i.e. shade, leaf litter and soil moisture) that favours mesic vegetation and suppresses xeric vegetation (Figure 27). Trees connected to groundwater are maintained by a specific regime where the rate of drawdown in groundwater and its maximum depth does not outstrip the ability of the trees’ rate and extent of root growth to maintain contact with groundwater stores. Moreover, deep-rooted trees have the potential to draw groundwater to the surface where it can become available to shallow-rooted mesic vegetation. The strength or existence of this effect, however, was deemed to be uncertain. Seasonal floods are generally shown to suppress xeric vegetation in the floodplain, to favour trees and mesic species, and to support a (as yet unspecified) floodplain community. Seasonal floods are important contributors to the recharging of groundwater, but excessive groundwater recharge could potentially contribute to saturated soil conditions where an anoxic root zone suppresses deep-rooted vegetation (i.e. trees connected to groundwater (TCGW) and mesic non-tree deep roots (MNTDR), Figure 27). It is uncertain, however, whether these floodplains could develop anoxic soil conditions, and existence of this link was also uncertain.

In summary, two uncertainties exist in the qualitative modelling. First, whether deep-rooted trees have the potential to draw groundwater to the surface where it can become available to shallow-rooted mesic vegetation. Second, whether there is a link between groundwater and the anoxic root zone. As a consequence of these uncertainties, four alternative models were developed (Figure 27, Figure 28, Figure 29 and Figure 30). In the first model (Figure 27, Table 21), both links are present. The second model (Figure 28, Table 22) lacks the link between trees drawing groundwater to the surface, but the link indicating the potential for groundwater to contribute to anoxic soils is present. In the third model (Figure 29, Table 23) the link between trees drawing groundwater to the surface is present but the link indicating the potential for groundwater to contribute to anoxic soils is not. In the fourth model (Figure 30, Table 24) neither link occurs.

Figure 27 Signed digraph model (Model 1) of the floodplain vegetative community associated with the 'Floodplain, terrestrial GDE' landscape group in the Galilee subregion zone of potential hydrological change

Model variables are: anoxic root zone (ARZ), floodplain community (FPC), groundwater depth and drawdown rate (GWDDR), decreased floods (DF), groundwater recharge (GWR), leaf litter (LL), mesic non-tree deep roots (MNTDR), mesic non-tree shallow roots (MNTSR), seasonal floods (SF), shade (Sha), soil moisture availability (SMA), trees connected to groundwater (TCGW), woodland community (WC), xeric shallow roots (XSR). A link ending in an arrowhead denotes a positive direct effect; a link ending in a filled circle denotes a negative direct effect. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Figure 28 Signed digraph model (Model 2) of the floodplain vegetative community associated with the 'Floodplain, terrestrial GDE' landscape group in the Galilee subregion zone of potential hydrological change

Model variables are: anoxic root zone (ARZ), floodplain community (FPC), groundwater depth and drawdown rate (GWDDR), decreased floods (DF), groundwater recharge (GWR), leaf litter (LL), mesic non-tree deep roots (MNTDR), mesic non-tree shallow roots (MNTSR), seasonal floods (SF), shade (Sha), soil moisture availability (SMA), trees connected to groundwater (TCGW), woodland community (WC), xeric shallow roots (XSR). A link ending in an arrowhead denotes a positive direct effect; a link ending in a filled circle denotes a negative direct effect. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Figure 29 Signed digraph model (Model 3) of the floodplain vegetative community associated with the 'Floodplain, terrestrial GDE' landscape group in the Galilee subregion zone of potential hydrological change

Model variables are: anoxic root zone (ARZ), floodplain community (FPC), groundwater depth and drawdown rate (GWDDR), decreased floods (DF), groundwater recharge (GWR), leaf litter (LL), mesic non-tree deep roots (MNTDR), mesic non-tree shallow roots (MNTSR), seasonal floods (SF), shade (Sha), soil moisture availability (SMA), trees connected to groundwater (TCGW), woodland community (WC), xeric shallow roots (XSR). A link ending in an arrowhead denotes a positive direct effect; a link ending in a filled circle denotes a negative direct effect. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Figure 30 Signed digraph model (Model 4) of the floodplain vegetative community associated with the 'Floodplain, terrestrial GDE' landscape group in the Galilee subregion zone of potential hydrological change

Model variables are: anoxic root zone (ARZ), floodplain community (FPC), groundwater depth and drawdown rate (GWDDR), decreased floods (DF), groundwater recharge (GWR), leaf litter (LL), mesic non-tree deep roots (MNTDR), mesic non-tree shallow roots (MNTSR), seasonal floods (SF), shade (Sha), soil moisture availability (SMA), trees connected to groundwater (TCGW), woodland community (WC), xeric shallow roots (XSR). A link ending in an arrowhead denotes a positive direct effect; a link ending in a filled circle denotes a negative direct effect. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Surface and groundwater modelling indicate potential impacts of coal mining to the regime for groundwater depth and drawdown rate (i.e. such that trees can maintain contact with groundwater through root growth), and decreased flood events. These projected changes may be ecologically meaningful. Based on all combinations of these impacts, three cumulative impact scenarios were developed for qualitative analysis of response predictions (Table 20).

Qualitative analyses of the four signed digraph models (Figure 27, Figure 28, Figure 29 and Figure 30) generally indicate a negative or ambiguous response prediction for all biological variables within the floodplain ecosystem (Table 21, Table 22, Table 23 and Table 24, respectively). The only variable that was predicted to respond positively to any of the cumulative impact scenarios was xeric vegetation with shallow roots. Response predictions for models 3 and 4, which lacked the link between groundwater and anoxic conditions in the root zone of the floodplain, generally had a higher level of sign determinacy for impacts that included a decrease in floods (i.e. impact scenarios C2 and C3, Table 23 and Table 24).

Table 20 Summary of the cumulative impact scenarios (CISs) for the ‘Floodplain, terrestrial GDE’ landscape group in the Galilee subregion zone of potential hydrological change

CIS

GWDDR

DF

C1

0

C2

0

+

C3

+

Pressure scenarios are determined by combinations of no-change (0), increase (+) or a decrease (–) in the following signed digraph variables: groundwater depth and drawdown rate (GWDDR), and decreased flood events (DF). Scenario C3 shows the expected impacts under the modelled coal resource development pathway in the Galilee subregion. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Table 21 Predicted response of the signed digraph variables (Model 1) in the ‘Floodplain, terrestrial GDE’ landscape group to (cumulative) changes in hydrological response variables

Signed digraph variable

(full name)

Signed digraph variable

(short form)

C1

C2

C3

Seasonal floods

SF

0

Groundwater recharge

GWR

0

Floodplain community

FPC

0

Groundwater depth and drawdown rate

GWDDR

0

Decreased floods

DF

0

+

+

Trees connected to groundwater

TCGW

?

?

Mesic non-tree deep roots

MNTDR

?

?

Mesic non-tree shallow roots

MNTSR

?

(–)

Xeric shallow roots

XSR

+

?

(+)

Woodland community

WC

?

?

Sha

?

?

Leaf litter

LL

?

?

Soil moisture availability

SMA

?

?

Anoxic root zone

ARZ

0

Qualitative model predictions that are completely determined are shown without parentheses. Predictions that are ambiguous but with a high probability (0.80 or greater) of sign determinacy are shown with parentheses. 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. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Table 22 Predicted response of the signed digraph variables (Model 2) in the ‘Floodplain, terrestrial GDE’ landscape group to (cumulative) changes in hydrological response variables

Signed digraph variable

(full name)

Signed digraph variable

(short form)

C1

C2

C3

Seasonal floods

SF

0

Groundwater recharge

GWR

0

Floodplain community

FPC

0

Groundwater depth and drawdown rate

GWDDR

0

Decreased floods

DF

0

+

+

Trees connected to groundwater

TCGW

?

?

Mesic non-tree deep roots

MNTDR

?

?

Mesic non-tree shallow roots

MNTSR

?

(–)

Xeric shallow roots

XSR

+

?

(+)

Woodland community

WC

?

?

Sha

?

?

Leaf litter

LL

?

?

Soil moisture availability

SMA

?

?

Anoxic root zone

ARZ

0

Qualitative model predictions that are completely determined are shown without parentheses. Predictions that are ambiguous but with a high probability (0.80 or greater) of sign determinacy are shown with parentheses. 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. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Table 23 Predicted response of the signed digraph variables (Model 3) in the ‘Floodplain, terrestrial GDE’ landscape group to (cumulative) changes in hydrological response variables

Signed digraph variable

(full name)

Signed digraph variable

(short form)

C1

C2

C3

Seasonal floods

SF

0

Groundwater recharge

GWR

0

Floodplain community

FPC

0

Groundwater depth and drawdown rate

GWDDR

0

Decreased floods

DF

0

+

+

Trees connected to groundwater

TCGW

Mesic non-tree deep roots

MNTDR

Mesic non-tree shallow roots

MNTSR

Xeric shallow roots

XSR

+

+

+

Woodland community

WC

Sha

Leaf litter

LL

Soil moisture availability

SMA

Anoxic root zone

ARZ

0

0

0

Qualitative model predictions that are completely determined are shown without parentheses. Predictions that are ambiguous but with a high probability (0.80 or greater) of sign determinacy are shown with parentheses. 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. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Table 24 Predicted response of the signed digraph variables (Model 4) in the ‘Floodplain, terrestrial GDE’ landscape group to (cumulative) changes in hydrological response variables

Signed digraph variable

(full name)

Signed digraph variable

(short form)

C1

C2

C3

Seasonal floods

SF

0

Groundwater recharge

GWR

0

Floodplain community

FPC

0

Groundwater depth and drawdown rate

GWDDR

0

Decreased floods

DF

0

+

+

Trees connected to groundwater

TCGW

Mesic non-tree deep roots

MNTDR

Mesic non-tree shallow roots

MNTSR

Xeric shallow roots

XSR

+

+

+

Woodland community

WC

Sha

Leaf litter

LL

Soil moisture availability

SMA

Anoxic root zone

ARZ

0

0

0

Qualitative model predictions that are completely determined are shown without parentheses. Predictions that are ambiguous but with a high probability (0.80 or greater) of sign determinacy are shown with parentheses. 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. GDE = groundwater-dependent ecosystem

Data: Bioregional Assessment Programme (Dataset 4)

Last updated:
4 January 2019