4.4 Prioritisation of Wetlands
4.4.3 Prioritisation based on Wetland Degradation and Onsite Rehabilitation103
4.4.4.3 Scenario Three: Prioritisation based on Wetland Degradation and
constraints of targeting only five wetlands with the occurrence of particular land-cover classes in their catchments, the improvement in the overall catchment water quality enhancement effectiveness score is indicative of the optimal water quality enhancement effectiveness of that catchment under these constraints.
4.4.4.3 Scenario Three: Prioritisation based on Wetland Degradation and Onsite
By hypothetically implementing the appropriate rehabilitation technique to the wetlands targeted for rehabilitation in this scenario, the intensity of impact score for each of these wetlands would theoretically be reduced. In running this scenario however, cognisance was taken of the fact that it is not possible to accurately predict the effect that onsite rehabilitation will have on the intensity of impact scores. However, from a hydrological health perspective it is possible to make some general assumptions on the effect of onsite rehabilitation on the intensity of impact scores for each land-cover class within a wetland, which are presented in Table 22.
As is reflected in Table 22, rehabilitation in forest plantations can be assumed to lower the intensity of impact score by 2, while for degraded vegetation, the intensity of impact score may be lowered to 1 or 2 (average of 1.5). Similarly, rehabilitation in cultivated land in a wetland can be assumed to lower the impact score to 2. Rehabilitation of dongas in wetlands generally involves preventing the advance of the gully into areas that have not yet eroded (generally natural areas) for which it can be assumed that the gully would have otherwise increased the impact score (i.e., the rehabilitation would keep the areas that are under threat of erosion in a natural state) (Kotze, 2009, pers. comm.). The areas that have already eroded and are then rehabilitated would likely reduce their impact score to 5. Rehabilitated residential rural and urban informal land-cover classes can be assumed to return to a score of 3 because they are generally not associated with major changes such as infilling and reconfiguration of drainage patterns (Kotze, 2009, pers. comm.). The intensity of impact scores for land-cover classes urban residential – high density; urban commercial; urban industrial/transport; and mines and quarries have remain unchanged, as the presence of these land-cover classes within the wetland would make rehabilitation unfeasible, as is discussed in Section 4.4.1.
In applying this scenario, the post-rehabilitation intensity of impact scores were applied to the land-cover classes present in the wetlands of the case study catchment, and the processes of determining final hectare equivalents of water quality enhancement and water quality impairment were undertaken, the steps of which are presented in Figures 12 and 13. These steps included the determination of magnitude of impact scores for each land-cover class present (Table 56), for catchment and within-wetland impacts; the resolution of catchment impacts; the application of the relevant equations from Tables 6 to 11 to determine the effect
of each impact on the provision of sediment trapping, phosphate trapping, nitrate removal, and toxicant removal; the resolution and scaling of onsite and catchment impacts to determine a final functional effectiveness score; and the subsequent calculation of functional hectare equivalents of water quality enhancement functionality (Table 57). Hectare equivalents of water quality impairment were also determined by utilising the relevant impact ratio from Table 25, and both hectare equivalents of water quality enhancement and hectare equivalents of water quality impairment were used to establish overall effectiveness of water quality enhancement (Table 58). Finally, as was conducted in Scenarios One and Two, newly determined water quality enhancement effectiveness scores, along with those effectiveness scores for wetlands that were unchanged, were then applied to the wetlands of the case study group and the wetlands were arranged in order of inflow so as to integrate the spatial configuration of the wetlands in the catchment (Table 59). By doing so, it was concluded that by targeting wetlands for rehabilitation based on wetland degradation, and by altering intensity of impact scores for the five most degraded wetlands in the case study catchment, overall effectiveness of water quality enhancement improved from -1561.17 to -1363.75.
While this value of -1363.75 is indicative of an improvement in overall effectiveness of water quality enhancement, it is noted that due to the difficulty in accurately predicting the effect that onsite rehabilitation will have on the intensity of impact scores, especially since all the scores are currently solely land-cover based, this may not be an accurate reflection of the positive effect that onsite rehabilitation may have on the effectiveness of water quality enhancement by each wetland.
Table 22. Intensity of impact scores to be used for within-wetland land-cover pre- and post-rehabilitation Land-cover
Category Intensity of impact score for within-wetland impacts
Pre-rehabilitation Post-rehabilitation
Increased direct water losses
Reduced surface roughness
Flow impediment
Flow enhancement
Increased direct water losses
Reduced surface roughness
Flow impediment
Flow enhancement
Natural 0 0 0 0 0 0 0 0
Forest
plantations 9 (forest) 7 (forest)
7 (heavy alien plant infestation)
5 (heavy alien plant infestation) 5 (modest
alien plant infestation)
3 (modest alien plant infestation) 3 (light alien
plant infestation)
1 (light alien plant infestation)
Cultivated, 5 2
irrigated Cultivated,
dryland 5 2
Dongas and sheet
erosion 9 5
Degraded
vegetation 3 1.5
Urban residential -
high density 7 7
Residential -
rural 5 3
Urban
commercial 9 9
Urban industrial/
transport
4*** (for area of wetland above road across a
wetland) 9
5 (for area of wetland above road across a wetland)
4*** (for area of wetland above road across a
wetland) 9
5 (for area of wetland above road across a wetland) Mines and
quarries 9 9
Urban
informal 5 3