4.4 Prioritisation of Wetlands
4.4.3 Prioritisation based on Wetland Degradation and Onsite Rehabilitation103
4.4.4.1 Scenario One: Prioritisation based on Land-cover
The first step in running this scenario was that land-cover classes present in each of the wetlands and each of their catchments was scrutinized, such that cultivated, irrigated;
cultivated, dryland; residential- rural; mines and quarries; and urban informal land-cover classes were highlighted, and the sum of the areal extents of these land-cover classes affecting each of these wetlands was determined. The wetlands with the five highest areal extents of these land-cover classes were subsequently targeted for land-cover rehabilitation (Table 45). These wetlands were Wetland FID 25, FID 28, FID 46(1), FID 58, and FID 62.
The next step was to apply the simplest hypothetical rehabilitation technique to these wetlands, so land-cover classes cultivated, irrigated; cultivated, dryland; residential- rural;
mines and quarries; and urban informal were removed and replaced with natural land-cover.
The process of determining overall water quality enhancement effectiveness of the catchment was reapplied to the changed land-cover classes. Magnitude of impact scores were determined for the altered land-cover classes (Table 46), and the application of relevant equations from Tables 6 to 11 were undertaken for both impacts arising in the wetland’s catchment and impacts arising within the wetland. Final functionality scores were determined
and resolved for sediment trapping, phosphate trapping, nitrate removal and toxicant removal, and hectare equivalents for each of these ecosystem services was established, which allowed for the determination of hectare equivalents of water quality enhancement (Table 47).
Hectare equivalents of water quality impairment were also determined through the application of the process described in Figure 13. Finally, overall effectiveness of water quality enhancement was determined by subtracting three times the determined hectare equivalents of water quality impairment from the hectare equivalents of water quality enhancement (Table 48).
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. The result was that overall water quality enhancement effectiveness was improved from -1561.17 to -1029.51, indicating an improvement in the enhancement of water quality by the collective wetlands in the catchment (Table 49).
This indicates that although the catchment was still not completely filtering the waters passing through it, there was an improvement in the quality of the water leaving the catchment after rehabilitation was undertaken. Given the constraints of targeting only five wetlands with the occurrence of particular land-cover classes in their catchments, the score of -1029.51 hectare equivalents of water quality enhancement effectiveness is indicative of the optimal water quality enhancement effectiveness of that catchment under these constraints.
4.4.4.2 Scenario Two: Prioritisation based on Effectiveness of Water Quality Enhancement In running this scenario, the overall effectiveness of water quality enhancement scores of the wetlands in their current state were scrutinized, and negative overall water quality enhancement effectiveness scores were highlighted. The wetlands with the five overall water quality enhancement effectiveness scores of greatest negativity were subsequently targeted for land-cover rehabilitation (Table 50). These wetlands were Wetland FID 25, FID 40, FID 42, FID 46(1), and FID 62.
The criterion upon which Scenario One was based (that only land-cover classes cultivated, irrigated; cultivated, dryland; residential- rural; mines and quarries; and urban informal be removed and replaced with natural land-cover due to the feasibility of their rehabilitation) was maintained, and these land-cover classes from the catchments of the aforementioned wetlands were hypothetically converted to ‘natural’ land-cover.
The calculation of the overall water quality enhancement effectiveness for the portion of the Goukou Catchment used for running these scenarios was then repeated for the ‘rehabilitated’
catchments. Magnitude of impact scores were determined for the altered land-cover classes (Table 51), and the relevant equations from Tables 6 to 11 were applied to the magnitude of impacts scores for both impacts arising in the wetland’s catchment and impacts arising within the wetland. Final functionality scores were determined and resolved for sediment trapping, phosphate trapping, nitrate removal and toxicant removal, which allowed for the determination of hectare equivalents of each of these ecosystem services analysed. The mean of these hectare equivalents was then determined to derive hectare equivalents of water quality enhancement for the catchment (Table 52). Hectare equivalents of water quality impairment were also determined through the application of the process described in Figure 13. Finally, overall effectiveness of water quality enhancement was determined by subtracting three times the determined hectare equivalents of water quality impairment from the hectare equivalents of water quality enhancement (Table 53).
The new water quality enhancement effectiveness scores from the targeted wetlands, along with those effectiveness scores for wetlands that were unchanged, were then applied to the wetlands of the case study group and in order to integrate the spatial configuration of the wetlands in the catchment, the wetlands were arranged in order of inflow. It was determined that overall water quality enhancement effectiveness was improved from -1561.17 to -927.53 (Table 54). This difference of 633.34 in the effectiveness of water quality enhancement by the collective wetlands in the catchment indicates an improvement in the enhancement of water quality due to land-cover rehabilitation for wetlands with a considerably negative initial overall water quality enhancement effectiveness score.
As in the case of Scenario One, the score of -927.53 indicates that although the catchment was still not completely filtering the waters passing through it, there was an improvement in the quality of the water leaving the catchment after rehabilitation was undertaken. Given the
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