4.3 The Application of Catchment Scale Analysis
4.3.5 Integrating the Spatial Configurations of Wetlands in a Landscape in
The overall effectiveness of water quality enhancement for each wetland in the case study was determined before cumulative analyses were conducted. As is described in Section 2.5.2, when considering the cumulative effects of wetland loss and degradation on the functional values of a catchment, it is important to recognise and determine the value of the component wetlands, “based on their relative contribution to the functioning of the entire landscape system” (Bedford and Preston, 1988, p567)
4.3.5 Integrating the Spatial Configurations of Wetlands in a Landscape in Catchment-scale
stream order are less likely to have other wetlands intercepting waters from upstream before reaching them. The waters reaching wetlands positioned at second or third-order streams however, are more likely to have been intercepted by upstream wetlands, which would have filtered the passing waters already, giving the higher ordered stream wetlands less function to perform. The problem with this idea is that if weightings are based solely on stream order, wetlands positioned in higher orders that are in fact servicing the lower order streams as well, will be discounted in weight unfairly. For example, two wetlands may be positioned on two different second order streams (Figure 15). Wetland A is servicing all of the area upstream of it, as there are no other wetlands positioned on the streams upstream. However while Wetland B is also positioned on a second order stream, Wetland C is already filtering passing waters from upstream, making the task of enhancing water quality easier for wetland B than it would be for Wetland A, given similar land-cover types in their respective catchments. As such, based on this idea of stream order is that of order of inflow.
Figure 15. Hypothetical example of wetlands positioned on streams of varying orders, with arrows indicating the direction of stream flow
Wetlands which are positioned in the landscape in such a way that there are no other wetlands intercepting waters upstream, may be considered as primary inflow wetlands. The functionality of primary inflow wetlands is not influenced by any other wetlands upstream. In Figure 15 for example, Wetland A and Wetland C are primary inflow wetlands. Alternatively, the waters of a secondary inflow wetland have been intercepted by one or more primary inflow wetlands (Wetland B in Figure 15 is a secondary inflow wetland). If the single upstream primary inflow wetland is removed, the secondary inflow wetland will then become the primary inflow wetland, as it will be servicing its entire catchment upstream. A tertiary inflow wetland is positioned in the landscape in such a way that the waters reaching it have already been intercepted by two or more secondary inflow wetlands upstream.
After having determined the overall effectiveness of water quality enhancement for each wetland in the landscape, the wetlands can be ordered in this way, and the ‘unassimilated’
hectare equivalents of water quality impairment (i.e. the negative values for overall effectiveness of water quality enhancement) from upstream will be ‘carried’ to wetlands downstream. Thus, by adding these ‘carried’ negative effectiveness values to the effectiveness values of the higher ordered inflow wetlands downstream, the unassimilated hectare equivalents of water quality impairment may either be assimilated by a higher ordered wetland with a high water quality enhancement effectiveness score, or will further decrease the quality of the water leaving the higher ordered wetland should that wetland not be effectively enhancing water quality. By continuing this process downstream, the water quality enhancement effectiveness value at the outflow of the catchment (marked as X in Figure 15) will be indicative of the overall effectiveness of the entire catchment at enhancing water quality.
To illustrate this concept, consider a catchment occupied by five wetlands (A-E) in various positions in the landscape (Figure 16). The black numbers in brackets indicate the overall water quality enhancement effectiveness score of each wetland, while the number in red indicates the level of inflow of the wetland (primary, secondary, etc). Without considering the spatial arrangement of the wetlands, the sum of the overall water quality enhancement effectiveness scores of all of the wetlands in the entire catchment may be taken as the overall water quality enhancement effectiveness of the catchment, which would total -46. However by considering the spatial configuration of the wetlands as described above, the overall water quality enhancement effectiveness of the catchment is calculated to be -59, indicating that the wetlands in this catchment are in fact assimilating fewer pollutants in comparison to if the wetlands had been treated as a single group entity, which, based on the logic above, is likely to be a more accurate representation of the reality of the water quality enhancement effectiveness of the catchment as a whole.
This value was determined in the following way: the overall water quality enhancement effectiveness score of Wetland A is negative, thereby implying that the water leaving this wetland has not been sufficiently enhanced. Wetland E is downstream of Wetland A, so the un-enhanced waters from Wetland A (with an overall water quality enhancement effectiveness score of -29) will be ‘carried’ to Wetland E. To the east of the catchment, the overall water quality enhancement effectiveness score of Wetland D is positive, implying that
the water leaving Wetland D has been totally enhanced. There is therefore no need for the higher ordered wetlands downstream of Wetland D to assimilate any unassimilated hectare equivalents of water quality impairment from the catchment of Wetland D. North of Wetland D is Wetland B, the overall water quality enhancement effectiveness score of which is negative, thereby implying that the water leaving this wetland has also not been sufficiently enhanced. These waters will be intercepted by second ordered Wetland C, which has a positive overall water quality enhancement effectiveness score. Therefore working from upstream to downstream, the overall water quality enhancement effectiveness score of -11 from Wetland B is added to the overall water quality enhancement effectiveness score of Wetland C (+16 + (-11)), which results in a total of +5. The waters leaving Wetland C will be intercepted by Wetland E downstream, but because the overall water quality enhancement effectiveness score from Wetland C is now positive 5, there is no need for Wetland E to assimilate any unassimilated hectare equivalents of water quality impairment. The only negative overall water quality enhancement effectiveness score that will therefore be ‘carried’
down to Wetland E is therefore that of -29 from Wetland A. The overall water quality enhancement effectiveness of the catchment is therefore -30 + (-29), which equals -59.
Figure 16. A hypothetical example of wetlands positioned in a landscape with black numbers in brackets indicating the overall water quality enhancement effectiveness score of each wetland and the number in
red indicating the level of inflow of the wetland
The negativity of the water quality enhancement effectiveness value at the outflow of the catchment is indicative of the fact that the impacts to the catchment’s water quality are greater than the ability of that catchment to totally enhance that water quality, resulting in water quality that is enhanced, but not to the degree that all impairments from surrounding land-cover have been totally assimilated. Should the result be positive, the wetlands of the catchment collectively (as a functional unit), are effectively enhancing the water leaving it, to the degree that all impairments from surrounding land-cover have been totally assimilated. It should be pointed out that a negative overall effectiveness of water quality enhancement score does not mean that the wetlands in the catchment are not enhancing water quality. The wetlands in the catchment may still be making a considerable contribution to water quality enhancement, but are just not filtering waters completely. It may be suggested then, that a less negative overall water quality enhancement effectiveness score indicates higher enhancement effectiveness than an overall water quality enhancement effectiveness score of greater negativity.
The most effective way to incorporate this method of integrating spatial configuration for multiple wetlands in a catchment was to ascertain the main drainage lines in the catchment, and to number them so that each drainage line and the wetlands feeding into that line could be dealt with separately from wetlands along another drainage line. Each drainage line will then have an effectiveness score reflecting the effectiveness of the wetlands along it. In this way, it was easier to determine the effectiveness score that was being carried downstream, and ultimately to the main drainage point of the catchment.
The drainage lines of the quaternary Goukou Catchment were generated using the ArcHydro application, and were then numbered for ease of analysis. The Goukou Catchment had two areas from which drainage occurred- from the east and from the west- and these two areas were subsequently numbered and dealt with separately (Figure 17).
Figure 17. Drainage lines feeding wetlands of the Goukou Catchment
Drainage lines from the west of the catchment were numbered W1 to W15 (indicated in Figure 17 in dark blue), while lines draining from the east of the catchment (indicated in Figure 17 in light blue) were numbers E1 to E7. Only those drainage lines that had wetlands feeding into them were numbered and analysed.
It was then determined which wetlands occurred along which drainage lines, and the order of inflow of those wetlands. The way in which this was conducted may be illustrated by considering just the eastern portion of the catchment featured in Figure 17. A simple diagram, Figure 18, shows how the wetlands of this portion of the catchment are related and can therefore be arranged in order of inflow.
Figure 18. The spatial relationship between the wetlands of the eastern portion of the Goukou Catchment, with drainage lines labelled E1 to E7
The numbered wetlands, with their effectiveness scores and orders of inflow were then arranged in an MS Excel spreadsheet within their drainage line and in ascending order of inflow (Table 20). The effectiveness scores in each column were added together, aside from the positive effectiveness scores of primary wetlands (because the waters from these wetlands had been completely enhanced). This resulted in each drainage line having an effectiveness score, all of which were added together to determine the overall effectiveness of the catchment in enhancing water quality.
Table 20. Example of how calculations were conducted for integrating the spatial configuration of wetlands for a portion of the Goukou Catchment
E1 E2 E3 E4
Wetland FID Effectiveness Order Wetland FID Effectiveness Order Wetland FID Effectiveness Order Wetland FID Effectiveness Order
27 -55.3719 1 39 -38.5172 1 28 -110.985 1 25 -272.27
47 -98.8663 1 42 -182.943 1
40 -113.43 2
TOTAL -267.669 -221.46 -110.985 -272.27
E5 E6 E7
Wetland FID Effectiveness Order Wetland FID Effectiveness Order Wetland FID Effectiveness Order
46(2) -23.3699 1 58 -96.48 1 62 -263.892 1
46(3) -35.8648 1
51 -52.6036 1
46(1) -216.579 2
TOTAL -328.417 -96.48 -263.892 -1561.17