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ONLINE SUPPLEMENTAL FILE
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APPENDIX 1 – PROCEDURE FOR WETLAND LAND COVER ASSESSMENT
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Riparian buffers play an important role in improving the overall water quality of wetlands
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by intercepting and filtering surface run-off containing nutrients and sediments (Skagen
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et al 2008). Two scores are calculated, first a riparian score for a 50 m buffer width,
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starting from the wetland itself and representative of the ability of its 50 m buffer strip to
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filtrate contaminants from the upland zone. This 50 m value is chosen as the The
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Planning and Development Act 2004 says that no construction must be done at 15 m
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buffer zone restriction with a further 35 m control zone. So, it is assumed that there will
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be mostly some form of natural vegetation existing within the 50 m buffer strip. The
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second score is calculated for a buffer width of 950 m, starting from the outer edge of the
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50 m buffer (Note that the distance of each buffer is bounded by the watershed of the
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wetland). This means that if a wetland has a watershed (i.e., an upslope drainage area) of
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only 100 m2 and maximum length 15 m, then the buffer cannot go past this maximum 15
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m length and in area, cannot exceed 100 m2. The watershed, in this case, is the limit to
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the surface water flow to the wetland. Any land cover outside a wetland’s watershed is
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thus assumed to not affect or contribute to surface water flow to the wetland. The
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drainage basins, flow direction and flow accumulations were obtained from Nigel and
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Rughooputh (2010b).
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Assessing land cover types and their threats to wetlands
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A land cover map of Nigel and Rughooputh (2010a) lists 11 land cover types obtained by
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partly digitizing the LU maps and its updated versions, coupled with supervised image
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classification on a SPOT 10 m pan sharpened multi-resolution image of 1994. The land
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cover map is in raster format with 10 m cell size. The 11 land cover (LC) types were
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assigned risk factors, indicative of the potential for a particular LC to either improve or
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impact negatively on the overall condition of a wetland and ranges from 0 (no impact)
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representing the lowest risk, to 5 (very high impact) or equivalent to highest risk. The 11
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land cover types and their associated risk factors for the 50 m buffer width and 950 m
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buffer width are: sugarcane (5 for 50 m, 4 for 950 m); sparse vegetation (2, 2); scrub (3,
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2); barren land (4, 3); food crops (5, 4); tea (4, 4); forest (1, 1); urban area (5,5). The
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remaining land cover types which are wetlands, water bodies and sand are given a risk
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factor of 0 (no impact) as they are assumed to be of no threat to the wetlands themselves.
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Generating Wetland Buffer Zones
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The ArcGIS™ software, ArcInfo™(9.3) was used to create wetland buffer zones using as
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main inputs 1) a GIS data layer containing the 14 wetland polygons and 2) the land cover
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map of Nigel and Rughooputh (2010a) at 10 m ground pixel size. In GIS, buffer widths
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can be automatically generated using the buffer analysis tool. After selecting the wetland
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polygon of interest from the wetland layer map, two buffer widths, one of 50 m, labelled
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riparian buffer zone and a second buffer width of 950 m (starting from the 50 m buffer
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zone) labelled upland buffer zone, were created for each polygon. A total buffer width of
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1 km was thus generated around each wetland. Adjustments to the 950 m buffer zone
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were needed in order to account for watershed boundary and upslope contributing areas
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only. As an example, La Prairie wetland’s buffer zones were trim to keep only upslope
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contributing areas that are inside the wetlands’ watershed boundary (Fig. 2). The river
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basins and watersheds of the island, which give the equivalent watershed perimeters for
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the studied wetlands, were obtained from Nigel and Rughooputh (2010a).
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Fig. 2 Buffer zones for La Prairie wetland
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Analysis of slope
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We also aim to determine the slope gradient of the buffer zones to assess their
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susceptibility to erosion, which affects wetland condition through surface run-off. Nigel
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and Rughooputh (2010a) found that nearly 31% of the Island’s surface area has slopes >
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8% which by soil erosion standards, are areas highly susceptible to erosion. The slope
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map used in this paper for slope analysis is at 25 m cell resolution, was produced in
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ArcGIS™ software from a 25X25 m Digital Elevation Model (DEM). The 25 m DEM in
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turn was produced in ArcGIS™ software with the topogrid tool, which uses the
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ANUDEM programme of Hutchinson (1989). Input elevation data for the DEM creation
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were 10 m interval contours from MHL (2005). The slope layers (in percent) for the 50 m
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and 950 m buffer zones were extracted for each 14 wetland using the ‘Extraction by
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mask’ spatial analyst tool in ArcInfo™(9.3), where in this case, the mask corresponds to
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the buffer zone being used. Mean slope and standard deviation for each slope layer was
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then obtained by enquiring after the ‘slope layer statistics’. Slope statistics are
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automatically calculated by the software and include mean, standard deviation, maximum
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and minimum values of the extracted slope.
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Calculating percentage land cover
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The method used was to first extract the land cover pattern corresponding to a particular
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buffer zone using the ‘Extraction by Mask’ spatial analyst tool in ArcInfo™ 9.3. This
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step generates a land cover layer which was saved as a layer file. The GIS software
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automatically generates an attribute table for each land cover layer created which consists
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of the number of ‘counts’ of cells (pixels) for each land cover feature, where one count is
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equivalent to one pixel size of 10X10 m (or 100 m2). The attribute table was then
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exported to a Microsoft Excel Sheet for calculation of percentage land cover within each
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buffer. To calculate the percentage of a land cover class within a particular buffer zone,
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for example sugarcane, the total number of counts for the buffer zone layer was first
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computed. The number of counts for sugarcane was then divided by the total number of
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counts to obtain percentage sugarcane. This procedure was carried twice for each 14
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wetlands, one for the upland buffer zone and the other for the riparian buffer zone.
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Final computation of Wetland Land Cover Assessment (WLCA)
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After obtaining all these information on land cover types, the following steps are carried
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out to calculate the WLCI:
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1.
Assign risk factors (0–5) to each 11 land cover types
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Calculate the percentage of each 11 land cover types (sugarcane, scrub, forest,
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urban area etc.) in riparian buffer zone for wetland of interest
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3.
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corresponding risk factor. This step is repeated for each 11 land cover types
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score (RIPSCORE)
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5.
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upland buffer score (UPLSCORE)
A value is then obtained by multiplying percentage land cover with the
The 11 values obtained for the wetland are then added to obtain a wetland riparian
Steps 1 through 5 above are repeated for the upland buffer zone to obtain an
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Addition of the UPLSCORE and RIPSCORE then yields a wetland score called
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WETSCORE which is then divided by 10 to obtain an overall Wetland Land Cover Index
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(WLCI) value between 0 and 1 for a particular wetland.
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WLCI 
UPLSCORE  RIPSCORE
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Equation 1
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The LCIs calculated are then grouped into categories to represent the degree of impact.
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These are: 0–0.29 (no impact), 0.30–0.45 (low impact), 0.46–0.61 (moderate impact),
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0.62–0.77 (high impact), and >0.77 (very high impact).
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The WLCA methodology developed in this study however may need some
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adjustment for specific wetland types such as floodplains and mangroves. This is because
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floodplains receive their water supply from catchments linked to the stream network such
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that local land use around the floodplain is relatively unimportant. In contrast, mangroves
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receive their water supply from streams and from the sea. It is thus proposed that in a
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future study, the land-cover assessment methodology is extended to the watershed scale
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for floodplains and not limited to the 1-km buffer. In the case of wetlands situated along
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the coastal plains and which are directly influenced by the sea, such as mangroves,
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further studies including modelling of current flow and contaminant movement in the
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lagoon is suggested to get a clearer picture of the overall impact brought about by land
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and sea pollution.
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