EVALUATION OF DIFFERENT METHODS FOR DELINEATION OF WETLANDS IN LIMPOPO RIVER BASIN USING LANDSAT ETM+ AND SRTM DATA

摘要

The International Water Management Institute (IWMI) has embarked on Global wetland Mapping project (GWMP) using remote sensing and secondary data. The overarching goal of the GWMP is to map, characterize, and classify the wetlands of the world at various scales or pixel resolutions through wide range of partnerships including the Ramsar Convention. Given the complexity and costs involved in mapping vast areas, the need to develop methods using freely available data was a top priority. In this paper different methods applied for wetland delineation will be discussed. Landsat ETM+ 30m and SRTM 90m data. The Landsat 30m data for nominal year 2000 and the SRTM data, both having global coverage, were selected so the methods developed at one location can be applied elsewhere. The methodology was developed for the Limpopo river basin in Southern Africa which is one of the major trans-boundary basins within the Southern African Region that occurs in four countries: South Africa, Mozambique, Zimbabwe and Botswana. Automated and semi-automated methods were investigated for delineating wetland boundaries. In the automated approach different threshold values of (a) slope derived from SRTM, (b) Tasseled-cap Wetness Index (TCVI), （c） Normalized Difference Water Index (NDWI), (d) multi-band vegetation indices (MBVIs), (e) two band vegetation indices (TBVIs), and (f) Normalized Difference Vegetation Index (NDVI) were used to separate wetlands from non-wetlands. However, the best of these indices provided only an accuracy of less than 30 percent with high levels of errors of omissions and commissions. Semi-automated techniques involved image enhancement, display, and digitizing techniques. Screen digitizing on wetland boundaries was performed on color enhanced images. Accuracy of such delineation, based on ground- truth data, was 86.4 percent; with an additional 7.7 percent of ground-truth points falling within 1 pixel (30m) of wetland area. The total wetland area in the Limpopo river basin was 5.2 million hectares (12.5 percent of total basin area which is 41.5 million hectares).