Application of Remote Sensing and GIS to Study Erosion in Tunisia

摘要

The objective of this study is to produce an erosion risk map and to identify areas, in the Bizerte lagoon catchment (Tunisia), needing urgent intervention in order to preserve agricultural soils. The ENIT prototype SAGATELE (Systeme d'Aide a la Gestion et l'Amenagement du Territoire pour la Lutte contre l'Erosion) was used. The data used included: cartographic data, Digital Elevation Model (DEM), satellite images that were spatially referenced and available in GIS format, a Global Positioning System (GPS) data and aerial photography. To insure the combination of the different thematic maps, we gave empirically weights to each parameter mapped in a layer of the GIS. The attribution of the weights is based on experience and dialogue with the water and soil conservation engineers and technicians. In its first version SAGATELE did not take into account the rain aggressiveness, only its quantity. It also did not take account of superficial geology, soil depth and fertility, the type of runoff (consequent or obsequent). We introduced these factors in the study. The field investigation and use of these factors improved the results. SAGATELE made possible four products: 1. An erosion sensitivity map, which combines rain erosivity, slope angle and erodibility. 2. An erosion risk map, this being the erosion sensitivity map with vegetation density (Vegetation index derived from Thematic Mapper is the indicator for vegetation). 3. A Map of areas in need of urgent intervention, this being the previous map to which has been added the current state of land erosion. 4. The final product is a map of the sub-catchment priorities for management. This is the previous map to which two layers have been added: the current management layer and the sub catchment perimeters.