Quantitative mapping of soil erosion risk using GIS/USLE approach at the kalaya watershed (North Western Morocco) [Cartographie quantitative du risque d'erosion des sols par approche SIG/USLE au niveau du bassin versant kalaya (Maroc Nord Occidental)]

Soil erosion by water has become a relevant issue at global scale. in morocco,particularly in recent decades,soil erosion knows a spectacular extension and causes effects,in- and off-site,increasingly worrying,as a result of the natural conditions and human impact especially in the rif's chain. it is therefore absolutely important and crucial to assess,spatially and quantitatively,the effects of soil erosion in order to face the phenomenon and to propose the best strategies for conservation and land management. this study aims to evaluate the risk of soil erosion by a gis/usle approach in the kalaya watershed (northwestern morocco). geographical information system (gis) techniques have been adopted to treat data obtained at watershed scale,within reasonable spatial resolution,for the application of the empirical model. this latter is a multiplication of five factors namely the rainfall erosivity,soil erodibility,slope steepness and slope length,vegetation cover and erosion control practices. each of these factors has been expressed as a thematic map. the resultant map of soil losses,with an average erosion rate of 34.74 t/ha/year,shows a small erosion rate (< 12 t/ha/year),covering 18.6% of the total area of the basin,and very high erosion (> 120 t/ha/year) that does not exceed 3.5% of the basin area. areas with erosion rate exceeding 30 t/ha/year cover more than 30% of area of the watershed. analysis of the erosion risk map,in comparison with different thematic maps of the various factors of the equation,shows a clear and significant influence of the vegetation cover on the erosion behavior of soils,followed by the topographic factor,particularly the slope steepness.