Mapping the risk of soil erosion using RUSLE, GIS and remote sensing: A case study of Zou watershed in central Benin

Abstract

Being aware that soil erosion is one of the major environmental problems of the 21st century, this research integrates the Revised Universal Soil Loss Equation (RUSLE) with a Geographic Information System (GIS) to assess erosion risk on Zou watershed in the centre Benin. RUSLE factor values (rain erosivity, soil erodibility, slope length and steepness, cover and support practices) for the study area were determined and spatialized. The rain erosivity (R-factor) was estimated based on the daily rainfall data from five rain gauges installed in the watershed. The slope length and steepness (LS-factor) were extracted from the Digital Elevation Model of the watershed. The soil erodibility (K-factor) calculation was based on the percentage of fine sand, silt, clay and organic matter in the soil as well as the water infiltration rate. 120 representative sampling points were taken into account in the different soil-mapping units of the watershed. The land cover factor (C factor) estimation was based on the Normalized Difference Vegetation Index (NDVI). The NDVI was calculated from a 30 m resolution Landsat-8 image. The adopted support practices on the watershed were investigated. The values of the P factor were attributed to each practice based on literature review. Data sets for the RUSLE factors, along with associated maps showing geographic distributions, were established on ArcGIS. ArcGIS was used to interactively calculate soil loss and map the erosion risk. The mean R-factor value was 915 MJ.mm.ha-1.year-1. The spatial distribution of the C-factor showed that 81.0% of the surface has low protection, 8.6% has medium protection and 11.0% had from good to very good protection against water erosion. The LS-factor value ranged from 0 to 39 in the watershed. The mean soil erodibility is 0.15 t.h.MJ-1.mm-1 over the watershed. The average erosion, for the watershed is 16.2 t.ha-1.year-1 which corresponds to a very high vulnerability level. The watershed area affected by low, moderate, high and very high erosion risk are respectively 3.1%, 22.4%, 22.9% and 51.6%. However, most of the eroded soil is from the area with low vegetation cover, high soil erodibility and with moderate and Gentle undulating. The findings of this study could constitute a baseline for any future development of the watershed.