Surface Runoff Variability on two Slope Positions and Land Use in Koupendri Catchment, Benin, West Africa: Observation and Modeling using SCS_CN Approach

Abstract:

Surface runoff contributes significantly to soil erosion by water which is a serious worldwide problem causing decline in water resources quality, agricultural and forest land productivity, alongside environmental and ecological degradations. A field experiment was carried out on two hillslope (2-7 %) positions (downslope (2.3%), upslope (5.8%)) and two land use types; maize-beans intercrop (MB); fallow shrub-grassland (SG) in Koupendri catchment in north-western part of Benin, West Africa using microplots. The objective was to evaluate the influence of slope positions and land use on surface runoff and their variability, and the ability of Soil Conservation Service Curve Number (SCS-CN) approach to simulate the measured runoff on both slope positions and land use types in Koupendri catchment. Six microplots (1m x 1m) were installed at 0.5m spacing, and on the same soil type in each slope positions and land use giving a total of 24 microplots. Surface runoff was measured after each rain event from September – October, 2014 and major runoff producing rain events were selected for analysis. The data obtained was subjected to analysis of variance (ANOVA) in RCBD. The results showed a very low variability (CV= 9%) for measured surface runoff on both slope positions and land use respectively. The surface runoff measured was significantly (p < 0.05) influenced by the land use with a total surface runoff of 340.1 mm in MB as against 250.3 mm in SG. However, the higher surface runoff depth obtained at the downslope (299.9 mm) compared to the upslope (290.5 mm) was due to shallow soil depth and saturation of the soil downslope but not slope position. Thus, land use must be given greater priority in future surface runoff and soil erosion related studies in the catchment or region. A good simulation (R²= 0.92 – 0.97) of the measured runoff depth was obtained using the SCS-CN approach on both slopes and land use types. A better simulation was obtained under the cultivated plot (R²= 0.96 -0.97). From the preliminary results of this study, we suggest a minimum of six erosion plots per location for future studies, and the development and application of SCS-CN approach for runoff simulation in the catchment or region.

Keywords: Surface runoff, ecological degradation, land productivity, water resources, hillslope, SCS-CN.