Estimating soil loss potential using rusle in GIS environment in Secoko sub-catchment, Rwanda

Abstract

Soil erosion is one of the major environmental degradation, which is caused by natural process like rainfall, high wind forces and also anthropogenic activities with improper utilization of lands with respect to agricultural activities, developmental activities and leading to severe erosion. The present study was conducted to quantify the soil erosion by water force. The Revised Universal Soil Loss Equation (RUSLE) into Geographical Information System (GIS) environment and Sediment delivery ratio by measuring total suspended sediment at Secoko outlet was adopted. The main objective of this study was to quantify the soil loss and the sediment delivered at Secoko outlet located at the mouth of reservoir of Nyabarongo Hydropower. The land use land cover cover (LULC) map of Secoko sub-catchment was extracted from the global LULC of 2020, published by ESRI in June 2021. The used Revised Universal Soil Loss Equation (RUSLE) model is composed of four factors namely: rainfall-runoff erosivity factor (R), soil Soil Erodibility Factor (K), Slope length and steepness (LS), and vegetation cover factor (C) and supporting conservation practice factor (P). The field works consisted of the measurement of flow and collection of water samples for analyzing the total suspended sediment (TSS) concentration at the outlet of the catchment and the sediment rating curve was developed. The results of RUSLE modeling show that the mean annual soil erosion was 25 t/ha /year with the minimum of 11 tons /ha/year and the maximum of 35 tons/ha/year which is equivalent to the detachment of 104,247 to 331,695 tons of soil from the whole sub-catchment of 9477ha. The field data collected for total suspended sediment concentration for the period of one year (June, 2020 to May, 2021) have shown that 66,178.14 tons of sediment were delivered at the outlet with a monthly average sediment delivery of 5,514.85 tons /months with a minimum of 2,743.42 tons in January and the maximum of 10,665.14 tons in April, 2021. The sediment rating curve developed resulted to R2 equivalent to 0.8634.The results of this study call for the decision makers and operators of Nyabarongo I hydropower to take appropriate measures to reduce the sediment delivered to the reservoir through proper catchment restoration measures and the special investment for the construction of check dam to capture the sediment generated from Secoko sub-catchment due to different causes including Mining activities and agriculture activities.