Assessment of Soil Erosion Susceptibility, Spatial Sediment Delivery Processes and Control Strategies at the Nzoia River Catchment in Kenya

Abstract

The Nzoia basin is among the most densely populated regions in Western Kenya, with an average density of up to 450 people per square kilometre. Cereal farming accounts for over half of the land usage in the basin. The massive conversion of land from forest to cropland and grassland in the region has significantly impacted soil erosion and reduced soil fertility. Several studies have been undertaken in the catchment, principally on potential soil loss; however, no extensive research has been conducted on the spatial sediment delivery processes or on establishing the best land management practices for the catchment. This study simulates the basin's susceptibility to erosion using the RUSLE model, examines the spatial sediment delivery process, and undertakes scenario analysis to establish the best erosion control practices for the catchment. The RUSLE factors were combined in ILWIS GIS to create functions for soil erosion and sediment transport capacity, which were applied to compute the potential annual soil loss for each pixel. The formulated sediment routing algorithm directed the sediments toward the river and the lake. The results from the study show that potential annual soil loss varied from 0.00 in the lowlands to 4,577 tonnes/ha/year in the highland areas, with the catchment's total potential soil loss estimated at 8,380,000 tonnes/year. The basin's sediment yield at Lake Victoria was 2,494,575 tons/year, translating to a sediment delivery ratio of about 29%. The cropland regions were the dominant contributors of eroded sediments. The scenario analysis applied practical agronomic and mechanical erosion control methods to croplands, demonstrating that adopting multiple soil erosion control strategies could effectively reduce soil loss across the catchment. Areas with high erosion are primarily found in the sloping regions of the catchment, especially around Mt. Elgon, Chereng’anyi Hills, Kipkaren, and Kapsokwony. The spatial soil erosion and deposition hazard maps generated in this study should be used as practical guides for combating land degradation in the Nzoia River catchment.