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.
