Catenary soil development influencing erosion susceptibility along a hillslope in Uganda

Abstract

Systematic variations of soil properties occur along the hillslope due to differences in water and energy conditions. Such catenary soil variations are often assumed, in turn, to influence erosion processes, but quantitative investigations dealing with the reciprocal relationship between catenary soil development and erosion processes are limited. This study models the influence of catenary soil development on erosion processes on a hillslope in Uganda. The Water Erosion Prediction Project (WEPP)- model was selected to determine the impact of spatial distribution of soil types on hillslope soil loss. A detailed soil survey confirmed a well-developed catenary sequence at the study site. Soils at the summit position had a thick solum due to the stable soil formation on the flat surface, whereas soils at the shoulder position had shallow A-horizons due to active erosion processes. Valley and footslope soils showed hydromorphic features and accumulation of soil material from upslope. The performance of the WEPP- model was evaluated by a sensitivity analysis, which proved that the model was sensitive to vertical changes in soil properties to a depth of 40 cm. High sensitivity to soil texture indicated that the catenary sequence at the study site may have a strong influence on model simulations. When the hillslope was modelled as a uniform soil-landscape unit using each individual soil profile separately, simulated outputs showed high variations with annual soil loss ranging between 2.5 and 9 t/ha. This variation was reduced by including an increasing number of distinct soil-landscape units representative for the individual slope sections. Simulations considering a catenary soil sequence showed a clear spatial demarcation between erosion and sedimentation zones, which was verified by soil investigations. This implies that simulations including a higher number of soil-landscape units generate a more realistic spatial distribution of erosion–sedimentation processes at a hillslope.