Modelling the impact of trenches on soil erosion control using OpenLISEM on Mount Elgon, Uganda

Abstract

Soil erosion, accelerated by anthropogenic activities, is a major driver of land degradation, resulting in the removal of fertile soil, humus and organic matter. On the steep slopes of the densely populated Mount Elgon region (Uganda), several soil and water conservation (SWC) measures, such as trenches (fanya juu/fanya chini), are adopted to counteract this challenge. Despite the adoption of SWC measures, soil erosion persists and the effectiveness of these interventions remains unassessed. This study characterises trenches in the upper Manafwa catchment, simulates their impacts on erosion and introduces a novel approach to integrating sub-grid structures through enhanced micro-depression storage. Trench dimensions were established through field surveys, and their impact on erosion at a sub-catchment level was evaluated using the hydrological and soil erosion model OpenLISEM. Because the actual trenches are (much) smaller than the DEM resolution used, trenches were parameterized by readjusting the random roughness of agricultural areas as a proxy to include micro-depression storage as provided by trenches in the field. Two likely future scenarios were considered: (1) no trenches (or abandonment of trenches) in any of the agricultural fields, and (2) implementation of trenches in all agricultural fields. This was compared with the current situation of the Ukha sub-catchment, where about 62% of the agricultural area has trenches. The field surveys revealed that trenches were mostly established on contour lines and the dimensions varied substantially, averaging 3 m in length, 0.7 m in width and 0.5 m in depth. Model simulations, calibrated against measured hydrograph data, estimated that non-adoption or abandonment of trenches (scenario 1) led to a 75.97% increase in runoff and a 35.12% increase in soil loss for the simulated rainfall event. In contrast, adopting trenches in all agricultural fields (scenario 2) reduced runoff by 40.45% and soil loss by 33.81% as compared to the current situation. Fostering the widespread adoption of trenches is therefore recommended for sustainable land management, reducing erosion and for ecosystem conservation in the Mount Elgon region.