Soil erosion and sediment yield estimation of Sange-wamala watershed

Abstract

Soil erosion and sedimentation are major watershed problems in most developing countries. The removal of top fertile soil and siltation of reservoirs by sediment are some of the problems in Ugandan watersheds. This study aimed at assessing the spatial distribution of sediment yield of the Sanga- Wamala watershed in a GIS and remote sensing environment using the Revised Universal Soil Loss Equation and Sediment Delivery Ratio (SDR) models. RUSLE framed with GIS and Remote sensing technique was employed to assess the amount of soil loss in the watershed for the years 2006, 2011 and 2016 by evaluating factors of soil erosion, that is, rainfall erosivity (R), soil erodibility (K), cover management (C) and topography (LS). The computed mean annual soil losses of the study area were found to be 5296 t ha-1year-1 in 2006, 6691 t ha-1year- 1 in 2011 and 7056 t ha-1year-1 in 2016. LS and K factors were the major soil loss contributors. Main stream channel slope-based sediment delivery ratio analysis was also carried out from an SRTM DEM to estimate the sediment yield. Sediment Delivery Ratio values ranged from 0 to 0.356, implying that the eroded material which passes to the channel system and contributes to sediment yield is up to 35.6%. The estimated sediment yield followed the same pattern as that of the soil lost with a gradual increase from 3.63 in 2006 to 8.08 tha-1year-1 in 2016. The percentage of sediment yield hotspots also continued to increase from 28% in 2006 to 37% in 2016. RUSLE and SDR framed with GIS have a strong capability of estimating soil loss and sediment yield. Results from the analysis show that critical sediment hotspots are situated in the steepest western lower end and the north western regions that are dominated by erosion susceptible soil types and with high steepness of the slope (areas with a high LS factor value). It is recommended that intensive soil conservation practices be emphasized in agricultural zones to ensure that the deposited soil is not carried away to the reservoirs.