ASSESSING THE EFFECT OF SOIL AND WATER CONSERVATION PRACTICES ON RUNOFF AND SEDIMENT YIELD ON HUNDE LAFTO WATERSHED OF UPPER WABI SHEBELLE BASIN

Abstract

Soil erosion is the major cause of land degradation and reservoir sedimentation. The Harerghe highlands of eastern Ethiopia, are among the highly affected areas by land degradation due to erosion. Hence, assessing the effectiveness of different Soil and Water Conservation (SWC) practices at watershed level is imperative. Soil and Water Assessment Tool (SWAT) model was used to simulate conservation practices implemented in the watershed based on different scenarios (base line, stone bunds and crop residue managements scenarios). Sensitivity analysis, model calibration and validation were made at daily time step. Statistical model performance measures coefficient of determination (R2) of 0.96, Nash-Sutcliffe efficiency (NSE) of 0.71 and percent bias (PBIAS) of 25% for calibration and 0.95, 0.76, and 20% respectively for validation. The simulated sediment yield shows that soil loss rate range from 0-76.9 t/ha/yr with annual weighted average rate of 45.4 t/ha/yr. The simulation result under crop residue management and parallel terraces scenarios reduced the sediment yield. Parallel terrace (stone bunds) scenario reduced the total sediment yield from 10,978.7 t/yr to 3,734.26 t/yr relative to base simulation, which is equivalent to 65.9% decrease and reduces the surface runoff by 27% from 410.4 mm to 299.5 mm, increase base flow by 23% and lateral flow by 22.6% at outlet of watershed. While simulation of crop residue management scenario reduced total sediment yield to 4,299.84 t/yr from base simulation, which is equivalent to 60.8% reduction and the surface runoff by 23.5% from 410.4 mm to 313.6 mm, increase base flow by 18.6% and lateral flow by 19.9% at outlet. From this result the benefits of crop residue management practices were more important in the watershed and recommended for farmers. Predictions of runoff and sediment yield support decision makers in developing watershed management plans for better soil and water conservation measures.