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.
