SWAT BASED RUNOFF - SEDIMENT YIELD MODELING AND DEVELOPMENT OF MANAGEMENT OPTIONS: (A CASE STUDY OF GIBE III WATERSHED, UPPER OMO-GIBE, ETHIOPIA)

Abstract

Surface runoff and sediment loading are immense problems that have threatened water resources development in the Omo river basin. Deposition of sediment in reservoirs reduces the storage capacity of the reservoir and they can cause serious problems concerning the operation and stability of the dam. The objective of the study is to model runoff-sediment yield for upper OmoGibe III Catchment, characterizes the runoff from catchment and associated sediment yield, to evaluate spatial distribution of sediment source areas, and identify hot spot areas, to assess the impact of different catchment management interventions on runoff and sediment yield and finally develop appropriate management options to control soil erosion and sedimentation problems in Omo-Gibe III watershed by using SWAT model. Thus, an insight into surface runoff /sedimentation mechanisms and mitigation methods plays an imperative role for the sustainable water resources development in the region. This paper presents daily flow and sediment yield simulations in the upper Omo-Gibe III under different Best Management Practice (BMP) scenarios. Scenarios applied in this paper are (i)maintaining existing / baseline conditions, (ii) introducing different width of filter strips, (iii) applying stone bunds (parallel terraces) with different slope length, and (iv) reforestation. The Soil and Water Assessment Tool (SWAT) was employed for modeling runoff and sediment yield in Upper Omo-Gibe III watershed and used to identify soil erosion prone areas and assess the impact of BMPs on sediment reduction. The model was calibrated and validated against measured flow and sediment data. Both calibration and validation results showed good match between measured and simulated flow and suspe nded sediment. For the existing condition scenario, the model results showed a satisfactory agreement between daily observed and simulated sediment concentrations as indicated by Nash-Sutcliffe efficiency greater than 0.84. The simulation results showed that applying filter strips, parallel terrace/stone bunds and reforestation scenarios reduced the current sediment yields by 60%, 50% and 73% respectively from the existing condition scenario both at the sub basins and the basin outlets i.e. sediment yield reduced to 7.09 t/ha/yr, 8.79 t/ha/yr, and 4.77 t/ha/yr by using filter strip scenario, conservation structure scenario and reforestation scenario respectively. However, a precise interpretation of the quantitative results may not be appropriate because some physical processes are not well represented in the SWAT model. Key Words: Modeling, SWAT, Stream Flow, Sediment Yield, BMP, Gibe-III Catchment.