Sileshi, Yilma (PhD)G/meskel, Fasil2018-07-312023-11-282018-07-312023-11-282012-03http://etd.aau.edu.et/handle/12345678/10721Sediment transport is a worldwide environmental problem that degrades soil productivity, water quality, causes sedimentation to the reservoirs and increases the probability of floods. Gefersa reservoir, one of the surface water supply sources of Addis Ababa city for the last 70 years, face this problem. The reservoir supplies an average of 30,000m3 of treated water per day to the city. Based on the 1979 and 1998 bathymetric surveys, with the assumption of linear yearly siltation rate there is 22,252 m3/year of sediment inflow to the reservoir. In terms of soil loss from the catchment area, this constitutes a loss of 575 tons/km2/year contributed by the catchment area. In this study, Soil and Water Assessment Tool (SWAT) was used to calibrate and validate a hydrologic component and sediment yield of Gefersa watershed. Back-calculation process was carried out to estimate the natural inflow to the Gefersa reservoir. Sensitivity analysis, model calibration and validation were also performed to assess the model performance. Nine highly sensitive parameters were identified of which curve number (CN2) was the most sensitive one. The coefficient of determination (R 2 ), Nash-Sutcliffe (E NS ) and the percent difference for a quantity (D) was used to evaluate model performance during calibration and validation. Results found were satisfactory and plausible for ungauged station i.e. R 2 = 0.78. E NS = 0.77 and D=-15.3 for calibration and R 2 = 0.72, E NS = 0.70 and D=-20.7 for validation period. Sediment were calibrated and validated on annual basis using D i.e.-1.37 for calibration and -6.9 for validation. Four scenarios are developed to observe the impact of land use changes. Based on this, change of 53% and 16% forest to agriculture resulted in 74.5% and 52.89% increase in sediment load. And change of 35% and 18% rangeland to agriculture land increase 40.47% and 29.51% increase in sediment load .Based on results of modeling, sediment reduction methods in the catchment as well as recovering storage capacity are proposed. Of the available reservoir sediment management approach, watershed management is the best method to reduce the yield of sediment and its entry into the reservoir. Periodic sluicing of sediments through operation of bottom outlet gates would also help to ease the problem. Key words: - Gefersa, Sediment, SWAT Watershed, Bathymetric surveyenGefersa; Sediment; SWAT Watershed; Bathymetric surveyThesis