SWAT BASED RUN OFF AND SEDIEMNT YIELD MODELLING (A CASE STUDY OF GUMARA WATERSHED IN LAKE TANA SUB BASIN), ETIDOPIA A THESIS SUBMITTED TO THE SCHOOL OF GRADUATE STUDIES,

Abstract

Land degradation is a series threat in the Ethiopian high lands in general and in Gumara watershed in particular reflected in the form of soil erosion and soil fertility decline. Sheet and rill erosion are considerably the most widespread accelerated water erosion and their combined effect significantly affect agricultural production and productivity. The objective of the study is to predict sediment yield and runoff, to identify spatial distribution of sediment source areas and propose high impact management options to control soil erosion and sedimentation problems in Gumara watershed. SWAT (Soil and Water Assessment Tool) was employed for modeling runoff and sediment yield in Gumara watershed .The model was calibrated and validated against measured flow and sediment data. Both calibration and validation i-�;ults showed good ,· match between measured and simulated flow and suspended sediment. The model prediction result indicated that about 72 % of the Gumara watershed is erosion potential area contributing high sediment yield exceeding the tolerance limit (soil formation rate) in the study area. Out of 30 sub watersheds, 18 of the sub watersheds were found to be critical contributing high sediment yield with an average annual sediment load ranging from 11 to 22 ton/ha/yr. The result of the model was verified by detail field observation that sub watersheds with average annual sediment yield exceeding the tolerable limit were identified as high erosion source area which additionally justifies the model's ability to replicate the erosion processes in the watershed. After calibration and validation of SWAT model, the impacts of watershed management interventions were tested to reduce sediment loads from critical sub watersheds. The simulation result indicated that implementing vegetation filter strip can reduce sediment yield by 58 to 74 percen