IMPACT OF LAND-USE/LAND-COVER CHANGE ON STREAM FLOW AND SEDIMENT YIELDS: A CASE STUDY OF GOJEB WATERSHED, OMO GIBE BASIN, ETHIOPIA

Abstract

Ethiopia’s primary water resources management challenges are due the extreme hydrological variability and seasonality and the international nature of its most significant surface water resources. The main objective of this study was to assess the impact of the Land use land cover changes on the stream flow and sediment yield of the Gojeb watershed, which is located in the Omo Gibe basin. The Soil and Water Assessment Tool (SWAT) is a watershed-based, semi-distributed hydrologic model for simulating hydrological processes at different spatial temporal scales. The land use land cover change analyses were performed using ERDAS Imagine 2014 which was used for further analysis of SWAT. Land use land cover changes for three different years of 1989, 2000 and 2013 land use scenarios with different management practices were used for estimation of stream flow and sediment yield. During the study period most parts of the Forest land were changed to cultivated land. An increase of cultivated land by 14.97% over 30 years period (1985 – 2015) resulted an increase of stream flow and sediment yield by 8.6 m3/s and 41.07t/km2 respectively. The Nash Sutcliff efficiency, coefficient of determination (R2) and PBIAS to the measured deviation of measured data (RSR) were used for evaluating the model performance. The model results has showed a good agreement and correlation with the observed data with NSE > 0.75, R2 > 0.81, and PBIAS < 0.07 values. Spatial variability of sediment done using the validated sediment yield results of 2013 land use showed high potential source areas were found at upstream, high elevation and slope lands. Hence, for the critical sub- watersheds the design and development of best management practices were performed. Three BMPs (best sediment management scenarios) S1 (filter strip), S2 (stone bund) and S3 (reforestation) were considered in this study. The results has showed a decrease of sedimentation by 22.9%, 29.4% and 32.3% sediment yield reductions implementing S1, S2 and S3 respectively. Therefore practicing S3 for Gojeb watershed should be developed and encouraged for efficient sediment reductions