SEDIMENT INFLOW ESTIMATION AND ITS SPATIAL VARIABILITY AT SUB BASIN SCALE (THE CASE STUDY OF TENDAHO DAM, AFAR, ETHIOPIA)

Abstract

Soil erosion or sedimentation is a major problem of reservoir operation in Ethiopia. Deforestation, overgrazing and poor land management practices are some which accelerate the rate of erosion and the topography of Ethiopia in general or Tendaho dam watershed in particular is full of ups and downs and local farmers commonly cultivate on the hilly sides causing easy topsoil wash away. Hence, this study has tried to determine the sediment yield at Tendaho dam reservoir, identify the high sediment source sub basins and check the applicability of SWAT model on Tendaho dam watershed. To go through these objectives, SWAT model was applied with methodology of collecting hydro meteorological data, sediment data, topographic, land use and soil map data by overlaying mechanism, the model run. Thus, it was successfully calibrated and validated for measured stream flow and sediment yield of Awash River at Wonji, Melka Werer and Tendaho stations. The model was found applicable in this watershed with the performance evaluation statistics (Nash-Sutcliffe model efficiency (ENS), coefficient of determination (R 2 )) and PBIAS in the acceptable range, (R 2 in the range of 0.75 to 0.93, ENS in the range of 0.74 to 0.89 and PBIAS in the range of -10 to -15). From the model simulated output, sub basins 27, 34, 29 and 50 were found the top four severely eroded sub basins with average annual sediment yield of 26.66t/ha, 24.22t/ha, 23.79t/ha and 19.13t/ha respectively. While, sub basins 31, 18 and 21 were found the least sediment source sub basins with annual averaged sediment yield of 0.02t/ha, 0.04t/ha and 0.04t/ha respectively. Generally, the annual averaged sediment inflow into Tendaho reservoir was 5.34t/ha and 59.08 million tons routed from the whole Tendaho dam watershed, in Ethiopia.