SEDIMENT YIELD ESTIMATION FROM UPPER AWASH WATERSHED: SEDIMENTATION IN TO KOKA RESERVOIR, ETHIOPIA

Abstract

Poor land use practices and improper management systems have played a significant role in causing high soil erosion rates, sediment transport and loss of agricultural nutrients. SWAT (Soil and Water Assessment Tool), the comprehensive and semi-distributed river basin model that requires a large number of input parameters, was used to simulate the sediment yield from the upper Awash watershed (Area 1156.048 km 2 ), located at central Ethiopia. The purpose of the study was to predict the sediment yield from the watershed and to evaluate the sediment inflow in to Koka reservoir. The simulation was done by dividing the watershed into 38 sub watersheds and by assigning a hydrological response unit based on multiple HRU definition. The manual and automated calibration processes were used to calibrate the model parameters using time series data from 1989 to 2000. Data from 2001 to 2005 were used to validate the model using the input parameter set. Time series plots and the statistical measures of coefficient of determination (R 2 ), Nash-Sutcliffe efficiency (ENS) and PBIAS were used to evaluate the performance of the model. The predicted and observed sediment yields generally matched well. The results of the model calibration and validation showed reliable estimates of monthly sediment yield with R 2 = 0.80, ENS= 0.79 and PBIAS=0.16 during the calibration period and R 2 =0.80, ENS=0.77 and PBIAS=0.19 during the validation period. The model was underestimated the sediment yield by 16% & 19% during calibration and validation period respectively. The result of the simulation was illustrates the potential sediment generating sub basins that needs urgent management or conservation measures.