Predicting Sediment Yield of Didessa River Basin and Sedimentation of Arjo Didessa Reservoi

Abstract

Predicting sediment yield is essential for design of dams, pollutant control and development of integrated watershed management practices. Poor land use practices and improper management systems have played a significant role in causing high soil erosion rates, sediment transport, reservoir sedimentation and loss of agricultural nutrients. Didessa river basin which is located in the southernmost part of the Blue Nile Basin, contributing roughly a quarter of the total flow of the Blue Nile as measured at Sudan border. It is the largest tributary of the Blue Nile in terms of its volume of water and the drainage area is nearly 27711.9km2 . The basin is geographically located between 360 02’ and 360 46’ longitude, and 70 43’ and 80 13 latitude. The Didessa river basin is characterized by high rainfall occurring during summer and moderate land use change from forest to agricultural land due to rapid population growth. The lower part of the catchment is characterized by Poor land cover coupled with hilly topography led to quick concentration of runoff and sediment into outlet. The general objective of this study was to predict sediment yield of Didessa river basin and sedimentation of Arjo Didessa Reservoir using physically based SWAT models. The SWAT model utilize GIS and DEM to delineate watershed and extract the stream networks. This study applied SWAT model to assess sediment yield at Arjo Didessa reservoir and Didessa river basin outlet. The simulated stream flow was calibrated for six years (1995 to 2001) and validated for five years (2001 to 2006) at Toba gauging station using SWAT-CUP to evaluate the performance of the model. In addition, Suspended sediment concentration was generated by rating curve from observed sediment sample and compared with simulated value to check the capability of SWAT model to simulate sediment yield and appreciable agreement was obtained. The trap efficiency and sedimentation rate of Arjo Didessa reservoir were also determined. The model was successfully calibrated and validated. The model performance for calibration and validation periods have been evaluated by using statistical parameters, Coefficient of determinant (R2 ) and Nash-Sutcliffe (ENS). For calibration R2 and ENSwere found to be 0.79 and 0.76 respectively, and 0.66 and 0.65 during validation. The sediment yield at Arjo Didessa reservoir and sedimentation rate found to be 0.99Mton/year and 0.86Mm 3 /year respectively. The reservoir trap efficiency found to be 98%. Sediment yield of the basin as measured at the outlet of the whole basin was found 8.29Mton/year. The erosion prone area which needs immediate watershed management in Didessa river basin were also identified. The middle part of the basin around Jimma Arjo, Limmu kosa, and downstream part of the area around Limmu, Gida and part of west Wollega have been identified as soil erosion prone area.