Runoff and Sediment Yield Modeling and the Evaluation of Best Management practices Using Arc SWAT. A Case of Meki River Watershed, Rift Valley Basin, Ethiopia.

Abstract

Runoff and sediment yield modeling was important in the Meki river watershed. Runoff and sediment yield modeling and the evaluation of best management practices is important to alleviate soil erosion and reservoir sedimentation. The general objective of this study was to model the runoff and sediment yield and to assess best management options to control soil erosion and sedimentation problems of the Meki River Watershed. The main materials and data used for input data preparation, analysis were, Arc GIS, Arc SWAT, SWAT CUP, Rainbow, excel stat and hydrological, meteorological, spatial data. The methodology of this work were the following: data collection, data preparation and quality assurance, data analysis and model setup, running model, model performance evaluation and model result interpretation. The total drainage area of the watershed was 2060.792 km 2 . Simulation of the streamflow of Meki River watershed situated in central rift valley basin for the period from 1993 – 2010 and sediment yield was computed by developing sediment rating curve for the comparison of sediment yield produced from rating curve and sediment produced by SWAT simulation. The model was calibrated by adjusting sensitive parameters for observed stream flow data from 1993 – 2003 and validation was done using observed data from 2004 – 2010. The highest flow sensitive parameter was the curve number (CN2), Available water capacity of the soil layer (SOL_AWC) and Manning's "n" value for overland flow (OV_N). The model performance was checked by statistical model performance evaluation such as the coefficient of determination (R 2 ), Nash – Sutcliffe model efficiency (NSE) and percent bias. From the results of stream flow calibration and validation R² = 0.81, NSE = 0.76, Pbias =18 and R² = 0.81, NSE = 0.74 Pbias =17.1 respectively. The total annual runoff and sediment yield produced by SWAT simulation was 367.95mm and 75.896 ton/ha/yr. respectively. The simulation results showed that applying scenario USLE_P, conservation structure (contour ploughing and terracing with contour) reduced the current sediment yield by 25.84% and 13.97% respectively and using scenario filter strips reduced the current sediment yield by 68.47%. More research was necessary to forecast sediment yield and runoff of each sub-basin for daily and monthly time step under different land use/land cover scenario to improve decision making of the stake holder.