IDENTIFICATION OF BEST MANAGEMENT PRACTICE OPTION ON SEDIMENT YIELD USING SWAT (CASE STUDY IN GUMARA WATERSHED, ETHIOPIA)

Abstract

The existence of soil erosion in a watershed is an indication of unsustainable land management practices. The major goal of the present study was to identify the critical erosion areas of an agricultural Gumara watershed and recommend the best management practices using a physical process based watershed scale model, soil water assessment tool (SWAT). The semi-automated Sequential Uncertainty Fitting (SUFI2) calibration process built in SWAT calibration and uncertainty program (SWAT-CUP) were used to calibrate the model parameters using time series of flow and sediment load data of 1994 to 2001 and validated with the observed data from years 2002 to 2004. The performance of the model was evaluated using graphical methods to assess the capability of the model in simulating the runoff and sediment yield for the study area. The coefficient of determination (R 2 ) and NSE values for the daily flow calibration using SUFI2 are 0.78 and 0.77 respectively for validation it was 0.73 and 0.72. For monthly sediment yield by using SUFI2 calibration technique the model evaluation coefficients R 2 and NS for calibration was computed as 0.76 and 0.74 respectively, for validation it was 0.71 and 0.70 respectively. The sensitivity analysis runoff and sediment producing parameters was also carried out and discussed. This paper presents sediment yield simulations in the Gumara watershed under different Best Management Practice (BMP) scenarios. Scenarios applied in this paper are (i ) Base scenario (ii) Applying (Terraces) (iii) Contouring (iv) Introducing strip cropping. The scenario results showed that applying terraces, contouring and introducing Strip cropping reduced sediment yields both at the sub watershed and the watershed outlets. Considering the critical sub watershed for the existing condit ions scenario (Base scenario), the model result indicates that simulated annual a verage sediment yield was 31.514 t/ha/yr. Depend on this from Base scenario terraces are saving 29.45 tons/ha/year of soil loss. Contouring was found to reduce soil erosion by 20.54 tons/ha/year. Strip cropping for agricultural fields in the watershed reduces erosion also by 18.75 tons/ha/year.