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.
