DEVELOPING RAINFALL-RUNOFF AND SEDIMENT YIELD RELATIONSHIP: CASE STUDY ON DIDESSA SUB CATCHMENT, ETHIOPIA

Abstract

The main aim of this study was to develop rain fall run off and sediment yield relationship for upper didessa water shed and also asses the spatial distribution of sediment for the catchment by using a physical process based watershed scale model, soil water assessment tool (SWAT). The SWAT model was used large number of input parameters to simulate flow and sediment yield of the watershed. The semi-automated Sequential Uncertainty Fitting (SUFI2) calibration process built in SWAT calibration. SWAT-CUP were used to calibrate the model parameters using time series of flow and sediment data of 1992 to 2003 and validated with the observed data from years 2004 to 2007. 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 (R2) and Nash Sutcliff efficiency (NSE) values for the monthly flow calibration using SUFI2 are 0.90 and 0.88 respectively and it was 0.77 and 0.73 for validation. For monthly sediment yield by using SUFI2 calibration technique the model evaluation coefficients R2 and NSE for calibration was computed as 0.88 and 0.86 respectively, for validation it was 0.78 and 0.74 respectively. The rain fall run off and sediment yield relationship of the catchment has been determined for each sub basin of the watershed. And also the spatial distribution of sediment yield, temporal distribution of rain fall runoff and sediment yields of the catchment was determined and the area which is sensitive soil erosion is identified during the study and the model result indicates that simulated annual average sediment yield for the catchment was 10.5 t/ha/yr.