Modeling of Stream Flow and Sediment Yield in Upper Awash River Basin by using SWAT Model

Abstract

Soil erosion by surface runoff is the current problem in Ethiopia, particularly in agricultural land. The upper Awash watershed also faces this problem. Large area of the upper awash watershed covered by agricultural land with poor land use management system. This watershed has hydropower reservoir at its downstream and eroded soil from the watershed may have high impact on the reservoirs. So, this study intended to model runoff and sediment yield in the upper awash watershed, Ethiopia by using Soil and Water Assessment Tool (SWAT) model. The data types used were spatial data, meteorological data and hydrological data. Upper Awash watershed was delineated in SWAT model based on Digital Elevation Model(DEM) drainage network. The upper Awash watershed was simulated for 31 years input meteorological data from 1987 to 2017. Model calibration and validation were done by using Soil and Water Assessment Tool- Calibration and Uncertainty Programs (SWAT-CUP) at the Hombole gauging station for both stream flow and sediment yield. The model was calibrated using stream flow data from 1991 to2001 and validated from2002 to 2008. The coefficient of determination (R²) and Nash–Sutcliffe model Efficiency (NSE) values were used to examine model stream flow performance and the R², NSE, Root mean Square Error Standard Deviation Ratio (RSR) and PBIAS of 0.84, 0.74, 0.51 and -13.72 for calibration respectively and R² of 0.789, NSE of 0.68, Root mean Square Error Standard Deviation Ratio (RSR) of 0.56 and PBIAS of -14.04 for validation, which shows very good agreement between monthly observed and simulated stream flow during both calibration and validation. The model was also calibrated using developed sediment data from1999 to2004 and validated from 2005 to 2008 and the values of R², NSE, RSR and PBIAS of 0.71 and 0.65 0.57 and -14.8 for calibration respectively and R² of 0.69, NSE of 0.62 RSR of 0.57and PBIAS of -15.02 for validation. Which was found to be satisfactory agreement between monthly developed and simulated sediment concentrations. In this watershed, 35.08 % of the total watershed have soil erosion hot spot or soil erosion prone area. Most of these soil erosion prone area have also high surface runoff and found at the northern parts of the watershed. Sub basins with high precipitation have high sediment yield, surface runoff and other hydrological parameters in this study area this shows that the main cause for the soil erosion in the study area is excess rainfall. High surface runoff and sediment loss were observed in July and August. The soil erosion prone area in the watershed need care and water resources planner and developer have to care attention in this watershed.