Evaluation of SWAT performance in modeling nutrients of Awash River basin, Ethiopia

Awash River basin has been recognized as exporting the highest amount of various pollutants in Ethiopia since the basin encompasses the main urban, industrial and agricultural centers of the nation. However, investigation of pollution level of the basin by nutrients is necessary for decision makers to safeguard Awash river and its end users, which has not been addressed yet. This study, therefore, evaluated performance of the Soil and water assessment tool (SWAT) by modeling nitrate and phosphate at the basin scale. First, the model was set up using digital elevation model, climate, soil, and land use data. Thereafter, overall performance of the model was assessed by linking its outputs to the Sequential Uncertainty FItting Version 2 procedure of the SWAT Calibration and Uncertainty Program. The most sensitive parameters for the flow and nutrients were identified using t stat and p values from global sensitivity analysis of the SWAT-CUP. The goodness-of-fit of the monthly calibration measured by coefficient of determination, Nash-Sutcliffe Efficiency, and root mean square error-observations standard deviation ratio were, respectively 0.79, 0.64 and 0.60 for flow; 0.73, 0.71 and 0.54 for nitrate and 0.77, 0.76 and 0.49 for phosphate. During validation, the objective functions were, respectively 0.81, 0.52 and 0.70 for flow; 0.68, 0.63 and 0.61 for nitrate and 0.82, 0.81 and 0.44 for phosphate. The results suggested that the simulated values of the variables fitted well with the observed ones and hence, SWAT is found to be promising to simulate nutrients in the basin.