Effects of Watershed Characteristics on River Flow for The Case Of Ribb and Gummara Catchments

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Addis Ababa University


In this study the effect of watershed characteristics on annual outflow contribution of Ribb Catchment is evaluated. The Gummara catchment, which is adjacent to the Ribb catchment and is equivalent in terms of area but a long year annual flow of more than double of Ribb was taken for comparison in the whole process of evaluation. ILWIS model was used for detail evaluation of the physical watershed characteristics (PCC) for both watersheds. The effects of physical catchment characteristics were analysed and characteristics which have high differences (Rainfall and soil) were identified. Distributed physically based hydrological model known as soil and water assessment tool (SWAT) has been applied for evaluation of PCC with significant differences. The model was calibrated and validated over the gauged upper reaches of catchments of Rib and Gummara. The model was calibrated for the period from 1998-2004 and validated for the period from 2005-2007. The performance of the model was evaluated on the basis of performance rating criteria, coefficient of determination, Nash & Sutcliff efficiency, and volumetric error. The overall performance of the two models gives satisfactory result. The physical catchment characteristics which have high differences (Rainfall and Soil) in the two watersheds were evaluated and tested by creating different scenarios in the calibrated and validated models and the result shows soil has a great effect in lower annual flow of Ribb watershed followed by rainfall.