In this study the effect of watershed characteristics on annual outflow contribution of RibbrnCatchment is evaluated. The Gummara catchment, which is adjacent to the Ribb catchmentrnand is equivalent in terms of area but a long year annual flow of more than double of Ribbrnwas taken for comparison in the whole process of evaluation. ILWIS model was used forrndetail evaluation of the physical watershed characteristics (PCC) for both watersheds. Therneffects of physical catchment characteristics were analysed and characteristics which havernhigh differences (Rainfall and soil) were identified. Distributed physically based hydrologicalrnmodel known as soil and water assessment tool (SWAT) has been applied for evaluation ofrnPCC with significant differences. The model was calibrated and validated over the gaugedrnupper reaches of catchments of Rib and Gummara. The model was calibrated for the periodrnfrom 1998-2004 and validated for the period from 2005-2007. The performance of the modelrnwas evaluated on the basis of performance rating criteria, coefficient of determination, Nashrn& Sutcliff efficiency, and volumetric error. The overall performance of the two models givesrnsatisfactory result. The physical catchment characteristics which have high differencesrn(Rainfall and Soil) in the two watersheds were evaluated and tested by creating differentrnscenarios in the calibrated and validated models and the result shows soil has a great effect inrnlower annual flow of Ribb watershed followed by rainfall.