The objective of this thesis work is to model stream flow at the outlet of the gauged Gumararnwatershed and analyze the associated uncertainty that can affects the accuracy in estimation ofrnthe stream flow. Having good certainty on simulated flow helps planners and policy makers tornimplement appropriate water resource management as Gumara catchment is getting threatened asrna result of increasing number of farmers using the river for different purpose, especially forrnirrigation. The Soil and Water Assessment Tool (SWAT) model was applied to estimate streamrnflow of the Gumara catchment and associated uncertainty with the simulated outputs. The SWATrnmodel was calibrated for the period of 1995 to 2003 and validated for the period of 2004-2009rnbased on ten parameters identified during sensitivity analysis. The uncertainty analysis was donernby using first SUFI-2 and to check parameter uncertainty using SWAT-CUP, which are bothrnpackages of SWAT CUP, were used to establish the uncertainty bounds of the model. Therncalibration and validation of the model was found satisfactory as performance rating criteriarnvalue of coefficient of correlation (R2) and Nash-Sutcliffe simulation efficiency (ENS) is found tornbe 0.63 and 0.56 for calibration and 0.71and 0.71 for validation ,respectively. In the same orderrnfrom the model uncertainties analysis the percentage of the simulated data within the uncertaintyrnbound is only 31% for calibration and 27% for validation, which shows that there is uncertaintyrnin the process. Then using SWAT CUP parameter uncertainty was tested and found with ENSrnvalue of 0.68 for calibration and 0.82 for validation. And this shows that the overall associatedrnuncertainty come from either conceptual or input or a combination of them but not fromrnparameter identification. Even though the predicted amount of flow of 1317.33MCM is almostrnequivalent to the latest study, the uncertainty might come due to either neglected abstractions orrnpoor quality of input data. Therefore, this simulated amount should not be used for any waterrnresource development works unless the correction of these cause of uncertainties are reduced asrnuncertainty in estimation of simulated flow will lead to wrong water resource managementrndecision.