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The Soil and Water Assessment Tool (SWAT) model was developed to be used in ungaged watersheds where no measured watershed response data are available for model calibration. The uncertainty associated with model parameters often limits the usability of SWAT in ungaged watersheds. This uncertainty was studied using SWAT model to simulate runoff from a small watershed in Northwest Arkansas (Moores Creek, Washington County). The SWAT model was validated using a stochastic procedure which transformed parameter uncertainty into model output uncertainty using probability density functions. A sensitivity analysis was performed to identify the most sensitive model parameters for flow in an agricultural watershed. A Monte-Carlo Simulation was performed using curve number (CN) as the most sensitive model parameter to quantify uncertainty in model output. The results indicated that model output uncertainty did indeed depend upon input parameter uncertainty. A decision regarding model acceptability can be made by placing confidence intervals on model output and using measured watershed response data and predetermined performance criteria on the model.