The use of hydrological distributed models to address water resource managementproblems has increased in recent years. Reasonable estimates of predictionuncertainty of hydrologic processes are valuable to water resources and otherrelevant decision making processes. Because of uncertainties associated withinput, model structure, parameter, and output, the model predictions are not acertain value, and should be represented with a confidence range. In this article theconceptual and semi-distributed Soil and Water Assessment Tool (SWAT) wasapplied for a semi-arid Nishabour watershed in Iran. Streamflow simulation isconsidered for 10 years period and Nishabour watershed modeling lead to 22subbasin and 146 Hydrologic Response Unit (HRU). SUfI2 approach was used forcalibration and uncertainty analysis of watershed modeling. Results showed thatcalibration and validation of the Nishabour watershed model are not verysatisfactory due to high conceptual model uncertainties such as dam structures,land subsidence and the complexity in hydrological system for arid regions.Furthermore, the results indicated that it is important to point out the importanceof evaluating the conceptual model uncertainty as well as the parameteruncertainty in hydrological watershed modeling.

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