As the use of hydrological models in water resources planning and management has become more prevalent, an increasing level of detail and precision has been demanded of them. Currently, it is not only indispensable to have reliable models that simulate the hydrological behavior of a basin, but it is also necessary to know the limits of predictability and reliability of the model outputs. The present study evaluates the influence on output uncertainty in a hydrological model produced by uncertainty in the main input variable of the model, rainfall. Using concepts of identifiability and sensitivity, the uncertainty of model structure and parameters was estimated. Then, the output uncertainty caused by uncertainties in i) the rainfall amounts, and ii) the periods of occurrence of these amounts, was determined. The main conclusion is that uncertainty in rainfall estimation during rainy periods produces greater output uncertainty. On the other hand, in non-rainy periods, the output uncertainty bands are not very sensitive to uncertainty in rainfall. Finally, uncertainties in rainfall during the basin filling and emptying periods (Apr. ¿ Jun., and Sep. ¿ Nov., respectively) produce an alteration in uncertainty of subsequent periods. Therefore, uncertainties in these periods could result in limited ranges of model predictability.