Rainfall data provide a prime input in nonpoint source pollution (nps) modeling. The sensitivity of model outputs to variations in the time step selected for rainfall data has been explored for two nps models, a field-scale continuous model, and an event-based watershed-scale model, for the temperate climatic conditions of Southern Ontario, Canada. This study has revealed that model outputs regarding runoff, soil loss and sediment yield, and calibrated parameters representing soil hydraulic properties and erosion characteristics are extremely sensitive to small variations in the rainfall time step. Model users must use caution therefore to take these variations into account during the calibration and application of such models.