The peak discharge rate of a runoff event is estimated from the runoff event volume, the duration of the causal rainfall event, and the catchment time of concentration. The runoff event volume is determined from the rainfall-runoff relationship developed by Guo and Adams . The average catchment time of concentration is used for all runoff events and is treated as a physical characteristic of the urban catchment, independent of input rainfall characteristics. Incorporating the exponential probability density functions of rainfall event characteristics, closed-form analytical expressions are then derived for the return period of peak discharge rate or flood frequency distribution of urban catchments. The derived flood frequency distributions compared favorably with sample distributions constructed from continuous simulation results for a range of urban catchments. Thus the derived analytical expressions or probabilistic models for peak discharge rates are proposed as an alternative to simulation modeling or regional analyses for the determination of flood frequencies for urban catchments. ¿ 1998 American Geophysical Union