A complete hydraulic routing model for watershed runoff based on flood routing technique of kinematic wave approach for overland flow and channel flow is developed. Physically-based geomorphologic expression for hydrograph characteristics of ungaged watersheds are derived. A comparison limited to a set of contrived watersheds with the existing methods available in the literature has revealed that the new expressions predict the hydrograph characteristics with much improved accuracy than the existing methods. A hydraulic routing approach is also used to develop alternate expressions for hydrograph characteristics. The geomorphologic expressions for hydrologic response are used in a probabilistic approach for developing parameterized flood frequency curves which employs the convolution of joint probability distribution function of rainfall intensity and duration. An existing space-time stochastic rainfall model provided synthetic sequences of rainfall as input to the hydraulic routing model both of which are used for deriving flood frequency curves via simulation.