Unlike the urban catchment hydrographs, rural catchment unit hydrographs do not calculate the time to peak TP as a function of the other variables. The TP parameter must therefore be deter-mined by the modeler. It should be noted that most methods of estimate TP, start by calculating the time of concentration, . Time of concentration is the time at which the centroid of the flow reaches the bottom of a catchment. TP is usually a fixed ratio of , depending on the unit hydro-graph chosen.
Over the past 40 years there have been numerous studies in both the United States and Canada in which empirical, semi-empirical, and mathematical relationships for have been derived. Most of the relationships state that is a function of catchment slope, catchment area, and ground cover. While no single method can be used for every situation, we have included the most common methods in this manual so that the modeler can choose what is appropriate for their situation.
Listed below are five methods for calculating TP. We have included both the source of the method as well as the context in which it was derived. This way the modeler should be able to choose a method that was derived for a similar situation as their own.