Time of Concentration and Travel Time in Watersheds

Abstract Time of concentration is usually defined as one of the following categories: The time that it takes for water to travel from the most distant part of a watershed to the outlet. Some literature appropriately emphasizes that the distance has a hydraulic base and not necessarily a geometric base. The wave travel time from the most hydraulically remote point to the outlet of a watershed. The time elapsed for which all parts of a watershed contribute to direct runoff at the outlet. In hydrograph analysis,Tcis defined as the time between the end of excess rainfall and the inflection point on the falling limb of the hydrograph. It is assumed that the inflection point indicates the end of direct runoff. The centroid of the rainfall hyetograph is sometimes defined as the beginning ofTc. Uncertainty in computingTcandTtrepresenting watershed time responses is due mainly to simplifications of complex rainfall–runoff processes. Watershed time response depends strongly on watershed static characteristics, such as shape, area, slope, length, roughness (neglecting long‐term changes), and drainage pattern. On the other hand, time response is a function of nonstationary factors such as spatial and temporal rainfall variations. Therefore, watershed time response is generally event‐dependent and not a stationary character of a watershed.