Some locations within a catchment are hydraulically closer to the discharge point than others. In other words, it may start raining right now, but it could be several minutes (or even hours) before the water that lands on some parts of the catchment arrive at the discharge point. Rational method hydrology is based on contributing flow from the entire catchment area. The time that it takes for water to go from the most hydraulically remote area to reach the discharge point is the governing time to be used in the Rational Method. This is called the time of concentration.