Run-off coefficient refers to the quotient of the total precipitation over that part of the precipitation that arrives at a discharge point. The data used have been collected from rain events that have been observed in the past. The run-off coefficient depends on the size and surface characteristics of the catchment area. The duration and intensity of the stormwater run-off also play a role. It should be remembered that the surface characteristics may change, depending on the duration and intensity of the rain event.