Runoff coefficient

In wet weather evaporations and partial infiltration of the collecting surfaces reduce the runoff volume. The runoff coefficient is a factor dependent on the collection surface, by which the accumulated rainfall is multiplied to obtain the actual rainfall runoff introduced into the drainage system. Alternatively, the runoff coefficient is the factor by which, depending on the type of surface, the effective impermeable area is calculated. For example, it is 0.9 for metal or tile roofs and up to 0.3 for green roofs.