In a simple definition, the runoff coefficient is the percentage of the water falling through precipitation that flows over the surface. This coefficient depends in each basin on the level of interception, infiltration and evapotranspiration. Runoff is a result of the difference between the rate of precipitation and infiltration, so that whenever precipitation overtakes infiltration there will be runoff. The detention coefficient refers to that part of precipitation which turns into infiltration, impoundment storage of interception and evapotranspiration. The amounts of runoff and detention are both parts of total precipitation and have contrary trends.
There are several methods for calculating the runoff and detention coefficients which depend on type of soil, land’s terrain and slope, vegetation and intensity of precipitation. Unfortunately, in many parts of Iran such studies are nonexistent or seriously incomplete. In this paper, it has been tried to present a method for calculation of detention and runoff coefficients which does not need such information while still providing acceptable precision. In this method by calculating hydrological reduction values, that part of precipitation which is not turned into runoff is determined; in other words, the amount of water which is initially kept and later disappears through evaporation or infiltration is calculated. In this way, the detention and runoff coefficients are obtained.