The ratio of the volume of runoff to the volume of precipitation over a given area and time period. It reflects how much rainfall becomes surface runoff versus being absorbed or evaporated.
The runoff coefficient (C) is a dimensionless ratio representing the fraction of rainfall that becomes surface runoff from a given area. In the rational method, which is widely used for designing stormwater systems for small urban catchments, the runoff coefficient relates peak runoff rate to rainfall intensity and drainage area. Values range from about 0.05-0.10 for flat, sandy, vegetated areas to 0.70-0.95 for impervious surfaces such as asphalt and rooftops. The coefficient integrates the effects of infiltration, depression storage, evaporation, and interception losses. For composite catchments with mixed land uses, a weighted average runoff coefficient is calculated based on the proportion of each surface type. The runoff coefficient is not truly constant but varies with storm intensity, antecedent conditions, and storm duration, making it an approximation that works best for the specific conditions under which it was calibrated. More sophisticated methods such as the SCS curve number approach provide a non-linear relationship between rainfall and runoff that better captures the effects of soil moisture and rainfall depth. Despite its simplicity, the runoff coefficient remains one of the most commonly used parameters in preliminary hydrologic design.