The coefficient of runoff, often referred to as the runoff coefficient and commonly denoted as C, is a dimensionless factor used in stormwater management and hydrologic analysis to represent the fraction of precipitation that becomes direct surface runoff. It expresses the relationship between the total rainfall over a given area and the portion of that rainfall that is not absorbed, infiltrated, evaporated, or otherwise retained, but instead flows over the land surface.
Mathematically, the coefficient of runoff is defined as the ratio of runoff volume to total precipitation volume for a specific area and storm event. A value of 0 indicates that all precipitation is retained or infiltrated with no runoff, while a value of 1 indicates that all precipitation becomes runoff with no losses.
In practice, the runoff coefficient is most commonly used in the Rational Method, a widely applied approach for estimating peak discharge from small drainage areas. Within that method, the coefficient represents the combined effects of surface characteristics on runoff generation, including imperviousness, soil type, land slope, vegetation cover, and land use.
Typical values of the runoff coefficient vary significantly depending on site conditions. Highly impervious surfaces such as rooftops and paved areas have coefficients approaching 0.9 to 1.0, meaning nearly all rainfall becomes runoff. In contrast, pervious areas such as forests or well-maintained grassed surfaces may have coefficients as low as 0.1 to 0.3, reflecting high infiltration and storage capacity.
The coefficient of runoff is influenced by several factors, including rainfall intensity and duration, antecedent moisture conditions, and seasonal variations such as frozen ground or dormant vegetation. Because of this variability, selected values are often based on empirical data, regional guidelines, or standardized tables provided in design manuals.
In stormwater management, the runoff coefficient is a critical parameter for designing drainage systems, sizing culverts and storm sewers, and evaluating the hydrologic impacts of development. It provides a simplified yet practical means of estimating how changes in land use, particularly increases in impervious cover, will affect runoff quantity and peak flow rates.
The coefficient of runoff is a key concept that encapsulates the hydrologic response of a drainage area, allowing engineers and planners to quantify and manage stormwater in both existing and developed conditions.