Factors Affecting Watershed Runoff Coefficients
The major factors affecting the rational method runoff coefficient value for a watershed are the land use, the soil type and the slope of the watershed. The physical interpretation of the runoff coefficient for a watershed is the fraction of rainfall on that watershed that becomes storm water runoff. Thus the runoff coefficient must have a value between zero and one.
Land Use: Surfaces that are relatively impervious like streets and parking lots have runoff coefficients approaching one. Surfaces with vegetation to intercept surface runoff and those that allow infiltration of rainfall have lower runoff coefficients.
Slope: All other things being equal, a watershed with a greater slope will have more storm water runoff and thus a higher runoff coefficient than a watershed with a lower slope.
Soil Type: Soils that have a high clay content don't allow very much infiltration and thus have relatively high runoff coefficients, while soils with high sand content have higher infiltration rates and low runoff coefficients. The U.S. Soil Conservation Service (SCS) has four soil group identifications that provide information helpful in determining watershed runoff coefficients. The four soil groups are identified as A, B, C, and D. Classification of a given soil into one of these SCS groups can be on the basis of a description of the soil characteristics or on the basis of a measured minimum infiltration rate for the soil.
The descriptive characteristics of the four SCS soil groups are summarized in the following list:
- Group A: Deep sand; deep loess; aggregated soils
- Group B: Shallow loess; sandy loam
- Group C: Clay loams; shallow sandy loam; soils low in organic content; soils usually high in clay
- Group D: Soils that swell significantly when wet; heavy plastic clays; certain saline soils
Following are the minimum infiltration rates in inches/hr for each of the SCS soil groups:
- Group A..............0.30 - 0.45 in/hr
- Group B..............0.15 - 0.30 in/hr
- Group C..............0.05 - 0.15 in/hr
- Group D.................0 - 0.05 in/hr