Runoff curve number
The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. The curve number method was developed by the USDA Natural Resources Conservation Service, which was formerly called the "Soil Conservation Service" or "SCS" — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an empirical analysis of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used and is an efficient method  for determining the approximate amount of direct runoff from a rainfall event in a particular area. Definition. The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group. The runoff equation is: In this equation, note that the values of formula_2 are not the same as the one used in estimating direct runoff with an formula_3 ratio of 0.20, because 5 percent of the storage is assumed to be the initial abstraction , not 20 percent. The relationship between formula_2 and formula_5 was obtained from model fitting results, giving the relationship: The user, then, must do the following to use the adjusted 0.05 initial abstraction ratio: