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.^[1] 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.

The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA^[1] indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group.

The runoff equation is

$Q = \frac{(P-I_a)^2}{{P-I_a}+S}$

where

Q is runoff ([L]; in)

P is rainfall ([L]; in)

S is the potential maximum soil moisture retention after runoff begins ([L]; in)

I_a is the initial abstraction ([L]; in), or the amount of water before runoff, such as infiltration, or rainfall interception by vegetation; and it is generally assumed that I_a = 0.2S

The runoff curve number, CN, is then related

$S = \frac{1000}{CN} - 10$

CN has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is.

Antecedent moisture condition (AMC) adjustment

Runoff is affected by the soil moisture before a precipitation event, or the antecedent moisture condition (AMC). A curve number, as calculated above, may also be termed AMC II or CN_II, or average soil moisture. The other moisture conditions are dry, AMC I or CN_I, and moist, AMC III or CN_III. The curve number can be adjusted by factors to CN_II, where CN_I factors are less than 1 (reduce CN and potential runoff), while CN_III factor are greater than 1 (increase CN and potential runoff). The AMC factors can be looked up in a reference table.^[1]

References

1. ^ ^{a b c} United States Department of Agriculture (1986). Urban hydrology for small watersheds. Technical Release 55 (TR-55) (Second Edition ed.). Natural Resources Conservation Service, Conservation Engineering Division. ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/tr55.pdf. 

See also

• Hydrological modelling
• Runoff model (reservoir)

External links

• SCS TR-55 Peak Discharge and Runoff Calculator
• Curve Number Calculator Online Free Curver Number Calculator
• Introduction to SCS Runoff Curve Number Method