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.
References, such as from USDA[1] indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group.
The lower the curve number, the more permeable the soil is.
As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met.
It is important to note that the curve number methodology is an event-based calculation, and should not be used for a single annual rainfall value, as this will incorrectly miss the effects of antecedent moisture and the necessity of an initial abstraction threshold.
The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table.
The table below presents curve numbers for antecedent soil moisture condition II (average moisture condition).
To alter the curve number based on moisture condition or other parameters, see Adjustments.
A curve number, as calculated above, may also be termed AMC II or
The AMC factors can be looked up in the reference table below.
Find the CN value for AMC II and multiply it by the adjustment factor based on the actual AMC to determine the adjusted curve number.
Since the history and documentation of this relationship are relatively obscure, more recent analysis used model fitting methods to determine the ratio of
In the model fitting done by Hawkins et al. (2002)[2] found that the ratio of
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: