Rational Method of Calculating Peak Rate of Runoff

The Rational method is one of the oldest methods of calculating the peak rate of runoff. The Rational method is considered simplistic, but it is still useful for small watersheds and using it explores the primary factors that affect runoff. The confidence in the output decreases as the watershed size and variability increases. For small areas that are relatively uniform, the Rational method will produce acceptable results for most needs. The Rational method equation is:

where Q = Peak rate of runoff (ft3/sec); C = Runoff coefficient; I = Rainfall intensity (in/hr); A = Drainage area, watershed (ac).

The accuracy of this equation as a predictor of the peak runoff rate is only as good as the accuracy of the numbers used for each of the variables. The following tables contain generalized values and should be used only as examples. When the calculations are critical, the actual values for a particular watershed should be obtained from the National Resource Conservation Service or the appropriate Web site.

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