Approach to confidence interval estimation for curve numbers

The runoff curve number (CN) is a widely used input to hydrologic designs. It is treated as a constant even though it is a random variable. The antecedent moisture condition CN adjustment is the only interval estimate available, but it is of questionable origin and not recommended for use. A general method is provided herein for developing confidence intervals for CNs. The procedure is illustrated for curve numbers to be used for designs reflecting annual maximum design storms. The quantity 100-CN was found to fit the gamma distribution, which was used to develop confidence intervals for CNs from 65 to 95. The confidence intervals can be used to assess the variation in CN. A hypothetical case is used to demonstrate the variation in the design of a detention basin due to variation in table CNs. Confidence intervals are also provided for the retention parameter, S.