Generalized skew coefficients of annual floods in the United States and their application

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Abstract
Generalized skew coefficients for use in defining flood‐frequency curves that follow log Pearson type 3 distributions are shown by isopleths on a map of conterminous United States. The generalized logarithmic skew coefficients range from 0.6 along the eastern seaboard to −0.5 in Indiana and Illinois. West of the one‐hundredth meridian the coefficients range from −0.3 to 0.2 except for a small area in Nebraska where the generalized skew goes as high as 0.4. The validity of the map values is verified by a split‐sampling procedure. In the west the discharge of 50‐ and 100‐yr peaks computed by using map values of skew is more accurate than that computed by using the observed skew of a sample of 30 annual peaks. East of the Mississippi River the accuracy is even higher and approaches the equivalent of 60 annual peaks along the east coast. An equation gives the adjustment by which a T‐yr peak computed by using map skew would have to be increased to give a discharge that has an average exceedance probability equal to 1/T.

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