Adjustment of logarithmic flood-frequency statistics for gaged California streams to minimize the time sampling error

Methods for adjusting logarithmic flood-frequency statistics for gaged streams to minimize the time sampling error that is inherent in short records are discussed. Statistical procedures for adjusting the mean and standard deviation of a short-term array of annual peak discharges are already well established; two standard sets of equations for utilizing the additional information contained in longer records of peak discharge observed at nearby gaging stations are given. A standard method for adjusting the coefficient of skew does not exist, and consequently a special technique for estimating the long-term value of that statistic was developed for this study. Regional equations were developed that relate the logarithmic skew coefficient to logarithmic transformations of mean annual basinwide precipitation and mean annual peak discharge per square mile. The technique appears to be satisfactory for use in the greater part of California, where over large areas the peak discharge in any year is usually associated with a single widespread general storm or with a series of such storms where snowmelt runoff is involved rather than with localized precipitation events.