Upper and lower frequency curves for peak rates of runoff

Abstract

Maximum annual peak rates of runoff plotted on extremal probability paper have many times been found to exhibit trends that may be defined by two straight lines designated as upper and lower frequency curves. Using a sample of 69 gaged watersheds with periods of record ranging from 26 to 48 years, a high degree of correlation is found to exist between the upper and lower frequency curves for 10‐ and 50‐year recurrence intervals. Tests are made to show that these relations may be used in conjunction with periods of record as short as ten years to materially improve the precision of estimates of peaks on the upper frequency curve. Evidence is presented that suggests that the dog leg in frequency curves is the result of sampling from two different populations of peak rates of runoff.