Author(s): Chuan Liang; Yongming Shen
Keywords: Nonparametric analysis; Hydrologic probabilityplotting position; Extraordinary values
Abstract: To reason the curve-fitting method in hydrologic frequency analysis a counter-traditional view is pointed out in this paper. It shows the basic problem of hydrologic frequency analysis should be understood as to estimate the probabilities of hydrologic events occurred rather than to determine the magnitudes of the events by some artificially regulated values of probabilities, according to the essence of the problem. It is also proved that the Weibull Formula of in plotting positions is not only an unbiased formula in terms of the estimation of excellence probabilities but also a special case of a generalized plotting position formula which is derived non-parametrically and based on order statistics analysis in consideration of an incomplete data sample containing both extraordinarily large and small historic values. A formula of probability's or horizontal curve fitting criterion and a table of percentage in excellence probability for Pearson Type III distribution are given following the view of the change ability of probability estimation.