CHOWDHURY AND STEDINGERS APPROXIMATE CONFIDENCE-INTERVALS FOR DESIGN FLOODS FOR A SINGLE-SITE

A basic problem in hydrology is computing confidence levels for the va lue of the T-year flood when it is obtained from a Log Pearson III dis tribution in terms of estimated mean, estimated standard deviation, an d estimated skew. In an important paper Chowdhury and Stedinger [1991] suggest a possible formula for approximate confidence levels, involvi ng two functions previously used by Stedinger [1983] and a third funct ion, lambda, for which asymptotic estimates are given. This formula is tested [Chowdhury and Stedinger, 1991] by means of simulations, but t hese simulations assume a distribution for the sample skew which is no t, for a single site, the distribution which the sample skew is forced to have by the basic hypothesis which underlies all of the analysis, namely that the maximum discharges have a Log Pearson III distribution . Here we test these approximate formulas for the case of data from a single site by means of simulations in which the sample skew has the d istribution which arises when sampling from a Log Pearson III distribu tion. The formulas are found to be accurate for zero skew but increasi ngly inaccurate for larger common values of skew. Work in progress ind icates that a better choice of lambda can improve the accuracy of the formula.