APPRAISAL OF REGIONAL AND INDEX FLOOD QUANTILE ESTIMATORS

Our results illustrate the performance of at-site and regional GEV/PWM flood quantile estimators in regions with different coefficients of v ariation, degrees of regional heterogeneity, record lengths, and numbe r of sites. Analytic approximations of bias and variance are employed. For realistic GEV distributions and short records, the index-flood qu antile estimator performs better than a 2-parameter GEV/PWM quantile e stimator with a regional shape parameter, or a 3-parameter at-site GEV /PWM quantile estimator, in both humid and especially in arid regions, as long as the degree of regional heterogeneity is moderate. As regio nal heterogeneity or record lengths increases, a-parameter estimators quickly dominate. Flood frequency models that assign probabilities lar ger than 2% to negative flows are unrealistic; experiments employing s uch distributions provide questionable results. This appraisal general ly demonstrates the value of regionalizing estimators of the shape of a flood distribution, and sometimes the coefficient of variation.