The partial duration series method in regional index‐flood modeling

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Abstract
A regional index‐flood method based on the partial duration series model is introduced. The model comprises the assumptions of a Poisson‐distributed number of threshold exceedances and generalized Pareto (GP) distributed peak magnitudes. The regionalT‐year event estimator is based on a regional estimate of the GP shape parameter. Analytical approximations of bias and variance of the regionalT‐year event estimator are deduced. The performance of the regional method is evaluated in the case of regional heterogeneity and intersite dependence. For small to moderate sample sizes, the regional estimator is superior to the at‐site estimator even in extremely heterogenous regions, the performance of the regional estimator being relatively better in regions with a negative shape parameter. When the record length increases, the relative performance of the regional estimator decreases, but it is still preferable to at‐site estimation in moderately heterogeneous and homogeneous regions for large sample sizes. Modest intersite dependence has only a small effect on the performance of the regional index‐flood estimator.