Variants of regionalization

In this monograph we have concentrated on regional frequency analysis using the index-flood procedure defined in Section 1.3 and the comparison of this method with at-site estimation. Several other regional frequency analysis procedures have been proposed; here we briefly describe them. For simplicity we consider them in the context of estimating a frequency distribution parametrized by its mean, its dispersion divided by its mean (typically L-CV), and one or more shape parameters (typically L-skewness). The estimators that each regional frequency analysis procedure uses for the parameters are summarized in Table 8.1. We are concerned with the question of which data are used in the analysis: at-site, regional, or some combination of the two. We do not consider the question of which statistical methods to apply to the data. We believe that methods based on L-moments are the best currently available; other approaches are reviewed by Cunnane (1988).

At-site estimation

For reference, we note here that at-site estimation involves the use of at-site estimates for all of the parameters of the distribution.

Regional shape estimation

If the mean and dispersion are estimated from at-site statistics, and the shape parameters are estimated by averaging the at-site shape measures for the sites in a region, we call the resulting procedure a “regional shape estimation” procedure. It is intermediate between pure at-site estimation and the index-flood procedure. It is discussed in more detail in Section 8.2.