Regional flood frequency analysis II: Multivariate classification of drainage basins in Britain

ABSTRACT The homogeneity statistic developed in the previous paper (Wiltshire, 1986) is applied here to geographical regions and to clusters of basins formed in a flow-statistic dataspace. Clusters are seen to offer several advantages over geographical regions and they are interpreted in terms of basin characteristics through the use of a multivariate linear discriminant analysis. The discriminant scores of each basin based on basin characteristics can be used to assess the performance of the original independent classification based on flow statistics. An ungauged basin for which a design flood is required can have a fractional membership of more than one cluster, and the desired quantile can then be estimated as a weighted average of quantile estimates from each cluster. Experiments with hypothetical ungauged basins show that the allocation of basins to clusters is in accord with physical reasoning.