Regionalisation of flood frequencies based on flood type-specific mixture distributions

The regionalisation of flood frequencies is a precondition for the estimation of flood statistics for ungauged basins. It is often based on either the concept of hydrological similarity of catchments or spatial proximity. Similarity is usually defined by comparing catchment attributes or distances. Here, we apply flood types in regionalisation directly to consider the type-specific aspects of similarity. The different flood types are classified according to their meteorological causes and hydrographs. Their probability distributions are modelled by type-specific distribution functions which are combined into one statistical annual mixture model afterwards. For regionalisation, we specified the parameters of each type-specific probability distribution separately with hierarchical clustering and regressions from catchment attributes. By selection of most relevant features, depending on the flood type, the specifics of flood-generating processes and meteorological causes were considered. The results demonstrate how this consideration of deterministic aspects can improve the transferability of distribution parameters to ungauged catchments. The type-specific regionalisation approach offers a higher degree of freedom for regionalisation as it describes the relationships between catchment characteristics, meteorological causes of floods and response of watersheds.