Regionalization of catchments in Sri Lanka for regional flood frequency analysis

In flood frequency analysis the objective is to estimate flood quantile magnitudes for different return periods at a station or at a number of stations in a river system. An understanding of the hydrologic characteristics of a catchment is essential in order to obtain a reliable estimate of the relationship between extreme flood quantiles and the associated return periods. An approach to catchment regionalization is presented, in which an agglomerative hierarchical clustering algorithm is used to define homogeneous regions that can be used for regional flood frequency analysis. Catchment similarity is expressed using seasonality measures derived from the mean date of occurrence of the annual maximum flood and its associated dispersion. Regions that are largely geographically contiguous are obtained by incorporating gauge latitude and gauge longitude to the clustering algorithm. The initial regions formed using the clustering algorithm are subsequently modified in an attempt to enhance the overall regional homogeneity. The approach is demonstrated through an application to a set of catchments in Sri Lanka