Classification of drainage basins according to their physical characteristics; an application for flood frequency analysis in Scotland

Abstract

If the standardised floods at all sites within a given region are assumed to be samples from the same distribution, all data may be combined to yield an estimate of a regional dimensionless flood frequency relationship. The identification of these hydrologically homogeneous regions can be achieved on the basis of the physical characteristics of the basin which control the hydrological processes. Classification of the basins independent of the discharge data avoids the problems of regionalisation based on the highly variable, individual site estimates of flood frequency. Classification of the basins can be performed using the NORMIX multivariate clustering algorithm. The homogeneity of the basins can be tested using a likelihood ratio test of whether standardised data for all basins within a region may be adequately represented by a single regional flood frequency relationship. The method was applied to 168 basins in Scotland and five regions were identified, corresponding to physically reasonable basin types and all but one of which yield homogeneous distributions of flood frequency.