Providing a Physical Basis for Statistical Homogeneity in Regional Frequency Analyses

To estimate the magnitude and frequency of flood flows at ungauged sites, space is traded for time using a regional frequency analysis. This requires the delineation of homogeneous regions in which the flood regime is sufficiently similar to allow the spatial transfer of information to ungauged sites. However, the ability to do so with adequate precision is hindered by our limited knowledge of the physical properties and mechanisms that produce flood flows. In addition, the actual delineation of regions is highly dependent on the similarity measures and classification techniques employed. It is generally accepted that similarity in the flood regime is indicated by similarity in basin characteristics, and thus these basin characteristics can be used to delineate regions and to classify ungauged sites. However, recent evidence suggests this is not the case. The objective of this research is to evaluate whether or not regions which are statistically homogeneous are also physically homogeneous. Preliminary results using data for 480 sites in the Southeastern U.S. are presented in this paper. Multivariate statistical methods were employed to delineate regions based on physical basin characteristics and at-site flood statistics. The delineated regions were then assessed from both a physical and statistical viewpoint.