Assessment of Hydrological and Physical Similarity of Southeastern Watersheds

Regional flood frequency analysis is widely used to overcome shortcomings of at-site analyses resulting from insufficient record lengths and to estimate flood quantiles at ungauged sites. This requires the delineation of homogeneous regions in which the flood regime is sufficiently similar to allow the spatial transfer of information. It is generally accepted that this hydrologic similarity is due to similarity in basins' physiological characteristics; however, because of nature's complexity and variability in flood producing mechanisms, this is often not the case. In addition, as currently practiced, the delineation of homogeneous regions is highly subjective, and delineated regions are highly dependent on the similarity measures and classification techniques employed. Moreover, current methods can only be applied with confidence to gauged basins as they allow subsequent testing for homogeneity. The goal of this research is to improve the delineation of homogeneous regions for regional flood frequency analyses by identifying the physical controls of hydrologic homogeneity. The study presented herein employs 480 USGS flow gauges located in the southeastern region of the United States. Multivariate statistical methods are used to identify key basin characteristics, and to quantify their individual or combined impacts on region formation. Preliminary results suggest mean basin elevation, soil moisture, and basin slope determine to a large degree the hydrological behavior of a watershed and therefore should be given considerable attention when forming hydrologically homogeneous regions in these states.