A novel procedure for delineation of hydrologically homogeneous regions and the classification of ungauged sites for design flood estimation

Regional flood frequency techniques are widely used to estimate flood quantiles when flow data is unavailable for the basin under study or the record length is insufficient for reliable analyses. Data from nearby gauged sites are pooled to compensate for the lack of at-site data. This requires the delineation of hydrologically homogeneous regions in which the flood regime is sufficiently similar to allow the spatial transfer of information. It is generally accepted that hydrologic similarity results from similarity among basins’ physiographic characteristics, and thus these characteristics can be used to delineate regions and classify ungauged sites. However, as currently practiced, the delineation is highly subjective and dependent on the similarity measures and classification techniques employed. Herein, a novel procedure for region delineation is proposed and evaluated using data for sites across the Southeastern United States. Key components of this procedure are a new statistical metric to identify physically discordant sites and a new methodology to identify the physical attributes that are the most indicative of extreme hydrologic response. The novel approach for region delineation is shown to produce regions which are more homogeneous and more efficient for quantile estimation at ungauged sites than those delineated using alternative physically-based procedures typically employed in practice. In addition, the identified physical attributes can be used to infer the flood regime and estimate quantiles at sites outside the extent of the area used for model development.