Mixed populations and annual flood frequency estimates in the western United States: The role of atmospheric rivers

More than 30 years have passed since the authors of Bulletin 17B recommended additional studies be performed: (i) to test whether the proper distribution and fitting procedures are different depending on watersheds and hydrometeorological conditions and (ii) to identify mixed distributions. The Bulletin 17B framework assumes that the annual peak flow data included in a flood frequency analysis are from a homogeneous population. However, flood frequency analysis over the western United States is complicated by annual peak flow records that frequently contain annual flows generated from distinctly different flood generating mechanisms. In particular, these flood series contain multiple zero flows and/or potentially influential low floods that substantially deviate from the overall pattern, as well as extreme flood events representing different hydrometeorologic events. Among the different flood generating mechanisms, atmospheric rivers are responsible for large, regional scale floods. This study examines the role played by atmospheric rivers in flood frequency analyses across the western United States. Results are based on U.S. Geological Survey streamflow-gaging stations with a record of at least 30 years, and highlight the areas of the western United States that are most affected by these events.