On the seasonality of flooding across the continental United States

This study examines the seasonality of flooding across the continental United States using circular statistics. Analyses are based on 7506 USGS stream gage stations with a record of least 30 years of annual maximum instantaneous peak discharge. Overall, there is a very strong seasonality in flooding across the United States, reflecting differences in flood generating mechanisms. Most of the flood events along the western and eastern United States tend to occur during the October–March period and are associated with extratropical cyclones. The average seasonality of flood events shifts to April–May in regions where snowmelt is the dominant flood agent, and later in the spring–summer across the central United States. The strength of the seasonal cycle also varies considerably, with the weakest seasonality in the Appalachian Mountains and the strongest in the northern Great Plains. The seasonal distribution of flooding is described in terms of circular uniform, reflective symmetric and asymmetric distributions. There are marked differences in the shape of the distribution across the continental United States, with the majority of the stations exhibiting a reflective symmetric distribution. Finally, nonstationarities in the seasonality of flooding are examined. Analyses are performed to detect changes over time, and to examine changes that are due to urbanization and regulation. Overall, there is not a strong signal of temporal changes. The strongest impact of urbanization and regulation is on the strength of the seasonal cycle, with indications that the signal weakens (i.e., the seasonal distribution becomes wider) under the effects of regulation.