Understanding Flood Seasonality and Its Temporal Shifts within the Contiguous United States

Abstract

Understanding the causes of flood seasonality is critical for better flood management. This study examines the seasonality of annual maximum floods (AMF) and its changes before and after 1980 at over 250 natural catchments across the contiguous United States. Using circular statistics to define a seasonality index, the analysis focuses on the variability of the flood occurrence date. Generally, catchments with more synchronized seasonal water and energy cycles largely inherit their seasonality of AMF from that of annual maximum rainfall (AMR). In contrast, the seasonality of AMF in catchments with loosely synchronized water and energy cycles are more influenced by high antecedent storage, which is responsible for the amplification of the seasonality of AMF over that of AMR. This understanding then effectively explains a statistically significant shift of flood seasonality detected in some catchments in the recent decades. Catchments where the antecedent soil water storage has increased since 1980 exhibit increasing flood seasonality while catchments that have experienced increases in storm rainfall before the floods have shifted toward floods occurring more variably across the seasons. In the eastern catchments, a concurrent widespread increase in event rainfall magnitude and reduced soil water storage have led to a more variable timing of floods. The findings of the role of antecedent storage and event rainfall on the flood seasonality provide useful insights for understanding future changes in flood seasonality as climate models projected changes in extreme precipitation and aridity over land.

River flood seasonality in the Northeast United States: Characterization and trends

“…Under these antecedent conditions, rainfall does not need to be extreme to produce flooding (and without these conditions, extreme rainfall often does not produce flooding; Ivancic & Shaw, 2015). This is likely true for HUC 02 watersheds too, although frozen ground and snow become even less significant factors further south in the Mid-Atlantic and high antecedent soil moisture associated with rainfall, and seasonally low evapotranspiration, become more important (Ye et al, 2017). Figure 8a shows how long-term median streamflow in southern VA increases steadily from late Oct through mid-late Mar.…”

“…Many aquatic and riparian area organisms are adapted to flood seasonality as are human uses of river channels and flood plains (Arias, Cochrane, Norton, Killeen, & Khon, 2013;Gardiner, 1994;Naesje, Jonssons, & Skurdal, 1995;Robertson, Bacon, & Heagney, 2001). Changes in flood seasonality may indicate changes in flood-generating mechanisms (Cunderlik & Ouarda, 2009;Ye et al, 2017), and their interactions, with implications for habitats, flood plain infrastructure, and human communities.…”

“…Villarini (2016) also investigated temporal trends in flood seasons, but the methods he employed did not assess trend direction. Ye et al (2017) improved the characterization of annual maximum flood seasonality in the United States, and associated interpretations of generating mechanisms, by focusing on a circular statistics measure of annual timing variability and comparing it with the annual timing variability of annual maximum rainfall. While not fully capturing the multimodal distributions of Northeast U.S. floods, their methods yielded valuable insights about the importance of antecedent soil moisture to flood generation in this region.…”

“…While not fully capturing the multimodal distributions of Northeast U.S. floods, their methods yielded valuable insights about the importance of antecedent soil moisture to flood generation in this region. They also analysed temporal trends in annual timing variability, including trend direction (Ye et al, 2017). Frei, Kunkel, and Matonse (2015) investigated the seasonality of trends in Northeast U.S. precipitation and streamflow extremes.…”

Spatial Patterns and Characteristics of Flood Seasonality in Europe

“…Parajka et al (2009) or Jeneiová et al (2016) for both Austria and Slovakia), while the detailed characteristics of monthly flood seasonality distributions has rarely been studied in Europe. However, if unimodal, bimodal or skewed seasonality distributions exist, the 20 mean date of the AMF can be misleading and mask important insights into the flood generating mechanisms (Ye et al, 2017).…”