05 September 2017

Seasonal Timing of Floods in the Eastern U.S. Has Become More Variable Since 1980

Scientists discovered processes influencing the timing of floods that could explain the recent decadal shift in flood seasonality.


Flood seasonality is an important signature that can help reveal the underlying mechanisms of flood generation. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory contributed to a study of the variability of flood occurrence, using data from natural catchments across the United States. Researchers found that, in catchments with closely synchronized seasonal energy and water cycles, seasonal precipitation strongly influences flood occurrence, but in catchments with loosely synchronized energy and water cycles, antecedent soil moisture has a more dominant influence over flood seasonality than rainfall.


Floods are the most common natural disaster and the leading cause of fatalities due to natural hazards. Understanding the dominant mechanisms behind flooding is important for improving flood prediction. Observations revealed a statistically significant shift of flood seasonality in catchments in the Eastern U.S. since 1980. Researchers attributed the more variable timing of floods in those catchments to a concurrent widespread increase in event rainfall amount and reduced soil water storage. These findings provide useful insights for understanding future changes in flood seasonality as Earth system models project changes in extreme precipitation and aridity over land.


Understanding the causes of flood seasonality is critical for better flood management. This study examined changes related to the seasonality of annual maximum floods (AMF) before and after 1980 at 259 natural catchments in the lower 48 United States. Researchers used circular statistics to define a seasonality index and analyzed the variability of flood occurrence date in the catchments. 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 is more influenced by high antecedent water stored in the soil, which is responsible for the amplification of the seasonality of AMF over that of AMR. This effectively explains a statistically significant shift of flood seasonality detected in some catchments since 1980. Catchments where the antecedent soil water storage increased since 1980 exhibited increasing flood seasonality, while catchments that experienced storm rainfall increases before the floods shifted toward more variable timing of flooding across the seasons.

L. Ruby Leung
Pacific Northwest National Laboratory
Ye, S., Li, H., Leung, L., et al. "Understanding Flood Seasonality and its Temporal Shifts within the Contiguous United States." American Meteorological Society 18, 1997-2009 (2017). [10.1175/JHM-D-16-0207.1].