Climate related changes to flood regimes show an increasing rainfall influence

Climate change can result in changes to floods and flood regimes, but the relationship between floods and climate change is complex and poorly understood. The flood regime influences the nature, severity, and timing of flood events for any watershed. Flood regimes should be viewed as a continuum with completely nival (snowmelt-driven) watersheds at one end of the continuum and purely pluvial (rainfall-driven) watersheds at the other. Flood regimes are important for the operation of flood defenses and to better understand how changes in the flood regime can affect the magnitude of flood events. Data from 46 long term streamflow gauging stations from reference hydrologic networks (RHNs) in Canada (23) and the US (23), are examined to detect changes that have occurred, or are continuing to occur, in flood regimes from watersheds responding only to climate variations and changes. Peaks over Threshold (POT) flood events are used to investigate changes in the timing of flood events. Seasonality measures, using circular statistics, illustrate changes in the nature of the flood regime based on changes in the timing and regularity of flood threshold exceedences. A nival fraction is determined for each watershed and for each year based on a clustering of the flood events into two groups, representing nival and pluvial events. Changes to the flood regime over time, and based on annual temperature, are determined from logistic regression of the number of nival and pluvial events for each year. While strongly nival watersheds show no changes, roughly 15% of the stations exhibit significant changes that represent shifts towards the pluvial end of the flood regime continuum, which has implications for design flood estimation and the operation of flood control infrastructure.