Groundwater shapes North American river floods

Abstract

The importance of soil moisture in triggering river floods is increasingly recognized. However, soil moisture represents only a fraction of the water stored in the unsaturated zone. In contrast, groundwater from the deeper, saturated zone, may contribute a significant proportion of river flow, but its effects on flooding are poorly understood. Here we analyze hydroclimatic records of thousands of North American watersheds spanning 1981-2018 to show that baseflow (i.e. groundwater-sustained river flows) affects the magnitude of annual flooding at time scales from days to decades. Annual floods almost always arise through the co-occurrence of high precipitation (rainfall + snowmelt) and baseflow. Flood magnitudes are often more strongly related to variations in antecedent baseflow than antecedent soil moisture and short-term (⩽3-day) extreme precipitation. In addition, multi-decadal trends in flood magnitude and decadal flood variations tend to better align with groundwater storage and baseflow trends than with changing precipitation extremes and soil moisture. This reveals the importance of groundwater in shaping North American river floods and often decouples the spatial patterns of flood trends from those of shifting precipitation extremes and soil moisture.