Groundwater shapes North American river floods

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.

Automated summary

The importance of soil moisture in triggering river floods is increasingly recognized. However, groundwater from the deeper, saturated zone plays a significant role in river flow and flooding, although its effects are not well understood. Analyzing hydroclimatic records of North American watersheds from 1981 to 2018, it is found that baseflow, which is groundwater-sustained river flow, influences the magnitude of annual flooding at different timescales. Floods are mainly caused by the combination of high precipitation and baseflow, with flood magnitudes more closely related to changes in baseflow than in soil moisture and short-term extreme precipitation. Additionally, long-term trends in flood magnitude and decadal variations align more with groundwater storage and baseflow trends rather than changing precipitation extremes and soil moisture, indicating the importance of groundwater in shaping North American river floods.