Connecting flood trends to groundwater

Soil moisture is increasingly recognized as shaping fluvial flood trends, but it only represents a fraction of subsurface water storage. In contrast, groundwater in the saturated zone often contributes a significant proportion of river flow, but its effects on large-scale flood trends are poorly understood. We analyzed streamflow and climate records of thousands of catchments 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 elevated baseflow. Consequently, trends and variations of flood magnitudes are often more strongly coupled to antecedent baseflow conditions than antecedent soil moisture and extreme precipitation. This reveals the importance of groundwater in shaping river floods and can decouple flood trends from shifting precipitation extremes and soil moisture.