Unraveling groundwater-stream connections at the continental scale

Abstract Groundwater is a critical component of the terrestrial water cycle, yet the extent and depth of its connections with other hydrologic components, such as streamflow, remain unquantified. Here, we conducted a backward particle tracking simulation based on high-resolution integrated hydrologic modeling across the contiguous United States (CONUS). We quantified the lateral length and vertical depth of groundwater flow discharged to streams as baseflow. Our simulation results suggest that water may travel underground almost 240km to emerge in a stream and that lateral groundwater flow lengths longer than 50 km are common across North America. Confined groundwater, aquifers typically 10–100m below the ground surface, significantly contributes to baseflow. Boundaries of surface water and groundwater watersheds rarely match and only align at continental divides. Our results suggest that about one-quarter of the baseflow over CONUS is inter-basin groundwater flow and that water-balance approaches underestimate these quantities due to concurrent groundwater exportation and importation of a watershed. This nested nature of groundwater flow is an important characteristic that determines how far and how deep water flows to supply streamflow. Our findings have fundamental significance for understanding the movement of groundwater and its interactions with other hydrologic components in the terrestrial water cycle.