Storage, Connectivity, and Stream Drying in Headwater Systems: Modeling Across Hydrogeomorphic Features

Abstract Headwater streams play a critical role in the physical, chemical, and biological functions of downstream waters. However, predicting their specific impact on a landscape scale remains a grand challenge, prompting the development of the river corridor framework to integrate the study of stream channels and hyporheic zones with that of adjacent hillslopes and riparian zones. To operationalize this framework and better explore hydrologic fluxes, the intermediate hydrogeomorphic feature (HGF) scale was recently defined to discretize the river corridor into quantifiable and repeatable compartments. In this study, we used process‐based modeling within and across HGFs to evaluate hydrologic connectivity, storage, streamflow, and drying. We used the Advanced Terrestrial Simulator to model hydrologic fluxes in an experimental watershed in the Coastal Plain of the southeastern United States, where we delineated incised, intact riparian, and wetland HGFs along three streams. Among HGFs, incised channels gained the most water from the subsurface, wetland HGFs maintained the highest water tables, and intact riparian HGFs frequently went dry. Incised and wetland HGFs generated the majority of discharge throughout the year, but dry season discharge was dominated by streamflow gains in incised HGFs. Together, these results suggest that stream incision increases downstream connectivity and export, whereas wetland HGFs increase watershed capacity for storage. Our findings highlight the role of geomorphic structure in driving spatial patterns of drying, storage, and connectivity. Given the pervasiveness of stream incision across anthropogenically impacted systems, our results provide a basis for identifying locations significant to watershed‐scale hydrologic export or retention.