THREE-DIMENSIONAL SURFACE WATER-GROUNDWATER CONNECTIVITY DRIVES EVENT AND SEASONAL RUNOFF AND CARBON EXPORT ACROSS WATERSHED SCALES (Invited Presentation)

At the aquatic-terrestrial interface in stream channels, surface water and groundwater connectivity varies along three spatial dimensions as well as through time. However, how lateral, longitudinal, and vertical surface water source areas integrate to control physical and biogeochemical fluxes at watershed outlets is not well understood. To address this, we collected chemical and hydrometric data from zero through second order catchments in the Piedmont region of North Carolina, USA to characterize spatiotemporal runoff and overland, shallow soil, and deep subsurface flow across characteristic landscape elements. We conducted 77 mapping campaigns of flow presence across the ephemeral-to-perennial stream network across a broad range of watershed moisture states. We determined that two superimposed runoff generation regimes, event-driven and baseflow-associated, produced distinct hydro-biogeochemical signals at the catchment outlets at two spatial scales. The variability and integration of shallow and deep groundwater contributions drove the partitioning of these runoff regimes, which controlled longitudinal expansion and contraction of streamflow as well as carbon export dynamics. The presence of shallow and deep groundwater flowpaths were mediated by both hydroclimatology as well as the structure and stratigraphy of the subsurface critical zone, specifically the presence of shallow clay-rich impeding soil layers that influenced the redistribution of precipitation across the watershed. Together, our results suggest that the balance of longitudinal, lateral, and vertical source area contributions, as dictated by both hydroclimatology and critical zone characteristics, can drive hydro-biogeochemical signals at larger watershed outlets.