Groundwater Flow and Exchange Across the Land Surface Explain Carbon Export Patterns in Continuous Permafrost Watersheds

Groundwater flow regimes in the seasonally thawed soils in areas of continuous permafrost are relatively unknown despite their potential role in delivering water, carbon, and nutrients to streams. Using numerical groundwater flow models informed by observations from a headwater catchment in arctic Alaska, United States, we identify several mechanisms that result in substantial surface‐subsurface water exchanges across the land surface during downslope transport and create a primary control on dissolved organic carbon loading to streams and rivers. The models indicate that surface water flowing downslope has a substantial groundwater component due to rapid surface‐subsurface exchanges across a range of hydrologic states, from unsaturated to flooded. Field‐based measurements corroborate the high groundwater contributions, and river dissolved organic carbon concentrations are similar to that of groundwater across large discharge ranges. The persistence of these groundwater contributions in arctic watersheds will influence carbon export to rivers as thaw depth increases in a warmer climate.Plain Language SummaryThis paper shows that groundwater processes have a dominant role in controlling carbon export from the land to streams in permafrost terrain. We use hydrologic models to show that microtopography on the land surface drives the rapid exchange of overland flow with shallow groundwater. In other words, the water (porpoises) from just above to just below the land surface and back again as it moves downslope. Combined with the rapid leaching of organic carbon from soils, these findings provide a mechanistic explanation for two decades of measurements showing high concentrations of carbon in soils and streams during high flow conditions for both spring snowmelt and summer storms. During drier time periods, groundwater contributions from the thin thawed layer make up the flow in streams and keep dissolved organic carbon concentrations high. The persistence of these groundwater contributions in arctic watersheds will influence carbon export to rivers as thaw depth increases in a warmer climate.Key PointsSurface‐subsurface water exchanges across the landscape control solute export from watershedsOverland flow water contains substantial amounts of groundwaterConstant stream DOC over large flow ranges and varied active layer thaw corroborate the role of surface‐subsurface exchanges in DOC exportPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145518/1/grl57780.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145518/2/grl57780_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145518/3/grl57780-sup-0001-Neilson_et_al_GRL_supplement.pd