Stable water isotopes suggest sub-canopy water recycling in a northern forested catchment

Stable water isotopes provide a means of tracing many hydrologic processes, including poorly understood dynamics like soil water interactions with the atmosphere. We present a four-year dataset of biweekly water isotope samples from eight fluxes and stores in a headwater catchment at the Hubbard Brook Experimental Forest, New Hampshire, USA. We use Dansgaard's deuterium excess (d) parameter to infer hydrologic processes that cause stable water isotope fractionation. Although we expected to observe a decrease in d from precipitation to soil water because of evaporation, instead we observed an increase, which suggests sub-canopy water vapour recycling (evapotranspiration and then re-condensation). However, the underlying mechanisms and spatial dynamics remain uncertain. The apparent recycling is most evident in the growing season; weak evidence suggests a similar process in the dormant season. Sub-canopy water recycling is a novel hydrologic process that should have implications for micro-meteorology and habitat provided by the forest sub-canopy environment.