Drivers of diurnal and seasonal dynamics of triple oxygen isotopes in atmospheric water vapor and precipitation at a Mediterranean forest site

Abstract

Triple oxygen isotopes are powerful tracers of hydrological processes, yet their variability in atmospheric water vapor and the underlying controls remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (δ18OV, d-excess V , 17 O-excess V ) measured below, within and above a downy oak forest canopy at the AnaEE platform O 3 HP in the French Mediterranean. This vapor dataset is complemented by isotopic data of precipitation (δ18OP, d-excess P , 17 O-excess P ) and groundwater, as well as monthly observations of stomatal conductance and transpiration. Seasonal variations in 17 O-excess V and d-excess V likely reflect changing evaporative conditions at oceanic moisture sources. d-excess P and 17 O-excess P show a similar seasonal pattern, enhanced by summer rain re-evaporation. However, no clear isotopic differences were observed in vapor or precipitation derived from different oceanic source regions and weather regimes, likely due to frequent mixing of multiple moisture sources. Diurnal variations in 17 O-excess V and d-excess V reflect a combination of vegetation-related processes, including local evapotranspiration. However, the impact of evapotranspiration was not evident at daily timescales. Although precipitation often deviates from isotopic equilibrium with near-surface atmospheric water vapor at the event scale due to incomplete equilibration and rain re-evaporation, equilibrium water vapor reliably approximates the near-surface isotopic composition of atmospheric water vapor at annual scale. Our results highlight the potential of 17 O-excess for understanding water exchange between the land and the atmosphere, regardless of climatic and vegetation conditions. They enhance the mechanistic interpretation of precipitation isotopes, which is essential for reliable paleoclimate reconstructions.