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 a powerful tracer of hydrological processes, yet their variability in atmospheric water vapor and the processes driving them remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (V) measured at four heights below and above a downy oak forest canopy at the AnaEE platform O₃HP in the French Mediterranean. This vapor dataset is complemented by isotope data from rainfall and groundwater, as well as monthly measurements of stomatal conductance and transpiration. Our results demonstrate that ¹⁷O-excess_V is principally driven by evaporation processes. Seasonal variations in ¹⁷O-excess_V ranging from 33 ± 9 per meg in winter to 25 ± 6 per meg in summer, reflect evaporative conditions in oceanic moisture sources. Diurnal variations, particularly pronounced in summer, with daytime maxima around 33 ± 6 per meg and nighttime minima around 16 ± 7 per meg, are linked to local evapotranspiration and isotope exchange between leaf waters and the atmosphere. On a monthly scale, precipitation is generally close to isotope equilibrium with atmospheric water vapor, except in summer when rain re-evaporation occurs. At event scale, large deviations from isotope equilibrium can occur due to raindrop evaporation and incomplete re-equilibration. Our findings enhance the mechanistic basis for interpreting precipitation isotopes in paleoclimate context, improves the robustness of isotope-based model evaluation, and highlights the potential of ¹⁷O-excess for better understanding of land-atmosphere water exchange across diverse climate and vegetation contexts.