Measurements of the isotopic composition of water vapor, δv, as well as measurements of the isotopic composition of evaporation and transpiration provide valuable insights in the hydrological cycle. Here we present measurements of δv in the surface boundary layer (SBL) in combination with eddy covariance (EC) measurements of the isotopic composition of evapotranspiration δET for both δD as well as δ18O over a full growing season above a managed beech forest in central Germany. Based on direct measurements of isoforcing IF and the height h of the planetary boundary layer (PBL), we provide an estimate of isoforcing-related changes in δv, revealing the influence of local evapotranspiration (ET) on δv. At seasonal time scales we find no evidence for a dominant control of δv by local ET. Rayleigh distillation could at most explain 35 % of the observed variability and we did not find indications for the influence of entrainment at seasonal time scales. Instead, we obtain a strong significant correlation (R2 ≈ 0.52; p < 10-35) of δv to temperature. We conclude that the observed seasonal variability of δv is neither dominated by Rayleigh processes, entrainment nor local ET but likely linked to other temperature-related processes such as fractionation during evaporation. At a diurnal time scale we find that even during summer, when transpiration is high and at a height of only 10 m above the canopy, ET is overruled by entrainment effects throughout the day from approximately 10 am to 4 pm. ET only dominates the diurnal cycle of δv in the mornings and evenings. Thus, from diurnal to seasonal time scale, ET does not dominate the measured δv at our field site, even if the measurements were carried out close to the canopy. We further conclude, that accounting for PBL height h is essential to understand drivers of δv. [ABSTRACT FROM AUTHOR]
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