Drivers of the variability of the isotopic composition of water vapor in the surface boundary layer

Abstract. Measurements of the isotopic composition of water vapor, δ_v, as well as measurements of the isotopic composition of evaporation and transpiration provide valuable insights in the hydrological cycle. Here we present measurements of δ_v in the surface boundary layer (SBL) in combination with eddy covariance (EC) measurements of the isotopic composition of evapotranspiration δ_ET 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 δ_v, revealing the influence of local evapotranspiration (ET) on δ_v. At seasonal time scales we find no evidence for a dominant control of δ_v 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 δ_v to temperature. We conclude that the observed seasonal variability of δ_v 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 δ_v in the mornings and evenings. Thus, from diurnal to seasonal time scale, ET does not dominate the measured δ_v 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 δ_v.