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 −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.

中文翻译：

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表面边界层中水蒸气同位素组成变异性的驱动因素

摘要。水蒸气同位素组成_δv的测量以及蒸发和蒸腾作用的同位素组成的测量为水文循环提供了宝贵的见识。在这里，我们δ的本测量_v在表面边界层（SBL）与蒸散的同位素组成δ的涡协方差（EC）测量组合_ET两个δD以及δ ¹⁸ ö在全生长季节管理山毛榉以上森林在德国中部。基于isoforcing IF和高度的直接测量ħ行星边界层（PBL），我们提供的在δisoforcing相关的变化作出估计_v，揭示了局部蒸散量（ET）对_δv的影响。在季节性尺度上，我们没有发现当地ET能够控制_δv的证据。瑞利蒸馏法最多可以解释35％的观测到的变异性，我们没有发现在季节时标上夹带的影响的迹象。相反，我们获得强的相关性显著（- [R ²  ≈0.52; p  -35）δ的_v至温度。我们得出结论，观测到的_δv的季节变化既不是由瑞利过程，夹带也不是局部ET所控制，而是可能与其他与温度有关的过程有关，例如蒸发过程中的分馏。在昼夜尺度上，我们发现即使在夏季，当蒸腾作用很高且在冠层上方仅10 m的高度时，从上午10点至下午4点，整天的夹带作用仍会排除ET。ET仅在早晨和晚上控制着_δv的昼夜周期。因此，从昼夜尺度到季节性尺度，即使在接近冠层的地方进行测量，ET也不占实地_δv的主导。我们进一步得出结论，考虑PBL高度h对于理解_δv的驱动力至关重要。