Transpiration (T) is perhaps the largest fluxes of water from the land surface to the atmosphere and is susceptible to changes in climate, land use and vegetation structure. However, predictions of future transpiration fluxes vary widely and are poorly constrained. Stable water isotopes can help expand our understanding of land–atmosphere water fluxes but are limited by a lack of observations and a poor understanding of how the isotopic composition of transpired vapour (δ_T) varies. Here, we present isotopic data of water vapour, terrestrial water and plant water from a deciduous forest to understand how vegetation affects water budgets and land–atmosphere water fluxes. We measured subdiurnal variations of δ¹⁸O_T from three tree species and used water isotopes to partition T from evapotranspiration (ET) to quantify the role of vegetation in the local water cycle. We find that δ¹⁸O_T deviated from isotopic steady‐state during the day but find no species‐specific patterns. The ratio of T to ET varied from 53% to 61% and was generally invariant during the day, indicating that diurnal evaporation and transpiration fluxes respond to similar atmospheric and micrometeorological conditions at this site. Finally, we compared the isotope‐inferred ratio of T to ET with results from another ET partitioning approach that uses eddy covariance and sap flux data. We find broad midday agreement between these two partitioning techniques, in particular, the absence of a diurnal cycle, which should encourage future ecohydrological isotope studies. Isotope‐inferred estimates of transpiration can inform land surface models and improve our understanding of land–atmosphere water fluxes.

中文翻译：

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混合落叶林中蒸散分区的同位素方法

蒸腾（T）可能是从陆地表面到大气的最大水通量，并且容易受到气候，土地利用和植被结构变化的影响。但是，对未来蒸腾通量的预测差异很大，并且约束较弱。稳定的水同位素可以帮助扩大我们的土地，大气水通量的理解，但由于缺乏观察和蒸腾水汽同位素组成（δ如何缺乏了解是有限的_牛逼）变化。在这里，我们提供了来自落叶林的水蒸气，陆地水和植物水的同位素数据，以了解植被如何影响水量预算和陆地-大气水通量。我们测量δ的变化subdiurnal ¹⁸ ö _Ť由三种树种组成，并利用水同位素来划分蒸发蒸腾（ET）中的T，以量化植被在局部水循环中的作用。我们发现δ ¹⁸ Ø _ŧ白天偏离同位素稳态，但未发现特定物种的模式。T与ET的比例在53％至61％之间变化，并且白天通常不变，这表明该位置的昼夜蒸发和蒸腾通量对相似的大气和微气象条件有响应。最后，我们将T与ET的同位素推断比率与另一种使用涡动协方差和汁液通量数据的ET划分方法的结果进行了比较。我们发现这两种分配技术之间存在广泛的共识，特别是缺乏昼夜循环，这将鼓励未来的生态水文学研究。同位素推断的蒸腾量估算值可以为陆地表面模型提供信息，并增进我们对陆地-大气水通量的了解。