An Isotopic Approach to Partition Evapotranspiration in a Mixed Deciduous Forest at the University of Michigan Biological Station, Pellston, MI (2017)

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 (δT) 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 δ18OT 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 δ18OT 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.

蒸腾作用（Transpiration, T）或许是陆地表面向大气输送水汽的最大通量之一，极易受到气候、土地利用与植被结构变化的影响。然而，未来蒸腾通量的预测结果差异显著，且约束性不足。稳定水同位素（stable water isotopes）有助于深化我们对陆气水通量的认知，但受限于观测数据匮乏，以及对蒸腾水汽同位素组成（δT）变化规律的理解不足，其应用仍存在瓶颈。本研究依托一片落叶阔叶林获取了水汽、陆地水与植物水的同位素数据，旨在厘清植被如何影响水分收支与陆气水通量。我们针对三种乔木树种的δ¹⁸OT开展了亚日尺度观测，并利用水同位素将蒸腾作用从蒸散发（evapotranspiration, ET）中拆分，以量化植被在局地水循环中的作用。研究发现，δ¹⁸OT在日间偏离同位素稳态，但未呈现物种特异性变化模式。蒸腾与蒸散发的比值介于53%至61%之间，且日内整体保持稳定，表明该站点的昼夜尺度蒸发与蒸腾通量对相似的大气及微气象条件产生响应。最后，我们将同位素方法推演得到的蒸腾-蒸散发比值，与另一种基于涡度协方差（eddy covariance）和树液通量数据的蒸散发拆分方法结果进行对比。结果显示，两种拆分方法在午间时段结果高度一致，且均未体现昼夜周期变化，这一发现将为后续生态水文同位素研究提供支撑。基于同位素推演的蒸腾估算结果可为陆面模型（land surface models）提供参考，进而深化我们对陆气水通量的理解。