Partitioning evapotranspiration - Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes
Partitioning evapotranspiration - Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes
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DOI：
10.1016/j.jhydrol.2013.05.033
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发表时间：
2013-07-24
影响因子：
6.4
通讯作者：
Werner, Christiane
中科院分区：
文献类型：
--
作者：
Dubbert, Maren;Cuntz, Matthias;Werner, Christiane
水的稳定氧同位素为生态系统内的水分运动提供了有价值的示踪剂，并用于估计蒸腾作用对总生态系统蒸散量（ft）的贡献。我们测试了克雷格和戈登方程对连续实地测量的蒸发同位素组成，并评估分区蒸散的影响。因此，蒸发（E）和它的同位素特征（δ O-18（E））裸露的土壤地块，以及蒸散（ET）和它的相应的同位素组成（δ O-18（ET））的草本层测量与腔衰荡光谱仪连接到一个土壤室在葡萄牙中部的现场。我们量化了δ O-18（E）的变化，这些变化是由于确定克雷格和戈登方程中环境输入变量的不确定性引起的：同位素特征（δ O-18（e））和蒸发地点的温度（T-e）以及动力学分馏因子（α（k））。因此，我们可以根据测量的Δ O-18（ET）和根据蒸发点观测的土壤水同位素组成模拟的Δ O-18（E）来量化ft（δ O-18（e）），并模拟了蒸腾作用的Δ O-18（δ O-18（T））的预测结果表明，预测δ O-18（E）值与实测土壤水同位素组成的预测结果基本一致。使用克雷格和戈登方程导致与测量的δ O-18（E）的良好一致性，假定土壤的温度和O-18同位素分布被彻底表征。然而，模拟的Δ O-18（E）对T-e和Δ O-18（e）以及α（k）的变化高度敏感。这显著影响了蒸腾和蒸发从总ET通量中的分配结果：使用不同的α（k）公式并假设稳态或非稳态蒸腾，蒸腾的分数（ft）变化很大。这些研究结果提供了第一个比较的激光为基础的和模拟同位素组成的蒸发的基础上的克雷格和戈登方程在现场条件下。这是特别感兴趣的研究，使用稳定同位素分离土壤蒸发和植物蒸腾通量，并强调需要一个彻底的表征微气象和同位素组成的上层土壤层，以定位蒸发锋的分辨率为几厘米的土壤深度。我们还呼吁更好地表征土壤蒸发的动力分馏因子。(C)2013爱思唯尔有限公司版权所有。
Stable oxygen isotopes of water provide a valuable tracer for water movements within ecosystems and are used to estimate the contribution of transpiration to total ecosystem evapotranspiration (ft). We tested the Craig and Gordon equation against continuous field measurements of isotopic composition of evaporation and assessed the impact for partitioning evapotranspiration. Therefore, evaporation (E) and its isotopic signature (delta O-18(E)) on bare soil plots, as well as evapotranspiration (ET) and its corresponding isotopic composition of (delta O-18(ET)) of an herbaceous layer was measured with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We quantified the variation in delta O-18(E) arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature (delta O-18(e)) and the temperature at the evaporating site (T-e, and the kinetic fractionation factor (alpha(k)). We could hence quantify ft based on measured delta O-18(ET), modeled delta O-18(E) from observed soil water isotopic composition at the evaporating site (delta O-18(e)), and modeled delta O-18 of transpiration (delta O-18(T)) from observed total soil water isotopic composition.Our results demonstrate that predicting delta O-18(E) using the Craig and Gordon equation leads to good agreement with measured delta O-18(E) given that the temperature and O-18 isotope profiles of the soil are thoroughly characterized. However, modeled delta O-18(E) is highly sensitive to changes in T-e and delta O-18(e), as well as alpha(k). This markedly affected the partition results of transpiration and evaporation from the total ET flux: The fraction of transpiration (ft) varied strongly using different formulations for alpha(k) and assuming steady or non-steady state transpiration. These findings provide a first comparison of laser-based and modeled isotopic compositions of evaporation based on the Craig and Gordon equation under field conditions. This is of special interest for studies using stable isotopes to separate soil evaporation and plant transpiration fluxes and highlights the need for a thorough characterization of the micrometeorological and isotopic constitution of the upper soil layer to locate the evaporating front with a resolution of a few cm soil depths. We also call on a better characterization of the kinetic fractionation factor of soil evaporation. (C) 2013 Elsevier B.V. All rights reserved.