Abstract. We developed a setup for a fully automated, high frequency in-situ monitoring system of the stable water isotopes Deuterium and ¹⁸O in soil water and tree xylem. The setup was tested for 12 weeks within an isotopic labelling experiment during a large artificial sprinkling experiment including three mature European beech (Fagus sylvatica) trees. Our setup allowed for one measurement every 12–20 minutes, enabling us to obtain about seven measurements per day for each of our 15 in-situ probes in the soil and tree xylem. While the labelling induced an abrupt step pulse in the soil water isotopic signature, it took seven to ten days until the isotopic signatures at the trees' stem bases reached their peak label concentrations and it took about 14 days until the isotopic signatures at 8 m stem height levelled off around the same values. During the experiment, we observed the effects of several rain events and dry periods on the xylem water isotopic signatures, which fluctuated between the measured isotopic signatures observed in the upper and lower soil horizons. In order to explain our observations, we combined an already existing root water uptake (RWU) model with a newly developed approach to simulate the propagation of isotopic signatures from the root tips to the stem base and further up along the stem. The key to a proper simulation of the observed short term dynamics of xylem water isotopes, was accounting for sap flow velocities and the flow path length distribution within the root and stem xylem. Our modelling framework allowed us to identify parameter values that relate to root depth, horizontal root distribution and wilting point. The insights gained from this study can help to improve the representation of stable water isotopes in trees within ecohydrological models and the prediction of transit time distribution and water age of transpiration fluxes.

中文翻译：

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木质部木质部水同位素的时间动态：原位监测和建模

摘要。我们为稳定的水同位素氘和¹⁸ O在土壤水和树木木质部中的全自动高频原位监测系统开发了一套设备。将该组装一个大型人工洒实验包括三个成熟欧洲山毛榉（在12周测试的同位素标记实验中水青冈）的树木。我们的设置允许每12–20分钟进行一次测量，从而使我们每天对土壤和树木木质部中的15种原位探针中的每一个进行约7次测量。虽然标记引起土壤水同位素特征的突变，但要花7至10天，直到树茎基部的同位素特征达到其峰值标记浓度，而大约8天后，茎8 m处的同位素特征才出现。高度趋近于相同的值。在实验过程中，我们观察到几次降雨事件和干旱时期对木质部水同位素特征的影响，木质素水同位素特征在上部和下部土壤层中观测到的同位素特征之间波动。为了解释我们的观察，我们将现有的根部吸水量（RWU）模型与新开发的方法相结合，以模拟同位素特征从根尖到茎基以及茎干的传播。正确模拟观察到的木质部水同位素短期动态的关键是要考虑树液的流速以及根部和茎部木质部内部的流路长度分布。我们的建模框架使我们能够确定与根深，水平根分布和萎wil有关的参数值。从这项研究中获得的见解可以帮助改善生态水文模型中树木中稳定的水同位素的表示，以及预测蒸腾时间分布和蒸腾通量的水龄。