The hydrologic cycle couples the Earth's energy and carbon budgets through evaporation, moisture transport, and precipitation. Despite a wealth of observations and models, fundamental limitations remain in our capacity to deduce even the most basic properties of the hydrological cycle, including the spatial pattern of the residence time (RT) of water in the atmosphere and the mean distance traveled from evaporation sources to precipitation sinks. Meanwhile, geochemical tracers such as stable water isotope ratios provide a tool to probe hydrological processes, yet their interpretation remains equivocal despite several decades of use. As a result, there is a need for new mechanistic tools that link variations in water isotope ratios to underlying hydrological processes. Here we present a new suite of “process-oriented tags,” which we use to explicitly trace hydrological processes within the isotopically enabled Community Atmosphere Model, version 6 (iCAM6). Using these tags, we test the hypotheses that precipitation isotope ratios respond to parcel rainout, variations in atmospheric RT, and preserve information regarding meteorological conditions during evaporation. We present results for a historical simulation from 1980 to 2004, forced with winds from the ERA5 reanalysis. We find strong evidence that precipitation isotope ratios record information about atmospheric rainout and meteorological conditions during evaporation, but little evidence that precipitation isotope ratios vary with water vapor RT. These new tracer methods will enable more robust linkages between observations of isotope ratios in the modern hydrologic cycle or proxies of past terrestrial environments and the environmental processes underlying these observations.

中文翻译：

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通过过程导向和同位素比示踪剂的配对增强对水文循环的理解

水文循环通过蒸发、水分输送和降水耦合地球的能量和碳预算。尽管有大量的观测和模型，但我们推断水文循环最基本特性的能力仍然存在基本限制，包括水在大气中的停留时间 (RT) 的空间模式以及从蒸发源传播的平均距离到降水汇。同时，稳定水同位素比等地球化学示踪剂提供了一种探测水文过程的工具，尽管使用了几十年，但它们的解释仍然模棱两可。因此，需要新的机械工具将水同位素比率的变化与潜在的水文过程联系起来。在这里，我们展示了一套新的“面向过程的标签，”，我们使用它在同位素启用的社区大气模型第 6 版 (iCAM6) 中明确追踪水文过程。使用这些标签，我们测试了以下假设：降水同位素比率响应地块降雨、大气 RT 的变化，并在蒸发过程中保存有关气象条件的信息。我们展示了 1980 年至 2004 年历史模拟的结果，受 ERA5 再分析的风影响。我们发现强有力的证据表明降水同位素比记录了蒸发过程中大气降水和气象条件的信息，但几乎没有证据表明降水同位素比随水汽 RT 变化。