The deuterium excess (d-excess) of precipitation, which tracks kinetic fractionations during water phase changes, has been used to trace the regions and conditions of oceanic moisture sources, in particular from polar ice-core records. Still, many observations suggest that precipitation d-excess varies significantly across terrestrial environments, both above and below the global average value 10. These variations are often interpreted to reflect either moisture recycling via terrestrial evapotranspiration or sub-cloud raindrop re-evaporation, respectively. Despite being frequently mentioned in literature, however, little work has been carried out to quantify these two competing effects on the widespread variations of d-excess. Here, we use a one-dimensional model of water vapor transport to interrogate the relative controls on d-excess of continental precipitation. We show that when the water vapor gradient is coupled with decreasing temperature, d-excess increases with net rainout and ${\delta }^{18}$O depletion along the model transect, while the magnitude of increase is controlled by the water balance, evaporation/transpiration ratio, and transport type. Raindrop re-evaporation functions as an additional flux of recycled moisture and further increases the d-excess downwind. Alternatively, when the water vapor gradient is coupled with decreasing relative humidity, d-excess may decrease along the model transect wherein upwind evapotranspiration is overwhelmed by local raindrop re-evaporation effects. This local effect becomes even stronger under a regime of turbulent eddy transport with high transpiration fractions, resulting in a pronounced decrease of d-excess without notable changes in ${\delta }^{18}$O. Finally, we demonstrate that model processes capture the isotopic variations in precipitation across the altitudinal gradient of the Andes as well as the South American low-level jet zone. Broadly, this study presents a novel framework for understanding the dynamical controls of precipitation d-excess and for linking spatial isotopic variations with ecohydrological fluxes and processes in both modern and paleo-environments.

降水的氘过量 (d-excess) 可跟踪水相变化过程中的动力学分馏，已被用于追踪海洋水分来源的区域和条件，特别是来自极地冰芯记录。尽管如此，许多观察结果表明，降水 d-excess 在陆地环境中存在显着差异，高于和低于全球平均值 10。这些差异通常被解释为分别反映通过陆地蒸散或亚云层雨滴再蒸发进行的水分循环。然而，尽管在文献中经常提到，但很少有工作来量化这两种对 d-excess 广泛变化的竞争影响。这里，我们使用水汽传输的一维模型来询问对大陆降水 d-excess 的相对控制。我们表明，当水汽梯度与温度降低相耦合时，d-excess 随净降雨量增加而增加。${\delta }^{18}$沿模型横断面的 O 消耗，而增加的幅度由水平衡、蒸发/蒸腾比和运输类型控制。雨滴的再蒸发起到了额外的循环水分通量的作用，并进一步增加了顺风的 d-excess。或者，当水汽梯度与降低的相对湿度相结合时，d-excess 可能会沿着模型横断面减少，其中上风蒸散被局部雨滴再蒸发效应所淹没。在具有高蒸腾分数的湍流涡流传输机制下，这种局部效应变得更加强烈，导致 d-excess 显着减少而没有显着变化${\delta }^{18}$O. 最后，我们证明模型过程捕获了安第斯山脉和南美低空急流区海拔梯度降水的同位素变化。从广义上讲，这项研究提出了一个新的框架，用于理解降水 d-excess 的动态控制，并将空间同位素变化与现代和古环境中的生态水文通量和过程联系起来。