Abstract Knowing how evapotranspiration (ET) is mediated by abiotic and biotic pathways is essential to understanding how water affects ecosystem productivity. Recent studies have investigated the average transpiration fraction (T/ET) across sites and biomes, but the temporal variability of the partitioning and its controls are less understood. Here, we examine how water availability may regulate the seasonal to interannual variability of ET partitioning. To do this, we leveraged four partitioning techniques and long-term (14 years) eddy-flux records from a semiarid grassland and riparian woodland site in southeastern Arizona USA that share the same climate but differ in water table depth. T/ET was sensitive to changes in water availability over space and time as evidenced by consistently lower and more variable T/ET at the grassland, where the vegetation does not access groundwater. Seasonally, a pronounced dry foresummer T/ET depression at the rainfall-dependent grassland resulted in a bimodal T/ET distribution. In contrast, T/ET was unimodally distributed at the groundwater-dependent woodland, where high T/ET levels were maintained throughout both the dry and wet summer periods. Annual transpiration at both sites closely tracked water availability as quantified by ET, whereas abiotic evaporation was nearly constant. All ET partitioning methods yielded similar patterns of seasonal and interannual T/ET, although they differed by approximately 5 to 10% in magnitude. At both sites, transpiration was most closely correlated to plant metrics (photosynthesis, leaf area index) and ET, whereas evaporation was most correlated to surface moisture variables (soil moisture, precipitation). However, substantial differences between monthly and annual correlations demonstrate variable controls on monthly partitioning throughout the year. These results provide novel insights into the effects of water availability on the seasonal and interannual variability of ET partitioning and illuminate mechanisms regulating terrestrial ecosystem carbon and water exchange.

中文翻译：

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水分可用性对蒸散分配的影响

摘要 了解非生物和生物途径如何介导蒸发蒸腾 (ET) 对了解水如何影响生态系统生产力至关重要。最近的研究调查了不同地点和生物群落的平均蒸腾分数 (T/ET)，但对分区及其控制的时间变异性了解较少。在这里，我们研究了可用水量如何调节 ET 分区的季节性到年际变化。为此，我们利用了美国亚利桑那州东南部气候相同但地下水位深度不同的半干旱草原和河岸林地的四种分区技术和长期（14 年）涡流记录。T/ET 对可用水量随空间和时间的变化很敏感，这可以通过草地的持续较低和更多变的 T/ET 来证明，植被无法进入地下水的地方。季节性地，依赖降雨的草地明显的前夏干旱 T/ET 降低导致双峰 T/ET 分布。相比之下，T/ET 在依赖地下水的林地呈单峰分布，在整个干燥和潮湿的夏季期间，T/ET 水平都保持在较高水平。ET 量化的两个地点的年度蒸腾作用密切跟踪可用水量，而非生物蒸发量几乎是恒定的。所有 ET 划分方法都产生了类似的季节性和年际 T/ET 模式，尽管它们在量级上相差大约 5% 到 10%。在这两个地点，蒸腾作用与植物指标（光合作用、叶面积指数）和 ET 的关系最密切，而蒸发与表面水分变量（土壤水分、沉淀）。然而，月度和年度相关性之间的显着差异表明全年对月度分区的可变控制。这些结果提供了关于水资源可用性对 ET 分配的季节性和年际变化的影响的新见解，并阐明了调节陆地生态系统碳和水交换的机制。