Land surface models (LSMs) play a crucial role in elucidating water and carbon cycles by simulating processes such as plant transpiration and evaporation from bare soil, yet calibration often relies on comparing LSM outputs of landscape total evapotranspiration (ET) and discharge with measured bulk fluxes. Discrepancies in partitioning into component fluxes predicted by various LSMs have been noted, prompting the need for improved evaluation methods. Stable water isotopes serve as effective tracers of component hydrologic fluxes, but data and model integration challenges have hindered their widespread application. Leveraging National Ecological Observation Network measurements of water isotope ratios at 16 US sites over 3 years combined with LSM-modeled fluxes, we employed an isotope-enabled mass balance framework to simulate ET isotope values (δET) within three operational LSMs (Mosaic, Noah, and VIC) to evaluate their partitioning. Models simulating δET values consistent with observations were deemed more reflective of water cycling in these ecosystems. Mosaic exhibited the best overall performance (Kling-Gupta Efficiency of 0.28). For both Mosaic and Noah there were robust correlations between bare soil evaporation fraction and error (negative) as well as transpiration fraction and error (positive). We found the point at which errors are smallest (x-intercept of the multi-site regression) is at a higher transpiration fraction than is currently specified in the models. Which means that transpiration fraction is underestimated on average. Stable isotope tracers offer an additional tool for model evaluation and identifying areas for improvement, potentially enhancing LSM simulations and our understanding of land-surface hydrologic processes.

中文翻译：

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使用启用同位素的质量平衡来评估现有的地表模型


陆面模型 （LSM） 通过模拟植物蒸腾和裸土蒸发等过程，在阐明水和碳循环方面发挥着至关重要的作用，但校准通常依赖于将景观总蒸散量 （ET） 和排放的 LSM 输出与测量的体通量进行比较。已经注意到各种 LSM 预测的分量通量划分存在差异，这促使需要改进评估方法。稳定的水同位素是组分水文通量的有效示踪剂，但数据和模型集成挑战阻碍了其广泛应用。利用国家生态观测网络在 3 年内对 16 个美国站点的水同位素比值的测量，结合 LSM 建模的通量，我们采用了支持同位素的质量平衡框架来模拟三个正在运行的 LSM（Mosaic、Noah 和 VIC）内的 ET 同位素值 （δET），以评估它们的分配。模拟与观测结果一致的 δET 值的模型被认为更能反映这些生态系统中的水循环。Mosaic 表现出最佳的整体性能 （Kling-Gupta 效率为 0.28）。对于 Mosaic 和 Noah，裸土蒸发分数与误差（负）以及蒸腾分数与误差（正）之间存在很强的相关性。我们发现误差最小的点（多站点回归的 x 截距）的蒸腾分数高于模型中当前指定的蒸腾分数。这意味着平均而言，蒸腾分数被低估了。 稳定同位素示踪剂为模型评估和确定需要改进的领域提供了额外的工具，有可能增强 LSM 模拟和我们对陆表水文过程的理解。