Abstract As the eddy-covariance technique enables intensive measurements of evapotranspiration (ET) at the ecosystem level, the interest in further partitioning ET into two main process-based components transpiration (T) and surface evaporation (E) □ is increasing. Although models for partitioning tower-measured ET have been developed, their reliability for different types of ecosystems still requires extensive validations. From 2001 to 2019, we measured CO2 and H2O vapor fluxes over an oak-grass savanna landscape from three eddy-covariance towers (i.e., one over an oak woodland; the other two over annual grasslands under tree canopy and in open area). Annual ET (± standard deviation) from the oak woodland, understory grassland, and open grassland was 419±85 mm, 167±36 mm, 324±43 mm, respectively. The differences between the above- and below-canopy ET indicated that oak canopy transpiration (Toak) was 281±48 mm year−1, accounting for 67±8% of the total ET of the woodland. The Toak/ET ratio varied in seasons, similar to the pattern of oak's leaf area index but opposite to that of soil moisture. We then tested two ET-partitioning models: Scott's long-term-regression-interception (LTRI) model (Scott and Biederman, 2017) and Zhou's quantile-regression-maximum-slope (QRMS) model (Zhou et al., 2016). Even though we expected that the two models would give divergent results since theiremo working principles, both models captured reasonable magnitudes and seasonal patterns of the T/ET ratio, as suggested by tower measurements. The study confirms that the LTRI and QRMS models are applicable for savanna ecosystems, but some modifications are necessary for tree dominated areas. In combination with field and modeling approaches, this study improves our understanding on the contributions of transpiration and evaporation to total ET from ecosystems with vertical vegetation layers.

中文翻译：

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加利福尼亚橡树草原的蒸腾和蒸发：现场测量和分区模型结果

摘要 由于涡流协方差技术能够在生态系统层面对蒸散量 (ET) 进行密集测量，因此将 ET 进一步划分为基于过程的蒸腾 (T) 和地表蒸发 (E) □ 两个主要成分的兴趣正在增加。尽管已经开发了用于划分塔测量 ET 的模型，但它们对不同类型生态系统的可靠性仍然需要广泛的验证。从 2001 年到 2019 年，我们测量了来自三个涡流协方差塔（即一个在橡树林地上；另外两个在树冠下和开放区域的一年生草原上）的橡树草稀树草原景观上的 CO2 和 H2O 蒸汽通量。橡树林地、林下草地和开阔草地的年 ET（± 标准差）分别为 419±85 mm、167±36 mm、324±43 mm。冠层以上和冠层以下ET的差异表明，橡树冠层蒸腾量（Toak）为281±48 mm year−1，占林地总ET的67±8%。Toak/ET 比率随季节变化，类似于橡树的叶面积指数模式，但与土壤水分的模式相反。然后我们测试了两个 ET 分区模型：Scott 的长期回归截取 (LTRI) 模型（Scott 和 Biederman，2017）和 Zhou 的分位数回归最大斜率（QRMS）模型（Zhou 等，2016）。尽管我们预计这两个模型会根据他们的工作原理给出不同的结果，但正如塔测量所建议的那样，这两个模型都捕获了 T/ET 比率的合理幅度和季节性模式。研究证实 LTRI 和 QRMS 模型适用于热带稀树草原生态系统，但是对于树木主导的区域，需要进行一些修改。结合实地和建模方法，这项研究提高了我们对蒸腾和蒸发对具有垂直植被层的生态系统总 ET 的贡献的理解。