Browsing by Author "Yuan, Yusen"

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    A modified isotope-based method for potential high-frequency evapotranspiration partitioning
    (Elsevier, 2022-02) Yuan, Yusen; Wang, Lixin; Wang, Honglang; Lin, Wenqing; Jiao, Wenzhe; Du, Taisheng; Earth Sciences, School of Science
    To better understand water and energy cycles, numerous efforts to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) have been made over the recent half century. One of the analytical methods is the isotopic approach. The isotopic composition of ET (δET) is a crucial parameter in the traditional isotope-based ET partitioning model, which however, has considerable uncertainty and high sensitivity. Here we proposed a modified T fraction in total ET (FT) calculation using Keeling plot slope (k), the atmospheric vapor concentration (Cv), and the isotopic composition of atmospheric vapor (δv), to avoid the direct use of δET. Following the traditional method, we used the Craig-Gordon model for the isotopic composition of evaporation (δE) and chamber method for the isotopic composition of transpiration (δT) in our modified method. The modified FT calculation method (FT (m)) can be applied at a 15-min time scale using the average values (FTi (mp)) and at a 1 Hz time scale for high-frequency method (FTi). The modified method was verified by both theoretical derivations and field observations. FTi (mp) was equivalent to those using the traditional isotopic method at a 15-min time scale. However, FTi eliminated the highly sensitive parameter δET, and redistributed the sensitivity of δET into three less sensitive parameters. Additionally, FTi has two main advantages. First, the high-frequency method avoids the extrapolation of the Keeling plot regression line intercept. Second, the high-frequency method can produce a 95% confidence interval of FT in a measurement cycle (e.g., 15 min). The calculated confidence interval was different from that of traditional uncertainty analysis. The high-frequency method might be useful when investigating evapotranspiration partitioning under short-term extreme weather events and flush agricultural irrigation.
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    Novel Keeling-plot-based methods to estimate the isotopic composition of ambient water vapor
    (EGU, 2020-09) Yuan, Yusen; Du, Taisheng; Wang, Honglang; Wang, Lixin; Earth Sciences, School of Science
    The Keeling plot approach, a general method to identify the isotopic composition of source atmospheric CO2 and water vapor (i.e., evapotranspiration), has been widely used in terrestrial ecosystems. The isotopic composition of ambient water vapor (δa), an important source of atmospheric water vapor, is not able to be estimated to date using the Keeling plot approach. Here we proposed two new methods to estimate δa using the Keeling plots: one using an intersection point method and another relying on the intermediate value theorem. As the actual δa value was difficult to measure directly, we used two indirect approaches to validate our new methods. First, we performed external vapor tracking using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to facilitate explaining the variations of δa. The trajectory vapor origin results were consistent with the expectations of the δa values estimated by these two methods. Second, regression analysis was used to evaluate the relationship between δa values estimated from these two independent methods, and they are in strong agreement. This study provides an analytical framework to estimate δa using existing facilities and provides important insights into the traditional Keeling plot approach by showing (a) a possibility to calculate the proportion of evapotranspiration fluxes to total atmospheric vapor using the same instrumental setup for the traditional Keeling plot investigations and (b) perspectives on the estimation of isotope composition of ambient CO2 (δa13C).