Kyounghee Kim Hometown: Seoul, South Korea Advisor: Xuhui Lee

Under conditions of the current global climate changes, there are rising questions and concerns about the terrestrial water cycle. For better understanding and precise estimations of each component in the hydrological cycle, stable isotopes (¹⁸O and ¹⁶O) in water, provide unique information due to discrimination between ¹⁸O and ¹⁶O during the evaporation from leaves and soil surfaces. This property produces different isotopic signals among rain, xylem, groundwater and soil water, which allow us to track the water movements inside plant systems. The isotopic enrichment of leaf water plays a significant role by delivering ¹⁸O to photosynthesis, plant tissues and global C¹⁸O₂-CO₂ budget. However, previous isotope studies are unable to fully explain the isotopic enrichment of leaf water during transpiration because lack of measurements of isotopic compositions of transpiration caused by technical limitations for isotope analysis. Existing models estimate the leaf water enrichment based on the Craig-Gordon model with substituting the isotopic composition of ambient vapor for the isotopic composition of transpiration, both of which have not measured continuously.

In my dissertation work, I propose measurements of the isotopic composition of transpiration using a tunable diode laser and a dynamic chamber. The objectives of the proposed studies are (1) to test the isotopic steady state assumption for leaves, (2) to provide in-depth information about isotopically enriched leaf water at the non-steady state, (3) to evaluate existing non-steady state models, (4) to build a leaf water content model using the ground based remote sensing, and finally (5) to suggest a model for the better predictions for isotopic compositions of leaf water at the non-steady state. The proposed study will provide the first real-time measurements of isotopic compositions of transpiration without the interfering ambient water vapor. These measurements will expand our knowledge especially about short time event such as transient time of day for the isotopic enrichment in leaves. The proposed studies will provide mechanistic insights into enrichment at the evaporating site, and, ultimately, will contribute to the study of the paleoclimate and global carbon budget as well as of the global water cycle.