Quantifying the Controls on Evapotranspiration Partitioning in the Highest Alpine Meadow Ecosystem

Quantifying the transpiration fraction of evapotranspiration (T/ET) is crucial for understanding plant functionality in ecosystem water cycles, land-atmosphere interactions, and the global water budget. However, the controls and mechanisms underlying the temporal change of T/ET remain poorly understood in arid and semiarid areas, especially for remote regions with sparse observations such as the Tibetan Plateau (TP). In this study, we used combined high-frequency laser spectroscopy and chamber methods to constrain estimates of T/ET for an alpine meadow ecosystem in the central TP. The three isotopic end members in ET (delta (ET)), soil evaporation (delta (E)), and plant transpiration (delta (T)) were directly determined by three newly customized chambers. Results showed that the seasonal variations of delta (ET), delta (E), and delta (T) were strongly affected by the precipitation isotope (R-2 = 0.53). The delta O-18-based T/ET agreed with that of delta H-2. Isotope-based T/ET ranged from 0.15 to 0.73 during the periods of observation, with an average of 0.43. This mean result was supported by T/ET derived from a two-source model and eddy covariance observations. Our overarching finding is that at the seasonal timescale, surface soil water content (theta) dominated the change of T/ET, with leaf area index playing only a secondary role. Our study confirms the critical impact of soil water on the temporal change of T/ET in water-limited regions such as the TP. This knowledge sheds light on diverse land-surface processes, global hydrological cycles, and their modeling. Key Points Laser spectroscopy and the chamber method were used to estimate T/ET in the central Tibetan Plateau Near-surface soil water content dominated the temporal change of T/ET, with leaf area index playing a secondary role Our study highlights the critical impact of environmental conditions on the temporal change of T/ET in water-limited regions