Rethinking isotopic evapotranspiration partitioning in extreme drylands: A multi-field coupled framework

Traditional isotopic evapotranspiration ( ET ) partitioning models systematically overestimate the transpiration fraction ( T/ET ) in desert succulent halophytes (e.g., Haloxylon ammodendron ) by relying on steady-state and pure-water assumptions. Using high-frequency in-situ laser spectroscopy over two growing seasons, we developed an “Anatomy-Physiology-Physics” coupled framework to rectify these structural biases. Our model explicitly corrects for four fundamental mechanisms: (1) volatile organic compound spectral interference; (2) thermodynamic salt effects on equilibrium fractionation driven by intracellular electrolytes; (3) radial Péclet effects specific to cylindrical branches; and (4) non-steady-state ( NSS ) isotopic hysteresis caused by high hydraulic capacitance. Neglecting these factors produced unrealistic T/ET estimates approaching 100%. Our fully coupled model successfully constrained peak summer T/ET to 70–80%, revealing a highly adaptable water-use strategy that is conservative during droughts but opportunistic following rain pulses. This integrated approach establishes a robust physical foundation for accurately quantifying dryland ecohydrological fluxes.