Decoupling ET from soil moisture: Tracing groundwater uptake in Mediterranean forests using stable isotopes to inform land surface models.

In Mediterranean-type ecosystems, droughts are altering the availability of soil water, forcing vegetation to adjust its water uptake strategies. When soil moisture is depleted, a critical survival mechanism for phreatophytes is the shifting of water uptake from shallow soil layers to groundwater. However, sustainable water resources management often overlooks this mechanism. Consequently, quantifying the evapotranspiration (ET) fluxes due to a direct link between the aquifer and the atmosphere represents an important advancement in hydrological modeling. We investigate this vegetation-induced water uptake at two sites with shallow water tables: San Rossore (Pinus pinea) and Castel Porziano (Quercus ilex). Preliminary analysis at these sites indicates a decoupling of ET from soil moisture availability during summer months, suggesting a contribution from deep taproots. To separate the sources of these fluxes, we combine high-frequency Eddy Covariance measurements with stable water isotopes analysis (𝛿18O, 𝛿2H ) of xylem, soil water, rainfall, and groundwater. We propose a framework where isotope-derived ET fractions are used to conceptualise and calibrate unsaturated zone models. This approach not only refines estimates of net groundwater recharge but also improves the representation of vegetation feedback in Earth System Models, ensuring that transpiration from groundwater is accurately represented in climate scenarios for adaptation and water management studies.