Assessing the seasonal compartmentalization of water fluxes in the soil-plant-atmosphere continuum of a high-elevation mountain grassland

Improving our understanding of snow–groundwater connectivity remains a key challenge in high-elevation mountain environments. This calls for a multidisciplinary and multimethod research framework that integrates different types of field observations, including the collection of water samples from diverse sources for stable isotope analysis. However, in remote alpine areas, the limited frequency of sampling hinders the generation of robust, data-driven insights into ecohydrological processes. Therefore, accurately modelling water movement and stable isotope transport through soil, vegetation, and groundwater recharge is essential for advancing our understanding of the hydrological functioning of high-altitude ecosystems. In this work, we combine a recently introduced snow isotope model with the HYDRUS-1D model to simulate water fluxes and isotope transport within the soil–plant–atmosphere continuum of a high-elevation mountain grassland located in the Aosta Valley, north-western Italy, over the period November 2017–February 2023. We use this modelling framework to: investigate the seasonal origin of two key water fluxes, namely transpiration and deep drainage (the latter assumed to contribute to groundwater recharge) clarify how seasonal water inputs and root water uptake patterns contribute to ecohydrological separation. The results demonstrate the effectiveness of the proposed modelling framework in accurately simulating volumetric water content (mean absolute error, MAE ≈ 0.03 cm ³  cm ⁻³ at 10, 20, and 40 cm), actual evapotranspiration (MAE = 0.62 mm d ⁻¹ ), soil (MAE = 1.6 ‰, 3.6 ‰, 3.7 ‰ at 10, 20, and 40 cm) and xylem (MAE = 1.85 ‰) isotope content at the study site. Based on the model outputs, a separation between the water used by plants and the water contributing to deep drainage is evident during the 2018–2020 growing seasons, when median Seasonal Origin Index (SOI) values of transpiration and deep drainage are positive and negative, respectively. Here, positive and negative SOI values indicate an overrepresentation of summer- and winter-derived water in the considered flux. This separation weakens in 2021, when drainage water exhibits a slightly positive median SOI value. Over intense snowmelt periods, meltwater (winter water) rapidly drains through the lower soil layers, whereas rainfall (summer water), which predominantly occurs after the snowmelt period, remains in the soil longer and sustains plant transpiration. The median SOI of transpiration remains consistently positive during the 2018–2021 growing seasons (SOI = 0.19–0.69) but shifts to a negative value in 2022 (SOI = −0.07), highlighting that winter-derived water becomes overrepresented in transpiration fluxes under snow drought conditions. This finding offers valuable insight into how mountain ecosystems may respond to projected increases in temperature and decreases in solid precipitation. Overall, this work highlights the hydrological conditions that drive the seasonal compartmentalization of water resources in a high-elevation alpine environment, with potential implications for similar mountainous regions worldwide.