Stable isotope ratios of water (δ18O, δ2H) have long been used for ecohydrological applications to answer questions such as: “What is the seasonal origin of water used by plants and streams?”. This issue is particularly relevant in high-altitude environments, where water pathways in soil and vegetation are mainly governed by snowmelt dynamics. These dynamics can vary significantly from year to year, depending on various factors, such as the amount of snowfall, the snow accumulation period, and the air temperature conditions. In this regard, it is crucial to determine the seasonal origin of water that supplies plants’ transpiration and groundwater recharge, as well as to assess whether there are interannual variations.

This study focuses on shedding light on the seasonal origin of water in a high-elevation grassland at 2550 m a.s.l. in the Aosta Valley, northwest Italy. We simulate 5 years of water fluxes and isotope transport through the soil-plant-atmosphere continuum of this ecosystem.

Although there are still uncertainties about the timing and distribution of infiltration during snowmelt and the variations in the isotopic composition of snow, the modeling approach applied in this work effectively reproduces observations of soil moisture, evapotranspiration, and isotope content at the study site.

Groundwater storage has resulted in being mainly recharged by winter-derived water (i.e., snowmelt), while plant transpiration is mainly supplied by summer-derived water (i.e., rainfall). Nevertheless, a change in hydrological functioning is observed during the 2022 European drought, where vegetation relied more heavily on winter-origin water to supply transpiration.

This result offers insight into how mountain ecosystems like that investigated in this study could adapt to future scenarios with warmer temperatures and less snowfall.