The study, led by Alessio Gentile of the University of Torino and Polytechnic University of Torino, focused on the Dora del Nivolet catchment in the Aosta Valley, a 16.99 square kilometer basin ranging from 2,390 to 3,430 meters in elevation. The site is snow-dominated: snow typically accumulates from November and persists until May, when melt begins, while summer brings intermittent rainfall. Between November 2017 and February 2023, the researchers combined field measurements — soil moisture sensors at 10, 20, and 40 centimeters, an eddy-covariance station tracking evapotranspiration, and monthly sampling of precipitation, soil water, plant xylem water, and spring water for stable isotope analysis — with a sophisticated modeling framework that couples a snow isotope model developed by Ceperley and colleagues with a modified version of the HYDRUS-1D soil physics model.

The scientific stakes here are considerable. For more than a decade, hydrologists have debated the Two Water Worlds hypothesis, which proposes that plants tap a portion of soil water that is partially disconnected from the mobile water that recharges aquifers and supplies streams. Evidence for this ecohydrological separation has accumulated from forests in the Mediterranean, temperate, and pre-Alpine settings, but the hypothesis remains one of the most controversial ideas in modern hydrology. Critics point out that apparent separation could be an artifact of sampling techniques, extraction biases, or unmodeled processes such as root water uptake dynamics and seasonality. Testing the idea in a high-elevation grassland — where snowmelt and rainfall arrive in sharply distinct seasonal pulses — offered an unusually clean natural experiment.

The key to the analysis was the Seasonal Origin Index, or SOI, a metric introduced by Allen and colleagues in 2019 that exploits the sinusoidal seasonal cycle of stable water isotopes in precipitation. Winter snow is isotopically depleted in heavy isotopes, summer rain is enriched, and any water flux — transpiration, deep drainage, streamflow — can be assigned a value between −1, meaning entirely winter-derived, and +1, meaning entirely summer-derived. Crucially, the SOI accounts for the fact that at a snow-dominated site, most precipitation falls in winter, so a flux fed indiscriminately by all precipitation would still carry a winter-leaning signature. The index isolates genuine seasonal preference from simple availability.

The modeling framework proved remarkably accurate. Simulated volumetric water content matched field measurements with a mean absolute error of roughly 0.03 cubic centimeters per cubic centimeter at all three monitored depths. Simulated actual evapotranspiration deviated from eddy-covariance observations by only 0.62 millimeters per day, and simulated soil and xylem water isotope compositions tracked the measured values with errors between 1.6 and 3.7 per mil. Because the model was calibrated on soil observations and then validated against independent atmospheric-flux measurements it had never seen — evapotranspiration and xylem isotopes — the researchers could be confident the good performance was not achieved for the wrong reasons.

The results during the 2018 to 2020 growing seasons were strikingly clear. Transpiration carried consistently positive SOI values, ranging from 0.19 to 0.69, meaning the water evaporating from grassland leaves was disproportionately summer rainfall. Deep drainage, the flux leaving the bottom of the 60-centimeter soil profile and assumed to recharge groundwater, carried negative SOI values, meaning it was disproportionately winter snowmelt. The physical mechanism is elegant: intense, sustained snowmelt between mid-April and mid-June saturates the soil profile, driving hydraulic conductivity and Darcy velocities to their annual peaks and creating vertical connectivity between pores that flushes meltwater rapidly past the root zone. Summer rain, arriving after the soil has drained, percolates slowly, lingers in the soil matrix, and is exactly where roots find it when transpiration demand peaks in July and August.

The isotopic field data told the same story from an independent angle. The median isotopic composition of plant water was enriched in heavy isotopes, typical of summer precipitation, while the median composition of the monitored spring — the Source, roughly 400 meters downslope — was markedly depleted, characteristic of winter snowmelt, and confined within a narrow range. Spring water remained winter-leaning even during winter months, suggesting subsurface mixing with snowmelt-recharged storage. The researchers also considered whether lateral redistribution on the steep 32-degree hillslope could complicate the picture, but three lines of evidence — the flat plateau where instruments sit, the low fraction of young water previously measured in the catchment’s streamflow and springs, and the agreement between modeled bottom-flux isotopes and spring water — all pointed to vertical infiltration dominating over shallow lateral flow.

Then came 2022, and the pattern broke. The winter of 2021/2022 brought a snow drought to the Italian Alps, with the snow line sitting hundreds of meters above its long-term average. Under these conditions, the median SOI of transpiration flipped negative, to −0.07, indicating that winter-derived snowmelt water became overrepresented in the water plants used. The explanation lies in the timing and intensity of melt: with a thinner, more ephemeral snowpack, meltwater arrived earlier and in smaller, more intermittent pulses. The soil never reached the saturation needed to establish vertical pore connectivity, hydraulic conductivity and Darcy velocities stayed low through summer, and the bottom flux — groundwater recharge — collapsed. Meltwater that would once have flushed through the profile instead lingered in the root zone, available to plants alongside summer rain. The team also noted a slight increase in actual evapotranspiration during the drought year, consistent with the so-called drought paradox reported elsewhere in the Alps, where vegetation can transpire more under warmer, drier summers as earlier snowmelt extends the growing season.

These findings carry a pointed message for how scientists should think about the Two Water Worlds hypothesis. The researchers tested a null hypothesis stating that snowmelt rapidly transits the soil to recharge groundwater while rainfall sustains transpiration — and found they could neither cleanly accept nor reject it, because the dichotomy itself is too rigid. Ecohydrological separation, they conclude, is better understood as a matter of degree, modulated by the time-variable character of seasonal water inputs and root water uptake. In years with concentrated, sustained snowmelt, separation is pronounced; in dry years with early, intermittent melt, it weakens or dissolves. This reframing aligns with recent work showing that separation is most likely after intense precipitation events that saturate the soil, and with the fill-and-spill conceptual model of runoff generation, in which connected vertical flow only emerges once available storage fills.

The broader implications stretch well beyond one Alpine valley. Snow-dominated mountains supply water to billions of people downstream, and climate projections consistently point toward warmer winters, less solid precipitation, and more frequent snow droughts. If reduced snowpacks mean that meltwater no longer flushes efficiently to groundwater but instead evaporates from grassland leaves, the consequences cascade: diminished aquifer recharge, altered baseflow to headwater streams, and shifts in which seasonal water reserves sustain mountain ecosystems through the growing season. The authors caution that their framework has limits — the degree-day snow model omits refreezing, compaction, and sublimation effects on meltwater isotopes, and the low temporal resolution of isotope sampling in remote terrain constrains parameter estimation. Still, the study demonstrates that a process-based coupling of snow isotope modeling and soil water and solute transport can turn sparse, hard-won field data from extreme environments into quantitative insight. The next step, the researchers suggest, is to feed climate change projections directly into the model, to anticipate how the delicate seasonal plumbing of mountain water towers will rewire as the snows retreat.

Subject of Research: Seasonal partitioning of snowmelt and rainfall between plant transpiration and groundwater recharge in a high-elevation Alpine grassland

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

Article References: Gentile, A., Gisolo, D., Brighenti, S., Zuecco, G., Marchina, C., Canone, D., Hamza, T., Ferrari, S., Bechis, S., & Ferraris, S. (2026). Assessing the seasonal compartmentalization of water fluxes in the soil-plant-atmosphere continuum of a high-elevation mountain grassland. Hydrology and Earth System Sciences, 30(19), 6131-6157. https://doi.org/10.5194/hess-30-6131-2026

Image Credits: AI Generated

DOI: 10.5194/hess-30-6131-2026

Keywords: ecohydrology, snowmelt, stable isotopes, Two Water Worlds hypothesis, alpine grassland, groundwater recharge, transpiration, HYDRUS-1D, Seasonal Origin Index, snow drought, Italian Alps, mountain water cycle