Gentile et al. (2026) Assessing the seasonal compartmentalization of water fluxes in the soil-plant-atmosphere continuum of a high-elevation mountain grassland

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-10-01
• Authors: Alessio Gentile, Davide Gisolo, Stefano Brighenti, Giulia Zuecco, Chiara Marchina, Davide Canone, Tanzeel Hamza, Stefano Ferrari, Stefano Bechis, Stefano Ferraris
• DOI: 10.5194/hess-30-6131-2026

Research Groups

• Interuniversity Department of Regional and Urban Studies and Planning (DIST), University of Torino and Polytechnic University of Torino, Torino, Italy
• Faculty of Science and Technology, Free University of Bozen-Bolzano, Bozen-Bolzano, Italy
• Eco Research, Bozen-Bolzano, Italy
• Department of Land, Environment, Agriculture and Forestry (TESAF), University of Padova, Legnaro (PD), Italy
• Department of Chemical Sciences (DiSC), University of Padova, Padova, Italy

Short Summary

This study used a coupled snow isotope and hydrological model to investigate the seasonal origin of water fluxes in a high-elevation mountain grassland. It found a clear ecohydrological separation where snowmelt primarily recharges groundwater and rainfall sustains transpiration, but this separation weakens under snow drought conditions, increasing winter water contribution to plant uptake.

Objective

• To investigate the seasonal origin of transpiration and deep drainage fluxes across multiple years with contrasting hydrological conditions (wet to extremely dry).
• To clarify how contrasting seasonal water inputs, typical of high-elevation environments, interact with root water uptake to assess the degree to which the Two Water Worlds (TWW) hypothesis holds.
• To test the null hypothesis (H0): "Winter precipitation (i.e., snowmelt) rapidly transits the soil profile recharging groundwater and streams, while summer precipitation (i.e., rainfall) remains available to sustain transpiration fluxes".

Study Configuration

• Spatial Scale: Dora del Nivolet (DOR) catchment, 16.99 square kilometers, located in the western Italian Alps (Valsavarenche, Aosta Valley), with elevations ranging from 2390 to 3430 meters above sea level. The study focused on a high-elevation mountain grassland site within the catchment, with a simulated soil profile depth of 60 centimeters.
• Temporal Scale: November 2017 to February 2023 (5 hydrological years), with daily resolution for model calculations.

Methodology and Data

• Models used:
  • Snow accumulation, melt, and isotope model (Ceperley et al., 2020)
  • HYDRUS-1D (modified version for isotope transport, Stumpp et al., 2012)
  • Modified Hargreaves–Samani equation (HSM) for potential evapotranspiration (Ravazzani et al., 2012)
  • Beer's law for partitioning potential evapotranspiration into potential evaporation and potential transpiration.
  • Craig and Gordon (1965) model for isotopic fractionation correction.
• Data sources:
  • Field observations: Volumetric water content and matric potential at 10, 20, and 40 cm depths; actual evapotranspiration from an eddy-covariance station; snow height measurements from an SR50AT sonic sensor; summer rainfall from a CS125 weather sensor; spring water levels from piezoresistive pressure sensors.
  • Stable isotope analysis: Water samples from precipitation, soil water (10, 20, 40 cm depths), plant water (xylem), and spring water. Samples were processed via cryogenic vacuum distillation (CVD). Isotopic compositions (δ¹⁸O, δ²H) were measured using cavity ring-down spectroscopy (Picarro L2130-i) for soil water, isotope ratio mass spectrometry (IRMS; Delta V Advantage) for plant water, and off-axis integrated cavity output spectroscopy (DLT-100, Los Gatos Research) for spring water.
  • Meteorological data: Air temperature from the monitoring station and three additional stations in the Valsavarenche valley.
  • Remote sensing: Leaf Area Index (LAI) time series derived from MODIS/061/MCD15A3H image collection using Google Earth Engine.
  • Site-specific pedological characterization: Soil pit analysis for sand, silt, and clay percentages.

Main Results

• The coupled modeling framework accurately simulated volumetric water content (mean absolute error, MAE ≈ 0.03 cm³ cm⁻³ at 10, 20, and 40 cm), actual evapotranspiration (MAE = 0.62 mm d⁻¹), soil isotope content (MAE = 1.6 ‰, 3.6 ‰, 3.7 ‰ at 10, 20, and 40 cm), and xylem isotope content (MAE = 1.85 ‰).
• During the 2018–2020 growing seasons, a clear ecohydrological separation was observed: transpiration was predominantly fed by summer rainfall (median Seasonal Origin Index (SOI) positive), while deep drainage (contributing to groundwater recharge) was mainly from winter-derived snowmelt (median SOI negative).
• Over intense snowmelt periods, meltwater (winter water) rapidly drained through lower soil layers, while rainfall (summer water), occurring predominantly after snowmelt, remained in the soil longer and sustained plant transpiration.
• This ecohydrological separation weakened in 2021, with drainage water exhibiting a slightly positive median SOI value.
• Under snow drought conditions in 2022, the median SOI of transpiration shifted to a negative value (SOI = −0.07), indicating an overrepresentation of winter-derived water in transpiration fluxes. This suggests that earlier and more intermittent snowmelt inputs, leading to lower soil saturation, allowed winter-sourced water to remain accessible to plants for a longer period.
• Soil isotope patterns showed that during winter, δ¹⁸O values were less depleted at 40 cm and progressively more depleted towards the surface, a pattern reversed during the growing season, supporting vertical filtration of snowmelt.
• The findings support the view of a "degree of ecohydrological separation" rather than a strict duality, with its magnitude modulated by time-variable seasonal water input and root water uptake patterns.

Contributions

• Provides novel insights into the seasonal compartmentalization of water fluxes and the dynamic degree of ecohydrological separation in a high-elevation alpine grassland.
• Demonstrates the effectiveness of a combined snow isotope and hydrological modeling framework (Ceperley et al. model + HYDRUS-1D) for process-based assessment of water flux partitioning and isotope transport in complex mountain environments.
• Quantifies the influence of contrasting seasonal water inputs (snowmelt vs. rainfall) and root water uptake patterns on ecohydrological separation across multiple years, including an extreme snow drought.
• Offers valuable insights into how mountain ecosystems may respond to projected increases in temperature and decreases in solid precipitation, particularly concerning shifts in water sources for transpiration under snow drought conditions.
• Advances the understanding of the Two Water Worlds (TWW) hypothesis by suggesting it operates as a continuum, with the degree of separation modulated by time-variable and site-specific hydrological dynamics.

Funding

• NODES (MUR–M4C2 1.5 of PNRR funded by the European Union–NextGenerationEU, grant no. ECS00000036)
• PRIN 2022 “Snow droUghts predictioN in the Alps: a changing climate assessmEnT: SUNSET” (Prot. n. 202295PFKP)
• PRIN 2017 “WATer mixing in the critical ZONe: observations and predictions under environmental changes-WATZON” (grant no. 2017SL7ABC)
• Funding 2023 Fondazione CRT “Valutazione della siccità nel territorio delle regioni Piemonte e Valle d'Aosta” (FERSCRT23_01–RIF. 2023.0369)
• Funding 2025 Fondazione CRT “Valutazione degli effetti del calo delle precipitazioni nevose sul bilancio idrico in Piemonte e Valle d’Aosta con l’utilizzo di modelli di machine learning” (FERSCRT25_01–RIF. 2025.0780)

Citation

@article{Gentile2026Assessing,
  author = {Gentile, Alessio and Gisolo, Davide and Brighenti, Stefano and Zuecco, Giulia and Marchina, Chiara and Canone, Davide and Hamza, Tanzeel and Ferrari, Stefano and Bechis, Stefano and Ferraris, Stefano},
  title = {Assessing the seasonal compartmentalization of water fluxes in the soil-plant-atmosphere continuum of a high-elevation mountain grassland},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-6131-2026},
  url = {https://doi.org/10.5194/hess-30-6131-2026}
}