Prima et al. (2025) Trees control hillslope subsurface flow: Insights from stemflow and throughfall experiments, geophysical surveys, and numerical modeling

Identification

• Journal: Journal of Hydrology
• Year: 2025
• Date: 2025-12-03
• Authors: Simone Di Prima, Gersende Fernandes, María Burguet, María Paz Salazar, Elisa Marras, Ilenia Murgia, Konstantinos Kaffas, Luca Peruzzo, Ryan D. Stewart, Majdi Abou Najm, Alessandro Comegna, Laurent Lassabatère, Giorgio Cassiani, Daniele Penna, Christian Massari, Filippo Giadrossich
• DOI: 10.1016/j.jhydrol.2025.134723

Research Groups

• Department of Agricultural, Forestry, Food and Environmental Sciences (DAFE), University of Basilicata, Potenza, Italy
• Department of Agricultural Sciences, University of Sassari, Sassari, Italy
• Institute of Agricultural Sciences, ICA-CSIC, Madrid, Spain
• Laboratorio de Física de Suelos (LaFiS), Facultad de Ciencias Agrarias y Forestales, UNLP, La Plata, Argentina
• Department of Agriculture, Food, Environment and Forestry, University of Florence, Italy
• Department of Science, Roma Tre University, Rome, Italy
• Department of Geosciences, University of Padua, Padua, Italy
• School of Plant and Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, United States
• Department of Land, Air and Water Resources, University of California, Davis, United States
• Université de Lyon, UMR5023 Ecologie des Hydrosystèmes Naturels et Anthropisés, CNRS, ENTPE, Université Lyon 1, Vaulx-en-Velin, France
• Forest Engineering Resources and Management Department, Oregon State University, Corvallis, United States
• Research Institute for Geo-Hydrological Protection, National Research Council, Perugia, Italy

Short Summary

This study investigated the effects of rainfall partitioning on subsurface water dynamics across multiple spatial scales on a forested hillslope in Central Italy. Findings reveal dual-permeability soil behavior, with throughfall promoting matrix infiltration and stemflow enhancing rapid macropore flow, which connects to deeper lateral pathways, controlling hillslope-scale groundwater fluctuations.

Objective

• To investigate the impact of rainfall partitioning (throughfall and stemflow) on subsurface water dynamics across multiple spatial scales (point, single-tree, plot, and hillslope) on a forested hillslope.

Study Configuration

• Spatial Scale:
  • Point scale: Single-ring infiltration tests (1.77 × 10⁻² m² surface area) at three depths (0, 0.2, 0.4 m).
  • Single-tree scale: Artificial stemflow events on seven beech trees, monitored with Electrical Resistivity Tomography (ERT).
  • Plot scale: 100 m² area, assessed by artificial throughfall events alone or combined with multiple artificial stemflow events, and Ground Penetrating Radar (GPR) surveys.
  • Hillslope scale: Piezometer data from the Re della Pietra catchment (2 km²).
• Temporal Scale:
  • Artificial infiltration experiments: July 26th and 27th, 2023.
  • Piezometer data: January 19, 2021, to January 20, 2022.
  • GPR surveys: Once before and once after the artificial events.

Methodology and Data

• Models used:
  • Fractional Wettability (FW) infiltration model (Di Prima et al., 2025) for point-scale infiltration tests.
  • HYDRUS-1D (van Genuchten and Mualem single porosity model) for matrix flow.
  • HYDRUS-1D and HYDRUS-2D/3D (Gerke and van Genuchten dual-permeability model) for fast-flow regions and combined throughfall/stemflow.
• Data sources:
  • Experimental: Single-ring infiltration tests, artificial stemflow events (water applied to stems, overland flow collected), artificial throughfall events (water sprinkled over plot, overland flow collected).
  • Geophysical: Time-lapse Electrical Resistivity Tomography (ERT) (72 electrodes, 1.2 × 1.2 × 1.2 m cuboid centered on a tree), time-lapse Ground Penetrating Radar (GPR) (900-MHz antenna, 50 ns time window, targeting top 4 m).
  • Monitoring: Piezometer data (groundwater levels measured every 15 min).
  • Soil characterization: Soil cores (dry bulk density at 0–0.1, 0.2–0.3, 0.4–0.5 m), hand auger samples (initial water content, particle size distribution at 0–0.2, 0.2–0.4, 0.4–0.6, 0.6–0.8, 0.8–1 m), water drop penetration time tests for soil water repellency.
  • Rainfall: Rain gauges for actual throughfall intensity.

Main Results

• The soil exhibits dual-permeability behavior, with distinct infiltration dynamics for throughfall and stemflow.
• Between tree stems (matrix flow): Throughfall promotes infiltration primarily into the soil matrix. Infiltrometer tests showed lower saturated hydraulic conductivity (Ks) values, ranging from 177.8 to 237.4 mm h⁻¹ depending on depth. Soil water repellency was strongest at the surface, temporarily impeding infiltration under dry conditions.
• Beneath tree stems (fast flow): Stemflow enhances rapid infiltration through macropores and fractures, reaching a mean steady-state rate of 1031.9 mm h⁻¹.
• Numerical modeling: Inverse modeling yielded consistent hydraulic parameters. The matrix region had a Ks of 217.5 mm h⁻¹. The fast-flow region beneath stems had Ks values ranging from 404.2 to 1636.5 mm h⁻¹ (mean 1032.9 mm h⁻¹), which was 4.7 times higher than the matrix. The fast-flow region occupied approximately 6% of the total system porosity.
• Geophysical surveys:
  • ERT detected predominantly vertical wetting beneath stems, confirming rapid infiltration pathways.
  • GPR revealed lateral subsurface flow at depths of approximately 1–2.5 m, indicating deeper flow pathways.
• Hillslope-scale response: Piezometer data identified fast-flow pathways at the hillslope scale, with rapid water table rises (e.g., within 5 minutes for intense events) explained by flow connectivity between vertical (stemflow-induced) and deeper lateral pathways.

Contributions

• Explicitly links point-, single-tree-, and plot-scale infiltration processes with hillslope-scale hydrological responses, providing a comprehensive understanding of subsurface water dynamics.
• Provides new insights into the mechanisms controlling rapid groundwater fluctuations on forested hillslopes, demonstrating how stemflow-driven fast-flow pathways connect with deeper lateral subsurface domains.
• Highlights the importance of integrated multi-scale approaches and explicit consideration of vegetation effects (rainfall partitioning) for accurate characterization of complex hydraulic functioning in forested ecosystems.
• Developed experimental protocols that can be used to inform and improve hillslope-scale mechanistic and semi-distributed catchment models, particularly in representing preferential flow dynamics.

Funding

• "Unravelling interactions between WATER and carbon cycles during drought and their impact on water resources and forest and grassland ecosySTEMs in the Mediterranean climate – WATERSTEM" (PRIN 2020, code: 20202WF53Z)
• "Methodological proposal for the Individuation of protection forests through LEgislation, geohazard assessment Tools and Ontology, MILETO" (PRIN 2022 PNRR; code: P2022587PM)
• "WATer mixing in the critical ZONe: observations and predictions under environmental changes – WATZON" (PRIN 2017, code: 2017SL7ABC)
• "Carbon and water cycles interactions during drought and their impact on WAter and ForEst Resources in the Mediterranean region − WAFER" (Italian National Research Council – CNR)
• "Hydrological controls on carbonate-mediated CO2 consumptions – HYDRO4C" (PRIN 2022, code: 2022PFNNRS)
• "Space It Up!" (ASI n. 687/2022, contract ASI N. 2024–5-E.0, master code: I53D24000060005, WP 7.6)
• RETURN Extended Partnership (European Union Next-GenerationEU, National Recovery and Resilience Plan – NRRP, Mission 4, Component 2, Investment 1.3 – D.D. 1243 2/8/2022, PE0000005)
• Geosciences for Sustainable Development project (Budget Ministero dell’Università e della Ricerca Dipartimenti di Eccellenza 2023–2027 C93C23002690001)

Citation

@article{Prima2025Trees,
  author = {Prima, Simone Di and Fernandes, Gersende and Burguet, María and Salazar, María Paz and Marras, Elisa and Murgia, Ilenia and Kaffas, Konstantinos and Peruzzo, Luca and Stewart, Ryan D. and Najm, Majdi Abou and Comegna, Alessandro and Lassabatère, Laurent and Cassiani, Giorgio and Penna, Daniele and Massari, Christian and Giadrossich, Filippo},
  title = {Trees control hillslope subsurface flow: Insights from stemflow and throughfall experiments, geophysical surveys, and numerical modeling},
  journal = {Journal of Hydrology},
  year = {2025},
  doi = {10.1016/j.jhydrol.2025.134723},
  url = {https://doi.org/10.1016/j.jhydrol.2025.134723}
}