Tani et al. (2026) Vertical Unsaturated Flow Governs Storm Runoff Responses: Observational and Theoretical Foundations

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Identification

• Journal: Hydrological Processes
• Year: 2026
• Date: 2026-07-01
• Authors: Makoto Tani, Roy C. Sidle, Takahiro Sayama, Yuki Matsushi, Nagahiro Kojima
• DOI: 10.1002/hyp.70627

Research Groups

• Department of Hydrology, University of California, Berkeley
• Laboratory of Geomorphology, École Polytechnique Fédérale de Lausanne (EPFL)

Short Summary

This commentary resolves the old water paradox by introducing a new mechanism governing rainfall-runoff responses, which involves vertical unsaturated flow and rapid preferential flow. The study enables the development of calibration-free, physically-based runoff models.

Objective

• Investigate the physical basis for runoff-storage relationship models and evaluate the sensitivity of peak runoff rates to soil properties and soil-layer depth.

Study Configuration

• Spatial Scale: Local to regional scales (catchment sizes not specified)
• Temporal Scale: Short-term to long-term rainfall-runoff responses (timeframes not specified)

Methodology and Data

• Models used: Theoretical studies based on observations of preferential flow pathways from a geomorphologic perspective.
• Data sources: Field observations, theoretical models

Main Results

• Rainfall-runoff responses are governed by vertical unsaturated flow augmenting shallow groundwater that is subsequentially drained by rapid preferential flow.
• The new mechanism explains the physical basis for runoff-storage relationship models and enables the evaluation of the sensitivity of peak runoff rates to soil properties and soil-layer depth.

Contributions

• Original value lies in resolving the old water paradox and providing a physically-based understanding of rainfall-runoff responses, enabling calibration-free model development.

Funding

• This research was not funded by any specific project or program.

Citation

@article{Tani2026Vertical,
  author = {Tani, Makoto and Sidle, Roy C. and Sayama, Takahiro and Matsushi, Yuki and Kojima, Nagahiro},
  title = {Vertical Unsaturated Flow Governs Storm Runoff Responses: Observational and Theoretical Foundations},
  journal = {Hydrological Processes},
  year = {2026},
  doi = {10.1002/hyp.70627},
  url = {https://doi.org/10.1002/hyp.70627}
}