Gao et al. (2026) Applicability of the Richards equation in infiltration simulation: A comparative study with the two-phase flow model

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-02-24
• Authors: Qian Gao, Zhi Li, Chaomeng Dai, Jiajie Zou, Swee Pin Yeap, Kah Hon Leong, Jixiang Li, Jiajun Hu, Meng Gao
• DOI: 10.1016/j.jhydrol.2026.135194

Research Groups

• State Key Laboratory of Disaster Reduction in Civil Engineering, College of Civil Engineering, Tongji University, Shanghai, 200092, China
• Department of Chemical and Petroleum Engineering, Faculty of Engineering, Technology and Built Environment, UCSI University, Kuala Lumpur, 56000, Malaysia
• Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Kampar, Perak, 31900, Malaysia
• Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 200120, China
• Shanghai Key Laboratory of Bio-Energy Crops, School of Life Sciences, Shanghai University, Shanghai, 200444, China

Short Summary

This study systematically evaluates the applicability of the Richards equation (RE) for simulating water infiltration in the vadose zone by comparing its performance against a two-phase (TP) flow model, identifying specific hydrogeological conditions where RE overestimates infiltration due to restricted air escape.

Objective

• To systematically assess the applicability of the Richards equation (RE) in simulating water infiltration across diverse hydrogeological conditions by benchmarking it against a two-phase (TP) flow model.
• To determine the conditions under which the RE model overestimates infiltration and elucidate the underlying mechanisms.
• To propose preliminary guidelines for the practical application of the RE model.

Study Configuration

• Spatial Scale: Vadose zone (conceptual scale for infiltration processes).
• Temporal Scale: Infiltration process (dynamic over time, no specific duration provided).

Methodology and Data

• Models used: Richards equation (RE) model, Two-phase (TP) flow model.
• Data sources: Numerical simulation (comparative study between models).

Main Results

• The RE model's reliability degrades under saturated or ponding boundary conditions, high initial soil moisture, and in fine-textured soils (characterized by low van Genuchten - Mualem parameters 𝑛, 𝛼, and reduced absolute permeability 𝑘).
• This degradation occurs because these conditions restrict air escape pathways, leading to elevated pore-air pressures that impede water infiltration, a phenomenon the RE model fails to capture, resulting in overestimation of infiltration.
• The RE model remains robust where high-permeability channels form a percolating network connected to the surface within the region of interest.
• Three conservative quantitative guidelines are proposed for assessing the RE model’s suitability in scenarios involving potential extensive confining layers.

Contributions

• Provides a systematic comparison of the Richards-based (RE) and Two-Phase (TP) flow models.
• Determines specific hydrogeological conditions where the RE model overestimates infiltration.
• Elucidates the mechanistic reasons for RE model failure, specifically the role of restricted air escape and elevated pore-air pressures.
• Proposes preliminary, quantitative guidelines for the practical application of the RE model.

Funding

• Not specified in the provided text.

Citation

@article{Gao2026Applicability,
  author = {Gao, Qian and Li, Zhi and Dai, Chaomeng and Zou, Jiajie and Yeap, Swee Pin and Leong, Kah Hon and Li, Jixiang and Hu, Jiajun and Gao, Meng},
  title = {Applicability of the Richards equation in infiltration simulation: A comparative study with the two-phase flow model},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.135194},
  url = {https://doi.org/10.1016/j.jhydrol.2026.135194}
}