J. Hydrol. Hydromech., Vol. 74, No. 4 - Early view, 2026, p. 1 - 10, doi: .
Scientific Paper, English

Katsutoshi Seki, Martinus Th. van Genuchten, Wolfgang Durner, Luwen Zhuang, Silvia L. B. Bermudez: Trimodal hydraulic models for unsaturated flow: Coupling triple-porosity water retention with general conductivity functions

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• We present trimodal hydraulic models that couple a triple-porosity water retention function (WRF) with a general hydraulic conductivity function (HCF) to predict unsaturated flow from saturation to dry conditions. The WRF is a linear combination of subfunctions representing macro-, meso-, and micro-pores such as, for example, tri-VG (VG+VG+VG), BVV (BC+VG+VG), or VVP (VG+VG+film flow) formulations, where VG and BC present the van Genuchten and Brooks-Corey type functions. The general HCF links K(h) to the measured WRF with only two additional parameters. Applied to soils, construction materials, and a trimodal sandstone, the models reproduce θ(h) and K(h) data across capillary and film-flow regimes and capture the sharp conductivity drop often observed just below saturation without ad hoc interpolation. The tri-VG and BVV models resolve three pore domains and match observed pore-size distributions where trimodality is evident, with VVP offering a compact alternative. The WRF, in practice readily measured over a wide pressure-head range, robustly identifies triple-porosity parameters, while only limited K data can calibrate the HCF. A limitation is that representing the near-saturated K drop requires a corresponding decrease in θ; without accompanying retention data, macropore parameters should not be over-interpreted. Overall, explicit macroporosity within a trimodal WRF plus a parsimonious HCF provides an efficient, unified framework for modeling unsaturated flow.
  
  KEY WORDS: Soil water retention; Unsaturated hydraulic conductivity; Triple-porosity model; Macroporosity; Film flow; General hydraulic conductivity function.
  
  Address:
  - Katsutoshi Seki, Natural Science Laboratory, Toyo University, 5-28-20 Hakusan, Bunkyo-ku, Tokyo 112-8606, Japan. (Corresponding author. Tel.: Fax.: Email: seki_k@toyo.jp)
  - Martinus Th. van Genuchten, Department of Earth Sciences, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, Netherlands. Department of Nuclear Engineering, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.
  - Wolfgang Durner, Soil Science and Soil Physics Division, Institute of Geoecology, Technische Universität Braunschweig, 38092 Braunschweig, Germany.
  - Luwen Zhuang, Center for Water Resources and Environment, and Guangdong Key Laboratory of Marine Civil Engineering, School of Civil Engineering, Sun Yat-sen University, Guangzhou 510275, China.
  - Silvia L. B. Bermudez, Department of Civil Engineering, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.

J. Hydrol. Hydromech., Vol. 74, No. 4 - Early view, 2026, p. 11 - 21, doi: .
Scientific Paper, English

Bernardo Gehlen, Ricardo Leite Martins Bazarin, Rafael Augusto Bastos Rodrigues Alves, Diogo Nardelli Siebert: An REV analysis of capillary pressure curves using the Full Morphology Method and the van Genuchten model

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• A Representative Elementary Volume (REV) analysis is fundamental to guarantee that simulations are performed in a representative domain. While the literature presents approaches to determine the REV of scalar properties, especially single-phase ones, identifying the REV for full capillary pressure curves remains challenging due to the computational costs of direct simulations and limited convergence metrics. This study proposes a hierarchical framework to evaluate the REV of drainage capillary pressure curves of digital rocks imaged via micro-computed tomography (micro-CT) using the Full Morphology Method (FM) and the van Genuchten model. Instead of adapting standard convergence criteria to the fitted parameters, whose non-linearity complicates a stability analysis, we introduce a statistical Mean Absolute Error (MAE) metric to quantify the convergence of the entire curve shape. The methodology was validated using four sandstone micro-CT samples. Results establish a preliminary hierarchy to identify the REV for the full curve at sizes at least double of the statistical REV for residual saturation. Crucially, the MAE-based approach diagnosed the lack of convergence of more heterogeneous samples for which a coefficient-of-variation-based metric failed. The framework provides a robust and physically representative criterion for defining simulation domain sizes, thereby preventing the use of non-representative data in reservoir characterization.
  
  KEY WORDS: Digital rock physics; Representative elementary volume; Full morphology method; van Genuchten model; Capillary pressure curves.
  
  Address:
  - Bernardo Gehlen, Federal University of Santa Catarina, Rua Dona Francisca, 8300, Joinville, Santa Catarina, 89219-600, Brazil. (Corresponding author. Tel.: Fax.: Email: b.gehlen@posgrad.ufsc.br)
  - Ricardo Leite Martins Bazarin, Federal University of Santa Catarina, Rua Dona Francisca, 8300, Joinville, Santa Catarina, 89219-600, Brazil.
  - Rafael Augusto Bastos Rodrigues Alves, Federal University of Santa Catarina, Rua Dona Francisca, 8300, Joinville, Santa Catarina, 89219-600, Brazil.
  - Diogo Nardelli Siebert, Federal University of Santa Catarina, Rua Dona Francisca, 8300, Joinville, Santa Catarina, 89219-600, Brazil.

J. Hydrol. Hydromech., Vol. 74, No. 4 - Early view, 2026, p. 22 - 31, doi: .
Scientific Paper, English

Felipe Eler, Martinus Th. van Genuchten, Paulo Couto: Uncertainty Assessment of Capillary Pressure Curves Measured Using Centrifuge Methods

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• The centrifuge technique (CT) is widely used across hydrology, petrophysics, and related disciplines to measure the water retention/capillary pressure curves, but standard data inversion methods lack uncertainty quantification and often cannot guarantee solution uniqueness. To address this, we present a novel Bayesian framework for inverting CT data that explicitly accounts for experimental uncertainties, provides confidence intervals, quantifies parameter covariance, and analyzes solution uniqueness. The framework was validated against synthetic first-drainage data (the desorption curve) across various curve shapes and noise levels, successfully recovering true capillary pressure curves. Application to experimental core samples of Indiana Limestone, Silurian Dolomite, and Upper Gray Berea yielded highly consistent results, demonstrating the framework’s practical reliability. The approach also facilitates crucial non-uniqueness analyses, identifies parameter correlations, and offers strategies for improved solution stability. To promote transparency, reproducibility, and cross-disciplinary collaboration, the complete framework is publicly available as an open-source code on GitHub.
  
  KEY WORDS: Capillary Pressure Curve; Centrifuge Method; Uncertainty Quantification; Solution Uniqueness; Bayesian Inference.
  
  Address:
  - Felipe Eler, Department of Civil Engineering, Federal University of Rio de Janeiro, UFRJ, Rio de Janeiro, RJ, Brazil. (Corresponding author. Tel.: Fax.: Email: felipe.eler@petroleo.ufrj.br)
  - Martinus Th. van Genuchten, Department of Nuclear Engineering, Federal University of Rio de Janeiro, UFRJ, Rio de Janeiro, RJ, Brazil. Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands.
  - Paulo Couto, Department of Civil Engineering, Federal University of Rio de Janeiro, UFRJ, Rio de Janeiro, RJ, Brazil.

J. Hydrol. Hydromech., Vol. 74, No. 4 - Early view, 2026, p. 365 - 381, doi: .
Scientific Paper, English

Koharu Tasaki, Nobuo Toride, Diederik Jacques, Shohei Koizumi, Ieyasu Tokumoto: Influence of Iron Minerals on the Fate and Transport of Substances in Re-duced Paddy Soils: Insights from Reactive Transport Modeling with HP1

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• A comprehensive reactive transport model was developed using the HYDRUS-PHREEQC (HP1) program to simulate the fate and transport of substances associated with aerobic and anaerobic soil organic matter (SOM) decomposition in paddy soils. The model incorporates complex biogeochemical processes, including sequential redox reactions, dissolution/precipitation, and ion exchange, while accounting for spatiotemporal changes in redox potential (Eh) and pH. Numerical experiments were conducted to evaluate the effects of iron mineral dissolution and precipitation on hydrogen sulfide (H_2 S) and methane (CH_4) formation under iron-sufficient and iron-deficient conditions. Distinct temporal and spatial differences in Eh, pH, and solute concentrations were observed between the surface-oxidized and deeper-reduced layers. Under the iron-sufficient condition, greater amounts of reduced Fe^(2+) generated from mineral dissolution were retained as exchangeable Fe^(2+), suppressing H_2 S formation through FeS precipitation, which also delayed CH_4 formation compared to the iron-deficient condition. This study revealed the critical roles of iron mineral dynamics and soil charge properties in regulating redox processes in paddy soils. By quantitatively representing the coupled interactions among redox transitions, mineral equilibria, and ion exchange, the developed model provides a robust framework for assessing biogeochemical processes and offers insights into the mitigation of methane and toxic sulfur formations from flooded soils.
  
  KEY WORDS: Paddy soil; SOM decomposition; Redox reaction; Reactive transport model; HP1.
  
  Address:
  - Koharu Tasaki, The United Graduate School of Agricultural Sciences, Kagoshima University, 21-24, 1 Korimoto, Kagoshima 890-8580, Japan.
  - Nobuo Toride, Graduate School of Bioresources, Mie University, 1577 Kuriyamachiya-cho Tsu, Mie 514-850, Japan
  - Diederik Jacques, Engineered and Geosystems Analysis Unit, Belgian Nuclear Research Centre, 2400 Mol, Belgium.
  - Shohei Koizumi, Research Section-?, Environmental Technology Research Department, Environment Research Laboratory, Advanced Technology Research Laboratories, NIPPON STEEL CORPORATION, 20-1 Shintomi, Futtsu-shi, Chiba 293-8511, Japan.
  - Ieyasu Tokumoto, Faculty of Agriculture, Saga University, 1 Honjo, Saga 840-8502, Japan. (Corresponding author. Tel.: Fax.: Email: yasu@cc.saga-u.ac.jp)

J. Hydrol. Hydromech., Vol. 74, No. 4 - Early view, 2026, p. 382 - 390, doi: .
Scientific Paper, English

Asha Nambiar, Gerrit Huibert de Rooij: Characterization of hysteresis and sigmoidal behaviour in the Rossi–Ippisch–Adaptation (RIA) of the soil water retention curve

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• The van Genuchten-parameterization (vG) of the soil water retention curve (SWRC) has been very popular for over four decades, but it has some physical inconsistencies at the dry and the wet end. The recently introduced Rossi–Ippisch–Adaptation (RIA) of vG resolves these by introducing an air-entry value and eliminating the residual water content. This study explores if a hysteric version of RIA is feasible. Expressions for its main drying and wetting curves are provided, showing that hysteresis in RIA necessarily induces hysteresis in the shape parameters α and n. A practical closed-form relation is proposed to estimate a hysteretic parameter set for the main wetting curve when measurements are only available for the main drying curve (without guarantee of accuracy). Using six soils spanning coarse to fine textures, we show that the wetting curves remain physically consistent and non-intersecting with the drying curves. Furthermore, the paper highlights RIA’s capability to transition smoothly from a sigmoidal to power-law shape. We elucidate how α determines the location, and even the existence, of an inflection point. Finally, we present a criterion to determine the limit at which the RIA curve converges to the more parsimonious Brooks–Corey power-law form.
  
  KEY WORDS: Soil water retention curve; Rossi–Ippisch–Adaptation; Soil hydraulic properties; Hysteresis; Unsaturated flow; Soil physics.
  
  Address:
  - Asha Nambiar, Helmholtz-Centre for Environmental Research - UFZ, Halle (Saale), 06120, Germany.
  - Gerrit Huibert de Rooij, Helmholtz-Centre for Environmental Research - UFZ, Halle (Saale), 06120, Germany. (Corresponding author. Tel.: Fax.: Email: gerrit.derooij@ufz.de)