STRADIVARI Project IV: Soil Hydrology and Biota Interactions: Beyond Static Properties

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While certain aspects of soil hydrology, regulated by Soil Water Retention Curves (Tubini and Rigon, 2022), are relatively well understood, even extending to their ability to describe soil structure beyond soil texture (Kosugi 1999), other areas remain less explored: the behavior of soils under non-equilibrium (Zehe et al., 2010), the evolution of hydraulic properties with soil evolution (Meurer et al., 2020), and the impact of underground vegetation on infiltration (Jones et al., 2022). According to Zehe et al. (2010), water transiently distributes across all pore sizes, moving more slowly toward the smallest pores, a dynamic which does not fit within the Mualem/vanGenuchten theory. A compromise approach often involves accounting for at least two independent pore domains, which exchange water while conveying it largely independently. However more physically sound mechanisms could be envisioned, such as allowing the SWRC to evolve dynamically over time. A key focus in understanding vegetation restoration effects requires comprehending soil biota impacts on hydraulic properties. Earthworm studies and preliminary modeling approaches classify porosity modifications using ordinary differential equations akin to population dynamics models or Hydrological Dynamical Systems (Meurer et al., 2020; Bancheri et al., 2019). Such equations could integrate into Richards-Richardson formulations using methods like Kosugi (1999), with improved modelling of root growth (Vanderborght et al., 2024).

A key focus in understanding effects of vegetation restoration lies in understanding the effects of soil biota on the hydraulic properties of soils. The actions of earthworms have already been studied (e.g. Meurer et al., 2020), and preliminary methods to incorporate these effects into hydrological models have been proposed. These approaches often classify porosity modifications induced by biota using systems of ordinary differential equations akin to population dynamics models or HDS (Calabrese and Porporato, 2015; Bancheri et al., 2019). Such equations could, in turn, be integrated into R2 formulations using methods extending the theory proposed by Kosugi (1999) that relates pore size with SWRC. The effects of root growth on soil structure and hydraulic properties has been already addressed (D'Amato and Rigon, 2025) and can be further improved (Vanderborght et al., 2024). A related aspect regards soil cover which is profoundly impacted by decaying plant material, particularly leaves, which contribute to mulching and alter soil moisture evaporation rates (Villegas et al., 2010). Often neglected is the energy budget of the soil which relates to all of the previously mentioned processes but, especially for what regards the soil evaporation, is often implemented in ways that do not coincide with the current understanding of the phenomena (Or et al., 2013).

Critical Gap: The modeling community faces a double challenge: despite mounting evidence that soil biota fundamentally alter hydraulic properties through macropore creation and soil aggregation modification (Weber et al., 2024; Fraccica et al., 2025), these biological agents remain absent from equations. Additionally, the energetic relationship between water potential and content needs revisiting in light of biological activity, extending beyond static parameter approaches. Furthermore, roots themselves occupy significant soil volume (Garré et al., 2011), creating flow patterns that diverge dramatically from bulk soil behavior, yet most models still treat root water uptake as a distributed sink term rather than acknowledging the three-dimensional flow fields roots create (Vanderborght et al., 2024).

STRADIVARI breakthrough: Extends proven WHETGEO from 1D and 2D to 3D with dynamically evolving hydraulic properties. Unlike episodic studies (Meurer et al., 2020) that suggest biota effects without providing implementable frameworks, STRADIVARI develops operational population dynamics equations coupled to Richards-Richardson formulations, creating the first systematic tool for investigating soil evolution effects on watershed hydrology under different soil management conditions. Experimental soil scientists can validate dynamic SWRC modifications through tomographic techniques (e.g., Yang et al., 2018) and controlled laboratory experiments, while STRADIVARI provides the modeling tools to explore the hydrological implications of observed biota-induced changes, under different soil management conditions. This collaborative approach enables hypothesis testing without requiring STRADIVARI to independently validate all biota-soil interactions.

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References - Soil Hydrology and Biota Interactions

• Bancheri, Marialaura, Francesco Serafin, and Riccardo Rigon. 2019. "The Representation of Hydrological Dynamical Systems Using Extended Petri Nets (EPN)." Water Resources Research 55(11): 8895-8921.
• Brinkmann, Pernilla E., et al. 2010. "Plant-Soil Feedback: Experimental Approaches, Statistical Analyses and Ecological Interpretations." The Journal of Ecology 98(5): 1063-73.
• Calabrese, Salvatore, and Amilcare Porporato. 2015. "Linking Age, Survival, and Transit Time Distributions." Water Resources Research 51(10): 8316-30.
• Fraccica, Alessandro, Enrique Romero, and Thierry Fourcaud. 2025. "Effects of Vegetation Growth on Soil Microstructure and Hydro-Mechanical Behaviour." Géotechnique 75(3): 293-307.
• Garré, S., et al. 2011. "Three-Dimensional Electrical Resistivity Tomography to Monitor Root Zone Water Dynamics." Vadose Zone Journal: VZJ 10(1): 412-24.
• Jones, Julia, et al. 2022. "Forest Restoration and Hydrology." Forest Ecology and Management 520(120342): 120342.
• Kosugi, K. 1999. "General Model for Unsaturated Hydraulic Conductivity for Soils with Lognormal Pore-size Distribution." Soil Science Society of America Journal 63(2): 270-77.
• Meng, Xia, et al. 2022. "The Current and Future Role of Biota in Soil-Landscape Evolution Models." Earth-Science Reviews 226: 103945.
• Meurer, Katharina, et al. 2020. "A Framework for Modelling Soil Structure Dynamics Induced by Biological Activity." Global Change Biology 26(10): 5382-5403.
• Or, D., et al. 2013. "Advances in Soil Evaporation Physics, A Review." Vadose Zone Journal 12.
• Tubini, Niccolò, and Riccardo Rigon. 2022. "Implementing the Water, HEat and Transport Model in GEOframe (WHETGEO-1D v.1.0)." Geoscientific Model Development 15(1): 75-104.
• Vanderborght, Jan, et al. 2024. "Combining Root and Soil Hydraulics in Macroscopic Representations of Root Water Uptake." Vadose Zone Journal: VZJ 23(3): e20273.
• Villegas, Juan Camilo, et al. 2010. "Ecohydrological Controls of Soil Evaporation in Deciduous Drylands." Journal of Arid Environments 74(5): 595-602.
• Weber, Tobias Karl David, et al. 2024. "Hydro-Pedotransfer Functions: A Roadmap for Future Development" 28(14): 3391-3433.
• Yang, Yonghui, et al. 2018. "Assessment of the Responses of Soil Pore Properties to Combined Soil Structure Amendments Using X-Ray Computed Tomography." Scientific Reports 8(1): 695.
• Zehe, Erwin, Theresa Blume, and Günter Blöschl. 2010. "The Principle of 'Maximum Energy Dissipation': A Novel Thermodynamic Perspective on Rapid Water Flow in Connected Soil Structures." Philosophical Transactions of the Royal Society of London 365(1545): 1377-86.

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