Advances in dynamic soil water retention behaviour and implications for vadose zone hydrology

In transient multiphase seepage, soil water retention behaviour deviates from equilibrium, leading to dynamic nonequilibrium effects in which suction and water content change asynchronously. This yields flow-rate-dependent soil water retention curves that critically affect predictions of fluid and solute transport in vadose zone and undermine the safety assessment of unsaturated soil slope stability. This brief review synthesises advances and challenges in understanding this behaviour through examining microscale multiphase physical mechanisms and macroscale influences of soil type and hydraulic history. Key experimental techniques, from instrumented soil columns to advanced electromagnetic and imaging methods, are evaluated alongside their limitations. There is also an analysis of continuum- and pore-scale numerical models, including those incorporating dynamic capillary coefficients, pore network models, and multiphase computational fluid dynamics. Despite progress, major challenges persist, including the empirical nature and scale-dependence of model parameters, path-dependent hysteresis, and the lack of a unified theoretical framework that couples dynamic capillarity with soil deformation. Future interdisciplinary efforts integrating advanced experimentation, multiscale numerical modelling, and multiphase physics-based constitutive theories are essential to develop predictive tools for more accurate vadose-zone hydrology and related engineering applications. Document Type: Invited review Cited as: Yan, G., Liu, B., Bore, T., Torres, S. A. G., Li, L., Scheuermann, A. Advances in dynamic soil water retention behaviour and implications for vadose zone hydrology. Capillarity, 2026, 19(1): 26-38. https://doi.org/10.46690/capi.2026.04.03