Unfrozen water content significantly affects the thermal‐hydro‐mechanical characteristics of frozen soil. Currently, theoretical explanations for the presence of unfrozen water include capillary action, surface effects, adsorption forces, and the electrical double layer. However, the relationships between unfrozen water and these actions are not well explained, except for capillary forces. In addition, although frost heave experiments indicate that the electrical double‐layer solution on a clay particle contains the main portion of unfrozen water, the electrical double‐layer theory which produces a cation solution has not previously been used to calculate unfrozen water content. In this paper, it is assumed that the residual unfrozen water at very low temperatures (below −18°C) is held within the adsorption‐layer solution and that the remaining unfrozen water within the influence of surface effects is the cationic solution in the diffuse layer. Based on these assumptions, a theoretical model of unfrozen water is established with independent variables of temperature, specific surface area, and electrical double‐layer parameters. While the input parameters of the theoretical model are numerous and difficult to obtain, the theoretical model is simplified as a parametric model. Results of the parametric model are strongly consistent with experimental data within a certain temperature range, supporting the hypothetical conditions. The similarity in the underlying mathematical structure of the derived theoretical and parametric models to current semi‐empirical models further suggests that the surface effects of clay are the main causes of unfrozen water in frozen soil.

Complex dielectric constant (CDC) of bound water determines the accuracy of the complex dielectric constant of wet soil. According to electrical double-layer structure and dielectric properties, the bound water on clay particle surface is divided into strongly bound water and weakly bound water. Based on this classification, models for the complex dielectric constants of bound water and soil are established taking into consideration factors such as temperature, moisture, texture, and microwave frequency. The results show that the fundamental reason why the complex dielectric constant of bound water is between that of ice and free water is the adsorption force which forms the electrical double-layer structure on the surface of clay particles. Low-concentration cationic solution could exist in free soil water and was found as the reason for the higher salinity and conductivity of free soil water, as compared to the measured soil solution. Results of soil CDC model are in good agreement with measured data across a wide range of microwave frequencies and soil temperature, moisture, and texture. The absolute root mean square error analysis also shows that the soil CDC model in this paper compared to the other models is more accurate.

The Tibetan Plateau is well‐known for its expansive wetland environments. Hydric soils, a fundamental component of these environments, exhibit diverse hydraulic characteristics attributable to their varied botanical and mineralogical composition and their inherent porous structures. Nonetheless, research on the hydraulic properties of such soils in Tibet remains notably underrepresented relative to European and Canadian regions. Consequently, in this study, we evaluate the effectiveness of different unsaturated hydraulic schemes in equilibrium and examine the parameter uncertainty of 14 undisturbed samples collected from four soligenous wetlands. The findings suggest that both the van Genuchten and Kosugi functions, when integrated with the Peters‐Iden‐Durner (PDI) model, yield a nearly consistent fit to experimental observations and demonstrate strong identifiability of parameters. This indicates that the PDI model can accurately characterize hydraulic properties across the complete moisture range of hydric soils. Analysis of samples with a low clay content and no sphagnum suggests that the intertwined, twisted, and hollow residues of herbaceous vascular tissues do not create a distinct, independent macro‐pore system. Therefore, the unimodal scheme integrating the PDI model is adequate. However, for samples that exhibit nonmonotonicity of the first‐order derivative of the retention curve, such as uncompacted samples containing sphagnum or samples rich in clay, the integration of the PDI model into the bimodal scheme boosts accuracy while having almost negligible impact on identifiability. The varied observed hydraulic properties of only 14 samples underscore the necessity for extensive hydric‐soil sampling and hydraulic analysis across the expansive and varied wetland landscapes on the Tibetan Plateau.

Soil freezing characteristics are predominantly governed by the mechanism of bound water, which essentially constitutes a multicomponent cations distribution within the electrical double‐layer (EDL) on clay particles. The freezing behavior of bound water is determined by two critical factors: (a) the distribution characteristics of cation solutions; (b) the quantitative relationship between cation concentration and freezing point. Although EDL‐based unfrozen water model has been proposed, the freezing characteristics of multicomponent cation solutions remain poorly understood. Our findings indicate that: (a) The synergistic effect of multicomponent cations increases the freezing point depression coefficient of bound water (i.e., the degree of freezing point lowering per unit concentration) by several‐fold compared to NaCl solution; (b) For typical mineral soils with low Na ⁺ content (<15%), a linear freezing point depression equation can accurately characterize the freezing process of multicomponent cation solutions; (c) typical mineral soils exhibit highly similar cation distribution characteristics. By integrating the freezing point depression equation with EDL theory, this study not only improves the EDL‐based unfrozen water model but also develops a parameterized model applicable to typical mineral soils, and elucidating the intrinsic mechanisms of the model's robustness. Validation using measured data from 12 typical soil types demonstrates that this parameterized model can accurately predict unfrozen water content in sands, silts, and clays with low to moderate clay content within the temperature range of −0.263°C to −20°C. The study establishes a theoretical framework distinct from conventional water potential theory, thereby deepening the understanding of freezing characteristics in frozen soils.

Unfrozen water affects the thermal-hydro-mechanical characteristics, microbial activity and freeze-thaw processes in frozen soils. This study found that part of the unfrozen water is formed by the coupling of adsorption and capillary action in soils with mid to low clay content, which is called bound-capillary water and located in nanopores between clay and sand (silt) particles. The nature of the bound-capillary water affects soil freezing characteristics, which varies with the initial water content. The influence of coupling effects arises from the adsorption effects on the clay surface and the capillary action between the clay-sand particles. The adsorption effects (or surface effects) establish an electrical double-layer structure for bound-capillary water. The capillary action (surface tension) forms bound-capillary water in a shape of meniscuses, which significantly increases its content. Here, we established four theoretical models and a parametric model of unfrozen water based on the coupling effects. The basic mathematical expressions of four theoretical models are almost identical to those of 10 existing unfrozen water semi-empirical models, demonstrating that the semi-empirical models are representative of empirical formulas describing the freezing characteristics of the bound-capillary water. A comparison of model results with the measured unfrozen water content of 9 soils verifies that the parametric model is suitable for soils with low to medium clay content.

The hydraulic properties of hydric soils in Tibet’s peatlands significantly influence hydrological processes and play a substantial role in carbon cycle. Soil hydraulic studies on the peatlands remain strikingly limited. By the comparison of six hydraulic parameterizations using either van Genuchten-Mualem (vGM) or Kosugi saturation function, the results indicate that the PDI (Peter-Durner-Iden) model remains an excellent choice for these multi-disperse systems composed of mixed components, despite substantial discrepancies in the micrometer-scale morphology of structure and composition of partially decomposed Sphagnum and vascular tissues and mineral particles. The bi-modal scheme does not adapt well in the later stages of the Evaporation Experiment (EE), as of the bi-modal scheme deviates in the dry range of the water retention curve (WRC). Nonetheless, Irrespective of the saturation functions, the bi-modal with PDI yields satisfactory results for the WRC and the hydraulic conductivity curve (HCC). We attribute this to the complementary effects of two uni-modals. But for bi-modal PDI scheme, vGM demonstrates better performance than the Kosugi in high clay content samples. These clay-rich samples exhibit a broader and more heterogeneous feature in the equivalent Pore Size Distribution (PSD), while other samples generally show a broad and relatively homogeneous feature. Therefore, we conclude that the WRC and HCC are jointly determined by structures and compositions of mixed components of lignocellulosic, humin and mineral materials. Intricate nature of these components underscore the necessity for extensive comprehensive field sampling and laboratory analyses to accurately capture the hydraulic characteristics in the peatlands of Tibet.