Rong Liu

Alpine wetlands on the Qinghai–Tibetan Plateau host vertically structured and highly contrasting pore systems that fundamentally shape land and atmosphere exchanges, yet their hydraulic expressions and process implications remain poorly quantified. This study provides the first process based and depth resolved characterization of these layered pore structures using soil physical analyses and laboratory evaporation experiments. The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts, with surface layers dominated by macropore connectivity and showing high θ _s and K _s and low b that promote rapid drainage and evaporation, mid layer domains with lower θ _s and K _s and larger b that enhance retention in finer pores, and deeper layers that act as stable and persistent storage reservoirs. These properties together generate a vertical regime of rapid near surface drainage, delayed mid layer release, and long lasting deep moisture storage. When implemented in Noah-MP, this hydraulic stratification systematically altered water and energy partitioning during wet and dry periods and showed that vertical hydraulic heterogeneity rather than a single layer parameterization governs the timing and magnitude of evaporation and heat fluxes. These findings provide the first quantitative evidence that pore scale structure regulates profile scale hydrothermal responses in Qinghai–Tibetan Plateau wetlands and establish a physically grounded basis for representing vertically heterogeneous hydraulic processes in land surface models.

By revisiting the physical consistency of existing soil hydraulic models in light of principles from interfacial science, we develop a physically constrained conceptual model that combines the van Genuchten–Mualem model with experimentally derived relationships that link pressure head to film thickness and film area. By explicitly incorporating film area and thickness, the model avoids reliance on the Fredlund–Xing–type noncapillary retention functions and surrogate parameters (e.g., residual and saturated adsorbed water contents) to represent adsorbed water. The model shows good fitting performance, with parameter identifiability remaining strong despite the introduction of film-area–related parameters. The model yields different predictions of dry‐range retention and conductivity compared with existing models, thereby offering an alternative modeling option for the dry end. Although the model constrains the pressure range of the capillary module, the physical interpretability of the capillary–film water-content partition and the prediction of conductivity may still depend on how strictly the conceptual capillary model is assumed to exclusively represent the capillary domain. Given the limited falsifiability of conceptual models, incorporating physically measurable film area may offer a useful basis for model validation. In addition, the limited success of empirical adjustments in smectite- and vermiculite-dominated clays further underscores the need for more adaptable parameterizations for full-range modeling in these soils. Overall, the proposed model attempts to integrate interfacial characteristics into full-range hydraulic modeling, offering complementary perspectives for characterizing the hydraulic behavior of noncapillary water.

Alpine wetlands on the Qinghai-Tibetan Plateau host vertically structured and highly contrasting pore systems that fundamentally shape land and atmosphere exchanges, yet their hydraulic expressions and process implications remain poorly quantified. This study provides the first process based and depth resolved characterization of these layered pore structures using soil physical analyses and laboratory evaporation experiments. The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts, with surface layers dominated by macropore connectivity and showing high θs and Ks and low b that promote rapid drainage and evaporation, mid layer domains with lower θs and Ks and larger b that enhance retention in finer pores, and deeper layers that act as stable and persistent storage reservoirs. These properties together generate a vertical regime of rapid near surface drainage, delayed mid layer release, and long lasting deep moisture storage. When implemented in Noah-MP, this hydraulic stratification systematically altered water and energy partitioning during wet and dry periods and showed that vertical hydraulic heterogeneity rather than a single layer parameterization governs the timing and magnitude of evaporation and heat fluxes. These findings provide the first quantitative evidence that pore scale structure regulates profile scale hydrothermal responses in Qinghai-Tibetan Plateau wetlands and establish a physically grounded basis for representing vertically heterogeneous hydraulic processes in land surface models.

Alpine wetlands on the Qinghai-Tibetan Plateau host vertically structured and highly contrasting pore systems that fundamentally shape land and atmosphere exchanges, yet their hydraulic expressions and process implications remain poorly quantified. This study provides the first process based and depth resolved characterization of these layered pore structures using soil physical analyses and laboratory evaporation experiments. The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts, with surface layers dominated by macropore connectivity and showing high θs and Ks and low b that promote rapid drainage and evaporation, mid layer domains with lower θs and Ks and larger b that enhance retention in finer pores, and deeper layers that act as stable and persistent storage reservoirs. These properties together generate a vertical regime of rapid near surface drainage, delayed mid layer release, and long lasting deep moisture storage. When implemented in Noah-MP, this hydraulic stratification systematically altered water and energy partitioning during wet and dry periods and showed that vertical hydraulic heterogeneity rather than a single layer parameterization governs the timing and magnitude of evaporation and heat fluxes. These findings provide the first quantitative evidence that pore scale structure regulates profile scale hydrothermal responses in Qinghai-Tibetan Plateau wetlands and establish a physically grounded basis for representing vertically heterogeneous hydraulic processes in land surface models.

Alpine wetlands on the Qinghai-Tibetan Plateau host vertically structured and highly contrasting pore systems that fundamentally shape land and atmosphere exchanges, yet their hydraulic expressions and process implications remain poorly quantified. This study provides the first process based and depth resolved characterization of these layered pore structures using soil physical analyses and laboratory evaporation experiments. The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts, with surface layers dominated by macropore connectivity and showing high θs and Ks and low b that promote rapid drainage and evaporation, mid layer domains with lower θs and Ks and larger b that enhance retention in finer pores, and deeper layers that act as stable and persistent storage reservoirs. These properties together generate a vertical regime of rapid near surface drainage, delayed mid layer release, and long lasting deep moisture storage. When implemented in Noah-MP, this hydraulic stratification systematically altered water and energy partitioning during wet and dry periods and showed that vertical hydraulic heterogeneity rather than a single layer parameterization governs the timing and magnitude of evaporation and heat fluxes. These findings provide the first quantitative evidence that pore scale structure regulates profile scale hydrothermal responses in Qinghai-Tibetan Plateau wetlands and establish a physically grounded basis for representing vertically heterogeneous hydraulic processes in land surface models.

Alpine wetlands on the Qinghai-Tibetan Plateau host vertically structured and highly contrasting pore systems that fundamentally shape land and atmosphere exchanges, yet their hydraulic expressions and process implications remain poorly quantified. This study provides the first process based and depth resolved characterization of these layered pore structures using soil physical analyses and laboratory evaporation experiments. The derived Clapp–Hornberger parameters reveal coherent hydraulic contrasts, with surface layers dominated by macropore connectivity and showing high θs and Ks and low b that promote rapid drainage and evaporation, mid layer domains with lower θs and Ks and larger b that enhance retention in finer pores, and deeper layers that act as stable and persistent storage reservoirs. These properties together generate a vertical regime of rapid near surface drainage, delayed mid layer release, and long lasting deep moisture storage. When implemented in Noah-MP, this hydraulic stratification systematically altered water and energy partitioning during wet and dry periods and showed that vertical hydraulic heterogeneity rather than a single layer parameterization governs the timing and magnitude of evaporation and heat fluxes. These findings provide the first quantitative evidence that pore scale structure regulates profile scale hydrothermal responses in Qinghai-Tibetan Plateau wetlands and establish a physically grounded basis for representing vertically heterogeneous hydraulic processes in land surface models.

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 compositions 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 equilibrium hydraulic schemes and examines 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 model, yield a nearly consistent fit to experimental observations and demonstrate strong identifiability of parameters. This indicates that the Peters-Iden-Durner 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 Peters-Iden-Durner model is nearly 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 Peters-Iden-Durner 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.

This work aims at evaluating the potential application of two cosmic-ray neutron sensing (CRNS) soil water content (SWC) retrieval methods and discusses the effective measurement depth of CRNS retrieval and vertical weights of different layers in a semihumid alpine meadow on the Tibetan Plateau. In this study, the widely used N0 method and the latest published method derived from physical principles named the universal transport solution (UTS) were used for SWC estimation during the whole nonfrozen season on the Tibetan Plateau. The N0 and UTS methods successfully retrieved the SWC on the Tibetan Plateau, and their variations were generally consistent. The effective measurement depths calculated by the N0 and UTS methods showed similar fluctuation patterns, ranging from 9.7 to 14.5 cm and 17.8 to 20 cm, with mean values of 12.0 and 19.2 cm, respectively. This work verified that the weighting methods of the N0 and UTS models provided credible performance for the effective measurement depths. The N0 method provided measurement depths more close to the surface, and the UTS method yielded slightly deeper averages. The UTS equation had a more sensitive response to periods of low precipitation. Including detailed Ultra Rapid Neutron-Only Simulation (URANOS) Monte Carlo simulations of the full time series, we find specifically an excellent agreement in the summer period. Our analysis suggests that CRNS is a promising innovation for SWC measurement and can be extended in its application at hectometer scales to monitor SWC in the Tibetan meadow ecosystem. Significance Statement: The purpose of this study is to better understand if cosmic-ray neutron sensing (CRNS) has the potential to sense soil water content (SWC) for the Tibetan meadow ecosystem with harsh climate conditions. Techniques for measuring SWC have evolved over many decades, from point-scale measurements of a few meters to satellite-based remote sensing methods of several kilometers, each with advantages and disadvantages. This is important because CRNS has emerged to fill the scale gap between point measurements and microwave remote sensing methods. The results are helpful in assessing the applicability of CRNS in alpine meadow ecosystems on the Tibetan Plateau.

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.

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.