Hydraulic Properties of Hydric Soils on the Tibetan Plateau: A Bayesian Evaluation of Unimodal and Bimodal Models Across the Full Moisture Range

Hydric soils on the Tibetan Plateau play a critical role in regulating alpine ecohydrological processes, yet their hydraulic properties across the complete moisture spectrum remain poorly characterized. In this study, we conducted evaporation and multi-step outflow experiments on nine representative sites, covering hydric meadow, marsh meadow, and slope soils, and obtained soil–water retention curves (SWRCs) and unsaturated hydraulic conductivity functions. A total of 45 undisturbed soil cores and 45 disturbed samples were analyzed under controlled conditions with parallel replications (n = 3). The experimental data were fitted using unimodal van Genuchten (VG), unimodal Kosugi, and bimodal particle discrete interaction (PDI) functions, and parameter estimation was performed through Bayesian inference. The results demonstrate that the bimodal-PDI scheme significantly outperformed unimodal models for sphagnum-rich and structurally heterogeneous soils. Compared with unimodal models, the bimodal-PDI improved the coefficient of determination (R²) from 0.91–0.92 to 0.96, reduced root mean square error (RMSE) by 30–40%, and achieved nearly identical corrected Akaike information criterion (AICc) values despite additional parameters. More importantly, Bayesian posterior analysis revealed that the bimodal-PDI reduced parameter uncertainty by approximately 40% and minimized parameter correlation, thus enhancing model robustness. Mechanistically, the bimodal representation effectively captured dual-domain pore systems, with macropores controlling near-saturation drainage and micropores governing residual retention at high matric potentials. This study provides the first systematic comparison between unimodal and bimodal models within a unified Bayesian–experimental framework for Tibetan Plateau hydric soils. The findings highlight the necessity of dual-domain modeling for accurately characterizing nonmonotonic SWRCs in organic-rich soils. The methodological advances presented here not only refine the hydraulic parameterization of alpine wetlands but also support improved land surface and climate modeling of the “Third Pole” region under changing environmental conditions.