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.

This study uses the brightness temperature at the given microwave frequency (18.7 GHz) from the Microwave Radiation Imager (MWRI) on-board the Fengyun-3B (FY-3B) satellite to improve the τ-ω model by considering the radiative contribution from waterbody in the pixels over the wetland of the Yellow River source region, China. In order to retrieve vegetation optical depth (VOD), a dual-polarization slope parameter is defined to express the surface emissivity in the τ-ω model as the sum of soil emissivity and waterbody emissivity. In the regions with no waterbody, the original τ-ω model without considering waterbody impact is used to derive VOD. With use of the field observed vegetation water content (VWC) in the source region of the Yellow River during the summer of 2012, a regression relationship between VOD and VWC is established and then the vegetation parameter b is estimated. The relationship is employed to derive the spatial VWC during the entire vegetation growing period. The VOD retrieved is invalid and failed in some part of the study area by using the previous τ-ω model, while the results from the improved τ-ω model indicate that the VOD is in the range of 0.20 to 1.20 and the VWC is in the range of 0.20kg/m² to 1.40kg/m² in the entire source region of the Yellow River in 2012. Both VOD and VWC exhibit a pattern of low values in the west part and high values in the east part. The largest regional variations appear along the Yellow River. The comparison between the remote-sensing-estimated VWC and the ground-measured VWC gives the root mean square error of 0.12kg/m². These assessments reveal that with considering the fractional seasonal wetlands in the source region of the Yellow River, the microwave remote sensing measurements from the FY-3B MWRI can be successfully used to retrieve the VWC in the source region of the Yellow River.

In this paper, the ground observations were compared to the ERA-Interim, NCEP-DOE AMIP-II Reanalysis, MODIS ET product, and emerging offline SEBS ET data sets in the Yellow River source region of the Tibet Plateau. In general, the slopes of linear least squares exhibit differences, with ERA-Interim, NCEP–DOE, MOD16, and SEBS slopes of 0.88 ± 0.05, 0.64 ± 0.07, 0.66 ± 0.17, and 1.24 ± 0.97 respectively. ERA-Interim was found superior with ground observations to others; therefore, it provided a good representation of the study area. Based on the ERA-Interim ET product, the Sen's slope estimator and the Mann–Kendall (MK) test were applied to quantify the significance of the shifts in trends, while the moving t-test and MK test characterized abrupt changes. The results show that the Yellow River source region experienced a statistical increase in evapotranspiration (ET) in the northern part and a decrease in the southern part of the region from 1979 to 2014 at rates of approximately 1.65 and −0.50 mm/yr, respectively. The shift in the annual ET trend was more pronounced, and abrupt changes were detected in the 1980s. Precipitation was the most dominant factor affecting ET variation, whereas surface temperature was the least influential.

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.