In the context of global warming, the soil freeze depth (SFD) over the Tibetan Plateau (TP) has undergone significant changes, with a series of profound impacts on the hydrological cycle and ecosystem. The complex terrains and high elevations of the TP pose great challenges in data acquisition, presenting difficulties for studying SFD in this region. This study employs Stefan's solution and downscaled datasets from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to simulate the future SFDs over the TP. The changing trends of the projected SFDs under different Shared Socio-economic Pathways (SSP) scenarios are investigated, and; the responses of SFDs to potential climatic factors, such as temperature and precipitation, are analyzed. The potential impacts of SFD changes on eco-hydrological processes are analyzed based on the relationships between SFDs, the distribution of frozen ground, soil moisture, and the Normalized Difference Vegetation Index (NDVI). Results show that the projected SFDs of the TP are estimated to decrease at rates of 0.100 cm/yr under the SSP126, 0.330 cm/yr under the SSP245, 0.565 cm/yr under the SSP370, and 0.750 cm/yr under the SSP585. Additionally, the SFD decreased at a rate of 0.160 cm/yr during the historical period from 1950 to 2014, which was between the decreasing rates of the SSP126 and SSP245 scenarios. The projected SFDs are negatively correlated with air temperature and precipitation, more significant under the higher emissions scenario. The projected decrease in SFDs will significantly impact eco-hydrological processes. A rapid decrease in SFD may lead to a decline in soil moisture content and have adverse impacts on vegetation growth. This research provides valuable insights into the future changes in SFD on the TP and their impacts on eco-hydrological processes.

Known as the roof of the world , 50%-56% area of the Qinghai-Tibet Plateau (QTP) is covered by seasonal frozen ground (SFG), which has an important impact on local and global climate change, terrestrial ecosystems, and regional energy and hydrological cycles. In this study, long-term observational data of air and soil water (precipitation and soil moisture) and heat [surface air temperature (SAT) and soil temperature (ST)] at 30 meteorological stations were used to study the temporal and spatial changes of SFG and their possible causes for the central-eastern QTP (CEQTP). The results showed that latitude and altitude are the key factors affecting the spatial distributions of seasonal freeze-thaw activities of CEQTP. The stations with deeper freeze depths and more freeze days are mainly located in high-altitude and high-latitude regions, and those with shallower freeze depths and fewer freeze days are mainly located in the low-altitude and low-latitude regions of the southern QTP. This may be the reason that latitude and altitude are the key factors determining the temperature distribution on the CEQTP. SAT, ST, precipitation, and soil moisture are all significant correlations with the freeze depth, freeze days, freeze start date (FSD) and thaw end date (TED), and the abrupt change years of them are also consistent; they are the important factors affecting the freeze-thaw changes (FTCs) of SFG. Among them, ST is the key factor influencing the FTCs of SFG, and the variations of monthly average soil temperature (MAST) at 0-320 cm depths are the inverse of those of the monthly average freeze depth and freeze days during the year. Using the MAST data at 0-320 cm depths and the 0? ST threshold, the soil freeze-thaw processes at different depths on the CEQTP are revealed. Affected by global warming, SAT and ST at different depths on the CEQTP have shown the upward trends since the 1980s. Additionally, precipitation and soil moisture have also increased substantially, especially since the late 1990s. Enhancement of warming and wetting conditions from the land surface to the deep soil have accelerated the thawing of SFG, and led to the delay of FSD and the advance of TED, which further caused the reduction of freeze depth and freeze days of SFG on the QTP, especially since the late 1990s.

Using 1980-2014 observation data of 378 meteorological stations, this study quantified the correlations between snow cover and the annual maximum seasonally freeze depth (MSFD) across China and the contributions of snow cover to MSFD. Snow cover exhibited a weak effect on MSFD across China, which may be related to the thinness and short duration of the snow cover. Regional differences in the effects of snow cover on MSFD were relatively large, and the effects of snow cover were more significant than those of air temperature (characterized by the airfreezing index) at stations in Northeast China, northeastern Inner Mongolia, and the north of the Tianshan Mountains. In regions with a thin snow cover or short snow cover duration (SCD), the cooling effect of snow cover can dominate, but it only slightly influences MSFD owing to the short SCD. With increasing average snow depth (ASD) and SCD, the relative contributions of snow cover to MSFD gradually increased, peaking at SCDs of 120-140 days or ASDs of 8-10 cm, which is attributable to the increasing insulating effect. However, with further increase in ASD and SCD, the insulating effect decreases because of high albedo and the latent heat effect of snow melting. This decrease resulted in declining MSFD-snow cover correlations and snow cover contributions to MSFD. Compared with SCD, ASD has a greater influence on MSFD. By adding an ASD variable to the Stefan formula, this modified formula outperformed the conventional formula in MSFD estimation at SCDs of 60-140 days. This study demonstrated the importance of snow cover variables for soil freeze depth analysis and simulation in areas with large snow cover and further elucidated the effects of snow cover on soil freeze depth.