Mechanisms and influencing factors of hydrothermal processes in active layer soils on the Qinghai-Tibet Plateau under freeze–thaw action

• The freeze–thaw cycle affected the soil hydrological process in active layer; • There were significant differences in soil hydrothermal processes at the slope scales and profile scales; • The redistribution of soil water during the freezing and thawing periods is affected by vegetation.” The Qinghai-Tibet Plateau (QTP) is a region with an extensive area of permafrost that is very sensitive to climate change. In this study, soil water and thermal dynamic processes in the active layer of the QTP permafrost area at the profile and slope scales were investigated. The results showed that the hydrothermal process in the active layer soil was strongly affected by freezing and thawing processes and external weather conditions. The temperature of the active layer fluctuated in tandem with air temperature. The soil water content was more stable during the freezing period and showed a “double-hump” trend during the thawing period. The correlation coefficient between soil and air temperature decreased with depth, from 0.896 in the surface soil layer to 0.082 in the deep soil layer; and the correlation coefficient between unfrozen water content and soil temperature during the freezing period showed an overall increasing trend with depth. The soil layers at different depths at the top and bottom of slope profiles differed in their hydrothermal processes due to the physicochemical properties and texture of the soil and vegetation types. The water-heat exchange of the surface soil is more frequent than that of the deep soil, and the frequency is more at the bottom than at the top of the slope. The soil water content at a depth of 0.25 m was the highest in the profile in association with a higher organic matter content and the blocking effect of dense roots. The changes of soil hydrothermal process in the active layer accelerated the hydrological cycle and the spatial–temporal variability in water resources in the frozen soil area. These effects might lead to a series of ecological and environmental problems, such as permafrost degradation and desertification in the QTP alpine meadow ecosystem.