Divergent effect of the freeze–thaw process on soil moisture dynamics in an alpine region of the northeast Qinghai-Tibet plateau

Abstract

The seasonal freeze–thaw process significantly influences the migration and distribution of soil moisture in cold regions. However, the heterogeneity of these influences among vegetation type and elevation remains understudied, thereby limiting our understanding of hydrological processes and streamflow generation in alpine regions. Here, we used the Simultaneous Heat and Water (SHAW) model to simulate the dynamics of soil temperature and moisture during the freeze–thaw process of four vegetation types (i.e., shrub meadow, coniferous forest, mountainous steppe, and desert steppe) in the Qilian Mountains catchment from 2015 to 2023. We analyzed the effects of the freeze–thaw process on soil moisture. Results indicated that the SHAW model could effectively simulate the dynamics of soil temperature and moisture, and the simulated data had a high consistency with the measured data. The interplay of altitude, vegetation types, and precipitation resulted in soil under coniferous forest and shrub meadow exhibiting lower temperature but higher moisture, whereas mountainous steppe and desert steppe exhibited higher temperature and lower moisture. During the freeze–thaw process, the rate and extent of liquid–solid-liquid transformation in the soil water of coniferous forest and shrub meadow was more pronounced. The maximum proportion of solid water in the soil of coniferous forest and shrub meadow peaked at 83.78 % and 78.71 %, while it reached 21.25 % % and 44.59 % in the soil of mountainous steppe and desert steppe. These results indicate that freeze–thaw process impacts soil moisture more significantly in coniferous forest and shrub meadow. Specifically, the soil freeze–thaw process prompts the liquid-to-solid water conversion during the freezing and the initial phase of completely frozen period, while prompting solid-to-liquid conversion during the thawing and the latter phase of the completely frozen period. These results emphasize the critical role of freeze–thaw processes in soil water dynamics and deepen our understanding of the mechanisms of soil water dynamics in alpine regions.