Impact of the soil freeze-thaw process on runoff generation and water balance in an alpine region of the northeast Qinghai-Tibet plateau

The soil freeze-thaw process profoundly influences runoff generation through complex and interconnected mechanisms, yet its quantitative impact remains poorly understood, particularly across different vegetation types and elevations. Addressing this issue is critical for improving hydrological predictions in cold regions. In this study, we established an integrated atmosphere-vegetation-soil observation system across varying elevations and vegetation types in the Qilian Mountains (QLM) and employed the Simultaneous Heat and Water (SHAW) model to quantitatively assess soil hydrothermal dynamics and runoff generation during different freeze-thaw stages from 2015 to 2023. The results demonstrate that the SHAW model could accurately simulate soil hydrothermal processes across all vegetation types (NSE > 0.80 for soil temperature; NSE > 0.69 for soil moisture) in an alpine region. Soil water content and water balance components varied significantly across both freeze-thaw stages and vegetation types. There was almost no surface runoff formed in desert steppe and mountainous steppe, and deep seepage was also low. In contrast, shrub meadow exhibited substantial deep seepage (89.29 mm) during the completely thawed stage and could be a major source of recharging to streamflow. The major water fluxes for the four vegetation types occurred during thawing and completely thawed stages, dominated by evapotranspiration. Evapotranspiration accounted for 93 %, 94 %, 81 %, and 62 % of annual precipitation in desert steppe, mountainous steppe, coniferous forest, and shrub meadow, respectively. While the component of evapotranspiration differed, it was dominated by soil evaporation in desert steppe (79 % of total ET) and mountainous steppe (92 %), and by vegetation transpiration in coniferous forest (59 %) and shrub meadow (78 %). These findings offer critical insights into water partitioning within the soil-vegetation-atmosphere continuum, enabling more accurate predictions of streamflow and water availability in alpine regions.