Scale effects and driving factors of soil hydrothermal processes in the deep vadose zone under seasonal freeze–thaw action

• Soil moisture exhibits significant vertical stratification under freeze–thaw actions. • UVWC and ST showed exponential coupling in FTL and linear in VTL and STL. • Frozen layer blocks the influence of most meteorological factors on deeper UVWC. • The VTL exhibit significant seasonal dynamic and hydrothermal regulatory capacity. Deep soil water plays a critical role in vadose zone hydrological processes. However, due to the scarcity of comprehensive deep hydrological data, the hydrothermal characteristics, scale effects, influencing factors, and underlying mechanisms in deep vadose zones under seasonal freeze–thaw actions remain unclear. To this end, an 8-meter-deep soil profile was established in a seasonally frozen soil region of China, where unfrozen volumetric water content (UVWC), soil temperature (ST), and heat flux were monitored. The spatiotemporal variability and characteristic scales of soil hydrothermal processes during the 2017–2019 freeze–thaw periods were investigated using multivariate empirical mode decomposition. Subsequently, gradient boosting regression trees and path analysis were employed to identify the dominant drivers and transmission pathways of hydrothermal processes across different soil layers. Results revealed that under freeze–thaw disturbances, UVWC in the deep vadose zone exhibited pronounced vertical stratification in spatial scale, and its coupling relationship with ST shifted from nonlinear to linear with increasing depth. This stratified pattern was closely linked to the vertical advancement of the freezing front, which promoted the formation of variant extension layer (130–390 cm, VTL) between the freeze–thaw layer (0–130 cm, FTL) and the stable layer (390–800 cm, STL). The VTL, serving as a transitional interface, plays a key buffering and regulatory role in the deep vadose zone: supplying soil water upward in winter and storing water from upper layers in summer. Additionally, the FTL blocked the influence of meteorological factors on deeper UVWC, making UVWC variability in FTL jointly driven by ST (52 %) and meteorological factors (48 %). In contrast, UVWC in the VTL and STL was primarily governed by ST alone (over 78 %). These findings challenge the traditional assumption that deep UVWC is static and deepen the understanding of hydrothermal dynamics in deep vadose zone under freeze–thaw actions.