Effect of freeze–thaw process on heat transfer and water migration between soil water and groundwater

Understanding the mechanisms of water migration and heat transfer in frozen soil is essential for the selection of water resource management measures and determination of irrigation schemes for winters. However, the dynamics of the groundwater level and temperature caused by freezing and thawing and their impact on soil water redistribution remain unclear. In this field study, the mechanisms of water migration and heat transfer in the soil during the freeze–thaw period under the combined action of the surface temperature change (upper boundary) and groundwater level fluctuation (lower boundary) were comprehensively considered with the theory and method of heat transfer, freeze–thaw characteristic curves of soil, and water balance model. The results showed that: (1) The soil temperature distribution and groundwater temperature dynamics can be interpreted intuitively by the soil temperature gradient and accumulated heat storage. (2) The freeze–thaw characteristic curves of soil indicated that the soil temperature dynamics caused a phase transition of the soil water. This further changed the soil water potential and permeability by 1–2 magnitudes and 1–5 magnitudes, respectively, which affected the direction and quantity of water migration. (3) The water balance quantitatively revealed the controlling effect of groundwater level changes caused by freezing and thawing on soil moisture dynamics. About 20% of the ice in the frozen layer was formed by the upward recharge of the groundwater during the freezing period. In the thawing period, most of this water returned to the groundwater (84%), while the rest (16%) increased the soil water content. These results systematically explain the mechanisms of heat transfer and water migration between the soil water and the groundwater during the freeze–thaw period, providing a theoretical reference for agricultural water management in seasonally frozen regions.