The driving effect of Freeze-Thaw action on the shallow groundwater level fluctuation by altering the hydraulic conductivity of surface soil

Abstract

The fluctuation of groundwater levels during the freeze–thaw period exhibits a close connection with the hydrothermal variations in the surface soil. However, the underlying mechanisms and the extent of their correlation have not been thoroughly explored, which is crucial for understanding the evolution of groundwater circulation in permafrost regions under the context of climate change. This study utilizes field observation data from different permafrost regions in the central Qinghai-Tibet Plateau, proposing a calculation method for soil water potential and permeability coefficients suitable for freeze–thaw environments. It quantifies the contribution of freeze–thaw action to the fluctuation of shallow groundwater levels and analyzes the changes in the hydraulic conductivity of surface soil and their driving effects on shallow groundwater level fluctuations. The results indicate that groundwater level fluctuations are synchronized with the freeze–thaw process. The movement of the freeze–thaw front exhibits a quadratic polynomial relationship with the cumulative changes in groundwater level, with the highest amplitude of fluctuation and fitting degree observed in the continuous permafrost regions of high mountains. During the freeze–thaw period, hydrothermal variations in the soil lead to changes in hydraulic conductivity (comprising permeability coefficient and water potential gradient), thereby driving groundwater level fluctuations by modifying the direction and efficiency of water transport. The extent of freeze–thaw effects on groundwater level decline during this period varies between 3.97 ± 1.66 % − 64.49 ± 35.17 %, and is governed by various factors including initial groundwater depth, soil particle size distribution, and lateral water recharge and discharge conditions. These research results are instrumental in evaluating the impact of climate change on water resources in cold regions, and offer a scientific foundation for water resource management and ecosystem conservation.