Response mechanism of groundwater dynamics to Freeze–thaw process in seasonally frozen soil areas: A comprehensive analysis from site to regional scale

Studying the dynamic mechanism of water-level variations during the freeze–thaw period in seasonally frozen soil regions is an important premise and foundation for winter agricultural irrigation and groundwater resource assessment. Freezing and thawing-induced groundwater-level variations have been observed in regions with a shallow water table depth. However, whether the groundwater level is affected by the freeze–thaw process and the extent of this effect should be verified and discussed on a wider spatiotemporal scale. To this end, this study selected the plain area of Jilin Province, China as an example, and based on a comprehensive analysis of dynamic groundwater-level monitoring data from 114 monitoring wells, the distribution of the meteorological data and soil types and two typical experimental monitoring sites in the area, the dynamic characteristics and influencing factors of groundwater-level variations during the freeze–thaw period were identified. Combined with a multi-factor statistical analysis of the monitoring data at the regional scale of the plain area of Jilin Province, China, the response mechanism of the groundwater level to the freeze–thaw process in the seasonally frozen plain area was summarized. The results showed that: (1) The dynamic phenomenon where the groundwater level falls in the freezing period and rises in the thawing period due to the freeze–thaw process was widespread in seasonally frozen soil regions; (2) The depth threshold of the groundwater level affected by the freeze–thaw process was the sum of the maximum frozen soil depth and the maximum capillary rise height; (3) The main factors controlling the groundwater-level variation during the freeze–thaw period were the initial water-level depth and the maximum snow cover thickness; based on a random forest model, the groundwater-level variation during the freeze–thaw process could be accurately calculated with these main controlling factors; (4) The groundwater-level dynamics during the freeze–thaw process was mainly controlled by the exchange of water between the groundwater and the vadose zone system, and the rise in the water level was only partially replenished by the snowmelt water. The research results have an important guiding significance for winter agricultural irrigation and groundwater recharge resource evaluation.