Factors controlling the rise and fall of groundwater level during the freezing-thawing period in seasonal frozen regions

Abstract

Freezing- and thawing-induced variations of groundwater level and the corresponding changes in water storage in the saturated zone have been observed in regions with a shallow water table. However, the vertical migration of water under the effect of the freezing-thawing process on shallow groundwater has not been accurately quantified and the influencing factors of groundwater level dynamics during the freezing-thawing period remain unclear. Reasonable representation of this process on the field scale is very important for improving our understanding of water budgets during the freezing-thawing cycle. In this study, we investigated the main factors controlling groundwater level fluctuations during the freezing-thawing period at two sites in Northeast China through comparative analyses of long-term field observations of air temperature, precipitation, snow depth, liquid soil water content, soil water temperature, and fluctuations of groundwater level. We found that the seasonal variations in groundwater table coincided with the soil frost freezing-thawing seasonal cycle, which exhibited a “V” shape. Freezing-induced groundwater decline occurred only when the maximum depth of effect of freezing and thawing overlapped with the maximum height of capillary rise in unconfined groundwater. Moreover, the decline was controlled by air temperature, snow cover, soil media properties, and soil water distribution. By comparing the change trends of groundwater level and calculating the water budget, the process of the rise of groundwater level could be further divided into three stages, during which groundwater was recharged by frost melt water and snowmelt water. Among them, snowmelt water had a limited contribution, accounting for less than 20% of the total recharge. The results have potential applications in the development of reasonable water-management plans for irrigation, using which soil salt can be washed out and suitable soil thermal and moisture conditions can be maintained for winter crops or farming in the next year.