Influence Of Freeze-thaw Process On Water And Heat Distribution In Vadose Zone In Typical Areas Of Changbai Mountain

The area of seasonal permafrost in China accounts for more than 50% of the national territory.The freezing and thawing process of soil in the permafrost region will directly cause the change in soil porosity and permeability and then affect the process of recharge of groundwater by atmospheric precipitation or surface water infiltration.The freezing and thawing of the soil in the permafrost region significantly influences the infiltration recharge process and directly affects the formation and evolution of groundwater in the permafrost region.On the other hand,the freezing and thawing process will lead to the redistribution of soil moisture and heat,which will significantly affect the dynamic characteristics of soil moisture and temperature in permafrost regions in early spring,which will have a significant impact on vegetation and crop growth in permafrost regions.Therefore,it is of great significance for the formation and evolution of regional groundwater,agricultural production,and the maintenance of the ecological environment to study the dynamic law of water and heat distribution in the vadose zone during soil freezing and thawing in permafrost regions and reveal its water and heat change mechanism.Based on the national key R&D plan project "Technology and Mode of Efficient Utilization of Groundwater and High-quality Mineral Water Resources(No.:2019YFC0409103)",this study selected dark brown soil and albic soil with the largest distribution area in Changbai Mountain as typical areas,and systematically carried out the research on water and heat transfer during freezing and thawing in the study area.The freezing and thawing process is divided based on the real-time monitoring data of soil moisture and temperature in the Changbai Mountain area.The spatial variation law of soil moisture and heat in different freezing and thawing stages is analyzed.According to the characteristics of water and heat change during field freezing and thawing,indoor soil freezing and thawing experiments were designed to further reveal the mechanism of water and heat migration during soil freezing and thawing.On this basis,combined with the natural environment and soil characteristics of the study area,a numerical model of water and heat transfer in the study area was established,and the temporal and spatial migration and evolution of soil water and heat and the recharge characteristics of groundwater under different scenarios were studied.The main achievements are as follows:1.Based on the field soil water and heat monitoring and field investigation,the soil freeze-thaw process in the study area was divided into stages,and the characteristics of the soil freeze-thaw process in the study area were clarified.It is the key to understanding the characteristics of soil freeze-thaw and its influencing factors to divide the freeze-thaw period considering the seasonal freeze-thaw process.According to the changes of air temperature and soil temperature and humidity in the study area,the soil freezing and thawing process in the study area is divided into four stages: unstable freezing period,stable freezing period,unstable melting period and stable melting period.In the process of soil freezing and thawing in the study area,soil temperature and water content are significantly affected by the atmospheric environment in the shallow range of 0-70 cm,and the complexity of dynamic changes in temperature and water content is weakened below 70 cm deep.The depth of soil freezing in the study area is about 0.9m,and the frozen depth of soil in different locations in the area is basically the same.2.Based on the distribution of soil in the study area,the influence of soil types on water migration during freezing and thawing is clarified by comparing the freezing and thawing characteristics of different soil types.There are nine types of soils in the study area,of which dark brown and albic soil are the most widely distributed.Dark brown soil accounts for 53.6%of the soil distribution area and has good permeability.Albic soil accounts for34.46% of the soil distribution area,with a dense soil layer and poor permeability.The redistribution of soil moisture before and after freezing and thawing is significantly affected by soil texture,and soil permeability affects the supply of soil moisture from the upper part to the lower part during melting.After freezing and thawing,the water content in the upper soil of albic soil increased by about4% compared with that before freezing and thawing,and the water content in the upper soil of dark brown soil decreased by about 4% compared with that before freezing and thawing.There is a significant difference in water storage between albic soil and dark brown soil before and after freezing and thawing.3.Through indoor freeze-thaw experiments,it is revealed that water migration in the soil freeze-thaw process is influenced by external temperature and its variation range,soil layer temperature,and soil permeability.The greater the temperature variation,the more significant the pressure difference between the upper and lower soil layers,and the faster the water migration quantity and speed.Low permeability soil dramatically affects the migration of frozen meltwater,which makes water accumulate in the upper part of the soil.4.A long-term numerical model of soil water and heat transfer across the soil freeze-thaw period is established using SHAW simulation software.The spatial change of soil moisture and its downward recharge process under different precipitation guarantee rates in the study area are analyzed.Under the influence of freezing,thawing,and precipitation,the vertical recharge process of groundwater in seasonal frozen soil areas is time-varying,and its main controlling factors change in different periods.In the non-freezing period,groundwater is mainly supplied by infiltration of atmospheric precipitation;During the freezing and thawing period,groundwater recharge is mainly affected by soil’s freezing and thawing process.There are two primary sources of soil moisture leakage and recharge to the deep layer in the study area during the year: precipitation recharge in the non-freezing period and frozen layer and snow melt water recharge in the stable melting period.With the increase of precipitation guarantee rate,the proportion of soil water infiltration in the stable melting period gradually increased in the total infiltration amount in the year.The contribution rate of water infiltration in this period was 5.2% in extremely wet years(the precipitation guarantee rate was5%).However,it could reach 33.8% in parched years(the precipitation guarantee rate was 95%).Melting water and snowmelt water in a frozen layer during a stable melting period is essential to recharge sources for groundwater.The research results enhance the understanding of water infiltration process during soil freezing and thawing in seasonally frozen areas,which provides an important basis for the study of groundwater resources replenishment in seasonally frozen areas,contributes to the protection of surface ecological environment,and also provides a scientific basis for agricultural production land consolidation in the study area.