| The Badain Jaran Desert is the second largest desert in China,with hundreds of lakes and huge dunes,attracting the research interests of scholars at home and abroad.The desert is located in the arid region,the annual average precipitation in the desert is less than 100 mm,while the annual potential evaporation is more than 1200 mm.Therefore,only the continuous discharge of groundwater is able to maintain the existence of the lake.The annual average temperature of the Badain Jaran Desert is around 10?C,so the desert is in the seasonally frozen zone and the freezing period is from December to March of next year.When predecessors studied about seasonally frozen soil in the shallow groundwater zone,it is generally believed that the freezing of soil water will enhance capillary suction,leading to groundwater migration upward,thus causing decline of groundwater level.Shallow groundwater also exists in the lakeside of Badain Jaran Desert.However,according to the observation data,the groundwater level does not decrease but rise during the freezing period.This paper is mainly aimed at this exceptional phenomenon to start the research.The study site of this thesis is Sumu Jaran lake area in Badain Jaran Desert.The study team sited an observation station,including an automatic meteorological station,lake water level monitoring probes,groundwater observing wells and soil water dynamics monitoring systems.In this study,meteorological data,soil water dynamics data,groundwater data and lake level data from 2016 to 2017 were collected and analyzed.According to the observation results,it was determined that the freezing period was from mid-December in 2016 to mid-February in 2017,and the maximum freezing depth of seasonally frozen soils was 33 cm,which was far less than the average maximum freezing depth in the same latitude regions.In this study,a numerical model of water vapor migration coupling wth phase transition and heat transport in the vertical one dimensional saturated – unsaturated zone was established with the program SHAW.We simulated and analyzed the freezing and thawing processes of soil water in the observed station and itsrelationship with groundwater.The simulation results show that the soil moisture increment per unit area is 0.64 cm,which is able to cause 5 cm groundwater level decline under the closed condition.It indicates that the rise of measured groundwater level in the frozen period is caused by external recharge around the lake.By the simulation of soil freezing and thawing processes under the different groundwater table conditions,it was found that the extinction depth of freezing induced groundwater table decline was 85 cm.Consistent with 2081-2100 temperature change and CO2 trend in the future,we simulated the periodic soil freezing and thawing processes in RCP2.6,RCP4.5,RCP6.0,RCP8.5 four kinds of climate scenarios.We predicted that the freezing period will decrease to 53 days,49 days,57 days,34 days and the maximum freezing depth will decrease to 31.0 cm,28.3 cm,26.9 cm,20.6 cm in 2100 in the study site.In this paper,the relationship between the change in soil water and groundwater in the freezing period of the Badain Jaran Desert is studied from a new angle,which we can be used as a reference for further research. |
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