Response Of Soil Freezing-thawing Process And Hydrothermal Change Of Shallow Soil In The Northwest Seasonal Frozen Soil Region To Climate

In arid and semi-arid areas of Northwest China,the physical and mechanical properties of shallow soil will be greatly changed by local climate,and the sta bility of soil structure will be affected,resulting in a series of Engineering problems.The strong water-heat exchange and interaction between atmosphere and surface is one of the main causes of the engineering problems.Therefore,the analysis and discussion of the law of water-heat transport under climate change conditions can reveal the mechanism of water and energy changes in soil,which has c ertain guiding significance for the geological hazards encountered in engineering.In this paper,taking the Loess Hilly and gully area in the western Weihe River Basin of the Loess Plateau as the research area,the field observation site was established,and the response of the freeze-thaw process of shallow unsaturated soil and the law of water and heat change to climate change in this area was studied and analyzed.Based on the water and energy balance equation and meteorological data,a one-dimensional numerical model was established to simulate the soil temperature and water content of the observation points in the study area.The main conclusions are as follows:(1)Soil temperature is positively correlated with air temperature,which can be divided into heat absorption stage in spring and summer and exothermic stage in autumn and winter.The influence of air temperature on soil temperature decreases with the increase of depth.The amplitude of ground temperature decreases exponentially with the increase of depth,and the phase of temperature wave lags behind with the increase of depth.The changing speed of s oil temperature at different depths decreases with the increase of soil depth,and the cooling process is generally slower than that of warming process.(2)The maximum frozen depth of bare surface soil in this sthudy area is about 20-50 cm;the frozen coefficient of soil at different depths decreases with the increase of soil depth,but the thawing coefficient is opposite;during the process of soil freezing,the unfrozen water of deep soil gradually migrates to the frozen layer,resulting in the gradual decrease of deep water content;during the thawing,the relative frozen water content of each layer increases;The daily minimum temperature at 5 cm below the surface occurs between 8:00 and 9:00 in the morning,and the daily maximum temperature appears near 16:00 in the afternoon.The precipitation will cause a significant increase in the water content of shallow soil.The water content within 50 cm below the surface is obviously fluctuated by precipitation,and the water content at 350 cm is almost unaffe cted by precipitation.(3)Soil water content and temperature change in a similar way,and their water content and temperature interact and restrict each other,controlling the hydrothermal stability of the whole soil.With the increase of soil depth,the range and coefficient of variation of soil water heat in different seasons showed a decreasing trend,and the degree of hydrothermal variation of shallow soil is more intense.(4)Based on the equation of water and energy balance,a one-dimensional soil hydrothermal migration model is established,combined with the theory of near-surface atmospheric dynamics,the boundary conditions of water are driven by meteorological data such as precipitation and evaporation,and the numerical model is established by finite element software,and the above model is solved.Combined with its field monitoring data,the accuracy of the model is evaluated quantitat ively by using relative root mean square error and average relative error,and the results show that the measured values of soil temperature and water content are consistent with the change trend of simulated values,but there are some errors in shallow so il,and the simulation effect of deep soil is good,and its model still has certain reliability.