| In seasonally frozen soil regions,climate,topography,soil structure and other factors will affect the water and heat transfer process in the shallow unsaturated soil,so the phenomena and mechanisms presented are extremely complex.In recent years,with the efforts of scholars at home and abroad,the research on the seasonal frozen soil freezing and thawing process as well as the water and heat transport in the freezing and thawing period has made great progress,but these studies are mostly concentrated in Shanxi,Inner Mongolia,Northeast China and Xinjiang provinces.There is a lack of relevant research specifically for the Loess Plateau.At the same time,the water and heat transport during the freezing and thawing of the shallow soil in the seasonally frozen soil area is closely related to the external climate change.However,there are few studies on the interactio n between water and temperature during the freezing and thawing process of unsaturated soil in seasonally frozen soil area,the influence mechanism of rainfall on soil water and heat transport,and the prediction of soil water and heat response in the futu re warm and humid background,which need to be improved.This paper selects the loess hilly and gully area in the west of the Weihe river basin on the Loess Plateau as the main working area.Conduct field observations by setting up field monitoring points t o study the effects of meteorological factors on the freezing-thawing process and the process and mechanism of hydrothermal migration of shallow soil,and analyze the change law of water and heat transport and the interaction mechanism between temperature and moisture during the freeze-thaw and dry-wet cycles of shallow unsaturated soil under atmospheric action.Based on heat transfer theory and unsaturated soi l seepage theory,using temperature and moisture as basic variables,combined with energy conservation equation and mass conservation equation,a water-vapor-heat coupling migration model considering water phase transition,water convection heat transfer and water vapor diffusion latent heat was established.On the basis of formulating the parameterized scheme,it was solved using finite element software,and the rationality of the model was verified by the measured data in the study area.Based on this model,the water and heat changes in the soil in the seasonally frozen soil are predicted under the climate model of warming and humidification in the future.The research results are expected to provide theoretical support for geotechnical engineering design and disaster prevention and mitigation in seasonally frozen areas,and are also expected to provide new ideas and methods for engineering design and construction in the Loess Plateau.The main conclusions are as follows:(1)The influence of air temperature on shallow soil is significant,and the change of ground temperature at each depth lags behind air temperature,which becomes more obvious with the increase of soil depth.During the freezing-thawing process,the temperature and water content present a coupling characteristic,and the variation trend is basically consistent.The temperature change in the soil will cause the redistribution of water content,and the change of water content also has hysteresis.The greater the depth,the more obvious this hysteresis.(2)The soil water content shows a "low-high-low" change trend with the increase of depth,forming a "water-rich area" at a depth of 120 cm,but as the depth continues to increase,the soil water content begins to decrease gradually.However,short-term rainfall only affects the temperature and water content of the shallow soil,and will break the coupling characteristics of temperature and water content in a local range,making it show a reverse change trend,while the deep soil will not be affected by rainfall in the short term.(3)There is no significant fluctuation in the temperature w ithin 30~90 cm of soil before and after rainfall.The increase and decrease of the temperature are relatively gentle,and the dramatic changes in the ground temperature are mainly manifested in the surface layer of 0~30 cm.The water content of shallow soi l presents a changing pattern from low to rapid increase and then to rapid decline in time series;This process is more vigorous in shal low soils,and the duration is shorter,and as the soil depth increases,the severity will gradually decrease,the duration will become longer and longer,and there will be a certain degree in time delay.(4)The increase of air temperature will reduce the water content of soil,so that the moisture component related to the temperature gradient increases,and the moisture component related to the water potential gradient decreases,and the water vapor flux of the water gradient may have a negligible effect on water transport;The influence of the field is significantly greater than that of the moisture field,and th e range of influence is also deeper;when the temperature increases,the heat conduction flux,latent heat of water vapor diffusion and water vapor convective heat flux in the soil will increase,while the liquid water convective heat flux will decrease.Heat conduction is dominant in soil energy transfer,but other energy components are also considerable during the warm season,and should be considered when performing energy transfer analysis.(5)The increase in annual rainfall will cause the soil temperature to decrea se and the internal water content to increase slightly,and the impact on liquid water convection is greater than that of water vapor.Humidification will cause the soil heat flux at the surface to decrease.The internal soil heat conduction flux,water vapor convection heat flux,and water vapor diffusion latent heat will all decrease,but the liquid water convection heat flux will increase.The decrease in heat conduction flux due to increased rainfall is greater than the increase in liquid water convection heat flux.The impact of increased rainfall on water transport and energy transfer gradually decreases with increasing soil depth. |
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