Simulation Of Water-heat Transfer In Seasonal Freeze-thaw Loess Under Different Vegetation Restoration Modes

The hilly and gully area of the Loess Plateau is one of the main areas of freeze-thaw erosion in my country.It generally has a deep and long-lasting seasonal frozen layer.Under the background of the ecological construction of returning farmland to forest and grassland in the loess area,various vegetation restoration methods have been carried out.The study of the water-heat transfer characteristics of soil during freezing and thawing has important theoretical significance and practical value for revealing the mechanism of freeze-thaw erosion and the control of regional soil erosion.This study takes the Xindian Yulin Gully in the Wuding River Basin of the Loess Plateau as the research object,and uses outdoor in-situ monitoring,indoor analysis and numerical simulation to systematically analyze the water-heat migration process of seasonal freeze-thaw loess under different vegetation restoration methods..Based on in-situ monitoring data,the dynamic changes of soil temperature of different vegetation types in the study area were analyzed,and the temporal and spatial distribution characteristics of soil unfrozen water content in different vegetation restoration methods were revealed,and the water and heat transfer of unsaturated loess based on freezing and thawing was established.Numerical model of the process.The main research results obtained in this paper are as follows:(1)The freezing depth of seasonal freeze-thaw soil is different under different vegetation restoration modes.The maximum freezing depth of each sample plot was as follows:leisure plot=arbor<shrub and grass mixed plot<agricultural plot=shrub mixed plot.The soil freezes downwards in one direction during the freezing period.In the process of melting,the frozen soil of mixed agricultural land,fallow land and shrub grass mainly melts forward,while the frozen soil of mixed shrub land and tree mainly melts backward.(2)The spatial and temporal variation of unfrozen water content at different soil levels was different under different vegetation restoration methods.The variation degree of unfrozen water content of the five sample plots was the largest in the 10cm soil layer.The unfrozen water content of the mixed soil of agricultural land,fallow land and shrub all reached a weak variation in the 70-100 cm soil layer.The unfrozen water content of the mixed soil of shrub and grass showed a weak variation in the 60-100 cm soil layer,and the unfrozen water content of the arbor soil reached a weak variation in the 30-100 cm soil layer.Moreover,the change of unfrozen soil moisture content of trees is much more stable in the whole seasonal freeze-thaw period.(3)the relative difference method is used to describe the soil is not the time stability of the frozen water content characteristic,get the relative difference method is not applicable to shrubs mixed,bringing their mixed and trees,but for the shallow not frozen water content and the average soil not frozen water content for regression analysis will be the best representative soil layer,and for bringing their mixed not frozen soil water content forecast is more accurate.After redundancy analysis of soil unfrozen water content and environmental factors in five plots,it was found that the main controlling factors of soil unfrozen water content under different vegetation restoration methods were different in the seasonal freeze-thaw period.(4)Using the HYDRUS-1D model to simulate the change process of soil temperature and moisture during the whole seasonal freeze-thaw period,the analysis shows that:In terms of soil temperature,the simulation effect of the direct inversion module is relatively stable,and the relative error is small.The simulated soil temperature can reflect the variation trend of soil temperature in different areas.After adding freeze-thaw modules,the simulation results are not stable.Among them,the fitting coefficient of soil temperature decreased with the increase of soil depth,while the shrub mixture and trees were small in the middle of the shallow layer(20?30 cm),and the simulation effect was relatively good at the top(10 cm)and the bottom(40?50 cm).In terms of soil unfrozen water content,the simulation results are obviously worse than that of soil temperature.The simulation effect of farmland as a whole was better than that of shrub mixing and trees.The simulation effect of trees as a whole was general,but the simulation effect of soil unfrozen water content at the top(10 cm)and the bottom(90?100 cm)was better than that of the middle.This simulation result was different from that of farmland and shrub mixing.