Study On Movement Mechanisms Of Water And Heat Under Freeze-thaw Cycles In Northwest Sichuan Plateau

The permafrost and seasonal frozen soil are widely distributed in the high cold regions of northwestern Sichuan province.However,the mechanisms of soil water and heat status during freezing thawing and their varied rules in the process of grassland degradation and soil desertification have still not been clearly illuminated.In this study,the characteristics of soil physical properties were simulated before and after freezing thawing through repeated freezing-thawing and long time freezing-thawing of sandy grassland and natural grassland soil.The characteristics of soil hydro-thermal migration were detected before and after freeze thawing,and the model quantitatively depicted the law of soil hydro-thermal movement,which have i mportant practical significance of determining the technical parameters of soil water and heat regulation during freezing thawing periods reasonably.The related results are meaningful for improving soil water productivity and promoting the sustainable dev elopment of regional economy and ecosystem in the northwest of Sichuan province.(1)Through different freeze-thaw cycles,the soil physical properties of natural grassland and sandy grassland were substantially different.After continuously repeated freeze thawing and long-term freeze thawing,the content of clay particles decreased in the soils of natural grassland and sandy grassland,but the contents of grain and sand increased.Especially,the content of sand in the sandy grassland soil layers was higher than that of natural grassland.Compared with before freeze thawing,the soil bulk density decreased slowly after freeze thawing.The capillary porosity and total porosity decreased,and the non-capillary porosity increased both in natural grassland and s andy grassland.Capillary porosity decreased with the increase of soil depth,and the non-capillary porosity increased with the increase of soil depth.Soil water holding capacity of sandy grassland and natural grassland decreased.With the increase of water suction,the soil water characteristic curve showed the trend of "rapid descent-slow descent-basically smooth",and the curves were similar in the shape,and the varied amplitudes were more consistent with each other.(2)Under different freeze thawin g time,temperature and frequency,the changes of moisture content and temperature at different depths of soil were investigated.The results showed were as following:After continuously repeated freeze thawing.There were some time lag between the each layer of soil moisture appeared swell and the wetting front went through,and the terminal time of reaching stabilization showed a trend of short-phase interval both in the sandy grassland and natural grassland.After a long period of freeze thawing,there were some differences between the soil water content in each layer abruptly increased and abruptly decreased,and the time of the soil water content in each layer abruptly decreased after freeze thawing were earlier than that before freeze thawing.Soil temperature showed a slowly decreasing trend before and after freeze thawing,while the soil temperature showed a fluctuating trend after freeze thawing.Natural grassland showed a slowly increasing trend before freeze thawing,but the temperature increased after freeze thawing.There was not obvious rule of soil temperature change in different layers.After long-term freeze thawing,the soil temperature of each layer of sandy grassland increased with the increasing of environmental temperature,and the soil temperature of each layer of natural grassland decreased with the increasing of environmental temperature.(3)The mathematical model of soil Hydro-thermal movement during freeze thawing cycle was constructed,and the Hydrus-1D software was used to solve the mathematical model.The mathematical model of Soil Hydro-thermal movement was determined and verified by the model.The results showed that: the continuously repeated freeze thawing cycles and long-term freeze thawing significantly affected on soil volum etric water content and did not impact on soil temperature.The soil volumetric water content decreased with increasing the soil depth from March to May,and increased with the increasing soil depth from June to September.The volumetric water content of n atural grassland is from March to September,and soil volumetric water content decreased with the increasing soil depth.The soil volumetric water content of natural grassland decreased with increasing soil depth from March to September.The soil temperatures in each layer of sandy grassland and natural grassland were the same as the annual change of temperature.The greater the temperature change from the surface to the surface,the highest soil temperature in each layer of natural grassland was slightly l arger than the soil temperature in the sandy grassland.