Study On Water-Heat Coupling Movement And Numerical Simulation In Snow Cover-Soil Union Of Seasonal Frozen Soil Region

Snow cover is a widespread and relatively stable coverage type on soil surface during the winter of vast areas of northeast, northwest and southwest China. Snow cover and seasonal freezing-thawing soil need to be studied unitedly for water movement and heat transfer of seasonal frozen soil have distinctive properties under the coverage condition of snow cover. Movement, storage and transformation of water and heat in snow cover and soil union are the important link of nature water cycle, play an important role in the systems of agriculture, water resources and environment and have significances to agricultural production. However, studies on water-heat movement in seasonal frozen soil under snow cover still have many problems at the aspects of theory basis, technical method, practical application, etc. to be solved, particularly in the studies towards agriculture.Aiming at the problems in the studies on water-heat movements in seasonal frozen soil under snow cover and combining with the project"Study on farmland soil water-heat coupling movements and numerical simulation under snow cover"(No.RC2010XK002013) subsidized by Harbin Science and Technology Innovation Talents Research Special Funds, the thesis taking the water and heat in snow cover and soil union as study objects, studied the change laws, mathematical model and numerical simulation of water-heat coupling movements in the union by using the methods of field experiments and theory analysis.The study contents, specific methods, analysis results and conclusions of the thesis are as follows:(1) On the basis of the data measured in field experiments, some basic topics were studied by using comparison and statistics methods, including the driving force of water movement in seasonal frozen soil, change law of soil surface temperature during seasonal freezing-thawing process, correlation between soil surface and profile temperature, variations of soil profile temperature, total and liquid moisture and the influence of snow cover on soil temperature and moisture. The results indicated that water in seasonal frozen soil moved along the direction on which soil-water potential decreased, soil surface temperature had high correlation with air temperature and strongly affected surface soil temperature, daily average soil surface temperature assumed an upward opening parabola while its daily changes have various laws in different periods, the changes of frozen soil total and liquid moisture have high correlations with soil freezing-thawing process, soil freezing-thawing action redistributed soil water, snow cover blocked the heat exchange between soil and atmosphere and affected surface soil heat condition significantly, but only in thawing period have significant effects on surface soil moisture. All the above results and conclusions can provide an important basis for making reasonable hypothesis, simplifying initial and boundary conditions, ascertaining water-heat movement parameters, etc. of model establishment and numerical simulation.(2) Spatial variation structures and distributions of soil profile total and liquid moisture during freezing-thawing process were studied according to semivariance and spatial distribution analysis of seasonal frozen soil water using geostatistics. The results indicated that seasonal frozen soil total and liquid moisture have good spatial structures and correlations, freezing-thawing process affected the spatial correlations and ranges of the two type soil moisture, the spatial distributions of the two type soil moisture have good laws affected by freezing-thawing process, after thawing the spatial distributions of surface soil moisture have major randomicity. Geostatistics can effectively characterize the spatial variation of seasonal frozen soil water and provide a new way to study the movement mechanism of frozen soil water.(3) Seasonal frozen soil water-heat coupling movement model was established based on water-heat movement mechanism, conservation of matter and energy, Darcy's law, heat conduction theory and some hypotheses. Then the process was simulated numerically using finite difference method and the results were tested. The results indicated that simulation values have some error with the measured values, but the average relative error between them is small and they have the same trend. Therefore, the seasonal frozen soil water-heat coupling model established in the thesis was reasonable. The methods of parameter confirmation and numerical simulation were reliable.(4) One-dimension snow cover and soil union water-heat movement model and parameterization scheme were established based on water-heat movement mechanism and previous research results. Then numerical simulation analysis and test were conducted. The results indicated that the simulated values have major error with the measured values, but the absolute error between them can be accepted and they have similar trend. Thus, the one-dimension snow cover and soil union water-heat movement model and parameterization scheme need to be improved further.To conclude, the study on laws, model and numerical simulation of snow cover and soil union water-heat coupling movement in seasonal frozen soil region can not only reveal the soil water-heat movement law with snow cover and characterize the water and heat conditions during soil freezing-thawing process, but also provide theory support and science basis for effective utilization of soil water and heat resources, evaluation of surface and underground water, soil and water conservation, environmental protection, etc. in seasonal frozen soil region.