Numerical Simulation Of Seasonal Freeze-thaw Black Soil Water Heat Transfer Under Different Snow Cover

Snow cover, as the most common upper boundary conditions during freezing and thawing period in the cold region of northern China, directly affect the soil moisture and heat distribution and migration, soil temperature and frozen depth, freezing rate, etc. Through field test, we dynamically observed soil moisture and temperature variation of seasonal frozen region of Harbin under different mulching conditions(bare land, natural snow, thickened snow, compacted snow). According to the field experimental data, comparative analysis and statistical methods to study were used to study variation law, snow thickness and snow variation, soil temperature, total water content and liquid water content, the effect of snow cover with different snow cover density and thickness on soil temperature and moisture content. SHAW model was used to simulate seasonal freezing thawing process of soil under different snow cover conditions. The change of water content and temperature in different depth from time and space were analyzed. The main conclusions are as follows:(1) In certain cases total snow, snow melting rate is closely related to temperature, solar radiation. For different thickness and density of snow, the difference of the ablation time is small, and the thickness of snow cover is weak, the density and thickness of snow had little effect on the ablation rate. Due to the low thermal conductivity of the snow, and snow cover on the soil has a good buffer, adiabatic and keep moisture role. In the presence of only the subsidence caused by snow density, the greater the thickness of snow, the better the protection effect, the response of soil moisture to temperature and the time of thawing of soil were delayed, and the degree of delay increased with the increase of soil depth; when the people change the density of snow, compared to simply increase the thickness of snow, denser snow can better protect the soil, and make the effect of temperature smaller. When the winter snowfall is small, you can consider using artificially compacted snow, strengthen the protective effect on soil. The snow can slow down the effect of soil freezing, affect soil moisture redistribution. In the soil freezing process, liquid water will migrate to the freezing front, due to freezing moving more slowly in the stable freezing stage, the freezing front at the total soil water content increased, water accumulation phenomenon is obvious.(2) When there is no snow cover, the porous medium have larger effect on soil temperature. When there is snow cover, the large specific heat capacity of snow cover and its absorption of the energy result in the decrease of the freezing period of the ambient temperature; during stable freezing period, The existence of snow saved energy for the soil, and the soil temperature was relatively high; during the thawing thawing stage, the snow cover has some reflection and storage effect, so the temperature rise of the soil is slower under the snow cover. Snow cover is isolated from the energy exchange between the soil and the environment, in the overwintering period maintained soil temperature, at the same time, the processing of natural snow, thick snow, compacted snow soil temperature curve in turn become gentle. The heat preservation effect of the compacted snow cover is the most obvious, which is superior to the thick snow cover, and it shows that the reflection, storage and heat conduction of snow cover to the energy of the snow density is greater.(3) The SHAW model can well simulate the seasonal freezing thawing soil water heat coupling migration process, accurately simulate the dynamic change of soil temperature in different snow cover conditions, the simulation of the change law of soil moisture content has also obtained good results. When changing the parameters of snow density and snow thickness, there will be uneven density and make the simulation error increases, but the variation of simulation results and the measured value is consistent.In summary, through systematic research of snow cover under different condition and simulation of variation of freezing thawing soil water heat based on the SHAW model, the results revealed the law of soil water heat coupling movement under different snow cover condition, described the dynamic changes of soil moisture and heat during freezing thawing process. The results has important theoretical and practical significance for improving the utilization efficiency of soil water and heat resources in winter season and making water conserving irrigation scheme, it can also provide theoretical support and scientific basis for the research on the use of surface water, groundwater resources, agriculture and environment and water conservation.