Coupled Heat-moisture Process Of Different Landscape Soil Under Soil Freeze-thaw Cycles On Middle Reaches Of Heihe River Basin

In our country, arid region has obvious characteristics, from the mountain glaciers, foreststeppe to plain oasis and the Gobi desert formed a compound ecological system. In water wasa terrible shortage of arid zone, any form of water supply is likely to bring positive influenceto local ecological system. At the same time, the interference of human activities increases theparticularity and the complexity of ecological and hydrological processes. Due to the harshenvironment, most of such areas are semi-open or undeveloped. As the population increasingrapidly and the water shortage growing sever, the development of arid regions becomes moreand more urgent. Therefore, the research of soil water and heat in frozen soil area will drawmore attention in our society. The research on permafrost regions began in the50s to60s ofthe last century. With the development of scientific technology, as well as the graduallyincreased researchers, the monitoring method of soil heat and water in China is becomingincreasingly accurate, and the research systematic.This study selected three most representative landscape units in the middle area of Heiheas the research objects, aiming to figure out the coupling process of soil water and thermalunder alternating freeze-thaw through observing the changing process of soil water andthermo in the three different landscapes collected by the monitoring devices in the fields. Atthe same time, collected soil samples and conducted the indoor simulation experiment tofurther explore the changes of soil hydraulic properties in the process of freezing and thawing.Finally, according to the field monitoring and indoor simulation results, the water movementfeatures were preliminarily made under different landscape units; At the same time, analyzedthe field capacity and the effectiveness of soil moisture, further providing the theoreticalsupport in exploring new ways of enhancing soil moisture. The results were as follows:1. Throughout the study period, obvious phenomenon of soil freezing and thawingoccurred in the farmland, shelter forest and desert landscape units, and soil freezing andthawing process can be roughly divided into freeze stage, freeze stability stage and meltingstage. In the freezing and thawing process, soil was frozen step by step down from the surface,but when the soil thaw, it occurred from the surface and the bottom to the middle part.2. Soil water content, soil temperature, soil freezing depth and underground water level changed correspondingly with the undergoing freezing and thawing process, and the changetrend of each factor showed certain regularity and correlation. The freezing and thawingprocess can effectively reduce soil moisture evaporation and leakage; poly quantity after the"frozen" phenomenon is very obvious, which was crucial to hold soil moisture and the growthof plants.3. During the freezing and thawing process, the different landscape showed obviousdifference. Farmland had the highest frozen level followed by the forest land and the desert.High intensity of soil freezing were characterized with lower soil temperature, deeper freezingdepth and longer time to freeze. This is mainly because the farmland and woodland’s initialsoil water content was high, so the thermal conductivity of the soil was significantly higherthan that of desert, which made the soil heat transferred more easily. At the same time,influenced by the fallen leaves on the surface, soil freezing and melting time of the forest waslater than that of the farmland; the freezing intensity was lower than that of the farmland.4. During the freeze-thaw process, soil bulk density changed. For the soil with highersoil bulk density, its soil bulk density decreased gradually with the times of freezing andthawing increased, and vise versa. No matter high or low bulk density, after certain timesfreeze-thaw process, the change of soil bulk density remained stable. Soil saturated watercontent was also affected by freeze-thaw alternation. Due to freeze-thaw process, the soilporosity increased significantly, causing soil saturated water content increased obviously. Butafter certain times of freezing and thawing, the increase rate decreased, and finally remainedstable.5. Long-term cultivation is beneficial to improve the structure of soil, increase soilerosion resistance, and make it more suitable for planting. During the freezing-thawingprocess, soil aggregate size changed obviously. Mainly embodied in the graded aggregatelarger and smaller to medium size aggregate micro aggregate has the direction of development,lead to improve soil structure, enhance the water stability, and additional water erosionresistance; Freeze-thaw effect of graded larger aggregate influence more dramatic. In addition,with the change of the soil aggregate, the mineralization rate of organic carbon in soilchanged significantly, at the same time, the organic carbon component is changed, and withthe soil organic carbon stability decline, thus increasing the carbon emissions in the soil.6. During the process of freezing and thawing, the saturated hydraulic conductivity ofthe farmland and desert first decreased and then increased, while the saturated hydraulicconductivity of the shelter forest appeared increase first and then decrease, reflecting theprocess of compaction. With the times of freezing and thawing increased, after6cycles (144h)all change trends appeared turning point; by influencing the soil hydraulic conductivity, the process of freezing and thawing also had a certain impact on soil water infiltration capacity.7. Freezing and thawing alternation was beneficial to improve the water-holdingcapacity of the soil. In the meantime, the freeze-thaw processes narrowed down the scope ofthe distribution of soil moisture, reducing the efficiency of soil water, which has a highpractical significance for the water serving as the dominant factor for the region.