The Change Of Soil Water Environment And Vegetation Construction In Guanzhong Plain

Aimed at the phenomenon of dried soil layer existing in loess plateau and the doubt to the results of vegetation construction by someone, a series of questions such as the determinant criterion, the formation mechanism, the distribution, the renew condition of the soil dried layer in loess plateau, the time-space diversity of the soil dried layer forming in difference area of Guanzhong Plain, and the goals of vegetation construction, had been discussed based on the data of soil moisture content obtained from the samples which sampled by the light manpower drill and measured by electronic balance. The main achievements and new cognition are as follow:1. The determinant criterion of soil-dried layers has been proposed firstly from two aspects of soil water supply and vegetation water demand, and the soil-dried layer has been taken into two difference types according to its restricting extent to vegetation growth. One is the dried soil layer, which range of soil moisture content is from soil stabilization humidity to soil elementary languish humidity, and indicate the difficulty to meet demand of soil water in vegetation growth. Another is the soil drought layer, which range of soil moisture content is from soil elementary languish humidity to 0 theoretically. We named weak soil drought layer that range of soil moisture content is between the elementary languish humidity and stabilization languish humidity which indicate the vegetation growth had been stress by lack of soil water, and appeared languish but can growth, and named the strong soil drought layer that range of soil moisture content is below the stabilization languish humidity which indicate the vegetation growth had been stress strongly by lack of soil water, and appeared languish, withered and died.2. The data, which obtained from measure of soil moisture content in Xianyang and Xingping area, indicate that there are soil-dried layer developing obviously under the manmade forest in middle ages and older, and illuminated that the phenomenon of soil -dried layer has spread to the southern loess plateau, Guanzhong plain, which nature conditions are the best in loess plateau, in the low water years and the average years. More study on Baoji city, Wugong country and Weinan city showed that the growthdegree of soil-dried layer has an enhanced trend gradually from western to eastern in Guanzhong Plain. The disappeared of soil-dried layer in abundant water year in Guanzgong plain indicate that the renew of soil-dried layer under the condition of more than 800mm in annual precipitation has been realized.3. Deduced and Confirmed the infiltration depth of gravity-capillary water at a loess profiles in Guanzhong plain is 200cm or so in usually, 300-350cm in the extreme, by use of the theory of depth in chemical composition transfer and the experimentation of infiltration depth of gravity-capillary water. Concluded that violence evaporation depth of air is 200cm or so in usually, 400cm in the extreme, and the water consumed depth of vegetation growth is more than 500cm, but its violence used depth is 400cm or so, by analysis the air evaporation and pattern of root system distribution.4. The condition of forming the soil dry layer has been proposed definitely that exist of the membrane water hosepipe is the necessary condition and the bury depth of vegetation and gas spending soil water is deeper than depth of gravity-capillary water infiltration, which are the sufficient condition for forming the soil dry layer. In loess plateau district, the moisture content in the membrane water hosepipe is lowness, and its transfer speed is very slow. Those make the membrane water hosepipe become the direct moisture factor of the development of the soil dry layer in this area. The facts we discovered that water retrieved in the membrane water hosepipe is very slowly once it's consumed. The thickness of the membrane water hosepipe and the gravity-capillary hosepipe is complementation. In the same soil profile, the thicker the gravity-capillary hosepipe, the thinner the membrane water hosepipe.5. The bio-climatic mechanism of soil dried layer forming has been proposed. The decrease of precipitation, which conduced shoaling the infiltration depth of gravity capillary water, and made deeper the depth of evaporation of soil and vegetation at the same time, results in climatic drought. On the other hand, the adaptation to climatic drought of vegetation is to develop the vegetation of perpendicularity spread root system pattern and to restrict the level spread pattern, which made deeper on the depth of soil water consume by vegetation. Integrated three factors above, the soil-dried layer must be formed once the depth of water use is deeper than the infiltration depth of gravity-capillary water, and the forming degree of soil-dried layer is stronger with the development of drought. So, the exist of soil-dried layer is respond of soil moisture...