| The loss of soil and water and the desertification of land are among the most outstanding ecological and environmental issues, to which the deforestation, opening up wastelands and cultivating land on steep slopes contribute greatly. Therefore, to carry out the policy of quitting cultivation and turning to reforestation, and of protecting and constructing the ecological environment of the West China strictly and completely will definitely make differences to the situation in which the ecological environment is increasingly worsening. This has become an urgent mission which has a great hold over the carry-out of the strategy of the West development, and over the construction of the modernization of China even the future of China.The policy of reforestation is the key to contain the loss of water and soil on the loess plateau, and turn the situation for better. And the principle is to exploit the land in a scientific way, in which the practical theories and suitable tree breeds are highly considered. The following are the main conclusion drawn based on the observation on the water content of the soil of Nihegou drainage area and the study on potted plants on campus.1. The basic characteristics of the water content of the soil taken from the drainage area(1). The variation of space1) The water content of soil in the center of the plateau is higher than in the margin; 2) the variation of the water content observes the following rule: water content of the soil on the northern slope >water content of the soil on the flat ground> that on the southern slope; 3) the water content varies this way in different parts of the river: the water content of the soil between the upper and the middle steam is comparatively lower and is higher between the middle and the lower stream.(2). The variation of timeAs far as the water content of the soil in the two years is concerned, the variation is not obvious except those in the soil layers on the surface varied greatly in different seasons which decrease in May, increase in September and reach the highest in October.2. Analyses on the influential factors of the water content of soil (1) The influence of rainfall on the water content1) The water content is relatively low because the rainfall is relatively low with the rise of the temperature and the increase of the evaporation; 2) after that, the content begins to increase with the more and more rainfall; 3) the content reaches its maximum in October for in August and September the rainfall is plenty and the temperature begins to fall.(2) The factor of geographyThe content of the soil at the foot of the hill>that on the plateau for>that on the slope while on the slopes it observes such a rule that the content of the soil at the hill foot of northern sidothat on the top of the northern side>that at the foot on the southern side>that in the middle of the northern side>that on the top of the southern side>that in the middle of the southern side.(3) The situation in which the land is exploitedIt's concluded that the water content in the acacia tree fields agrees with that on the grass ground of wasteland with the variation of soil depth, the change of seasons and the variation of the places and directions of the slopes through observation on the acacia tree fields. However, with the increase of the depth, the water content of soil from the acacia tree fields, where so called dry soil layer occur, is lower than that from grass grounds of the waste lands.3. The characteristics of the water content of soil in the planting seasons Generally, the water content of soil in autumn is higher than in spring, so in autumnthe survival rate of the seedling hopefully will be higher.4. It's concluded that Prunus Armeniaca, Daospyros lotus, Amorpha fruitcosa. Sophora japonica, Zizyphus jujuba, Platycladus orientalis, Juglans cathayensis and Pinus sylvestris Var. mongoleca are quite resistant to drought through the experiment in which 21 nurserygrown plants were potted and the vitality of their roots were compar...
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