Forest Vegetation Impacts On Water Conservation And Flood At Small Watershed Scale In Three Gorges Reservation Basin

The three Gorges Project in the Yangtze River is one of the largest water conservancy projects in the world. The government has always attached importance to the problem of environment influences. From the beginning of the three Gorges Project in 1950's and the different layout phases and the process of scheme argumentation later, the wide discussion and study had have been developed accordingly. A case study in the Xiangshuixi small watershed in the three Gorges reservoir area has been carried out in this paper. The evaluation index system has been put forward and the water-conserving and flood-controlling protection forests has been filtrated on the basis of analyzing the eco-environment characteristics and studying the mechanism of water-conserving and flood-controlling protection forests. The forest hydrology model has been conceived on the level of small watershed and catchment area. The function that the forest vegetation of different scales impact on the flood has been discussed in the paper, which provide theory base for optimum structure technique for the project of protection forest of the "Returning Cultivated Land to Foresf'and "Natural Forest Protection". The main result are as followings:According to the data from field investigation, lab experiment and slope run-off plot observation, the hydrologic functions of forest canopy, lower shrub, litters and soil layers have been studied. The canopy precipitation interception of different vegetation types is different. The results of same precipitation process indicate that the broad-leaf forest and coniferous and broad-leaf mixed forests are better than phyllostachys pubescens forest and shrubs in canopy rainfall interception. The amount of the largest litters water-holding change from 25.19t/hm2 to 59.75t/hm2 and the rate of the largest litters water-holding change from 283% to 479%. The water-holding ability of different standard samples are closely related to soil depth and soil non-capillary porosity. The water-holding amount 1713.3 to 349.4t/hm2, initial soil water infiltration rate range from 13.7 to 43.7 mm/min and stable soil water infiltration rater range from 0.6 to 6.3 mm/min. the regression function of soil water infiltration shows power form. The coefficient of regression function is 0.83-0.99. The simulant effect of function is better. The coefficient of surface roughness rate is 0.0182~0.2575.According to the horizontal and vertical geographic distribution characteristic and the quantity and the influence degree of constructive species of vegetation community, twenty vegetation communities have been carved up. And the distributing atlas of altitude grades made based on the altitude atlas. The method of Analytic Hierarchy Process was applied to establish the hierarchy structure model and evaluation index system of vegetation community . The method of expert assessment and integrative grades were applied to assess the water-conserving and flood-controlling function of vegetation community. The index value ranges from 7.3 to 1.1. Based on the altitude atlas distributing discipline of life form of vegetation community, the water-conserving and flood-controlling function of vegetation community shows as follows: In the ranges of altitude above1000m, Cunninghamia lanceolata Hook, Finns massoniana Lamb and broad-leaf mixed forest > Lithocarpus hancei forest > Cephalotaxus fortunei and Pinus massoniana Lamb mixed forest > Pinus massoniana Lamb and broad-leaf mixed forest > Cunninghamia lanceolata Hook forest > Cephalotaxus fortunei Forest > Cryptomeria fortunei forest; In the ranges of altitude lower 1000m. Phebe chinensis forest > Cunninghamia lanceolata Hook, Pinus massoniana Lamb and broad-leaf mixed forest > Castanea mollissima Blume >Sassafras tzumu Forest > Wasteland.Base on the collection and arrangement of the materials of precipitation of each county in Chongqing, the characteristics of precipitation process has been analyzed: Precipitation is unevenly distributed yearly. The maximum precipitation reach 1400~2000mm, while mi...