A Study On The Relation Of Between Runoff And Sediment Variations And Deforestation In The Upper Of Minjiang River

The relationship between forest and water is nowadays a key problem that forestry and ecology. Impacts from the vegetation of forest on hydrology course are the important ecological function of the vegetation and Impacts from different climatic zones and different yardstick of forests on hydrology are different. It is a problem to be solved how to know the effect of the vegetation of forest on the hydrology course on greater yardstick. It is also a focal point of present research. The upper reaches of Minjiang River covers 25,000 square kilometers with enormous relief of the earth's surface and is known as a typical alp gorge district. The special physiognomy condition causes comparatively complicated climate, vegetation and soil in the district with distribution characteristic of area. The research on forest and hydrology of the complicated large basin of the upper reaches of Minjing river has certain academic meanings.The upper reaches runoff of Minjiang River is a source of irrigation for the Chengdu plain, with great influence on agricultural ecological environment of middle reaches and lower reaches. From the founding of the state to the eighties, having followed the exploitation of large scale of forest in this area, the agricultural ecological environment of the Chengdu plain receives serious influence, which is mainly shown as the water amount from the upper reaches decreases gradually;the flood flow increases in wet season and the low water flow reduces. The three are apt to cause difficulty in rice seed and rise of the silt content of river. Therefore, Studying relationship between deforestation and variations of runoff and sediment of the upper reaches of Minjiang River has certain realistic meanings, too.This paper uses deforestation materials and the upper reaches hydrology data of Mingjiang River collected and combines the precipitation data to have studied the relationship between variations of runoff and sediment and deforestation in the upper reaches of Minjiang River and drawn the following conclusions:1. The upper reaches deforestation of Minjiang River and the change in the water amount from the upper reaches present positive correlation. It is the deforestation that is the main reason to reduce the water amount from the upper reaches of Minjiang River. Under the special geology, physiognomy and climate, forest has the function of conserving source of water and increasing annual runoff.2. The upper reaches deforestation of Mingjiang River presents passive correlation with the change in the flood flow and presents positive correlation with the probability of the great floodthat causes disaster. The upper reaches forest of Minjiang River divides into better structure the jungle layer can hold back the precipitation in rainy season and the withered fallen leaves layer and soil can absorb rainy season precipitation under the forest. Therefore, it has the function of weakening flood flow. However, after the previous precipitation in the basin, the withered fallen leaves layer and water content of soil have already reached the situation of the saturation. At this time, precipitation comes again. Accordingly, the second precipitation decreases the function of retaining flood of forest, even causes it to disappear totally , and results in previous precipitation stored by forest to superpose to the precipitation runoff, which aggravates the probability causing great flood.3.The upper reaches deforestation of Minjiang River presents positive correlation with the change of low water flow, which proves it is the main factor that generates decrease of runoff in dry season.4. The upper reaches deforestation of Minjiang River presents not relevant with the flood and dried ratio within the year. It shows the deforestation of the upper reaches of Minjiang River influences decrease of forest function adjusting the water amount, causes the water amount to assign more and more uneven within a year, generates the flood disaster to aggravate in flood season, leads to arid consequence already before flood season and results in extremely unfavorable influence on agricultural ecological safety in middle reaches and the lower reaches.5. The upper reaches deforestation of Minjiang River presents positive correlation with the change in the silt content of river. It proves the deforestation of the upper reaches of Minjiang River reduces ecological function of forest preventing soil erosion in this area, manifolds silt source to enter river and is the key reason causing silt content of the upper reaches of Minjiang River to increase.