Evolution Laws of the Ecological Environment and Their Driving Forces in the Arid Areas of Northwest China during the Historical Periods

The basic evolution laws of ecological environment are that the diverted water volume is increased in the fore-mountain zones,the areas of artificial oases are enlarged,the ecological environment is improved,but the soil salinization becomes more serious;in the middle and lower reaches of rivers,the stream flow is reduced,the decline and fall of paleo-oases occur,desertification becomes more severe,and the ecological environment is degenerated.The evolution laws of landscapes are the “two enlargements and four reductions”,that is,under the enlargements of both deserts and oases,the areas of natural waters,woodlands and grasslands as well as the populations of the wild animals and wild plant communities in the ecotones between the oases and deserts are reduced.The driving forces resulting in the evolution of ecological environment are mainly that the regional distribution of water resources is changed due to the social and economic development,moreover,the evolution of ecological environment is also related to the natural factors,such as the climatic fluctuations,change of river courses and blown sand erosion and deposition.