| Oasis is a unique geographical view in arid areas, which directly affects the economic and social stability of the region. Hexi region, located in the northwest of Gansu Province, is one of the most prosperous oasis in China. As the important part of silk road,Hexi region was an important military and economic strategic position in the ancient times. After the reform and opening-up policy,the development of the oasis strength has entered an unprecedented stage. The massive expansion of the hexi region, caused water use conflicts and the destruction of the natural oasis also brought serious ecological environment problems. The study of the hexi region oasis change is the basis of local governance and oasis ecological environment construction, which has great research value.Firstly, the author has carried on the induction summary to the concept of "oasis", then get the currently accepted the basic characteristics of oasis and based on the landsat remote sensing image features, puts forward the strong operational oasis extract rules. Secondly,through the ndvi threshold segmentation and the method of visual changes, the "vegetation", "water" and "construction land" layers are extracted and get 7 samples from 1986 to 2014 years of oasis distribution data. Based on the quantitative analysis model, kernel density analysis model, grid handling analysis model, the author analyzes in detail the characteristics of spatial and temporal variation from 1986 to 2014 a and put forward the oasis change mode of seven. Conclusions are as follows:1. Look from the spatial distribution pattern of oasis, Oasis in hexi area are mainly distributed in shiyang river basin, heihe river basin, and the main stream of shule river basin, on both sides of the shiyang river in heihe river basin is more concentrated, shule river basin is more dispersed, on a smaller scale.2. The Hexi corridor has been in a state of expansion from 1986-2014 a. The oasis area is 10707.75 and 14950.11 km2 in 2014. The area of the oasis increases 4242.36 km2, accounting for 40% of the 1986. From the time, the intensity of the change of the oasis has experienced "low(1986-1990)- high(1990-2005)- low(2005-2010)- high(2010-2014)" process. Oasis transition strength decreased year by year, the oasis stability.3. Overall, the transformation of oasis and the oasis intensity decreased year by year, maintain at a low level, after the overall stable oasis. Oasis area expansion is the largest period is 1990-1995, 1995-2000, 2000-2005, back to the period of the most 1986-1990 and 2005-2010.4. From 1986 to 2000,more sever areas concentrated in the Hexi corridor’s central and eastern regions. After 2000,the heihe river basin and shule river region oasis chang began to increase. According to change of adjacent time cumulative intensity, the oasis is divided into "no change", "intensity" and "sharp" in general, "a sharp" four types. The "no change" as the main body, followed by "sharp" in general areas.5. Oasis from the pattern of oasis change, can be divided into stable variant not early, late appear, disappear, a flash in the pan, stage stable and stable stage, fluctuations in seven kinds of model, the proportion were 57.2%, 26.7%, 3.2%, 26.7%, 3.1%, 3%, 4.1%. The proportion of late emergence type oasis was 26.7%,which show that developed oasis in general can ensure adequate supply of natural and socio-economic conditions, more stable.6.From the point of view of the river basin scale, shule river, shiyang river oasis change drastically, less change in heihe river basin. Look from the city level scale, jiayuguan, jiuquan city, wuwei oasis change more violent and zhangye and Jinchang have weak singularity of oasis change. |
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