| Ejina Oasis, a typical tail oasis continental river, locates in downstream of Heihe River which is the second largest inland river in China. This region is in the desert area, with its climate belonging to extremely arid continental climate. In recent decades, for upper and middle reaches of Heihe River over exploitation, the ecological environment of the oasis is becoming worse and worse, such as vegetation degradation, species decline, oasis shrink, the underlying surface changes and so on. Studying spatial-temporal changes and analyzing driving mechanism has important significance in protecting and utilization oasis.Aim to accurate extraction the border of oasis, we choose object-oriented segmentation algorithm or automatic threshold method to extract oasis and adjust result adopting visual interpretation, utilizing long time series and multi-source remote sensing data (high-resolution satellite photo in1963,1968,1980, Landsat MSS images in1973,1977, landsat TM/ETM+images in1986,1990,1993,1996,1999,2002,2006,2009), in the support of RS and GIS. In the study, we have analyzed spatial-temporal changing process, characteristics, laws, and divided changing process into stages of the Ejina oasis, and gotten nine types of spatial-temporal variation patterns (permanent pattern, transient pattern, changeable pattern which from oasis to others, changeable pattern which from others to oasis, stable to chaos pattern, chaos to stable pattern, single peak pattern, wavy pattern, and capricious pattern), and analyze stability. Results of study are:Firstly, in view of area change and process, the total areas of oasis tend to expand fluctuately. The area changes was divided into five periods,"low fluctuation-rapid expansion-high volatility-sharp decrease-stable increase", by year of1977,1986,1996and1999as the cut-off points. The pure expansion of oasis mainly appear before1990with large fluctuations, and other periods expansion areas are small except several periods with slightly large expansion area after1990. The pure decrease is regular, which decreases from15053.0hm2in1968to295.1hm2in1986, afterwards the decreased area is very small.Secondly, in view of space distribution pattern, the oasis mainly distribute in south of East River and West River, West River coast, and the Delta region Ejina basin. It is broken distributed along the East River. Oasis area increased region are mainly distribute in the west of East River upstream and Heihe River, east of West River upstream and middle, and delta northwest, northeast and southeast. In the meanwhile, oasis area decreased regions locate in West River coast and delta east and north.Thirdly, in view of the oasis changing pattern, oasis variation can be divided into nine models, permanent pattern, transient pattern, changeable pattern which from oasis to others, changeable pattern which from others to oasis, stable to chaos pattern, chaos to stable pattern, single peak pattern, wavy pattern, and capricious pattern. Their proportion of the total area are8.54%,23.61%,6.36%,0.62%,5.39%,15.27%,2.80%,16.97%and20.43%, respectively. The transient pattern accounts for the largest share and the second is capricious pattern, which means Ejina oasis is unstable in recent50years.Fourthly, according to the stability, the oasis is divided into stable, unstable and highly unstable, and ratios are33.48%,41.58%and24.94%, respectively. The stable oasis is in alluvial delta plain of Heihe River downstream, West River flow region and the reservoir region of the west of Heihe River. Peripheral oasis is unstable.Fifth, discharged water change directly leads to the oasis development, affecting entire study phase. Intensified human activities, unscientific overgrazing and deforestation are the major reason for oasis development. Climate dryer and warmer cause and exacerbate natural disasters, and influence oasis development. The main factors differ in difference period. In planned economy period, factors are policy, population, upstream inflow and natural disasters. In market economy period, factors are policy, upstream inflow, human destruction, economic development and land salinization. Upstream inflow, economic benefits and relevant policy about ecological construction are factors in ecological construction period. |
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