Hysteresis effect of runoff of the Heihe River on the vegetation cover in Ejina Oasis

In recent years, the recession of the oasis areas appeared to be extensive in Northwest China. The Heihe River basin is the second largest inland river basin in China, covering an area of 128, 283 km2. With the continuous increase of the Zhangye area, which is the middle stream area of the Heihe River basin, the water consumption recently increased gradually. It caused a decrease of incoming water in the downstream area resulting in a shrinking of the Ejina oasis area downstream. As a result, in that region the eco-environment is degrading and it causes a series of environmental problems like land desertification and part of the area even became the cradleland of sandstorms.
The Ejina oasis was chosen to be the study area in this study. With extremely arid climate, the annual precipitation is 40.76 mm and the annual evaporation is 3755 mm in Ejina Oasis. The vegetation growth mainly depends on the groundwater and the incoming water (runoff) of the Heihe River is the only recharge resources of the groundwater. Therefore, research on the relation between the runoff and the vegetation cover has great significance in protecting and optimal use of the oasis resources, the sustainable development of regional economy and social stability of the inland area.
On the basis of background research and GIMMS NDVI data, the quantitative relationship between runoff of the river and the mean annual NDVI from 1989 to 2002 was investigated. The stepwise regression was analyzed using mean annual GIMMS NDVI as the dependent variable and the runoff of the current year, the runoff of the previous year and the runoff of the previous two year as the independent variable. The result indicated that the runoff of the previous year and the mean annual NDVI of the current year have good linear correlation. The time lag of impact of the runoff of the river on the vegetation cover of the Ejina oasis was one year.