干旱区生态环境知识资源中心

The desertification problem aggravated by increasing human activities threatens the ecosystem of Xihu oasis in Gansu Province, China, where the ancient Dunhuang Civilization was located. It is crucial to explore the vegetation characteristics and dynamics before developing appropriate strategies to mitigate the ecosystem degradation. In this paper, remote sensing images are used to obtain vegetation cover data, which are considered to show ecosystem statuses in the Xihu oasis during 1986-2010. Both the land use dynamic change (LUDC) model and proposed transitional area ratio (TAR) index are used to analyze the vegetation dynamics among four vegetation cover types (i.e. no, low, medium and high) and possible driving factors. Results indicate that: (1) the vegetation degrades during 1986-1999 and then has been recovered since the early 2000s; (2) the introduced LUDC model shows that the low and medium vegetation covers are the most unstable types because they can easily transfer into the other types under external disturbances; (3) the proposed TAR index agrees with the results from LUDC model in the overall change trend of vegetation cover during 25 years, and is a good representation of vegetation stability; and (4) the correlation analyses between TAR index and the runoff and groundwater in the upper river basin present that the oasis ecosystem is affected mostly by human activities, and the impacts of runoff and groundwater in upper river basin on vegetation cover in the Xihu oasis can reach approximately 30 and 15 years, respectively. To recover the Xihu oasis ecosystem to the status in 1986, the continuous annual runoff at the Panjiazhuang hydrologic station should be more than 2.9 x 10(8) m(3) for 30 years, or the depth of groundwater level in the Yumen basin should be higher than 5.3 m for 15 years.