The Vegetation Cover over Last 20 Years in Yellow River Basin
近20年来黄河流域植被覆盖变化分析

Vegetation cover is a major feature for ecological system. The change of vegetation cover in large area is mainly caused by climate and human activities. The Yellow River, the second largest river in China with a basin of 752,443 km2, includes semi-arid and arid environment. Therefore, it is necessary to study the change of vegetation cover in the Yellow River Basin, which will help us understand climate influences and impacts of human activities. In order to understand the change of vegetation cover in the Yellow River Basin and its influencing factors, we use the data of AVHRR-NDVI and the information of 76 meteorological stations in the Yellow River Basin covering 1982-1999 to analyze their spatial distribution and dynamic change in spring, summer and autumn. On the basis of calculating the NDVI mean deviation and Moisture Index (MI) mean deviation, we study the feature of MI mean deviation, the speed of NDVI mean deviation and the correlation between them. Finally, some conclusions are drawn. Firstly, the climate was relatively humid from 1982 to 1992 and relatively dry since 1993. The environment in the Yellow River Basin tended to be desiccated, but the vegetation cover is increasing. Secondly, the correlation between NDVI mean deviation and MI mean deviation demonstrates that the humid climate was helpful for vegetation cover. Thirdly, the average speed of vegetation cover increase was 0.58%. The speed of vegetation cover increase in the loess region and catchment area is 0-3.8%, which shows the achievement of ecological measurements in China. The vegetation cover in desert and riverhead region is decreasing, the speed of which is 0-3.0%, a reflection of the climate change.