Study On The Temporal-spatial Changes Of Vegetation Cover In Shiyang River Basin Based On MODIS Data

Surface vegetation has functions with slowing down runoff, combatng desertification, preventing soil erosion and so on. It plays an important role in the process of energy exchange, biogeochemical cycles and hydrological cycle in land surface. MODIS vegetation index is a factor of the best indicators between state of vegetation growth and vegetation cover. At the same time it is the important indicator about the season of change and human activities affect. Vegetation coverage is a very important eco-climatic parameters and it is also a comprehensive quantitative indicators of covering the surface condition of plant communities. We have studied the estimation of vegetation coverage and changes, which will play an important role in environmental change hydrology, ecology, global change and other areas.In this paper, the study area is Shiyang river basin which is a typical inland river basin of the arid and semi-arid north-west. Firstly, we extract MODIS-NDVI and MODIS-EVI in this basin which is different in time and space resolution through the GIS software in the basisof MODIS dates and when w e analyzes the temporal and spatial variation of vegetation index characteristics. And then we estimated the vegetation coverage in this basin of different time and space resolution through improved vegetation cover estimation model. We also analyzed the distribution of vegetation cover characteristics and the dynamic changes. Finally, we discussed the driving factors of changes in watershed vegetation cover and reached the following main conclusions:Firstly, the spatial distribution of MODIS vegetation indexes in the basin was affected by the natural conditions and the land use, showed significant geographical differences. There are three different geographical distribution: The Southern Qilian mountains, central plains district, and the northern desert area. Vegetation indexes in various regions have different characteristics and change rules.Secondly, for the entire basin, vegetation index curves of the year are similar in shape with downward opening second-degree parabolas, whose changes were in line with the biological characteristics of vegetation and the changes in phenology, which showed obviously seasonal pattern. The precipitation and the temperature affect the vegetation index year changes are the leading factor. Further more, during 2000～2007, the vegetation changes in the basin are primary by stable and restores slightly, returning farmland to forest and the grass coexist with the destruction of forests and the grass, ecological restoration coexist with ecological degradation. The proportion of restoration is stronger than the proportion of degradation of vegetation cover in different regions.Finally, the spatial difference of the vegetation cover is closely related with terrain and river system. The vegetation will be impacted by the human activities at an altitude of about 1800m and the river distance ranged of 4.5～5.0km.The water resources is an important factor in vegetation cover. The environmental protection and watershed management may take with the integration of administration and watershed management, and the administration should be subject to watershed management.