Spatio-temporal Changes Of Vegetation Under The Surface Water Allocation Policy In Heihe River Basin

Heihe River Basin is the second largest inland river basin in Northwest China, which is located in the central part of Hexi Corridor. It is a natural laboratory for surface vegetation of different geographical units in Northwest China. However, human activities such as urbanization exerted a series of negative effects on the Heihe River Basin especially the lower reaches in the 1980s. For the sake of improving the eco-environment, and deterring the trend of further degradation of the oasis, comprehensive measures including the surface water allocation policy have been taken since 2000.Taking the vegetation of Heihe River Basin as the research subject, EOS MODIS NDVI as the data source, this paper focuses on the spatio-temporal changes of vegetation, the differences and connections of these changes among different reaches, as well as the rules of vegetation change response to the surface water allocation policy over the past 15 years by means of trend analysis and correlation analysis. Thus to complement the existing research and provide decision basis for the efficient and sustainable utilization of water, ecological security and social and economic development. The findings are as follows:(1)Vegetation growth in Heihe River Basin was overall improved in a fluctuant way from 2000 to 2014, which had a significant seasonal variation during the years.(2)The structure of vegetation coverage in Heihe River Basin was getting optimized from 2000 to 2014. The vegetation coverage decreased gradually from south to north, with the high density of vegetation coverage distributed in the upper reaches and oases in the middle and lower reaches, the medium and low density distributed in the southern regions of the upper and middle reaches, and non-vegetation areas such as bare land and water distributed in the northern regions of middle reaches as well as most part of lower reaches.(3)Key trends showed the basically unchanged or slightly improved vegetation in 2000-2014. The area of improvement was much larger than that of degradation. The vegetation variation increased first and then decreased from south to north, while the magnitude in the middle reaches was bigger in inter-annual fluctuation and led to poor stability.(4)The result showed that the monthly NDVI was influenced by the monthly benefit water in both the middle and the lower reaches while only the annual NDVI of the lower reaches made a positive response to the annual benefit water. Specifically, the surface water allocation policy had an obvious positive impact on the medium and high density of vegetation coverage in the lower reaches.(5) In addition, vegetation changes in Heihe River Basin was also influenced by natural factors such as temperature, precipitation, elevation, slope, and human factors such as Fencing, Grain for Green, the adjusted planting structure and enhanced irrigation technology.