| Vegetation coverage can not only measure the growth and changes of vegetation,but also provide the necessary data for meteorological,hydrological,ecological and other studies.Vegetation coverage and its changes have been widely used in specific application studies such as vegetation degradation,desertification evaluation or disaster monitoring in ecologically fragile areas.The research on the spatiotemporal changes of vegetation coverage in the source region of the Yellow River is of great significance for in-depth understanding of the response of vegetation growth to climate change and human activities in the permafrost regions of the Qinghai-Tibet Plateau,as well as to guide the ecological environmental protection and governance of the Sanjiangyuan National Park.This paper first uses Landsat annual time series images of the vegetation growth seasons in the source regions of the Yellow River in 2000,2004,2008,2013,and 2015,supplemented by MODIS MOD09 GQ time series data and MVC(Maximum Value Composite)method to determine the maximum vegetation coverage date in the same year.Pixel-by-pixel reconstruction of the annual maximum value of Landsat NDVI in the source area of the Yellow River based on unary quadratic polynomials;then use the reconstructed annual maximum value of Landsat NDVI to invert the maximum vegetation coverage in the source area of the Yellow River and finally analyze the maximum vegetation coverage in the source area of the Yellow River from 2000 to 2015 The dynamic change characteristics of the degree and its influencing factors.The main conclusions of this paper are as follows:(1)The results indicate that the fractional vegetation cover inversion method using Landsat NDVI annual maximum reconstruction results in the source region of the Yellow River has weakened the differences in vegetation growth due to inter-annual phenological changes and image acquisition time differences,guaranteeing the comparability of inter-annual vegetation coverage.Compared with the commonly used MVC products,the reconstruction results can accurately reflect the detailed characteristics of vegetation cover.(2)The overall vegetation coverage in the source region of the Yellow River is relatively poor,mainly due to the low to medium vegetation coverage.The spatial distribution shows a trend of decreasing from southeast to northwest;the stability of vegetation coverage shows that the northern region has the most dramatic changes,followed by the central region and the southern region The characteristics of the smallest change;the vegetation coverage in the source region of the Yellow River generally shows an improvement trend,but the vegetation coverage in areas that are more affected by human activities,such as river valleys,lakes,and alluvial plains in the front of the mountain,is significantly reduced.The key area for the monitoring of the ecological environment of national parks.The vegetation coverage in the source area has deteriorated from 2000 to 2004,mainly due to the conversion of medium,medium,high and high vegetation coverage types to low vegetation coverage types;the trend of improvement in vegetation coverage from 2004 to 2015 is obvious,and the area ratios of evolution and degradation trends are 38.25% and 16.77%.(3)The interaction between vegetation coverage and terrain and climate factors is strong,and the influence of terrain factors is mainly reflected in altitude.Through experimental analysis,it can be seen that the altitude is in the range of 4256-4799 m,and the vegetation coverage shows an increasing trend with the increase of altitude and terrain slope.There is a significant correlation between changes in temperature and precipitation and vegetation coverage,and it is the most important hydrothermal factor affecting vegetation coverage. |
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