Characteristics of NDVI Temporal and Spatial Variation in the Source Area of the Yangtze River and Its Response to Hydrothermal Conditions

Abstract

vegetation cover change monitoring is the basis of regional resources and environmental carrying capacity research. Based on the data set MODIS-NDVI 2001∼2018, this paper analyzes the spatial and temporal characteristics of annual NDVI in the Yangtze River source area by using maximum synthesis method and various statistical methods. The effects of surface hydrothermal conditions on space and time were discussed. The results show that:1) the NDVI of the source area of the Yangtze River shows a significant increase trend, and only some areas of Dongyuan NDVI show a downward trend. In spatial distribution, the vegetation coverage in the source area of the Yangtze River is mainly low and medium vegetation coverage, the vegetation coverage increases gradually from northwest to southeast, and the vegetation coverage of Yushu Tibetan Autonomous region is generally higher than that of Hercynian Mongolian Tibetan Autonomous region. At above 4000 m, the vegetation coverage increases more obviously with the increase of altitude. 2) precipitation in the source area of the Yangtze River decreases gradually from southeast to northwest, the low value area is located in the north of the source area of the Yangtze River, and the high value area is located in the eastern part of the Yushu Tibetan Autonomous region. However, the spatial trend of annual mean temperature and precipitation in the source area of the Yangtze River is opposite. The precipitation tendency rate in the source area of the Yangtze River shows obvious spatial difference, the precipitation in the south source generally shows a decreasing trend, and the precipitation in the north source generally shows an increasing trend. The southwest source and east source NDVI of 3) Yangtze River source area are negatively correlated with precipitation, while NDVI and temperature are positively correlated in most areas of the Yangtze River source area, and the effect of temperature on interannual variation is stronger than that of temperature. With the increase of temperature and precipitation, the NDVI also shows an increasing trend in spatial distribution, but precipitation has a stronger effect on the spatial heterogeneity of NDVI. From this we can see that the influence of hydrothermal conditions on the interannual variation of NDVI depends on the temperature, while the effect on spatial distribution mainly depends on the precipitation.