Spatial heterogeneity of changes in vegetation growth and their driving forces based on satellite observations of the Yarlung Zangbo River Basin in the Tibetan Plateau

Changes in vegetation growth may influence the availability of water resources. Located on the Tibetan Plateau, the Yarlung Zangbo River Basin (YZRB) is a major freshwater source for Tibet, China and downstream South Asian countries, with high spatial heterogeneity in altitude and climate from the upstream to downstream regions. In this study, we combined satellite-based gridded datasets of the normalized difference vegetation index (NDVI), topography, precipitation, and temperature to investigate how vegetation growth has changed in the YZRB over recent decades (1982-2010), and to determine their driving mechanisms. Although a statistically significant trend in growing season NDVI was not detected at the basin scale, at the pixel scale 16.8% and 5.3% of the area of the YZRB was found to exhibit increasing and decreasing trends, respectively. The greatest increases occurred in the Nyang subbasin. Variations of NDVI values along elevation, precipitation, and temperature gradients revealed the greatest variation over the middle ranges of the three variables. Partial correlation analyses indicate that in both arid and semi-arid regions of the YZRB precipitation is positively correlated with NDVI. The relationship between NDVI and temperature varies with aridity, reflecting different effects of snow-melting processes on vegetation growth. Temperature positively correlates with NDVI in the arid region. In addition, a negative correlation between NDVI and temperature is detected in the semi-arid region. This is the first comprehensive study to explore climate-vegetation-hydrology relationships in the YZRB at a spatial resolution finer than those derived from sparse in-situ meteorological stations or low resolution global climate datasets. Our results are valuable for understanding ecohydrological processes in both arid and semi-arid regions of this internationally important river basin.