Spatial-temporal variations in NDVI in vegetation-growing season in Qinghai based on GIMMS NDVI 3g.v1 in past 34 years

Under the influence of climate change and human activities, the ecological environment in Qinghai Province has changed markedly. In this context, taking GIMMS NDVI3g.v1 as the data source, we analyzed temporal and spatial changes in NDVI of the vegetation coverage area in Qinghai Province from 1982 to 2015, using the Theil-Sen Median trend analysis and Maan-Kendall test. Both the trend analysis and R/S analysis were superimposed to study the persistent characteristics of NDVI change during the vegetation growth season, and reveal the regularity of vegetation response to climate change and human activities. The results showed that: 1) Vegetation NDVI in Qinghai Province increased from the Northwest to Southeast in the past 34 years, and the coefficient of variation indicated that a larger volatility region was concentrated in the lower NDVI region of the Qaidam Basin and northwest of the Qingnan pastoral area; the volatility was concentrated in the eastern part of the Qilian Mountains, eastern agricultural area, and southwest of Qingnan pasture area with high vegetation NDVI. 2) In the past 34 years, the overall NDVI of vegetation in Qinghai showed an increasing trend; growth rate was 0.38%·10 a-1, the change in NDVI was obvious in phases, and there were two abrupt changes in 1994 and 2000. 3) The area of vegetation improvement in Qinghai Province (75.4%) from 1982 to 2015 was much larger than that of vegetation degradation (24.6%), of which the significant improvement area represented 40.9% of the area covered by vegetation, and the degradation zone showed a significant shift in space over time. 4) The Hurst index showed that the reverse trend of vegetation change in Qinghai Province was remarkably persistent; the trend analysis and Hurst index obtained by superposition showed that 13.7% of the area affected by degradation was enhanced from degradation to improvement, and 44.3% of the area from improvement to degradation; future trends of 41.5% of the area could not be determined, and only 0.2%-0.3% of area was between continuous improvement and sustained degradation. 5) The NDVI of vegetation growth in Qinghai Province was affected by climate change and human activities, and the response of different vegetation types to climate change was quite different.