Abstract:

Google Earth engine （GEE） is the most advanced platform for analysis and visualization of geographic big data， which makes remote sensing monitoring surface vegetation break through the limitations of data acquisition， local storage and low processing efficiency. Based on GEE cloud platform to calculate the NDVI of Landsat series satellite images， this paper studied the spatial and temporal variation of NDVI in Minqin oasis from 1987 to 2019， analyzed and predicted the variation trend of NDVI in Minqin oasis through unvariable linear trend analysis， stability analysis and Hurst index. The results shows that： （1） From 1987 to 2019， the annual average NDVI value of Minqin Oasis increased from 0.172 to 0.230 at the speed of 4.6%?（10a）^-1， and the multi-year average value was 0.244， showing an upward trend in 1989-2007 and 2010-2016， and a downward trend in 1987-1989， 2007-2010 and 2016-2019. During the year， NDVI ranged from -0.017 to 0.333， and the highest value was from April to October every year. In terms of spatial changes， NDVI in the periphery of the oasis has increased obviously， and the oasis has expanded to desert. （2） From 1987 to 2019， most areas of NDVI in Minqin Oasis remained basically unchanged. The periphery of the oasis in dam region and Quanshan region， the south of the lake region and Qingtu Lake showed an increasing trend. The urban area of Minqin County in the middle of the dam region and the edge of the oasis in the north of the lake region showed a decreasing trend. Basically unchanged， increasing and decreasing areas account for 81.90%， 16.25% and 1.85% of the total area respectively. （3） From 1987 to 2019， the areas with high NDVI change stability in Minqin oasis are mainly located in the southeast of the dam region， the east of Quanshan region and the north of the lake region； the regions with high volatility are mainly located in the periphery of the oasis in dam region and Quanshan region， the south of the lake region and Qingtu lake. （4） Hurst index shows that the trend of NDVI change in most areas of Minqin Oasis remains unchanged in the future. In the periphery of the oasis in the dam region and Quanshan region， in the north of the lake region and in Qingtu Lake， NDVI is likely to increase. In the middle of minqin County and the north of the lake region， NDVI is likely to decrease. The areas of NDVI basically unchanged， increased and decreased accounted for 71.62%， 12.96% and 15.42%， respectively.

Key words: GEE, NDVI, coefficient of variation, Hurst index, Minqin Oasis