Monitoring of vegetation changes using multi-temporal NDVI in peripheral regions around Minqin oasis, Northwest China

Minqin Oasis encircled by the Badain Jaran Desert in north and Tengger Desert in east to south lies in the lower reach of Shiyang River Watershed, Northwest China. The peripheral desert ecological system around the oasis is quite important and absolutely necessary to the oasis economic systems in terms of protecting the oasis from desertification and ensuring sustainable development. However, the peripheral eco-system disturbed by human activities has been changed greatly since 1960s, bringing the great attention to the environment deterioration problems. In this study, Time series of normalized difference vegetation index (NDVI) derived from seven Landsat TM/ETM+ satellite images were used quantitatively to monitor the vegetation change in peripheral regions around the Minqin Oasis. Based on field observation and re-adjustment, the TM-based NDVI values from 0 to 0.21 were classified as natural and artificial vegetation such as herbage, desert shrub, and landscapes such as semi-fixed sand dunes, fixed sand dunes and sand dunes, while the NDVI values more than 0.22 as the crops and shelter belt beside the crops. The variations of NDVI values less than 0.21 indicating the natural and human planted vegetation were analyzed. Results show that the areas of NDVI values ranging from 0.090.14 indicating semi-fixed sand dunes planted perennial herbage and desert shrub have decreased gradually. The human planted shrubs (NDVI 0.15-0.21) have reduced in short time occurring during 1987-1991. While the areas indicating shifting sand dunes and inter-dunes planted annual or perennial herbages and scrubs (NDVI less than 0.09) fluctuated. The correlation analysis between NDVI and annual precipitation shows that NDVI is statistically correlated with the variations in the annual precipitation, implying that the precipitation-control led natural vegetation coverage in the margins of the Minqin oasis has overshadowed the made-planted vegetation in dominating the ecological landscape since late 1980's.