Study of Human Impact on Vegetation in Desertified Regions based on Rainfall Use Efficiency:A case study of Horqin region, Inner Mongolia

Rainfall use efficiency (RUE) is defined as integrated NDVI divided by annual precipitation data. The optimum time length representing annual plant growth is decided by comparing standard deviation of RUE under different composite time length which is from 30 to 360 days. It is found that the maximum RUE deviation occurred in time span from April to October which is also the growing season in local place. RUE median in each year, that is about 0.484～0.606 DN per millimeter precipitation, is selected to represent the RUE without human disturbance. Based on the linear relationship between precipitation and NDVI, the value of RUE median multiplying precipitation is used to predict the expected NDVI corresponding with local climate. Residue between predicted NDVI and smoothed NDVI data is used to indicate human influence. Variance analysis demonstrates that human impact on vegetation in different area has a significant difference. In the southern part of Horqin region in Inner Mongolia, human activities averagely decrease NDVI by 0.114, whereas human activities exert positive influence on vegetation in the middle-north part and the northwest part, and human activities slightly increase NDVI by 0.055 and 0.076 respectively. Finally, economical development, land use, precipitation and NDVI are incorporated to evidence the rationality of the method.