Ziwei Xu¹, Shaomin Liu¹, Xin Li², Tao Che³, Zhiguo Ren³, Yang Zhang³ and Junlei Tan³, (1)Faculty of Geographical Science, Beijing Normal University, Beijing, China, (2)Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China, (3)Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China