1 Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth Environmental Sciences, Lanzhou University, Lanzhou 730000, China. Electronic address: djzhang@lzu.edu.cn.
2 Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth Environmental Sciences, Lanzhou University, Lanzhou 730000, China; Key Laboratory of Alpine Ecology (LAE), Center for Excellence in Tibetan Plateau Earth System Sciences, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China.