1 Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China.
2 Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China; College of Urban and Environmental Sciences, Yancheng Teachers University, Yancheng 224002, China.
3 Key Laboratory of Geographic Information Science of Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200241, China.
4 Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China. Electronic address: panbt@lzu.edu.cn.