1. School of Economics, Lanzhou University, Lanzhou, China
2. School of Economics, Minzu University of China, Beijing, China
3. Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China
4. Key Laboratory of Remote Sensing of Gansu Province, Heihe Remote Sensing Experimental Research Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China