1. Ph.D. Student, College of Geography and Environmental Science, Northwest Normal Univ., Lanzhou 730070, China.
2. Professor, College of Geography and Environmental Science, Northwest Normal Univ., Lanzhou 730070, China (corresponding author).
3. Lecturer, College of Geographical Science, Inner Mongolia Normal Univ., Hohhot 010022, China.
4. Lecturer, College of Geography and Environmental Science, Northwest Normal Univ., Lanzhou 730070, China.