1. School of Resource and Environmental Sciences; Wuhan University; Wuhan 430079 PR China
2. College of Urban and Environmental Sciences; Peking University; Beijing 100871 PR China
3. China Land Surveying and Planning Institute; Ministry of Land and Resources; Beijing 100035 PR China
4. Cold and Arid Regions Environmental and Engineering Research Institute; Chinese Academy of Sciences; Lanzhou 730000 PR China