1. Key Laboratory for Environmental Pollution Simulation and Control, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Gansu Province, P. R. China
2. College of Agricultural and Forestry Economics & Management, Lanzhou University of Finance and Economics, Lanzhou 730101, P. R. China
3. Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, P. R. China