Zhanxiang Wang, Lanzhou University, Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou, China, Hong Gao, Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, China and Jianmin Ma, Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, China