1. College of Resources and Environment; Northwest A&F University; Yangling Shaanxi 712100 PR China
2. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau; Institute of Soil and Water Conservation; Chinese Academy of Science and the Ministry of Water Resources; Yangling Shaanxi 712100 PR China
3. Key Laboratory of Ecosystem Network Observation and Modeling; Institute of Geographic Sciences and Natural Resources Research; Chinese Academy of Sciences; Beijing 100039 PR China