1. College of Natural Resour. and Environ. Northwest A&F Univ.Yangling712100China
2. State Key Lab. of Soil Erosion and Dryland Farming on the Loess PlateauNorthwest A&F Univ.Yangling712100China
3. Key Lab. of Ecosystem Network Observation and ModelingInst. of Geographic Sciences and Natural Resources Research Chinese Academy of SciencesBeijing100101China