1. College of Natural Resources and Environment Northwest A&F Univ. Yangling 712100 China
2. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau Northwest A&F Univ. Yangling 712100 China
3. Natural Science Laboratory Toyo Univ. Tokyo 1128606 Japan
4. College of Water Resources and Civil Engineering China Agricultural Univ. Beijing 100083 China
5. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources Yangling 712100 China