**Acknowledgements**

The research was supported by the National Natural Science Foundation of China (Grant No. 40901138),National Basic Research Program of ChinaGrant No. 2007CB407201and also supported by State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Pla‐ teauInstitute of Water and Soil ConservationChinese Academy of Sciences and Ministry of Water Resources (Grant No. 10501-283).
