1. School of Soil and Water Conservation; Beijing Forestry University; Beijing P. R. China
2. USDA-ARS Grazinglands Research Laboratory; El Reno OK USA
3. Center for Spatial Analysis; University of Oklahoma; Norman OK USA
4. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau; Northwest A&F University; Yangling P. R. China