DU She-ni1,2,GENG Gui-jun2,BAI Gang-shuan1,YU Jian3(1.Institute of Soil and Water Conservation,Northwest A & F University,Yangling,Shaanxi 712100,2.Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,3.Institute of Inner Mongolia Hydraulic Research,Huhhot,Neiminggu 010020)