Runoff erosion formation and sediment yielding intensity on slopes of the Loess Plateau of China show apparent vertical zonation because of the complicated topography and the special climate conditions of the region.To reveal mechanism of the formation of runoff erosion and the characteristic of vertical zonation,an experiment was carried out with a simulated slope-gully system,which was composed of a 4-meter long slope(20°)and a 3-meter long gully slope(50°),and four different fluxes of runoff(2.1 L min^-1,3.2 L min^-1,5.2 L min^-1,and 7.2 L min^-1).Impacts of flux,silt content and impetus of the runoffs on sediment yield were explored.Based on the findings,a mathematical model was established for depicting sediment yielding process in the system.Results show that intensity,silt content and flowing power per unit volume of the runoff flowing down the upper slope were the major factors affecting sediment yield in the gully.Sediment yield in the gully was exponentially related to intensity of the runoff,and linearly to silt content and flowing power per unit volume of the runoff.Based on the concept of flowing power per unit volume,a value defined to be critical to sediment yielding in the gully was figured out to be 0.632.And based on the modified Yalin equation and factors affecting sediment yield,mathematical models were worked out for sediment yield on the slope and in the gully,respectively.Results of the calculation using the models were found to be satisfactorily comparable to the measured ones.