Based on the field plots data from simulated rainfall experiments, a theoretical model for calculating the kinetic energy of a single raindrop was developed in the Loess Plateau of China Formulae for both single raindrops and rainfall events are presented in this paper.Analysis shows that overland flow and the area producing runoff on the land surface increase with the duration of rainfall, while the effects of raindrop impact on the surface of the hillslope decrease. The rate of interrill erosion decreases gradually with the rainfall duration. A physically-based formula to calculating the area of runoff production is also given in this paper The overland flow and canopy affect jointly the interrill erosion processes. The paper presents an equation to quantitatively evaluate the effect. It is concluded thats (1) the peak rate of interrill erosion is related to rainfall intensity, rainfall kinetic energy, and slope; and (2) the average rate of interrill erosion during an individual time period for variable rainfall intensity varies with the rainfall intensity, rainfall duration and runoff producing area. Based on the field plots data from simulated rainfall experiments, a theoretical model for calculating the kinetic energy of a single raindrop was developed in the Loess Plateau of China Formulae for both raindrops and rainfall events are presented in this paper. Analysis shows that overland flow and the area producing runoff on the land surface increase with the duration of rainfall, while the effects of raindrop impact on the surface of the hillslope decrease. The rate of interrill erosion decreases gradually with the rainfall duration. A physically-based formula to calculating the area of ​​runoff production is also given in this paper The overland flow and canopy affect jointly the interrill erosion processes. The paper presents an equation to quantitatively evaluate the effect. It is concluded thats (1) the peak rate of interrill erosion is related to rainfall intensity, rainfall kinetic energy, and slope; and (2) the average rate of interrill erosion during an individual time period for variable rainfall intensity varies with the rainfall intensity, rainfall duration and runoff producing area.