Soil erosion is caused by the interaction between rainfall and soil factors. Despite the considerable complexity of interaction between these factors, there is a need to reveal their mechanisms, and to quantitatively predict the rate of soil loss. In this study, the relationship between the rate of soil loss and factors upon which this loss depends was investigated using dimensional analysis. Erosion processes caused by the effects of raindrop impact were represented by a simple mathematical model which has been used in bed-load studies in the open channel. This model contains two parameters ξ_h, η_F, the first represents the effect on the detachability of soil by raindrops, the second represents the effect on soil transport by surface flow. Regarding to the simulated laboratory experimental data, it was found that the raindrop impact causes the transportation of soil by surface flow even if the flow is below the critical tractive force and the value for ξ_h increased in the begining and then decreased with increasing depth of water flow. The value for η_F increased linearly with increasing the water depth.