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Highlights • Physical erosivity factor is the effective energy of rainfall and surface runoff. • Work of interrill erosion shows the highest correlation with effective energy. • In vegetated slopes sediment yield depends on effective kinetic energy. • On bare soil sediment yield depends on effective potential energy. • Energy efficiency for interrill erosion decreases with vegetation coverage. Abstract The process of interrill erosion is complex by interaction of raindrop impact and sheet flow. Their relative contribution to interrill erosion is difficult to be evaluated even on bare soil. This study presents the new erosivity factor to evaluate the interrill erosion on steep vegetated hillslope with the more relevant understanding of the physical processes. The effective energy, the erosivity factor, is defined as the sum of the effective kinetic energy of rainfall and effective potential energy of surface runoff based on the energy balance. The effective kinetic energy of rainfall is determined by the horizontal component for slope of kinetic energy deducting energies dissipated by structure of vegetation canopies and a litter layer. The effective potential energy of surface runoff is equal to potential energy of the available surface water following rain-mass allocations of interception and infiltration. The data from experimental field plots with various vegetation coverage after wildfire were used to verify the effective energy equation. On densely vegetated slopes sediment yield depended greatly on effective kinetic energy of rainfall, while they from hillslopes having sparse coverage were dominated by effective potential energy of surface runoff. The dissipated energy due to interrill erosion showed the highest correlation coefficient with the effective energy under various cover conditions. The kinetic energy of raindrops was greatly reduced by the litter layer and the potential energy of rainwater decreased predominantly due to infiltration. The ratio of effective potential energy of surface runoff to total effective energy was the highest at 71.2% in the plots with low vegetation coverage. The energy efficiency for interrill erosion increased with decreasing vegetation coverage and reached maximum 1.35% in extreme rainfall event under low vegetation coverage. The constant and exponent of power-law functions between the effective energy and the soil erosion work were strongly correlated with gravel ratio and litter coverage, respectively. The results indicate that the effective energy is useful erosivity factor to evaluate the interrill erosion occurred by the complicated interaction of rain splash and sheet flow on vegetated hillslopes. [ABSTRACT FROM AUTHOR]