Abstract: For studying the runoff characteristics of purple soil slopes in Three Gorges reservior region, a series of simulated rainfall experiments under the different gradients（10°，15°，20°，25°，30°，35°）and the different rainfall intensities（1.0 mm/min，1.5 mm/min，1.8 mm/min） were conducted on a test zone plot with 5.0 meters long and ?1.5 ?meters width. The erosion characteristics, sediment yield mode, runoff generation process and the percentages of the surface runoff and the subsurface runoff accounting for the total precipitation are discussed in this paper. The results show as follows: The runoff generation mode belongs to saturated runoff. The percentages of the surface runoff and the subsurface runoff holding the total precipitation rainfall vary with the different rainfall intensities and the gradients. Under the same slope gradient, the percentage of the subsurface runoff possessing the total precipitation increases with the rainfall intensity decrease; under the similar rainfall intensity, the percentage of the subsurface runoff occupying the total precipitation rainfall increases with the slope gradient increase. The subsurface runoff is not only the dynamical erosion force but also takes a considerable effect in the progress of the slope erosion by analyzing the sediment yielding in various experiments, especially in the steeper slope, the erosion? forming a debris flow so as to lead to gravity erosion because of the abundant subsurface runoff. The amount of erosion caused by the subsurface runoff is larger than that caused by sheet erosion and rill erosion on slope. This result is different from that gained by previous, so account should be taken much into the erosion caused by subsurface flow in the harnessing the slope erosion.

Key words: purple soil , erosion , runoff and sediment yield, subsurface runoff , surface runoff