Simulated rainfall experiments were conducted under 1.5and 2mm/min rainfall intensities

with fourslope gradients(10°，15°，20°，25°)and two slope lengths(5

10m).Purified water was used for the simulationexperiments.To study the effects of slope gradient on rill erosion under different surface conditions

a threedimensional laser scanning system was used to monitor the subtle changes on slope surface before and aftereach rainfall experiment.The results indicated that the time of runoff initiation showed no significant trendwith slope gradient

drop-pits appeared earlier with greater slope gradient.Both the initiation time for runoffand drop-pit were mainly controlled by the rainfall intensity.And the increase of the slope length promotedearlier initiation of runoff.The runoff rate was first climbing up and then declining with increasing slope gradients.No significant variations were found in the total runoff under different rainfall intensities with thesame volume of rainfall.The sediment yield increased with slope gradient

which was further enhanced by increasing rainfall intensity but weakened by increasing slope length to some extent.Moreover

during therainfall process

soil erosion first increases with slope rapidly and the increasing rate reduced after a criticalgradient was reached

which could be further enhanced by both rainfall intensity and slope length.