Abstract: Based on the long-term observation and from the viewpoints of water balance and runoff-and sediment generation,this paper studied the generation processes of runoff and sediment on two typical woodlands, artificial P.tabulaeformis and secondary natural P.dadidiana,and uncultivated slopeland in Loess Plateau under hypo-rainfall.The results showed that within the range of 5.0～50.0 mm rainfall,the total interception of canopy and litter was 15.45%～56.80％ for P.tabulaeformis and 20.56%～47.81％ for P.dadidiana,and decreased with increasing rainfall.Woodlands had a higher soil water infiltration capacity than uncultivated slope-land,especially in 0～20 cm soil layer.Both the two woodlands did not generate runoff under regular rainfall.Under the assumed rainfall of 2.5 mm·min^-1 intensity and 30 min duration,P.dadidiana stand did not produce runoff,but the runoff velocity and sediment-carrying capacity of uncultivated slope-land were 23.5 times,and runoff shearing stress and energy were 8 times as much as P.tabulaeformis stand.The runoff-and sediment generation on P.tabulaeformis stand decreased by 87.6% and 99.4%,respectively,compared with those on uncultivated slope-land,which was well accorded with the average observed value in runoff plots during 1988～2000The theoretical analysis on the generation mechanism of woodland runoff and sediment may be effective to evaluate the benefits of forest in soil and water conservation.

Key words: Net photosynthetic rate, Transpiration rate, Kelimeris integrifolia, Grassland