Abstract:Basde on observations from Chinese fir watersheds before and after artificial distur-bance,“excess storage model”was developed in the light of specific conditions of forest watersheds，the features and mechanisms of runoff yield and confluence analyzed and a comparative study of runoff dynamics under the influence of clearcutting and thinning made．The model was of high accuracy by which interflow could be prceisely separated from groundwater flow．The accuracy of modelling runoff at one—day intervals was 95．7％ in the clearcutting watershed and 92.3％ in the thinning watershed．The water holding capacity of crown and undergrowth amounted to more than 1—2mm．The final infiltration rate of surface water was 3.4—14.Omm/min.,most of which then turned in-to interflow and groundwater，while surfaceflow was only 0.4％ of total runoff．Simula-tion results showed that after clearcutting the storage capacity in the zone of aeration de-creased，time of throughflow concentration shortened，final infiltration capacity of groundwater dropped，and total storm runoff increased，while after thinning the water holding capacity in the zone of aeration decreased，time of throughflow concentration shortened slightly，infiltration rate of groundwater increased，thus its effect of storage control was greater than that in the clearcut watershed．The Chinese fir plantation reduces the flood flow and peak flow，and prolongs the time of concentration by means of aboveground storage control，transformation of direct runoff into groundwater and delay of throughflow confluence．