Abstract:In order to evaluate the impacts of forest vegetation on rainfall-runoff in a forest watershed in area of 834 km2, two quantitative indexes of forest cover, cover ratio and timber volume were adopted,and they were divided into two grades: (1) forest cover ratio ≥ 60% and timber volume ≥5000 (m3/km2); (2) forest cover ratio <60% and timber volume <5000 (m3/km2). Different parameters were selected in the proposed mode in accordance with the two quantitative indexes. The simulation technique of filled-up runoff generation was applied to the calculation of runoff, and the two-layer simulation of evapotranspiration was applied to the calculation of soil water storage in watershed. An exponential equation i.e. a=1-eKP was proposed to describe the relationship between the water storage capacity and the relative filled-up area in watershed. The different runoff components were considered in order to calculate the flood hydrograph, therefore, two routing methods were utilized , namely, the technique of equal time line of flow with regulation of linear reservoir and the simplified triangle hydrograph were adopted for surface and underground runoff of concentration simulation respectively. Comparing the simulation results with the observed data indicates that the proposed model not only is high in accuracy, but also reflects the effects of the two different forest indexes on water conservation and the flood regulation on the whole. The model is practical and will play important rule in farther study of the forest hydrological function.