Impact of forest and grass ecosystems on the water budget of the catchments

The characteristics of the temporal-spatial distribution of the surface runoff,the percolation from the root wne,the evapotranspiration and the soil water storage of the four forest and grass ecosystems (the natural grassland,the board leaved forest land,the mixed forest land and the coniferous forest land) in the hilly region of red soil are studied with the soil and water assessment tool (SWAT) 99.2 model.The outputs of SWAT model are calibrated and validated against the observed surface runoff.Comparing the simulated monthly surface runoff with the observed one, we gain a good result with a deterministic effciency of 0.90 and an simulation efficiency of 0.74.The results shows the variation of the rainfall within a year is the rmst important factor that leads to the difference of the monthly surface runoff,the percolation and the soil water storage in the red mil area.And the annual fluctuation of weather conditions,especially the rainfall,results in the fluctuation of the water budget.Of the water outputs, the surface runoff and the percolation are influenced most by the rainfall,then the evapotranspiration.The annual fluctuation of the soil water storage is very little.Compared with the natural grassland,the forest land reduces the surface runoff more effectively,in which the board leaved forest land and the mixed forest land are preferable.The water infiltration property of the forest land is better than the natural grassland.And the evapotranspiration of the forest land is much larger than that of the natural grassland.The results show that land use mostly influences the short-term water budget in the region scale.