A physical process based per storm soil erosion model to predict
small watershed sediment production in the hilly loess region
is developed.Due to the complex topographical conditions,a wide range of processes and subprocesses of erosion produce patterns of downslope changes in soil loss which are more variable than the systematic changes generally recognised by existing models.This model is divided into three submodels:the
hillslope submodel,the gully slope submodel and the channel submodel.Spatial unit considered in this model are parcels of land with generally homogeneous in characteristics,i.e.slope zones.In the hillslope submodel,rill detachment and transport are explicitly modelled.Estimated runoff was first compared with the rilling threshold.If the threshold is exceeded,estimation of flow detachment is based on a separate relation.The gully slope submodel is further divided into four components:erosion on the loess gully slope,active gully wall gravitational erosion,gully head tunnel erosion,and red earth gully landslip erosion.