A practical method for numerical calculation of base flow has been developed to clarify the relation between parameters of a runoff model and charactenstics of draughty discharge based on the following assumptions．(i) The parameters of storage model for analyzing flood and long term runoff are identified in an investigation basin. (ii) The recharge of delayed subsurface flow and groundwater runoff is evaluated by KWST model which is an applied model of the storage model for analyzing flood and long term runoff proposed by Kadoya and Nagai. (iii) When a rainfall is ended，the recharge of base flow is over at the same time. (iv) In the period of no rainfall, the evapotranspiration is estirmated as a function of the valley storage.
A runoff cycle is divided into five stages. The first and second stages are related to rainless periods. The recharge of base flow occurs during other stages. The equations of runoff height of base flow for each stage are derived by the use of model parameters. Recession curves of base flow, which is correspond to normal recession curves, are simulated by the two identified runoff models and the characteristic values of base flow are evaluated. It is pointed out that a normal recession curve has a fluctuating tendancy with years in the same way as a duration curve of river discharge.