The hydrological principles of applying the flow hydrograph produced by precipitation are introduced, and a flood and waterlogging index calculated only with precipitation data and a quantitative method for forecasting the flood and waterlogging intensity over a river drainage area are presented. The method uses the conventional precipitation data to calculate the effective integrated areal rainfall in the valley (considering different contributions of successive areal rainfall days in the valley during the former period). Through checking the relationship between flood and waterlogging data (such as precipitation, water level, etc.) and the effective integrated areal rainfall, the magnitude of the effective integrated areal rainfall for various levels of flood and waterlogging index in the valley can be determined. In the operational forecasting, the actual and forecasted areal rainfall are used to determine the effective integrated areal rainfall for a certain future day, and then by comparing it with the given effective integrated areal rainfall corresponding to the flood and waterlogging indexes of various levels, the level and intensity of flood and waterlogging can be estimated.