The paper presents a procedure for the on-line estimation of the φ-index and rainfall excess as a storm progresses in time. This enables the application of a unit hydrograph model in real-time river flow forecasting. The rainfall and runoff readings up till the time of forecast are used to obtain the current estimate of the φ-index and rainfall excess. The rainfall excess thus computed is convolved with the unit hydrograph to formulate the forecast. The success of the suggested procedure is demonstrated on two Indian catchments.