In recent years, several serious water-related disasters which surpassed the design standard have occurred in Japan. It raised an urgent need of considering risk management in river planning. On the other hand, how to study the basic natural of uncertainty in rainfall-runoff system had always been an important topic in hydrology. Based on the above, Yoshimi and Yamada have recently suggested a theory of using the Ito's stochastic differential equation to study the uncertainty of runoff rate due

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... to the random fluctuation in rainfall. The method can be used in the risk analysis of rainfall-runoff process and also gives a clear physical explanation to the uncertainty of the process. The present study is based on Yoshimi and Yamada's work, aimed at using the theory to practical problems. As a result, the present study firstly applied the method to Kusaki dam basin, unlike the deterministic analysis method, stochastic method gives a result of prediction interval, which is very important for risk management. Secondly it discussed the fractional natural of the rainfall uncertainty and developed a method to consider this kind of uncertainty using stochastic differential equation. The result of the new method is consistent with ensemble method.