Abstract
Improvement in flood control and water availability by multipurpose dams has been called for as a part of the strategies for adapting to climate change. Improvement in the accuracy in flood forecasting is one such strategy. The runoff of a river depends on the moisture condition in the watershed. A runoff calculation method that takes into consideration the moisture condition in the watershed is useful in improving the accuracy and stability of flood forecasts. This paper quantifies a basin's storage by evaluating the long-term water cycle in the basin and proposes a method of calculating the runoff percentage using the resulting quantities. When the proposed method was applied to 25 floods in the subcatchments of five dams in the Ishikari River System, the calculations corresponded closely to measurements. The proposed method is thought to be effective for calculating general runoff.
