Specifications and Accuracy of Rainfall Forecast Required for Pre-Release at Multi-Purpose Reservoirs in Japan

Pre-release is the discharge from a reservoir before a flood to enhance flood control capability. Its success depends on the performance of rainfall forecasting. However, there is little information regarding the causal relationship between its performance and the success of pre-release. Therefore, the rainfall forecast required for pre-release at 326 multi-purpose reservoirs in Japan is shown quantitatively in this paper. In our analysis, pre-release was simulated based on tentative rainfall forecasts made using observed rainfall data from a period of 17 years (2006 to 2022) with some processing. Then, outputs were evaluated in terms of two risks: not avoiding emergency spillway gate operation and no recovery of water use capacity. The results of five elements were reached: (1) the characteristics of situations requiring pre-release, the required (2) forecast length and (3) spatial resolution, the required accuracy of (4) the rainfall amount, and (5) the position of rainfall zone. For (1), pre-release is required nationwide in typhoons or stationary fronts at a frequency of four instances per year. For (2) and (3), assuming perfect accuracy, the current specifications of rainfall forecast in Japan: forecast length of 84 h or more and the combined use of 5 km and 20 km spatial resolution are generally effective in themselves. For (4) and (5), possible uncertainties in the rainfall amount and the position of rainfall zone needs to be decreased by one digit for avoiding emergency spillway gate operations, while excessive pre-release tends not to result in no recovery of water use capacity.