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Title
Japanese: 
English:Rainfall Forecast Accuracy required for Pre-release at Multi-purpose Dams in Japan 
Author
Japanese: 藤田隼人, 鼎信次郎.  
English: Hayato Fujita, Shinjiro Kanae.  
Language English 
Journal/Book name
Japanese: 
English: 
Volume, Number, Page        
Published date Dec. 15, 2022 
Publisher
Japanese: 
English: 
Conference name
Japanese: 
English:AGU Fall Meeting 2022 
Conference site
Japanese:U.S.A. 
English:マコーミックプレイス 
Official URL https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1076738
 
Abstract Dam pre-release is a discharge before projected flood arrival based on rainfall forecast to temporarily enhance flood control capability. However, rainfall forecast still has large uncertainties. In such a situation, we revealed how much accuracy of rainfall forecast is required for dam pre-release. We analyzed all multi-purpose dams from first class rivers of Japan (326 dams) and 1 km grid data of observed rainfall during the past 16 years (2006 to 2021). Target spatiotemporal scale is small catchment area (a few to hundreds km2), small reservoir capacity (hundreds of thousands to hundreds of millions m3) and hourly forecast up to next several days because of the rapid increase of their inflows within several hours. We focused on four controversial components in recent flood disasters and their forecasts in Japan: 1. relationship with synoptic scale disturbance, 2. lead-time, 3. spatial resolution, and 4. position accuracy of rainfall zone. In our analysis, we assessed the effect in “the time required for pre-release” under discharge at harmless flow rate. This time is computed by integrated rainfall-runoff model (the tank model similar to JMA system) for inflow calculations using the rainfall data, and reservoir operation model for calculations of the amount exceeding capacity when stored by regular rules from the inflow. Results suggest that typhoon, stationary front, and their combination should be prioritized to improve the forecast. Required lead time is within a day and a half in typhoon cases and reaches 2-3 days in front cases. This means high quantitative accuracy is required until the end of JMA Meso-Scale Model (MSM) having forecast length of mainly 39 hours, and the forecast length should be also extended further. It is also indicated that the combined use of 5 km and 20 km spatial resolution models (similar to JMA MSM and Global Spectral Model) is generally sufficient, and the position error of rainfall zone should be smaller than current JMA forecast by one digit. This study can be utilized for similar small-scale dams in upstream mountainous areas or small rivers all over the world.

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