Discussion on the important key factor of storm transposition method under long duration Typhoon

The south region of Taiwan was struck by the rainstorm caused by Typhoon Morakot by which the 48 hour rainfall not only closed to the 48 hour design of PMP but also edged closer to the world record of 48 hour rainfall. Therefore, it is necessary to re-evaluate the impact of long duration typhoon on Tsengwen reservoir. This study aims to select two important factors (representative duration and meteorological station) used in storm transposition method to estimate moisture maximization factor and probable maximum precipitation (PMP) under long duration typhoon. After the comparison of each moisture maximization calculated with 6, 12, 24, 36, and 48 hour representative duration, the result suggests using 48 hour representative duration which is more conservative to estimate design of PMP. As for the meteorological station selection, World Meteorological Organization (WMO) suggests stations located between the direction of moisture and typhoon when it comes to moisture maximization estimation. Therefore, this study used wind direction and rainfall data of 5 typhoon events which influenced catchment of Tsengwen Reservoir to determine the wind direction range of strong rainfall. The meteorological stations in the wind direction range are the most reasonable meteorological stations, as in this case, Tainan meteorological station.