Design hyetograph estimation of different durations in the Fushan area of northeastern Taiwan

Estimating stromflow hydrographs with hydrologic simulation models at upstream watersheds of Taiwan is difficult because of the lack of input information of rainfall characteristics. Analyses of storm pattern characteristics, establishment of rainfall depth-duration-frequency relationships, development of design hyetographs and their estimated equations were carried out in this study with rainfall records of the Fushan weather station in northeastern Taiwan. Results showed a middle peak storm pattern for hyetographs of 6, 24, and 72-hour durations, front peak storm pattern for hyetographs of 12-hour duration, and back peak storm pattern for hyetographs of 48-hour duration at the Fushan area. Rainfall frequency analyses of different hour durations were performed by the log-Pearson type 3 distribution method and an estimated equation of rainfall depth-duration-frequency was also established. Based on error analyses, the modified ranking method showed a good result for solving the design hyetograph of different durations. The time to rainfall peak, expressed by the percentage of rainfall duration, for design hyetographs of 6, 12, 24, 48, and 72-hour duration were 50.0%, 33.3%, 50.0%, 58.3%, and 55.6%, respectively. The rainfall peak ratio, expressed by the percentage of total rainfall, for design hyetographs of 6, 12, 24, 48, and 72-hour duration were 31.08%, 18.60%, 12.45%, 10.44%, and 9.02%, respectively. Meantime, the estimated equations of exponential type to estimate design hyetographs for different hour durations were established snd showed quite good agreement. Therefore, the design hyetographs of different frequency years and durations can be estimated using only two input parameters of frequency year and duration.