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Extreme rainfall causes floods and landslides, and so damages humans and socioeconomics; for instance, floods and landslides have been triggered by repeated torrential precipitation and have caused severe damage in the Kyushu region, Japan. Therefore, evaluating extreme rainfall in Kyushu is necessary to provide basic information for measures of rainfall-induced disasters. In this study, we estimated the probability of daily rainfall in Kyushu. The annual maximum values for daily rainfall at 23 long-record stations were normalized using return values at each station, corresponding to 2 and 10 years, and were combined by the station-year method. Additionally, the return period (RP) was calculated by fitting them to the generalized extreme value distribution. Based on the relationship between the normalized values of annual maximum daily rainfall and the RP, we obtained a regression equation to accurately estimate the RP up to 300 years by using data at given stations, considering outliers. In addition, we verified this equation using data from short-record stations where extreme rainfall events triggering floods and landslides were observed, and thereby elucidated that our method was consistent with previous techniques. Thus, this study develops strategies of measures for floods and landslides.

Evaluating how the changes in rainfall caused landslides is important to prevent sediment disasters. Using the soil water index (SWI) and a three-layer tank model, this study examined the relationship between the occurrence of landslides and long-term changes in rainfall in Miyagawa Village, Mie Prefecture, Japan, where shallow and deep-seated landslides have occurred at different times. The three-layer tank model detected the effects of long-term changes in rainfall on landslides. We educated that the change in the first storage tank layer indicates the potential risk of shallow landslides, while that in the second and third storage tanks indicates the potential risk of deep-seated landslides. Our method could be used to examine the effects of long-term changes in rainfall on landslides in the same region at different times, or over a wider area. Besides, these results are valid because the three-layer tank model used to calculate the SWI is applied throughout Japan, and its effectiveness has been verified.