Long-Term Change in Regional T-year Rainfall

Abstract

Long-term changes in 10-year daily, hourly, and 10-minute rainfall in Japan were examined by applying Gumbel distribution to regional annual maximum rainfalls, where regional maximum rainfall is defined as the largest value in all rainfall records at rain gauging stations in a region. Here Japan was regionalized into 11 hydro-climatic regions. The objective record length varies by objective rainfall durations, as 105 years for daily rainfall, 58 years for hourly and 47 years for 10-minute. The daily rainfall record was divided into four periods, and the hourly and 10-minute rainfall records into three periods, respectively, and then change in 10-year rainfall for each recording interval was examined by comparing that estimated in the first period and that in the last period. Beside, change in estimated parameters of the applied Gumbel distributions was also examined. The result shows that the numbers of regions where 10-year regional rainfall has increased by more than 10 % were eight out of eleven for daily rainfall, three for hourly and two for 10-minute. The result also clarified that change in parameters of Gumbel distribution mainly arises from increase in standard deviation of annual maximum rainfall, not from increase in its average. It suggests that increase in 10-year rainfall is mainly due to larger fluctuation in annual maximum rainfall.