Abstract
This study examines records of annual maximum floods on six major rivers in the Kanto and Hokkaido regions of Japan between 1960 and 2002. Analysis of the Kanto river records indicates that the study period can be divided into two sub-periods. Period 1 (1960 to the early 1980's) was a period of smaller average floods and fewer large floods and Period 2 (the early 1980's to 2002) was a period of larger average floods and more frequent large floods. Analysis of the Hokkaido river records indicates that the differences in flood magnitudes are more limited.
Annual maximum floods in Kanto can be attributed to typhoons and heavy rain during the summer rainy season while those on Hokkaido are mainly due to spring snowmelt. Of these three causes, typhoons are usually responsible for the largest magnitude floods. The limited effects of typhoons and heavy rains during the summer rainy season on rivers in Hokkaido accounts for smaller differences in flood magnitudes between the two periods.
For Kanto, analysis of the record of typhoon tracks and daily precipitation (1-day and 3-day totals associated with the typhoons responsible for annual maximum floods and the 30-day totals prior to the events) suggest that there have substantial increases in rainfall associated with typhoons responsible for annual maximum floods in Period 2.
We also provide data suggesting that the increase in rainfall amounts brought by typhoons can be linked to major shifts in global atmospheric circulation patterns such as ENSO and the PDO.
