Abstract
When the soil is wet enough, only the rainfall in the flood arrival time affect to the peak discharge. In Japan, plan rainfall duration is too long while flood arrival time is generally short.
Firtsly we investigated in the flood arrival time of the upper areas of the discharge observatory in A-class rivers in Japan, and researched the relationship with geographical characteristics; and we revealed that the mean flood arrival time can be represented by the length of river channel. Nextly we compared the flood arrival time and the plan rainfall duration in each basin, and found out that in around 90% basins plan rainfall duration is twice as long as the mean flood arrival time.
