Abstract
Year-to-year variation of the average residence time of fresh water in Ariake Bay from 1990 to 2000 is investigated using observed salinity and river discharge data. Average residence time of fresh water mainly depends on the river discharge, that is, it becomes long as river discharge decreases in Ariake Bay. However, under the same river discharge the average residence time was longer in early 1990 than in late 1990. This is considered to be due to the decrease of tidal current amplitude in late 1990. Because of the decrease of tidal current amplitude the estuarine circulation was intensified and the average residence time of fresh water became shorter, that is, water exchange has become better in recent years in Ariake Bay.
