Definitions on the onset and ending date of East Asian summer monsoon were reviewed and a new method by use of effective precipitation data only proposed.
At the onset day of summer monsoon, the daily value of available water resources that named effective precipitation should be larger than year average. two or more consecutive rainy days should appear also. Through the monsoon period, the effective precipitation should be larger than the sum of year average and one standard deviation.
Ending day is the last rainy day before two or more consecutive days that not only have no considerable rainfall but also are less than threshold value of effective precipitation.
The method was tested with the synoptic chart and daily rainfall distributions. As a result, it showed better objective results on defining the space and time boundary of monsoon area and duration