In the recent years,with the global warming,the number of extreme drought events is increasing.During the period of the previous study,we just focus on drought climate changes that caused by the decline of precipitation.However,the impact of drought events on agriculture is determined not only by precipitation but also by the number of persistent rainless days.The rainless days is a significant index indicating drought and it is necessary to do some research about it.However,we find few researches on the ra...