Analysis on the Extreme Precipitation Events in the Flood Season of Yan'an City for Nearly 50 Years

According to the daily precipitation data in Yan'an City from 1961 to 2010,the changes of extreme precipitation events during the flood season were analyzed.As indicated by the results,the rainfall amount during the flood season from May to September in Yan'an plays a decisive role in the precipitation of the whole year.The daily threshold value of extreme strong rainfall was 32.6 mm,and the extreme strong rainfall amount declined at a rate of 10.65 mm/10a.The relative threshold(R95p and R90p) showed a similar change,presenting a declining tendency.While the absolute thresholds(R25 mm and R50 mm) kept in the similar step with the change of R95p and R90p.In the 21st century,the flood season rainfall increased,the extreme strong rainfall and extreme precipitation intensity tended to increase with a low speed.However,the extreme precipitation days and heavy rain days tended to reduce,indicating that the rainfall of Yan'an city is more concentrated and there is a trend of increase in droughts and floods disasters.