Investigative Trend of Commencing and Ending Date of 0.1 and 5 mm Rainfall Thresholds of in Selected Stations of Iran

The global warming process during the last century not only has affected meteorological elements, but it also affected onset and end of meteorological elements. For studying probability changes in time series of onset and end date of rainfall thresholds 0.1 and 0.5 mm and more in the country level and deciding on the kind and its direction, daily rainfall data during past 45 years (1962-2006) of 29 synoptic meteorology stations has been used. First, onset and end date of rainfall in Julius code has been implied. The homogeneity of time series was tested by run-test missing value and was constructed by auto-correlation. For distinguishing random data and trend, Mann-Kendal method was used. The type and commencement time trend was calculated and the changes were also calculated on daily basis with Mann-Kendal graphical test and moving average of 5 years. The findings of this research show that there are trends at onset and end date of rainfall in some stations. On the other hand, in some meteorological stations, onset date of rainfall shifted forward and end date of rainfall shifted backward and the rainfall period length in some stations has been decreased.