investigating the changes of seasonal rainfall pattern in hamedan province

abstract one of the most variability of atmospheric parameter is precipitation, which changes seasonally. it is important to understand exact characteristic of rainfall seasonal variable. therefore, it is unavoidable to study rainfall season in hamedan province, because hameden is one of the important centers of agriculture in iran. in this research, the seasonal variability of rainfall is considered in hamedan province. for this object, precipitation, onset and cessation data were extracted and the trend and seasonal variability of rainfall were analyzed during 30 years (1979-2009), so observations were divided to three separated decades. the result of this study show the duration of rainfall is from early falls until end of spring in the first decade. this result indicated that the duration of rainfall is long. in the second decade, there is no significant change in rainfall season, but only in some part of the province, the rainfall duration is shorter and in the other part it is longer. nevertheless, in the third decade there is considerable movement. the onset of rainfall duration has been in winter and cessation of rainfall duration has been in summer. it means that rainfall season has moved to summer slowly in recent years. the trend of rainfall duration has increasing trend. on the other hand, the duration length of rainfall is longer than before. in order to classify, rainfall seasonal variability is interpolated and the cluster analysis is done base on the “oghlidos distance” and “ward” integrated method. according to this method, hamedan province is classified to 3 groups. the first group indicates the shortfall in the most parts, the second group indicates rainfall season onset is moved to winter, and in the third part the rainfall season has not had any changes.