The Changes of Precipitation Frequency Distribution in Iran

In this study, annual precipitation of 232 synoptic and climatology stations of Iran during 1961 to 2003 has been analyzed. The precipitation was mapped based on Kriging method. Then a gamma distribution function was employed for every mapʹs pixel. Finally, the shape and scale parameters of distribution were analyzed and explained. Since 1961, three phases of precipitation have occurred in Iran. The first phase took place during 1961 to 1967 with high asymmetry of precipitation distribution in some places and low scale parameters almost all over the country. The second phase happened in 1968 and continued to 1986; this phase is characterized by its high frequency of precipitation of extreme aridity and humidity in the mid-areas of the country extending from West to East. The third phase started in 1986. At this time, the rain fall occurred around the long-term average, but the range was lower compared to the previous phase. In this phase, the shape and scale parameters even in the moist parts of the country had remarkably changed.