The Changes of Precipitation Frequency Distribution in Iran

In this study, annual precipitation of 232 synoptic and climatology stations of iranduring 1961 to 2003 has been analyzed. the precipitation was mapped based on krigingmethod. then a gamma distribution function was employed for every map's 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 tookplace during 1961 to 1967 with high asymmetry of precipitation distribution in someplaces and low scale parameters almost all over the country. the second phasehappened in 1968 and continued to 1986; this phase is characterized by its highfrequency of precipitation of extreme aridity and humidity in the mid-areas of thecountry extending from west to east. the third phase started in 1986. at this time, therain fall occurred around the long-term average, but the range was lower compared tothe previous phase. in this phase, the shape and scale parameters even in the moist partsof the country bad remarkably changed.