بررسي تابع چگالي احتمال بارندگي هاي ماهانه و سالانه ايستگاه هاي قديمي ايران

Probability distribution functions as applied to monthly and annual precipitation of old stations in Iran

Monthly and annual precipitation data in Iran were explained by a variety of statistical distributions. For a certain station precipitation distribution had been reported differently by different researchers. Range of annual precipitation in Iran was from 25 mm for Shahdad to 2000 mm for Tawalesh-e Gilan in coast of Caspian sea. In this paper, normal, two parameter lognormal,three-parameter lognormal,two parameter gamma, Pearson type III, log Pearson type III and type I extremal (Gumbel) distributions were fitted to the monthly and annual precipitation data of the oldest stations in Iran including Bushehr,Isfahan,Mashhad, Tehran and Jask.To test the goodness of fit of distributions Kolmogrov-Smirnov test,mean relative deviation,and mean square relative deviation were used and among which the log-Pearson type III was the best for fitting to monthly precipitations and we couldnʹt choose a unique distribution for fitting to annual precipitations.