Document Type : Research Paper
Analyzing the changes in the characteristics of daily precipitationis very important in water resources planning and cultivation patterns. In this research, the changes of the most fitting frequency distribution and its parameters over time are investigated. To this end,thenetworked daily precipitation data of the Middle East Aphroditeprecipitation database with a latitude and longitude of 0.25, 0.25 degreeswere used.The daily precipitation data of this database in Iran, during the period of22.03.1951 to 20.03.2007 (20453 days) was extracted using Grads software. To track the changes over time, the data was divided into two 28-year periods. The precipitation data of the first period from 22.03.1951 to 20.03.1979 was placed in an array with a dimension of 10226*2491 and the data of the second periodfrom 21.03.1979 to 20.03.2007 was placedin an array with a dimension of 10227*2491(the rows and columns represent the number of days and cells respectively).The Kolmogorov-Smirnov Goodness-of-Fit test was used to identify the most fitting distribution of rainy days. The normal and gamma type distribution functions onindividual cells of the precipitation were fitted by programming in Matlab software environment. The theoretical two- parameter and exponential gamma distribution function has been able to attain the required statistical conditions for the Goodness-of-Fit test at a 95% confidence level as the most fitting distribution in both periods.The extensive spatial and temporal changes in precipitation distribution are not confirmed as one of the components of climate change tracking. The calculation of the most fitting distribution parameters by linear torque method shows slight variations in the spatial dispersion pattern of these parameters. But, these changes do not follow a specific pattern. The mapping of the standard deviation variation indicates that in a vast part of central and western Iran, the confidence in precipitation has decreased in recent decades.