Analysis of Changes in the Frequency Distribution of Four Decades of Iranian Daily Precipitation

The aim of this research is to investigate temporal-spatial variations in the frequency distribution of daily precipitation in Iran. For this purpose, data from daily precipitation were interpolated using 1427 synchronization, climatic and rain-gauging stations in Iran through Kriging method for the time interval of 1961/03/21 –2004/03/19 and were divided into two periods. Programming in MATLAB software environment with normal family distribution functions (standard normal and log-normal) and gamma (two-parameter and exponential) on a single 7187 pixells were fitted separately in each period to determine the fittest frequency distribution. The results show that, with 95% confidence level, gamma and exponential distributions were statistically able to obtain the conditions required for test. Extensive spatial and temporal changes in precipitation distribution are not confirmed as one of the components of climate change tracking. Calculating the fittest distribution parameters using linear momentum, this parameter indicates the slight changes in spatial distribution pattern; for example, the standard deviation parameter shows that in 63% of the country's area, including central and south of Iran and the confidence in rainfall in the second period has decreased compared to the first one. This increase in uncertainty is a hazard to the vulnerability of sensitive and fragile ecosystems in these areas.