Statistical analysis of precipitation uncertainty in Iran

The average annual rainfall in Iran is about 250 millimeters that is the small than one third of the average annual rainfall of the world. Imbalanced and non-uniform distribution of the low rainfall also increases the rainfall uncertainty in the country. In this study, rainfall uncertainty index (UI) based on statistical data from 48 synoptic stations in a period of 30 years from 1979 to 2008 has been investigated. In UI index, contrary to other methods, rainfall regime or temporal distribution and characteristics of rainfall are considered. The trend and significant status in each station through Mann Kendall and Sen.’s Slope Estimate non-parametric tests and at the 95% confidence interval has been determined. Finally, based on index values obtained, the country’s is classified in different zones using IDW and Kriging interpolation methods. The results show that the average rainfall uncertainty in Iran is equal to 2.2, or about 36%. The level of rainfall uncertainty in humid areas – where rainfall starts earlier and rainfall season is longer – is more, and in arid regions – where rainfall begins later and rainfall season is shorter – is less. However, unlike the results of other works rainfall uncertainty in Caspian humid region and semi-humid North West region of Iran in in high level. In contrary, the lowest level of rainfall uncertainty belongs to dry southern coastal region of Iran. However, Except the Bandar Length station which has an upward slope of 0.04 in a year, no significant trend weren’t observed in other stations.