Investigation of Annual Rainfall Trends in Iran

This paper presents an analysis of annual precipitation trends in Iran. Mean annual rainfalls were collected from 30 synoptic stations with a reasonable geographic distribution and with data equal to or less than 50 years. Trend analysis was investigated using a "regression line slope" method (annual rainfall as a dependent variable and year as an independent variable). The results showed that for the entire period, and at a 95% level of significance, seven stations showed a negative trend, while a positive trend was found at six stations. The same data over the period of last 40 years demonstrated that four and 8 stations had negative and positive trends, respectively. Decreasing the record length, up to the last 30 years, resulted in fewer stations with any significant trend. The results identified that, in any case, the trend did not depend on the mean annual rainfall but rather record length could have some effects on it