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 distribu-tion 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 identi-fied that, in any case, the trend did not depend on the mean annual rainfall but rather re-cord length could have some effects on it.