Long term precipitation trend in Iran

Daily precipitation data from 38 stations in Iran are analysed for the period of 1960-2001 to evaluate the possible long term trend in annual and seasonal precipitation. Trends are evaluated for the station records and regional average series. The results show that the annual precipitation has a negative trend in the west, northwest and southeast and a positive trend in most of the stations in other parts of the country. The regional results show that the number of rainy days during the year and in winter has increased in all regions (95% significant). Trend in the spring precipitation is negative in most regions, but significant (99%) only in the north. Autumn precipitation shows a positive trend over the country and in all regions, except for the west and north-west. The analysis of trend has also performed for 10 classes of intensities of mean daily precipitation. The results show that the type of variations of the total precipitation is mainly determined by the trend in heavy precipitation. In the south and south-west, winter contribution trend shows a shift from lightest deciles to the heaviest ones. In the west and north-west, trends in the spring time heavy precipitation are negative. In the south and south-west regions, trends of annual light and medium deciles are negative, while those of heavy deciles are positive. In the west and north-west, annual light precipitation deciles show weak increasing trends, but annual heavy deciles show strong decreasing trends.