Study of Variation of Extreme Precipitations over Iran

Changes in heavy and extreme precipitation and increase in frequency of extreme events such as flood and drought is one of the results of global warming. Based on recent report of Untied Nations Environmental Programme (UNEP), frequency of flood and other extreme climatic events are doubled and their hazards of them increase to $ 150 milion every year.These impacts will cause a major variation on society and ecosystem. Then in recent decades, survey on precipitation variation and other extremes such as snow, hail, heavy precipitation and ect. have a key role in policy of agricultural planning and water resources management. Therefore, countries have begun to study monthly and seasonal–scale climate variabilty and also variation of extreme values. Studies on monthly and annual precipitation have been done in Iran. National and regional investigation on extreme values of temperature and precipitation variation have begun on the basis of daily data and new extreme indices. In this paper, we try to introduce some of indices for precipitation and temperature and to investigate their variation in some stations. A few indices such as, no. of frost days (FD), Percent of time Tmin greater than 90th percentile of daily minimum temperature (Tn90), for Consecutive Dry Days (CDD), No. of days with precipitation equal or greater than 10 mm(R10), Maximum 5 day precipitation total (RX5day), Simple Daily Intensity Index (SDII) and Fraction of annual total precipitation due to events exceeding the 1961-90 95th percentile, are presented for 5 stations in 1951-2003. The result shows that the analyzed stations have significantly fewer frost days per year. There is a statistically significant increasing in warm nights but the result for precipitation indices shows a mixed pattern of positive and negative changes which are needed to be considered in agriculture planning and water resource management.