Trends in total precipitation and magnitude-frequency of extreme precipitation in Iran, 1969-2009

Spatiotemporal changes in total precipitation as well as the magnitude and frequency of extreme precipitation events in Iran are assessed using 187 gauging stations with at least 41 years of records until 2009. The spatial distribution of extreme precipitation is evaluated based on the generalized extreme value (GEV) distribution fitted to the annual/seasonal maximum daily series at each location. Temporal trends of the magnitude of extreme precipitation events are also analysed using the annual/seasonal maximum daily series, while temporal trends of the frequency of extremes are assessed based on records exceeding the 99th percentile threshold. Results show an overall declining trend of the annual precipitation in particular in regions located on the north, west and northwest of Iran. Seasonal analysis shows the largest contribution of winter to this declining trend. In addition, precipitation has significantly decreased in the northwest during spring. Although the changes in the magnitudes of extreme precipitation events are insignificant, with increasing trends in 50% of the stations, the overall frequencies show significant declines in particular during winter. The magnitude and frequency of extreme precipitation have also significantly declined in the northwest region during spring.