DAILY PRECIPITATION EXTREMES IN IRAN: DECADAL ANOMALIES AND POSSIBLE DRIVERS 1 Hossein Tabari and Patrick Willems 2 ABSTRACT: This study focuses on the empirical statistical analysis of the anomalies in daily precipitation extremes by applying the quantile perturbation method (QPM) to data from 31 Iranian weather stations during the period between 1961 and 2005. The possible causes behind the anomalies in precipitation extremes are iden- tified by analyzing their relationship with the anomalies in eight atmospheric indices (i.e., NAO, SOI, PDO, AMO, NCP, DMI, WeMO, SSN). In terms of decadal oscillations, the country was generally wet in the 1960s and 1970s with most stations exhibiting periods of higher quantile perturbations, whereas lower quantile per- turbations were dominant in the 1980s and 1990s. The highest perturbation in extreme precipitation quantiles prevails in Central Iran during the early 1980s, in which the quantiles are about 50% higher than the ones based on the full time series. The frequency of significant precipitation anomalies for winter season was greater than that for spring and autumn seasons. For the summer season, the humid region in North Iran demonstrates strong positive anomalies. The results highlight the noticeable role of large-scale climatic factors in the anoma- lous behavior of precipitation extremes in Iran. The atmospheric drivers of the quantile anomalies in extreme precipitation were found to differ from one season to another. (KEY TERMS: high precipitation extreme; decadal analysis; anomaly; quantile perturbation method; atmo- spheric drivers.) Tabari, Hossein and Patrick Willems, 2016. Daily Precipitation Extremes in Iran: Decadal Anomalies and Possi- ble Drivers. Journal of the American Water Resources Association (JAWRA) 1-19. DOI: 10.1111/1752-1688.12403 INTRODUCTION One of the most significant potential consequences of climate changes may be alterations in regional hydrological cycles and subsequent changes in river flow regimes, for example, floods or low flows (Zhang et al., 2008). The magnitudes of change in mean and extreme precipitation are different (Dominguez et al., 2012). An increase in the frequency of extreme precipi- tation events as a result of climate change has been pointed out, which can cause severe damage to agricul- ture, ecology, infrastructure, and even loss of human lives (Garcia et al., 2007). In the past 25 years, five billion people were affected by natural disasters resulting in approximately US$1 trillion of economic losses around the world (Stromberg, 2007). These nat- ural hazards are becoming more dangerous as popula- tion and infrastructure continue to increase and occupy areas exposed to higher flood risk (Garcia et al., 2007). Study of change in extreme climatic events is more important than that in mean values, because small changes in mean values can be associated with large 1 Paper No. JAWRA-15-0070-P of the Journal of the American Water Resources Association (JAWRA). Received May 26, 2015; accepted December 22, 2015. © 2016 American Water Resources Association. Discussions are open until six months from issue publication. 2 PhD researcher (Tabari) and Professor (Willems), Hydraulics Division, Department of Civil Engineering, KU Leuven, Kasteelpark Aren- berg 40, BE-3001 Leuven, Belgium; and Professor (Willems), Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brus- sel, 1000 Brussels, Belgium (E-Mail/Tabari: tabari.ho@gmail.com; hossein.tabari@bwk.kuleuven.be). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA 1 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION AMERICAN WATER RESOURCES ASSOCIATION