Probability changes of observed extreme precipitation events over Iran from 1962 to 2019

Global warming increases flood risk and the intensity and the frequency of extreme precipitation events. Temporal and spatial evolution and changes of extreme precipitation events probability is very important for regional flood control and water resource management. This study aims to analysis the probability of extreme precipitation events and their changes over Iran. A gamma function was fitted to each grid's daily data for each year, taking into account non-zero 24-h total precipitation events (precipitation ≥0.1 mm) for the study period 1962 to 2019. The extreme precipitation event is defined by the 90th percentile of the gamma distribution, measuring the extreme tail of the precipitation distribution. The modified Mann-Kendall test was applied to look for trends of the gamma parameters (α and β) and the probabilities of daily precipitation intensities. It is shown that across the country, the shape parameter generally tends to show a decreasing trend while the scale parameter exhibits an increasing trend. As a consequence, the trend of the probability of daily precipitation with intensities of ≥5, ≥ 10, ≥ 15, and ≥ 20 mm/day is positive with slight spatial differences, mainly in the southwestern and western parts of Iran. In contrast, most areas in the western part of country have encountered negative trends in the probability of daily precipitation with intensities of <5 mm/day. This study provides important guidance for water resource planning and coping with changes in the probabilities of extreme precipitations over Iran, as well as in other countries around the globe that are similarly facing extreme precipitation events.