The combination of global warming and increasing water demand in recent years has posed significant challenges to water resource management. In particular, increasing the probability of extreme precipitation due to global warming has made it difficult to supply and use rainfall. Identifying the measure and severity of hydrometeorological phenomena could help improve water resources management, especially in droughts, and dried regions such as Iran. Also, these phenomena investigation could be beneficial in flood management. One such phenomenon that is known as a significant one in heavy precipitation and catastrophic floods is the Atmospheric River (AR). Atmospheric rivers are long and narrow filaments that transport water vapor across the Earth's middle latitudes, originating from tropical moisture sources, play a significant role in local and global hydrology. In this study, ARs were identified over Iran during a seven-month period (November to May) from 1983 to 2020. The identification was based on Integrated Vapor Transport (IVT) which obtained from the ERA5 reanalysis dataset provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The IVT values was integrated from the ground surface to 300 hPa, and the 90th percentile of IVT values for each grid point during 1980–2020 was used as the threshold (IVT₀) to notice the locality of ARs. Additional geometric criteria—such as a length exceeding 2000 km and a length-to-width ratio greater than 2—were also applied. These conditions helped distinguish ARs from other forms of moisture transport. The results show that Atmospheric Rivers (ARs) occur most frequently in March, whereas May and November have the lowest frequencies. To further characterize the ARs, the average Integrated Vapor Transport (IVT) within the AR region is calculated to determine their strength and intensity. These ARs are then classified into categories based on the quartiles of the intensity data, using a localized intensity calculation within a defined region.
The most intense ARs – which are in the first quartile- were observed in January, February, and March, with an average intensity of 269 kg·m⁻¹·s⁻¹. The weakest ARs' frequently occurred in May and November. Precipitation analysis showed that the highest amount of precipitation due to ARs occurs in December and some other characteristics, except intensity, such as AR duration (ranging from 1 to 5 days), and the frequency of this phenomenon also play an effective role in the amount of precipitation of each month. These results suggest that longer and more frequent AR events contribute significantly to water input during the cold season. Further more, the AR behavior was assessed across dry, normal, and wet years using the Nietzsche index classification. In classifying the surveyed years, due to the Nietzsche index, the highest intensity ARs happened in wet years. The wet years received all of their ARs in strong and extreme categories, about 40 and 60 percent of occurrences, respectively. In dry years, the ARs occurred in all four categories of intensity and most of them (40 percent) happened in median intensity, the normal years, experience about 75 % of ARs in the strong category. These findings underscore the pivotal role of ARs in seasonal precipitation patterns and water resource dynamics in Iran. Understanding their characteristics, variability, and intensity across different hydrological regimes offers a valuable tool for improving flood forecasting. further research into ARs across this region could significantly contribute to more adaptive and informed water management strategies, especially in flood forecasting.