Seasonal variations and trends of very wet days in Iran

The changing frequency and intensity of extreme precipitation events has raised concerns for ecosystems, agriculture, and water resources. While global analyses indicate an overall intensification of extreme precipitation, subregional deviations can occur due to local climate variability, highlighting the need for detailed regional assessments to evaluate risks and develop effective mitigation strategies. Despite Iran’s vulnerability to climate extremes, few studies have examined seasonal variations in extreme precipitation and the occurrence of very wet days. This study analyzes the spatial distribution, intensity, and frequency of very wet days across Iran using daily data from the Global Precipitation Climatology Project (GPCP) for the period 1997–2023. Extreme precipitation thresholds for each season were defined using a percentile-based approach. Results indicate that precipitation is highest in western Iran and the Caspian Sea region, while eastern Iran remains drier throughout the year. In spring, very wet days are most frequent in northern and northwestern Iran. During summer, most regions experience a decline in precipitation due to the dominance of subtropical high pressure, except for the southern parts of southeastern Iran, which are influenced by the Indian monsoon. Trend analysis reveals a pronounced decline in both the intensity and frequency of very wet days in southwestern Iran during winter, reflecting local and regional influences despite the global trend toward intensifying extreme precipitation. In contrast, increasing trends are observed in parts of southern and southwestern Iran during autumn. These findings demonstrate that the response of extreme precipitation to climate change can vary markedly across subregional scales and seasons, underscoring the importance of region- and season-specific assessments for effective water resource management and climate adaptation. • Very wet days in Iran show pronounced seasonal and regional variations, with highest precipitation in western Iran and along the Caspian Sea coast • Southwestern Iran experiences a declining trend in winter very wet days, whereas parts of southern and southwestern Iran show increasing trends in autumn • Extreme precipitation responses to climate change can vary across regions and seasons, highlighting the need for targeted climate assessments.