Abstract

The rainy season (RS) constitutes a fundamental component of the hydroclimatic system in arid and semi-arid regions, where water availability is highly sensitive to changes in precipitation timing and persistence. In a country such as Iran, where large areas are characterized by limited and irregular rainfall, variations in the onset, cessation, and duration of the rainy season can exert profound impacts on water resources management, agricultural productivity, food security, and ecosystem sustainability. Despite its importance, comprehensive assessments of long-term changes in rainy season characteristics across different spatial scales in Iran remain limited. The present study aims to investigate spatiotemporal variations in the timing of the rainy season—including onset, cessation, and length—as well as changes in the proportion of rainy season precipitation relative to annual precipitation. These variations are examined for both the widespread rainy season and regional rainy seasons across Iran, with a particular focus on contrasting their behavior during two distinct climatic phases: a relatively wet period (1960–1997) and a predominantly dry period (1998–2023). To achieve these objectives, daily precipitation records from 72 synoptic meteorological stations distributed across Iran were analyzed. The selected stations provide long-term observational records ranging from 32 to 63 years, ensuring adequate temporal coverage for detecting climatic shifts. Rainy season indicators were derived using consistent criteria applied across all stations, allowing for a robust comparison of temporal changes and spatial patterns. In addition, multivariate regression analysis was employed to assess the influence of key geographical factors—longitude, latitude, and altitude—on the spatial variability of rainy season timing. The results indicate pronounced changes in rainy season characteristics during the second climatic phase. Specifically, the onset of the widespread rainy season has been delayed in many regions, while its cessation has occurred earlier, leading to a statistically significant shortening of the rainy season length. These changes are most evident in the northeastern, southeastern, and parts of the southwestern regions of Iran. Furthermore, the explanatory power of spatial variables varies across the country, highlighting strong regional contrasts in the controls governing rainy season dynamics. At the regional scale, the rainy season has experienced reduced rainfall continuity, accompanied by an expansion of dry intervals within the season. The most substantial declines in continuity are observed in the northeastern and southeastern regions, which are already vulnerable to water scarcity. Overall, the findings reveal structural changes in the seasonal rainfall regime of Iran, reflecting modifications in the behavior of rain-producing atmospheric systems. These changes are consistent with the broader signals of recent climate change and global warming, underscoring the need for adaptive strategies in water resources planning, agriculture, and climate-resilient development.