Precipitation is the most critical atmospheric indicator of Iran’s climate, exhibiting significant variation over time and across different regions. This variability is influenced by terrain configuration and Iran's extensive longitudinal and latitudinal expanse. In recent decades, Iran's precipitation patterns and spatial distribution have experienced significant changes, largely due to global warming and climate change. This study used daily gridded precipitation data from the Asfazari database, with a 0.25 × 0.25 degrees resolution from 01/01/1962 to 31/12/2019. The precipitation concentration index (CI) was utilized to assess the variation in precipitation over Iran. The results indicated that the average CI for Iran is 0.86, demonstrating a high precipitation concentration (PC) across the country. CI is higher in the southern, southeastern, and central regions of Iran compared to the northern part. In specific areas of the southeast and south of the country, the CI can reach as high as 0.95. As the number of rainy days increases, the precipitation becomes more evenly distributed over time, leading to a decrease in CI. However, the study period revealed a trend of increasing CI over most of the territory. Additionally, there has been a significant increase in the variation of PC values across most of Iran (74.4%) during the study period. The contribution of different precipitation intensities and the frequency of rainy days demonstrates an almost uniform spatial distribution throughout the country. As a result, in the central and eastern regions of the country, precipitation ranging from very low (P ≤ 1) to moderate (P 5–10) intensity plays a more significant role in the total precipitation. Conversely, in the western half, southwest, and northern coastline, severe intensity (P 10–15) to very heavy (P > 30) precipitation contributes more substantially. The trend in precipitation amounts and the frequency of rainy days is generally declining across most parts of Iran, ranging from very low to moderate levels. However, there is a notable exception regarding heavy to very heavy precipitation, which shows a significant upward trend in the western and southwestern regions of the country. Furthermore, the annual precipitation rates have increased, while the frequency of rainy days, particularly in the southwestern region, has decreased. This indicates PC in specific areas. These findings have significant implications for water resource management and the mitigation of atmospheric hazards.
