A comprehensive analysis of tempo—spatial changes in Iran's precipitation

Abstract

This study analyzed Iran's trends and variations in annual precipitation (AP) from 1971 to 2015. The findings indicate that 80.9% of the nation’s territory has experienced a declining trend in AP, while only 19% has exhibited an increase. Notably, the pronounced decreasing trend is observed in approximately 33.53% of the country. Regions demonstrating statistically significant increasing trends represent merely 2.93% of the total area and appear as dispersed patches. The correlation coefficients (CC) between precipitation trends and geographical coordination-topographic variables (GCTV) were found to be relatively low. In contrast, the CC between AP trends and the amount of AP was more significant; suggesting that large-scale atmospheric forcing predominantly influences long-term trends. Furthermore, a slight decrease in the long-term trend of days with normal and extreme precipitation was identified. These types of precipitation, particularly those below the 10th and 25th percentile, appear to be more influenced by local geographic features than by large-scale atmospheric systems, resulting in a negligible relationship between their trends and the overarching trends in AP. The analysis delineates three distinct phases within the AP time series—1971–1982, 1983–1999, and 2000–2015—highlighting a recorded decline in national AP from the first phase to the final phase. Additionally, a decrease in the month-to-month coefficient of variation (MCV) of precipitation, which indicates a reduction in precipitation during the wet months, generally aligns with the spatial pattern of AP; however, some variability in the CC between GCTV and MCV was observed. The study suggests large-scale atmospheric forcing and the local features (e.g., elevation) play a significant role in Iran's AP trends and phases of AP in Iran. © The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature 2024.