Trends in river flow at national scale in Iran remain largely unclear, despite good coverage of river flow at multiple monitoring stations. To address this gap, this study explores the changes in Iranian rivers’ discharge using regression and analysis of variance methods to historically rich data measured at hydrometric stations. Our assessment is performed for 139 selected hydrometric stations located in Iranian data-rich basins that cover around 97% of the country’s rivers with more than 30 years of observations. Our findings show that most of the studied Iran’s rivers (>56%) have undergone a downward trend (P value < 0.1) in mean annual flow that is 2.5 times bigger than that obtained for the large world’s rivers, resulting in a change from permanent to intermittent for around 20% of rivers in Iran’s subbasins. Given no significant change observed in the main natural drivers of Iranian rivers’ discharge, these findings reveal the country’s surface fresh-water shortage was caused dominantly by anthropogenic disturbances rather than variability in climate parameters. It may even indicate the development of new river regimes with deep implications for future surface fresh-water storage in the country. This research’s findings improve our understanding of changes in Iranian rivers’ discharge and provide beneficial insights for sustainable management of water resources in the country.

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