Climate change assessment on river flows and downstream lakes from mountainous basins: A SWAT and CMIP6 scenarios modeling

This study examines the impacts of climate change on mountainous river basins and their downstream hydrological systems, with a specific focus on the Haraz River Basin in northern Iran, a key tributary draining into the Caspian Sea. Historical records of precipitation, temperature, and river discharge from 2000 to 2022 were analyzed for trends using regression analysis and the Mann–Kendall statistical test. The results reveal a statistically significant decline in river discharge at the downstream gauging station over the historical period. Among the CMIP6 global climate models, the EC-Earth3-VEG model was selected for future projections based on its lower error indices. Future changes in precipitation and temperature for the 2025–2050 period were downscaled using the CMHyd tool under two Shared Socioeconomic Pathways: SSP2-4.5 and SSP5-8.5. Projections indicate mean temperature increases of 1.1°C and 1.4°C, corresponding to rises of 7% and 10% relative to the historical baseline, respectively. These climatic forcing data were then fed into the SWAT hydrological model to simulate future river discharge. The model projects average annual flow reductions of 8% under SSP2-4.5 and 15% under SSP5-8.5. Collectively, these findings underscore the growing vulnerability of the Caspian Sea to diminishing inflow from its tributaries, including the Haraz River. Such reductions are expected to contribute to declining water volume and surface elevation of the Caspian Sea, with far-reaching implications including ecological degradation, shoreline retreat, and transboundary environmental challenges.