Diverging futures: The overriding influence of climate model generation (CMIP6 vs. CMIP5) on projected river-reservoir water quality and quantity under land-use change

Study region The Tajan watershed and Shahid Rajaei Dam reservoir, Northern Iran. Study focus Climate change, along with land-use and land-cover (LULC) variations, significantly impacts hydrological processes and water quality. Advancing climate science necessitates exploring how the latest CMIP6 scenarios improve impact projections compared to previous generations. This study evaluates the combined effects of climate and LULC changes on water quantity and quality within a river-reservoir system across various SSP and RCP scenarios. To establish a comprehensive framework, the methodology integrates the SWAT and CE-QUAL-W2 models, using downscaled CanESM2/CanESM5 outputs and annual LULC maps to combine CMIP phase comparison with river–reservoir modelling, thereby addressing the underexplored interplay of external factors in the receiving water body. However, downscaling constraints precluded uncertainty assessment of projections through multi-GCM ensembles. New hydrological insights for the region Results indicate a consistent temperature increase across all scenarios, despite variable precipitation trends during 2023–2043 compared with 2003–2022. Among non-aquatic land covers, shrublands are predicted to expand by 24.7%, while croplands shrink by 20.7%, marking the greatest changes between 2023 and 2043. Annual average streamflow is projected to increase by 4.3–19.2% across RCPs, whereas SSPs show mixed changes ranging from –5.9% to +4.3%. Similarly, nitrate loads increase by 3.9–15.1% across RCPs but change heterogeneously within the range of –6.5% to +1.6% across SSPs. Reservoir simulations show greater water-temperature increases under RCP pathways (8.4–9.4%) compared to SSPs (5.6–6.9%). Nitrate concentrations also rise by 1.9–8.2% under RCPs, while SSPs show variations ranging from –3.7% to +0.5%. Overall, RCPs exhibit more pronounced positive effects on water quantity but exert more severe impacts on water quality than SSPs, highlighting the need for mitigation strategies in dynamic, climate-LULC-driven systems.