Assessing the Sustainability of Instream Flow Under Climate Change Considering Reservoir Operation in a Multi-Dam Watershed

Sustaining instream flows is becoming increasingly critical due to the combined pressure of climate change and intensive reservoir operations in multi-dam watersheds. This study evaluates instream flow sustainability in the Seomjin River basin by integrating the SWAT and K-WEAP models with CMIP6-based climate scenarios. Two contrasting dam operation strategies—firm and deficit supply—were assessed over multiple temporal scales, including hydrological seasons and agricultural activity. Sustainability was quantified using the Sustainability Index (SI), which integrates reliability, resilience, and vulnerability. The probabilistic assessment revealed that the relative performance of the two strategies varied depending on the season and flow conditions. The firm supply generally exhibited higher sustainability under drought and low-demand periods, effectively reducing the probability of unsustainable outcomes. In contrast, the deficit supply often achieved higher sustainability under wet conditions or peak agricultural demand, although it was occasionally linked to extremely low SI values. These findings underscore the importance of season-specific, risk-informed dam operation planning over reliance on a single strategy and emphasize the need for flexible management frameworks capable of responding to diverse hydrological futures.