Hydropower is critical to South and Southeast Asia’s energy security, yet climate change threatens its sustainability through altered hydrological cycles. This study assesses the impacts of climate change on hydropower capacity factors across 12 countries (86% of regional installed capacity) from 2020 to 2100. Using an ensemble of 60 climate-hydrological models (5 GCMs, 4 GHMs, 3 RCPs), we evaluate shifts in precipitation, glacier melt, and extreme events on seasonal and annual hydropower performance. The objectives are to 1) quantify regional and sub-regional capacity factor trends, 2) identify drivers of variability, and 3) propose adaptive strategies for resilient energy systems. Results indicate a regional mean decline in hydropower capacity factors by 4.8% (2020-2059) and up to 5.4% (2060-2099) under high-emission scenarios. The Indian Subcontinent faces severe reductions (6.9% by 2100) due to drier winters and monsoon shifts, while Mainland Southeast Asia declines by 5.9%. Conversely, Himalayan regions (Nepal, Bhutan) show transient declines (？2.4% by 2059) followed by recovery post-2060 as glacial melt temporarily offsets precipitation deficits. Maritime Southeast Asia exhibits mixed trends, with capacity factors rebounding under high warming. To mitigate risks, we recommend integrating hydropower with floating solar photovoltaics, optimizing reservoir operations via AI-driven forecasting, and strengthening transboundary water-energy governance. These strategies can enhance grid resilience, offset seasonal variability, and support decarbonization goals. Policymakers must prioritize region-specific adaptations, particularly in climate-vulnerable zones like the Indian Subcontinent, to ensure energy security amid escalating hydrological uncertainties.
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