Article Open access Saritha Padiyedath Gopalan , Naota Hanasaki , Adisorn Champathong & Taikan Oki npj Natural Hazards 1 , Article number: 48 (2024) Cite this article Metrics Abstract Adaptation is critically important for coping with climate change. However, quantitative studies on which adaptation measures should be taken to maintain the present water risk level in the context of climate change have been explored little, particularly at large basin scales. Here, we devised three adaptation portfolios composed of combinations of measures to alleviate floods and drought with explicit basin-wide modelling in the Chao Phraya River basin, Thailand. Two portfolios mitigated future water scarcity to the present level but failed to eliminate extreme floods. The remaining portfolio with basin-wide reforestation substantially reduced the number of future flooding days but enhanced the number of drought months to 3–6 months a year, resulting from increased evapotranspiration by 7–11%. Overall, future flood adaptation remains challenging even in highly regulated rivers. We also observed that adaptation effects differ substantially by sub-basins. It highlights the necessity of spatio-temporal detailed impact modelling, including multiple adaptation measures. Introduction Climate change will intensify flood and drought events and alter their frequency in conjunction with population growth and economic development 1 , 2 , 3 . Inevitably, to respond to the negative impacts of climate change, there is a persistent need for adaptation planning and implementation, which have continued to increase in at least 170 countries via the formulation of national adaptation plans and policies to enhance their adaptive capacity 4 , 5 , 6 . Despite this significance, quantitative climate change impact studies, including concrete adaptation measures to combat flood and drought events, are limited due to the poor portrayal of human–water interactions in most hydrological models. In existing hydrological simulation studies, adaptation measures are focused on single […]