Future changes in water resources, floods and droughts under the joint impact of climate and land-use changes in the Chao Phraya basin, Thailand

Global Climate change and local human activities have profoundly affected the regional hydrological cycle and water resources. It is imperative to explore the potential changes in future water resources and water-related hazards at the regional scale under global warming and local socioeconomic development, while a scientific assessment of future hydrological risks requires reasonable projection of future climate, land use and vegetation changes. In order to improve the traditional statistical downscaling method, this study combines the machine learning and quantile mapping methods to project future climate under four shared socio-economic pathwayrepresentative concentration pathways (SSP-RCP) of the CMIP6. Future land use is projected jointly with the future climate by the CA-Markov model, and the vegetation dynamics are simulated by the Biome-BGC model. Then we employ a physically-based distributed hydrological model to simulate the future hydrological changes in the Upper Chao Phraya basin under the interaction among climate and land use changes and the vegetation dynamics. The results show that under the joint impact of climate and land-use changes, the study area may face increasing water scarcity and more frequent floods and droughts in the future. Water scarcity will reach the worst in the mid-21st century (water resources per capita decrease 34.2% compared to the 2010 s). By the end-21st century, the 100-year historical flood and drought in the study basin will increase by 1.63 times and 0.59 times, respectively, under the SSP126 scenario (the most sustainable pathway), and by 4.55 times and 1.56 times under the SSP370 scenario (the most pessimistic rocky-road pathway). Results demonstrate that climate change is the major cause for more frequent floods and droughts in the future, while afforestation or more sustainable land use management will mitigate the adverse effects of climate change to some extent. This finding is helpful to the local government in managing future water resources, floods, and droughts in the study basin.