Assessing Climate-Change Impacts on Hydrological Processes and Blue–Green Water Dynamics Using Multi-Model Ensembles Under Shared Socioeconomic Pathways in the Hanjiang River Basin, China

This study offers a novel perspective by assessing the impacts of climate change on hydrological processes, with a specific focus on the spatial and temporal dynamics of blue and green water components in the Hanjiang River basin in southern China under various shared socioeconomic pathway (SSP) scenarios. Twelve global climate models (GCMs) from the coupled model intercomparison project phase 6 (CMIP6) were downscaled using the delta change method, producing future climate scenarios for near (2031–2060) and far (2061–2090) future time windows under four SSP scenarios (i.e., SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). These downscaled climate variables, along with the historical control period data, were used to drive the calibrated SWAT model. The model was then employed to analyse blue and green water characteristics and assess the potential changes in hydrological processes under future climate scenarios. The findings reveal that surface runoff constitutes the dominant component of blue water in the Hanjiang River basin. Moreover, the multi-GCM ensemble mean predicts an increase in green water (evapotranspiration and soil water content) over the basin. For blue water, the ensemble mean suggests change patterns similar to those of precipitation, with decreases or slight increases expected in the northeastern part of the basin and larger increases in the southwestern part under most SSP scenarios. Compared to the historical control period, blue water is projected to experience the greatest increase (13.1%) in the southwestern part under SSP1-2.6 and the largest decrease (8.8%) in the northeastern part under SSP3-7.0, both in the far future. The findings have broad international relevance, as the methodology and insights can be applied to other regions worldwide facing similar challenges. This work contributes to a better understanding of hydrological processes in the context of global climate change and supports global efforts to enhance sustainable water resource management and climate resilience.