Asymmetry of blue and green water changes in the Yangtze river basin, China, examined by multi-water-variable calibrated SWAT model

Renewable freshwater is a fundamental natural resource for sustaining human social development and the health of terrestrial ecosystems. While blue water (BW), which can be directly utilized by human beings, has received significant attention, the importance of green water (GW), crucial for ecosystem safety, has often been overlooked in water resource assessments. To date, there has been a lack of integrated research incorporating the detection and attribution of changes in both BW and GW in the Yangtze River basin (YRB), China's largest river basin, resulting in a deficiency of comparative and interactive analyses between them. In this study, we examined the historical trends and long-term memory of BW and GW flows across 272 sub-basins of the YRB using a multi-water-variable calibrated SWAT model, which was validated by streamflow, evapotranspiration, and terrestrial water storage change. We determined the individual contributions of climate variability and land use changes to BW and GW changes. Additionally, we identified the responses characteristics of BW and GW changes in four different ecosystems primarily characterized by alpine grasslands, cropland, forests, and river–lake systems, respectively. The results indicate a non-significant decreasing trend in precipitation and BW in most regions, while an increasing trend is observed in GW, which is expected to persist in the future. Climate variability played a predominant role in BW and GW changes, while land use change also appeared to have significant impact on monthly GW changes. GW showed a positive correlation with precipitation/humidity index in relatively dry and moderately humid areas, but a negative correlation in highly humid areas. Moreover, a stronger correlation was found between the gross primary production (GPP) of terrestrial ecosystems and GW compared to GPP and BW. The findings contribute to guiding the harmonious utilization of water resources between humans and ecosystems in the YRB, as well as optimizing water allocation.