Perspectives of human-water co-evolution of blue-green water resources in subtropical areas

Blue-green water resources and their transformation play a vital role in crop production and ecosystem services regulation, and their distribution and utilization are affected by human activity, such as policy-driven green water resources development projects. However, it is unclear how policy-driven green water resources development projects, such as the Grain for Green Project (GGP) in China, affect eco-environmental quality and socio-economic development, that is, human-water co-evolution. Therefore, considering the GGP in a subtropical ungauged basin in southern China as an example, based on remote sensing hydrological station (RSHS) technology and multisource remote sensing data, this study analyzed the impact of the GGP on human-water co-evolution from the perspective of blue-green water resources. The results show that: (1) From 2001 to 2020 the GGP transformed more blue water resources into green water resources, resulting in an increase in green water resources of 282.29% compared to that of blue water resources. (2) Due to the initial success of the GGP in 2005, there was a distinct positive correlation between the green water coefficient (GWC) and basin development index (BDI) from 2005 to 2020 (R-2 = 0.70, p < 0.05), indicating that the GGP did not cause conflict between the sustainability of socio-economic development and the health of the ecological environment. (3) In the GGP scenario, in which the cultivated land area maintained 50% of the current decreasing rate, the overall efficiency index maintain a continuously increasing trend at an average rate of 0.001 year(-1) from 2021 to 2050, which not only improved the eco-environmental quality but also met cultivated land demand. This research reveals the role of GGP in promoting the transformation from blue to green water resources, and proves the feasibility of the simultaneous promotion of GGP and socio-economic development, providing significant insights for the promotion of policy-driven green water resources development projects and long-term blue-green water resources planning and management in other countries or regions.