Study on the spatiotemporal evolution of blue and green water security in hilly regions with hydrological variations

Blue water and green water are vital freshwater resources for ecosystems and human activities and are particularly crucial for effective water management in hilly areas. However, their availability and security are threatened by climate and surface conditions, impacting sustainable development. This study utilized the Soil and Water Assessment Tool (SWAT) hydrological model to estimate the blue and green water resources in four hilly areas of the Haihe River Basin from 1998 to 2019. A comprehensive differential split-sample test was conducted to classify the climate periods into wet, normal, and dry years, highlighting the spatiotemporal evolution of these water resources. A risk assessment framework was developed to evaluate the security of blue and green water resources in each climate zone. Key findings are as follows: (1) SWAT models demonstrated strong accuracy (Nash–Sutcliffe efficiency coefficient of >0.70; percentage bias <20%) and (2) green water was predominant (approximately nine times more than blue water), with notable spatial differences across climate periods; (3) the blue and green water scarcity index increased from wet to dry years, indicating a higher risk of blue water. The research results can provide a reference for the sustainable management of regional water resources.