The grey water footprint of the Guangdong-Hong Kong-Macao Greater Bay Area, China: Spatial patterns, driving mechanism and implications

The grey water footprint (GWF) quantifies the environmental impact of socio-economic activities on water resources. Facing the dual challenges of rapid economic growth and significant water pollution, the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) urgently requires precise assessments of its water pollution situation. This study provides a systematic analysis of the spatial and temporal distributions of the GWF in the GBA from 2008 to 2021, and utilizes the Random Forest model to investigate its driving mechanisms. SHapley Additive exPlanations method was applied to examine the dynamic contributions of driving factors to GWF, while partial dependence plot analysis quantified the relationships between these factors and GWF at various thresholds. By identifying the impact of each factor at different levels, this combined approach provides valuable insights for developing targeted policy recommendations to reduce GWF across cities in the GBA. The results showed that GWF peaked at 103.15 billion m³ in 2011, then declined due to improved water management efficiencies. Spatial analysis revealed higher industrial GWF in central and eastern regions, in stark contrast to the predominantly agricultural GWF in the west. GDP and population were significant determinants, explaining 18.78 % and 17.72 % of GWF variability, respectively. As GDP grew from 371 to 730 billion, GWF surged by 2.21 billion m³; similarly, a population increase from 7.84 to 8.94 million resulted in a GWF rise of 1.8 billion m³. Contributions of P and Ind 3 to the GWF fluctuated significantly, while other factors remained stable. Recommendations advocate for the widespread adoption of advanced aquaculture technologies and reduction of domestic wastewater to mitigate GWF effectively. These insights establish a scientific basis for robust water resource management and sustainable practices within the GBA, offering essential guidance for relevant government agencies.