Synergizing conventional and unconventional water to promote water resources spatial equilibrium: An integrated multi-system compatibility evaluation framework.

Unconventional water, as an important supplement to traditional water sources, plays a crucial role in promoting water resources spatial equilibrium (WRSE). From the perspective of the synergistic use of conventional and unconventional water, this study examines the formation, measurement, and improvement of WRSE. Methodologically, it integrates a compatibility index model, set pair analysis, a coupling coordination degree model, interpretable machine learning, and an obstacle degree model to reveal the dynamics of WRSE and identify potential improvement pathways. The results show that: (1) From 2010 to 2023, China's overall WRSE increased by approximately 46.15%, with positive growth maintained throughout 2021-2023. Among the four subsystems, coordination between conventional and unconventional water improved most rapidly. (2) The synergy between conventional and unconventional water was substantially stronger in northern China than in southern China. In many northern regions, synergy scores ranged from 49 to 60, whereas those in many southern regions remained below 12, indicating that northern provinces relied more heavily on the coordinated use of both water sources. (3) The GWRF-SHAP results indicate that ecological protection, wastewater reuse, industrial efficiency, and the expansion of unconventional water use are critical factors for improving WRSE. In contrast, the obstacle degree model reveals that insufficient coordination between conventional and unconventional water remains the primary constraint, particularly in northern China.