How Does Water Quality Reshape the Water–Energy–Carbon Nexus? Evidence from the Yellow River Basin

The water–energy–carbon (WEC) nexus is central to sustainability, yet the role of water quality as a potential moderator within this nexus remains empirically underexplored. Using a fixed-effects panel regression model applied to provincial-level data from the Yellow River Basin over the period of 2007–2022, this study provides systematic evidence that water quality significantly moderates the WEC nexus. The empirical findings indicate that (1) water quality negatively affects the WEC nexus significantly; (2) the moderating effect presents spatial heterogeneity, showing greater carbon reduction potential in middle and lower reaches; and (3) the marginal impact of water cycle energy consumption on carbon emissions declines monotonically with improvements in water quality, with a sample-specific estimated critical point identified at a compliance rate of 88.159%—below which the marginal effect remains positive and above which it turns negative. (4) The moderating effect exhibits pronounced spatial heterogeneity, being significantly positive in the middle and lower reaches but insignificant in the upper reaches, and is significantly strengthened after the implementation of China’s strictest water resource management policy in 2013. These results suggest that water quality serves as a strategic lever for synergizing WEC governance. The findings offer theoretical and practical insights for adjusting the tightly coupled WEC nexus in the Yellow River Basin, with implications for region-specific and threshold-oriented policy design.