Evolution of water clarity across the Beijing-Tianjin-Hebei region over the past four decades (1984–2024): Driven by hydrological expansion and anthropogenic intervention

Given escalating water scarcity and rapid urbanization, characterizing the long-term evolution of surface water environments across the Beijing-Tianjin-Hebei (BTH) region is vital for coordinated regional development. Using Landsat imagery spanning 1984–2024, this study quantified Secchi disk depth ( Zsd ) dynamics in 1105 water bodies across the BTH region and identified the dominant drivers through correlation analysis and generalized linear models (GLMs). Results show that water bodies in the BTH region are predominantly small (1.33 ± 10.32 km 2 ) and characterized by a generally low baseline water clarity (mean Zsd = 0.62 ± 0.22 m). Nevertheless, a pervasive long-term clearing trend was detected: 60.0% of water bodies ( N = 663) exhibited significant increases in water clarity over the past four decades, whereas only 4.2% ( N = 46) showed significant declines ( P < 0.05). GLM analysis revealed that water area expansion was the primary driver of Zsd variability, consistently explaining more than 45% of the long-term variations. This expansion was closely linked to precipitation variability in Hebei Province, while in Beijing and Tianjin it was primarily driven by the South-to-North Water Diversion Project. Land-cover transitions acted as secondary drivers, accounting for 27.2% of the observed variance, whereby reductions in barren land and increases in impervious surfaces together with strengthened environmental governance, effectively mitigated sediment loading. In contrast, intensive cropland in certain regions, particularly Hebei, may still exert a localized influence, where agricultural runoff can partially limit improvements in water clarity. Overall, these findings suggest that strategic water diversion and ecological restoration have been associated with improvements in regional surface water quality, while localized agricultural pressures require more targeted management interventions. This study provides new insights into how both natural and anthropogenic drivers interact to shape long-term water clarity dynamics in this highly water-stressed region and offers a robust scientific basis for informed water resource and land-cover management, and sustainable regional development.