Spatiotemporal Evolution of Structural Hydrological Connectivity in Baiyangdian from 1990 to 2025 and Its Associations with Water Environmental Factors

Structural hydrological connectivity characterizes the spatial organization and potential linkage of water bodies and provides an important physical basis for water exchange, material transport, and habitat connectivity. To characterize its long-term spatiotemporal evolution in Baiyangdian, annual representative water-patch maps were constructed from Landsat imagery for 1990–2025, and structural hydrological connectivity was quantified using the integral index of connectivity (IIC) and probability of connectivity (PC) at the whole-system and regional scales. Associations with water level, the Water Quality Index (WQI), and reed area were further examined. Results showed that: (1) Temporally, mean IIC and PC were 0.078 and 0.109, indicating weak connectivity. Neither index showed a significant trend, but both shifted from high early values to a period of decline and low-level fluctuations. Relative wavelet power was mainly concentrated at 2–8-year scales during 1990–2001 and weakened markedly after 2003, indicating a transition from strong multiscale oscillations to weak periodic variability. (2) Spatially, connectivity was highest in the Central region and lowest in the Southern region; estimated IIC and PC values increased in all five regions during 2021–2025 relative to 1998–2020, with the largest increases in the Yellow River Diversion region, by 82.9% and 78.2%, respectively. (3) Environmentally, water level was positively associated with both indices, whereas WQI was negatively associated with them; reed area showed moderate positive associations, but its recovery after 2021 lagged behind connectivity improvement. Overall, structural hydrological connectivity has recently improved but remains relatively weak and spatially uneven. These findings provide a long-term characterization of structural hydrological connectivity in Baiyangdian and offer spatial information relevant to water-system restoration and coordinated water-environment management.