Runoff reduction impacts riparian zone ecological security in the terminal reservoir of the central line of the South-to-North Water Diversion Project, China

The Miyun Reservoir (the terminal reservoir of the central line of the South-to-North Water Diversion Project, SNWDP) and the upstream watershed of the Miyun Reservoir (uwMR) in China. Under the influence of runoff, water intake and water diversion, the water level of the terminal reservoir of the interbasin water diversion project greatly fluctuates, which poses a threat to the ecological security of the riparian zone. Therefore, the impact of runoff reduction on the ecological security of the riparian zone of the terminal reservoir must be evaluated. This study focuses on the Miyun Reservoir and the uwMR, and the impact of upstream runoff reduction on the ecological security of the riparian zone is systematically evaluated. Runoff in each subbasin of the uwMR is decreasing, with climate change as the primary driver (66.69 %). As water consumption increases (0.44 × 10 8 m³/a) and water diversion reaches its upper limit (12.4 × 10 8 m 3 ) by 2028, the riparian zone of the Miyun Reservoir will exhibit significant exposure (45.08 km 2 ) by 2041. However, vegetation restoration is slow, requiring 19 years to stabilize, which poses a threat to riparian zone ecological security. Therefore, enhancing vegetation monitoring and implementing active restoration measures are essential. The findings offer valuable guidance for protecting the riparian ecosystems of terminal reservoirs of interbasin water transfer projects. • Inflow runoff of the Miyun Reservoir has decreased by 0.075 × 10 8 m 3 /a. • Climate change dominates the reduction in runoff. • It took 19 years for the riparian vegetation to reach a stable state of recovery. • Future riparian zone exposure will threaten the ecological security of this area.