Groundwater connectivity driven watershed boundary evolution leads to lake shrinkage in the Inner Mongolia Plateau

While extensive research has explored the responses of lakes to climate change and human activities, the process of watershed evolution driven by altered hydrological connectivity, and the mechanisms by which it drives lake shrinkage, remain poorly understood. We reconstructed groundwater evolution (1985–2020) in the Chahannur Basin using a SWAT-MODFLOW model constrained by drilling and geophysical data. To quantify the process, we developed a novel Regional Groundwater Connectivity Index (RGCI) based on spatial autocorrelation to reveal how watershed evolution drives lake shrinkage. Our results show that under pressure from agricultural pumping, groundwater levels declined continuously, causing the RGCI to drop from 0.46 in 1985 to 0.33 in 2020 and triggering a stepwise boundary evolution. The initial disconnection of the Yannur sub-basin in 1997 marked a shift to instability, followed by a systemic collapse during 2005–2007 as other major sub-basins disconnected, sharply reducing the watershed area from 6757 km 2 to 4136.4 km 2 . We reveal that arid-zone watersheds are dynamic entities controlled by groundwater connectivity, not static units. This discovery provides direct mechanistic evidence challenging the static-watershed paradigm, and offers a new paradigm for understanding lake shrinkage while providing precise, actionable targets for groundwater conservation in similar arid basins.