Groundwater is widely regarded as a critical buffer that sustains evapotranspiration (ET) and streamflow under hydrologic stress. However, whether this buffering capacity persists under sustained increases in ET demand remains unclear. Here we test whether sustained increases in ET demand can reorganize subsurface connectivity and undermine effective groundwater buffering, using controlled hillslope simulations with integrated hydrologic modeling and particle tracking. Under baseline semi-arid forcing, ET and outflow exhibit coexisting young and older age components. Following late-summer groundwater drawdown, intermediate-age flow paths weaken, eventually producing a temporary age gap that separates shallow and deep sources. Streamflow responds more abruptly than ET due to hydraulic disconnection at the outlet. Warming and vegetation greening amplify this intrinsic seasonal tendency. Intermediate-age contributions collapse earlier and recover more slowly, amplifying the polarization between shallow and deep water pools and further suppressing older groundwater inputs. Streamflow becomes increasingly dominated by very young water, indicating strengthened groundwater–surface decoupling. These results suggest that sustained hydrologic stress structurally reduces effective groundwater connectivity, weakening subsurface buffering and shortening hydrologic memory. This tendency persists across parameter perturbations. As a consequence, water cycling shifts toward shallower and faster pathways. Progressive groundwater decoupling therefore represents not merely a change in source depth, but a structural transition toward a more rapidly recycled and potentially less predictable mode of terrestrial water cycling under sustained increases in terrestrial water use.