Extreme drought and declining groundwater tables are exacerbating water scarcity in semiarid regions, threatening the sustainability of revegetation efforts. To quantify how canopy transpiration (E-n) of Mongolian pine (P. sylvestris) responds to these dual stressors, we conducted artificial precipitation reduction experiments at sites with contrasting groundwater table depths of 2 m (WT2) and 11 m (WT11). At each site, three precipitation treatments were established: 0% reduction (CK, ambient precipitation), 30% reduction (-0.3P, light drought), and 60% reduction (-0.6P, severe drought). From May to October 2023-2024, we continuously monitored soil water content (SWC), sap flow, groundwater table dynamics, and meteorological variables. Results showed that SWC decreased with increasing precipitation reduction at both sites, with more pronounced declines at WT11 than WT2. E-n in the WT2-CK, WT2-0.3P, and WT2-0.6P treatments exceeded those at WT11 by 41%, 42%, and 28%, respectively, indicating that shallow groundwater effectively sustains high transpiration and partially mitigates drought stress induced by precipitation reduction. However, E-n decreased significantly with intensifying water stress at both sites (p < 0.05), demonstrating groundwater's limited capacity to offset precipitation-driven soil water losses. As water stress intensified, SWC replaced meteorological factors as the primary regulator of E-n. Concurrently, canopy conductance and its sensitivity to vapor pressure deficit decreased significantly under drought conditions, a flexible stomatal regulation strategy that minimizes excessive water loss. Notably, the ratio of E-n to precipitation (E-n/P) increased significantly in the WT11-0.6P treatment, indicating that severe drought led to a greater fraction of effective precipitation being consumed by transpiration, resulting in continuous SWC decline and subsequent water deficit. Our findings highlight that revegetation in water-scarce regions must integrate both prevailing groundwater table depth and precipitation regimes. This study provides critical insights for optimizing afforestation management and enhancing ecosystem resilience under future climate change scenarios.c