The drought intensification–phenological extension paradox is primarily explained by the lagged propagation of groundwater drought and the legacy effects of flooding

In the context of accelerating global climate change, conventional wisdom holds that intensified drought universally shortens vegetation phenology in the Northern Hemisphere's arid regions. However, the paradox of "intensified drought-phenology extension" challenges this view. Using nearly 40 years of two remote sensing vegetation phenology datasets and records from 319 ground-based phenology sites, we identified two distinct phenological zones: the Synergistic Phenology Zone (0–51.5°N) and the Trade-off Phenology Zone (51.5–66°N). We confirmed the dominant role of hydrological factors in spring (SOS) and autumn (EOS) phenological changes (44.3% for SOS, 29.8% for EOS). We innovatively proposed the Drought-Flood Trade-off and Synergy Framework (GDFTS Framework), precisely quantifying the lagged propagation and legacy effects of drought (SOS + 1.81 d/yr, EOS - 1.63 d/yr, peaking in the fourth year) and flood’s positive legacy effects (SOS - 3.34 d/yr, EOS + 1.88 d/yr), alongside their interactive legacy effects (SOS - 0.14 d/yr, EOS + 1.04 d/yr). This framework explains the paradox in phenological changes. Additionally, we developed the Hierarchical Memory-Threshold Response Ecological Groundwater Drought Risk Model (HMT-EGDI) and dataset, revealing that ecological groundwater drought risk is increasing in 57% of the regions. Notably, in the Ecological Water Conveyance Project (EWCP) case, the GDFTS interactive legacy effects were significant, substantially reducing ecological groundwater drought risk, offering new insights into the potential of human intervention to mitigate ecological risks.