Spatiotemporal Responses of Groundwater Levels to Drought–Flood Abrupt Alternation in the Poyang Lake Area: Mechanisms and Multi-Factor Contribution Quantification

Groundwater responses to drought-flood abrupt alternation (DFAA) depend on both rainfall input and antecedent dry-wet conditions. We incorporated the Standardized Drought-Flood Abrupt Alternation Index (SDFAI) into a geographically weighted regression (GWR) model for the Poyang Lake Area. The model performed well (mean local R2 = 0.895). SDFAI showed the largest coefficient-based contribution, accounting for 56.48% under drought-to-flood (DTF) and 56.52% under flood-to-drought (FTD) conditions, compared with 12.36% and 11.64% for precipitation. Despite similar relative shares, the two regimes exhibited contrasting groundwater-response pathways: DTF coefficient patterns were consistent with rapid recharge after antecedent drought and preferential infiltration through clay-rich deposits, whereas FTD coefficient patterns were consistent with delayed aquifer recession and enhanced lateral groundwater discharge as lake stage declined. Alternating SDFAI signs along the lake fringe further indicated a dynamic lake-groundwater hydraulic boundary. SDFAI therefore complements rainfall amount in characterizing groundwater response to abrupt dry-wet transitions.