Lagged contributions and compound effects of meteorological and surface water droughts on groundwater drought

Previous studies have not precisely quantified the relative contributions of different drought types during propagation. This study used lagged feature importance derived from a random forest model to quantify the contributions of meteorological and surface water droughts to groundwater drought. Conditional probability and spatial analyses are combined with Copula functions to assess joint exceedance probabilities under different scenarios and reveal the conditional and multidimensional responses of groundwater drought. The results show that: (1) The optimal propagation time from meteorological drought to groundwater drought is 3 months, compared with 24 months for surface water drought. Under their combined effects, the groundwater drought index (GDI) is best explained by standardized precipitation index (SPI) lags of 0–3 months and standardized runoff index (SRI) lags of 0–9 months (R2=0.600). (2) Meteorological drought propagated to surface water mainly within 12 months, whereas groundwater responded more slowly. Groundwater drought areas generally overlapped with earlier surface water deficits, and longer lags occurred at higher elevations. (3) Under high joint probability scenarios (0.66–1.00), the average duration, severity, and affected area of groundwater drought are markedly greater than those under low and medium probability levels. These findings improve the understanding of drought propagation and support watershed water management and drought risk assessment.