Spatial-temporal dynamics of meteorological and groundwater drought in Northwest China: Propagation, threshold, recovery time, drivers

Northwest China Understanding the propagation mechanism from meteorological to groundwater drought is crucial for groundwater management and drought early warning. However, scant research exists for mechanism of the unseen groundwater drought propagation. This study applied drought indices including the Standardized Precipitation Index (SPI) and Groundwater Drought Index (GDI), and utilized methods such as run theory, convergent cross mapping (CCM), Copula function, and Bayesian network, as well as several open-source data sources to analyze the drought characteristics, propagation rule, threshold, and recovery time of meteorological-groundwater drought in Northwest China (NWC) from 1960 to 2024. Specifically, the 'compound meteorological-groundwater drought event' is defined as the period from the onset of groundwater drought to the end of meteorological drought, aiming to highlight the full system response time from the initiation of deep water deficit to shallow water recovery. The contributions of driving factors were further quantified using XGBoost-SHAP, a game theory-based approach for interpreting model outputs and quantifying feature importance. The number of meteorological-groundwater drought events is lower than that of meteorological droughts but higher than that of groundwater droughts, with the shortest average duration (2.29 months) and the lowest severity (3.94). The propagation time (PT) of meteorological-groundwater drought is 4.69 months. The average probabilities of the meteorological drought triggering mild, moderate, severe, and extreme groundwater droughts are 30.27 %, 20.60 %, 9.63 %, and 5.50 %. The propagation threshold is dominated by extreme meteorological drought, accounting for 55.69 %. The recovery time for compound meteorological-groundwater droughts reached up to 3.05 months, exceeding that of individual meteorological or groundwater drought events. ENSO has the strongest influence on the groundwater drought. The interaction between climate change and human activities has the largest average contribution at 64 %, with Digital Elevation Model (DEM), precipitation (Pre), soil moisture (SM), and Gross Domestic Product (GDP) being the primary factors. These findings highlight the importance of drought monitoring and differentiated groundwater management in arid and semi-arid regions. After 1990, the GWSA showed a significant downward fluctuating trend. Propagation thresholds are primarily governed by extreme meteorological drought. Meteorological-groundwater compound droughts exhibit the longest recovery time. XGBoost-SHAP reveals DEM, Pre, SM, GDP as key drivers.