A probabilistic approach to enhance drought modelling in alpine regions: Transition from meteorological to hydrological droughts.

Droughts, particularly in alpine regions, pose significant challenges to water resources management and drought mitigation strategies. This study investigated the transition of meteorological to hydrological droughts in the Qilian Mountains (QLM) of northwest China, using monthly streamflow and meteorological data from 1982 to 2015 across 19 alpine basins. The results revealed that hydrological droughts, though less frequent than meteorological droughts, were more severe and prolonged, with an average duration ranging from 4.64 to 10.43 months and severity from 3.86 to 7.91. The response time of transition decreases as drought duration increases, with net response times averaging 3.84, 3.58, 2.84, and 1.58 months for 1-, 3-, 6-, and 12-month timescales, respectively. Strong linear correlations (R > 0.7) were observed between the duration and severity of meteorological, hydrological, and compound droughts, indicating a consistent pattern of drought intensification. The Gumbel copula function combined with the Bayesian method effectively modelled drought transition, with glacier coverage and soil water variation identified as the most significant factors influencing drought transition, explaining over 20 % of the variability. The study also highlighted that the transition ratio increases with drought severity, suggesting a decline in the resilience of the QLM's water regulation capacity. These findings underscore the importance of understanding drought transition mechanisms to enhance early warning systems and improve water resources management in alpine regions.