The propagation from meteorological to hydrological drought and its potential influence factors in a typical inland river basin

The thresholds and influencing factors that trigger the transition between meteorological drought and soil drought are poorly understood, which hinders the effective establishment of drought early warning systems and preventive measures. We selected a typical inland river basin with an alpine-oasis-desert ecosystem, the Shiyang River Basin in Northwest China, toidentify the drought events from 1980 to 2020, and then used the Copula function to assess the risk and threshold of the propagation from meteorological drought to soil drought. The Drought Intensity Propagation Index (DIP) and the Drought Duration Propagation Index (DDP) were used to quantify the propagation process of drought characteristics. Furthermore, we identified and quantified the drivers of drought propagation characteristics. The results showed that: (1) the duration of meteorological drought events extended in 85% of the regions and the intensity of drought increased in 26% of the areas from 1980 to 2020, while the duration of soil drought events decreased and the intensity of drought increased in almost all regions; (2) the propagation of drought has spatial heterogeneity, that is, from the upper mountain to the lower oasis, the probability of propagation  from meteorological drought to soil drought decreased, the degree of drought intensity propagation weakened, and the degree of drought duration propagation intensified; (3) the cumulative precipitation deficit thresholds triggering soil drought were -11mm/ month, -7.5mm/ month and -5mm/ month, respectively; (4) precipitation and evapotranspiration were the main driving factors of the drought propagation characteristics, which explained 44% of the drought intensity propagation upstream and explained 60% of the drought duration propagation downstream, respectively. This study further reveals the law and mechanism of drought propagation, which is helpful for the early monitoring and warning of drought events in river basins, and provides new ideas for the construction of drought propagation models.