Drought propagation characteristics across China: Time, probability, and threshold

Clarifying drought propagation characteristics is crucial for grasping the mechanism of drought development and evolvement and also for identifying the early warning signs of droughts. Currently, the studies examining drought propagation characteristics are primarily focused on two of the three types of droughts (meteorological, agricultural, and hydrological droughts) and are concentrated at the river basin scale; however, few studies to date have examined all three types of droughts, particularly at a large spatial scale. In this study, the Standardized Precipitation Evapotranspiration Index, Standardized Soil Moisture Index, and drought severity index of groundwater storage anomalies were used to characterize meteorological, agricultural, and groundwater droughts, respectively. Based on the copula probability model and Bayesian network analyses, time, probability, and threshold indicators were comprehensively utilized to reveal drought propagation characteristics in China. The results indicated that (1) the average propagation times from meteorological to agricultural, agricultural to groundwater, and meteorological to groundwater droughts in China were 4.1, 15.1, and 24.1 months, respectively. The propagation time in most river basins in northern China was longer than that in southern China. (2) Moderate and above meteorological droughts (agricultural drought) exhibited a much higher probability of triggering mild and moderate agricultural droughts (groundwater drought), and thus, more attention should be focused on drought early warning during this period. Statistics indicated that the average probability of extreme meteorological drought (agricultural drought) triggering mild agricultural drought (groundwater drought) was as high as 61.93 % (59.41 %), and the affected regions were primarily concentrated in the southern and northeastern areas of China. (3) When the meteorological drought level reached severe drought, it triggered mild agricultural drought in 64.3 % of the valid area. The Pearl River Basin exhibited the smallest absolute value of threshold from meteorological to agricultural drought in China, while the propagation threshold of extreme groundwater drought triggered by meteorological or agricultural drought was relatively high. Overall, the results have great implications for the framework construction of comprehensive drought monitoring.