Conditional probability analysis of flash droughts in the Yangtze River Basin under high temperatures

Based on ground meteorological observations and land surface reanalysis data, flash drought and high temperature events in the Yangtze River Basin from 1950 to 2021 were identified, and the spatiotemporal distribution characteristics of these two extreme events were analyzed. Combined with the Copula function, the joint probability distribution and conditional probability distribution functions of high temperature and flash drought were constructed, and the onset time periods when the soil moisture content percentile reached moderate, severe, extreme, and excessively extreme droughts under different temperature conditions were estimated. The results show that the times of occurrence of flash droughts and days of high temperature have significantly increased since 1990. The onset time period of droughts is significantly negatively correlated with daily maximum temperature, and shortens as the temperature increases, indicating the higher likelihood of flash drought occurrence at higher temperatures. The joint probability of high temperature and flash drought in the upper and middle reaches of the Yangtze River Basin, including Yunnan, Sichuan, Chongqing, Guizhou, and Hubei, is higher than that in the downstream areas in typical events in 2006, 2011, and 2013. Under low temperature conditions, the onset time period of droughts in the source region of the Yangtze River Basin is twofold of that in the middle and lower reaches. With the rising temperatures, the onset time period of droughts in the source region and the upper reaches shortens. When the daily maximum temperature exceeds 30℃, the development of droughts shows a spatial pattern of rapid onset in the upstream and delayed onset in the downstream.