Re-intensification of flash drought in western China over the past decade: Implications of fluctuating wetting trend

Climate changes and human activities have led to a rise of frequency and intensity of the global flash droughts, resulting in severe consequences for ecosystems, agriculture, and human societies. However, research dedicated to flash droughts in the dryland of western China is relatively limited, leaving their evolutionary characteristics and development processes of these phenomena unclear. To bridge this gap, this study analyzed the spatiotemporal characteristics of flash droughts in western China from 1981 to 2020, based on the standardized evapotranspiration stress index. Additionally, we investigated the development mechanisms by taking meteorological conditions and soil moisture into account. The findings revealed that the northern Qinghai-Tibet Plateau, western Qilian Mountains, and western and southern Loess Plateau are hotspots of flash droughts, characterized by rapid development rates. Across most of the study area, flash drought events persisted between 25 and 30 days. Adequate precipitation is necessary before the onset of flash droughts in western China, while water scarcity and high temperatures played crucial roles in driving the mid-stage of flash droughts. Within the context of the observed "warming and wetting" trend, the average flash droughts occurrence from 2011 to 2020 was approximately 16 % lower than that from 1981 to 1990, and there was a significant annual decrease in spatial coverage of 0.01 % per year. However, in the "wetting in west, drying in east" trend, the spatial coverage of flash droughts has shifted from a declining trend to an insignificant increasing trend since 2000 in the study area, with significant regional differences between the western and eastern regions. Over the past decade, flash droughts had once again intensified in the central Qinghai-Tibet Plateau and the Loess Plateau due to warming and fluctuating wetting trends, raising significant concerns for future ecosystem and agricultural water management in these regions.

A reversal in global occurrences of flash drought around 2000 identified by rapid changes in the standardized evaporative stress ratio

Flash drought is characterized by a rapid rate of onset and intensification within a few weeks. It usually accompanies exhausted soil moisture and high-temperature stress and exerts detrimental impacts on the growth of crops and the ecosystem. However, the global occurrence characteristics of flash drought in the recent four decades remain unclear. This study analyzes the spatiotemporal variability of flash drought identified by rapid decreases in the standardized evaporative stress ratio (SESR) from 1981 to 2020 and investigates their meteorological drivers. Results show that the flash drought mainly occurred in middle and low latitude areas. The coverage of flash drought showed a statistically significant decrease during 1981-2020. With the year of 2000 as a turning point, the coverage of flash drought trend reversed from a significant decline to a significant rise. Flash drought has no noticeable seasonal change. With the increase of the intensity of flash drought, the proportion of flash drought gradually decreased. Slight flash drought (FD_1; 50.9 %) is seven times of extreme flash drought (FD_4). The analysis of the evolution of hydro-meteorological variables concurrent with the global flash drought shows that flash drought was more triggered by abnormally low precipitation, soil moisture, evapotranspiration, and high temperature. In addition, the anomaly gradually increases with the increase of intensity. Water deficit is an important factor affecting the occurrence of flash drought, and only 10.9 % of flash drought events occurred in both positive soil moisture and precipitation anomalies. The results reference future research on flash drought on various spatial scales under a changing climate.