Technical note: A stochastic framework for identification and evaluation of flash drought

Abstract. The rapid development of droughts, referred to as flash droughts, can pose serious impacts on agriculture, ecosystem, human health, and society. However, its definition using pentad-average soil moisture could result in low accuracy of assessing the drought occurrence, making it difficult to analyze various factors controlling the formation of flash drought. Here we used a stochastic water balance framework to quantify the timing of drought as the first passage time of soil moisture dropping from a higher level to a lower one. Based on this framework, we can theoretically examine the nonlinear relationship between the timing of drought and various hydrometeorological factors and identify possible flash drought risks caused by less rainfall (e.g., long dry spells), higher evapotranspiration (e.g., extreme heatwaves), lower soil water storage capacity (e.g., deforestation), or a combination thereof. Applying this framework to the global datasets, we found that flash drought risks also exist in humid regions such as southern China and the northeastern United States, calling for particular attention to flash drought monitoring and mitigation.