Flash droughts are characterized by the rapid drying of soils over durations lasting weeks to a couple of months. Given their rapid onset and intensification, flash droughts are often unexpected and challenging to predict. In particular, agricultural flash droughts affect vegetation when there is a soil moisture deficit and plant water requirements are not met, especially during the critical growth period of crops.

In our recent publication in NPJ Climate and Atmospheric Science, we propose a flash drought indicator that relies on soil water availability and can couple rapid depletion rates of soil moisture in the root-zone with impacts on vegetation health. Using this method, our study assesses the prevalent life cycle of agricultural flash droughts, highlighting global similarities. We analyze atmospheric and surface drivers throughout this cycle and discuss the impact of these droughts during the critical growth period of crops.

Proposed method: coupling rapid soil moisture depletion and plant water stress

The proposed method utilizes the root-zone soil moisture and two soil hydraulic properties, namely field capacity and wilting point, to capture the rapid depletion of soil moisture, along with vegetation stress that severely impacts agriculture and ecosystems. To detect such occurrences, we suggest using the well-known Soil Water Deficit Index (SWDI) along with a threshold range within the transitional regime. The upper SWDI threshold marks the initiation of the soil moisture deficit, while the lower SWDI threshold indicates water stress conditions on crops. This approach is illustrated schematically in Figure 1.

Our main findings

Our results suggest that agricultural flash droughts are most frequent in croplands of southern China, central-eastern Europe, southern Russia, India, southeastern South America, and the central-eastern USA (see Figure 2f). The transition belt between the Sahel and the tropical forests in central-western Africa, northern South America, and southeastern Asia are also identified as agricultural flash drought hotspots.

We also find that agricultural flash droughts tend to occur during critical growth period of crops, as indicated by the analysis of their seasonal frequency. In line with their definition, agricultural flash droughts often occur during the growing season, mainly in spring and extending into summer. During this period, soil moisture is largely influenced by increases in evapotranspiration rates that exacerbate precipitation deficits.

Agricultural flash droughts exhibit similar evolution of relevant atmospheric and surface variables regardless of the geographical location or climatic regime (Figure 2). A precipitation deficit is the main driver for rapid soil moisture depletion. Before the flash drought onset, there is sufficient soil moisture (energy-limited regime). The favorable conditions of abundant soil moisture, combined with a decrease in precipitation, allow evapotranspiration to increase considerably, intensifying and accelerating soil moisture depletion. During the flash drought’s intensification period, soil moisture becomes insufficient to supply more water for evapotranspiration, which in turn decreases. In this water-limited regime with decreased evapotranspiration, energy is transferred to sensible heat flux, i.e., to an increase in temperature that is crucial in the persistence of flash drought events and which may conduct to subsequent heat waves.

Closing insights

Our research provides insights into the characteristics and life cycle of agricultural flash droughts, which are essential for accurately predicting them. We expect this straightforward approach to clarify the physical processes involved in agricultural flash drought development worldwide.