A new indicator reveals frequent flash droughts with a common physical evolution in different agricultural regions worldwide

Flash droughts negatively impact agriculture and natural ecosystems worldwide. However, there is still no flash drought indicator that can couple rapid soil moisture depletion and its impacts on vegetation health. Moreover, the literature describes several indicators that may not reliably represent flash drought evolution or its drivers in varying climatic regimes. Our study introduces a robust and straightforward approach to identify flash droughts based on ERA5 root-zone soil moisture along with soil properties (field capacity and wilting point) to reflect the soil moisture deficit and plant water stress conditions across different climate regions. Using this approach, we identify the regions in the world prone to flash droughts, and their seasonal frequency. This study also analyzes the processes involving atmospheric and surface drivers during the drought lifecycle.

The proposed indicator captures the rapid soil moisture depletion by assessing the decline in the soil water deficit index (SWDI) from an upper threshold to a lower threshold over a period of 20 days (4 pentads). The upper threshold (SWDI=-3) ensures an increase in evaporative stress as it is close to or at the critical soil moisture value, which differentiates energy- and soil moisture-limited evapotranspiration regimes. The lower threshold (SWDI=-5) is the readily available soil water limit for vegetation growth. Below this point, plants begin to experience water stress. Although soil water is theoretically available for plants before reaching the wilting point, the plant water uptake is reduced well before that value.

Our findings show that the main flash drought hotspots are found to be located in southeastern South America, southern China, India, central-eastern Europe and southern Russia, and the central-eastern United States. Most flash drought occurrence hotspots are found in agricultural regions. Additionally, the analysis of the seasonal flash drought frequency indicates that most flash drought events impact the critical growth periods of crops.

Our results reveal that all flash drought hotspots exhibit similar evolution of key atmospheric and surface variables regardless of the location or climatic regime. Thus, the physical processes involved in flash droughts appear to be similar worldwide. As expected, a precipitation deficit is the main driver for rapid soil moisture depletion. Temperature also plays an important role in the persistence of flash drought events. The evolution of evapotranspiration during flash droughts modulates the precipitation effects: evapotranspiration increases until the onset of the flash drought (energy-limited regime) and then decreases during the intensification period due to water stress (soil moisture-limited regime).