Not all droughts are the same.  In some cases, drought rapidly intensifies at subseasonal to seasonal scales with significant impacts to agriculture and water resources along with the increased propensity for heatwaves and wildfires.  Soil moisture is a critical drought variable, and the desiccation of the terrestrial surface is an effective resource for monitoring drought development and associated impacts.  During flash drought, soil moisture can play two critical roles: (1) drought enhancement via dry soils, enhanced sensible heat flux, reduced evaporation, and enhanced vapor pressure deficit and, (2) drought resistance via moist soils that cool the surface via evaporation and decreased vapor pressure deficit.  Thus, a fundamental question is at what point during flash drought development does the environment transition from drought resistance (a negative feedback) to drought enhancement (a positive feedback) and vice versa? Further, how do surface-layer processes impact the development of the planetary boundary layer (PBL) during this transition?  Finally, what is the overall relationship between atmospheric demand, evaporative stress, terrestrial desiccation, and precipitation in the progression of flash drought.  This study provides a conceptual framework that captures the critical processes that drive flash drought progression (and moderation) through a relative parameter space approach. Using this framework, the onset, development, intensification, moderation, and termination of flash drought can be diagnosed by the complex interactions between terrestrial and atmospheric variables.

How to cite: Basara, J. and Christian, J.: A Framework for Flash Drought Progression, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16659, https://doi.org/10.5194/egusphere-egu23-16659, 2023.

As global population continues to rise, the associated demand for agriculture is expected to significantly increase over the next several decades. Furthermore, projected increases in climate variability due to global climate change will impact the cropland expansion and agricultural intensification required to meet the demand in the coming decades. Flash drought presents a unique challenge within the realm of weather and climate extremes. Given their rapid development, drought mitigation strategies are challenging to implement during flash drought because these events often develop with limited warning, while leading to wide-ranging impacts across the land surface. As such, this study seeks to address two key questions on flash drought: 1) What are the trends in flash drought frequency across the globe in a warming climate and 2) how does the risk to cropland from flash drought change in the future? These questions are addressed by identifying flash drought events from six CMIP6 models using the standardized evaporative stress ratio (SESR) and soil moisture. Historical simulations capture the period spanning 1850-2014, while three scenarios are used to project flash drought development under different socioeconomic pathways and radiative forcing levels for the years 2015-2100 (SSP126, SSP245, and SSP585). We find that flash drought occurrence is expected to increase globally among all scenarios, with the sharpest increases seen in scenarios with higher radiative forcing and greater fossil fuel usage. Regionally, the largest projected increases in flash drought occurrence are in Europe and the Amazon. Flash drought risk over cropland is expected to increase globally, with the largest increases projected across North America (change in annual risk from 32% in 2015 to 49% in 2100) and Europe (32% to 53%) in the most extreme SSP585 scenario. Following conservative and medium scenarios compared to high end scenarios indicates a notable reduction in annual flash drought risk over cropland.

How to cite: Christian, J., Martin, E., Basara, J., Furtado, J., Otkin, J., Lowman, L., Hunt, E., Mishra, V., and Xiao, X.: Global Projections of Flash Drought in a Warming Climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2880, https://doi.org/10.5194/egusphere-egu23-2880, 2023.

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).

How to cite: Lovino, M. A., Berbery, E. H., Pierrestegui, M. J., Müller, O. V., and Müller, G. V.: A new indicator reveals frequent flash droughts with a common physical evolution in different agricultural regions worldwide, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10623, https://doi.org/10.5194/egusphere-egu23-10623, 2023.

Flash droughts are a highly impactful, subseasonal to seasonal phenomenon that pose serious risk to several groups of stakeholders, including agricultural producers. One major example includes the 2012 flash drought event over the United States, which resulted in tens of billions of dollars of crop loss and caused longer-lasting effects to the overall US economy. This study aims to quantify impactful flash drought events that have occurred over two agricultural regions of the central United States over a 40 year period from 1981 to 2020. These two regions, referred to as the Southern Great Plains and Midwest, were selected as they are agriculturally-dense areas that are located spatially close to one another, yet are in distinctively unique climate regions. 

Using the standardized evaporative stress ratio (SESR) for flash drought identification, events are selected based on several factors, including spatial coverage and spontaneous nature of onset and spread across the region. Given the sudden nature of rapid drought intensification with these events, there is less time for farmers to implement mitigation strategies and remain resilient to the droughts. We define these events as abrupt agricultural flash droughts (AAFDs). Initial results show changes to the timing and frequency of AAFD events over the Southern Great Plains and Midwest. More specifically, over the last 20 years, AAFDs have increased in frequency by 100-200% across critical regions of agricultural growth, suggesting these events are likely to pose increased risk to agricultural producers in the near future.

How to cite: Fellman, B., Basara, J., Christian, J., and Tye, M.: Abrupt Agricultural Flash Drought: Investigating Flash Drought Events over Agricultural Regions of the United States, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4530, https://doi.org/10.5194/egusphere-egu23-4530, 2023.

Flash drought is a unique natural hazard due to its rapid rate of intensification with potentially detrimental effects on agriculture production. So far few work has been done to investigate the space and time behavior of flash droughts across Central Europe. In this study, we examine the space and time characteristics (Frequency, Rate of intensification, Severity, and Extent) of flash drought events in Central Europe between 1970 and 2020. The weekly averaged top layer (1-10cm) soil moisture (SM) data set of the Global Land Data Assimilation System (GLDAS) has been used as an indicator for identifying flash drought events. Here, we adopted a new definition for flash droughts by considering the intensity, duration, and persistence of flash drought. The results of this study reveal that the occurrence of flash droughts over Central Europe increased rapidly in the last two decades. The intensification rate and severity of flash drought show positive correlation. Moreover, the areal extent of flash drought events increased since 1970 and their centroid shifted towards the southern part of Central Europe. This study provides a better understanding of flash drought process and its dynamics over Central Europe.

How to cite: Rahim, A., Markonis, Y., and Ruth, B. J.: Space and Time Characteristics of Flash Drought Over the Central Europe, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-443, https://doi.org/10.5194/egusphere-egu23-443, 2023.

A mega drought occurred in the summer of 2022 over the Yangtze River basin in southern China, and affected 2.85 million hectares of crops and water supply for 4.73 million people. With the strongest rainfall deficit and hottest temperature since 1961, the drought developed from less than 20% of the basin at the beginning of July to more than 80% of the basin in the mid-August, which is an unprecedent flash drought in the Yangtze River basin. Here, we investigate multi-scale causes of the mega-flash drought including the land-atmospheric dry coupling at sub-seasonal time scale, the link with La Niña and PDO at interannual to decadal scales, and the effect of anthropogenic climate change. The subseasonal-to-seasonal (S2S) climate forecast models and the NMME seasonal prediction models will be used to assess the forecast skill of the drought onset, and a copula-based method will be implemented to predict the probability of drought recovery. Lastly, the future risk of the mega-flash drought will also be investigated by using CMIP6 models.

How to cite: Yuan, X. and Wang, Y.: Causes and predictability of the 2022 mega-flash drought over the Yangtze River basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2169, https://doi.org/10.5194/egusphere-egu23-2169, 2023.

Flash-droughts events are characterized by the fast depletion of soil moisture in the top soil layers. Given their rapid onset and intensification, flash droughts entail severe impacts on ecosystem productivity. Thus understanding their initialization mechanisms is important for improving the skill of drought forecasting systems. Here, we examined the role of antecedent meteorological conditions that lead to flash droughts across Europe over the last 70 years (1950--2020). We found two major flash-drought types based on sequence of development of antecedent hydro-meteorological conditions. The first one is characterized by a joint occurrence of two mechanisms, a continuous decline of precipitation in conjunction with an increase of the evaporative demand, both occurring before the onset of a flash drought event. The second type, on the contrary, is characterized by high precipitation preceding the start of the event followed by a sudden precipitation deficit combined with an increase in evaporative demand at the onset of the drought. Both drought types showed increased occurrence and higher spatial coverage over the last 70 years, the second drought type have increased at a much faster rate compared to the first one specifically over the Central Europe and the Mediterranean region. Overall our study highlights the differences between the two types of flash droughts, related to varying antecedent meteorological conditions, and their changes under recent climate warming. 

How to cite: Shah, J., Kumar, R., and Rakovec, O.: On the role of antecedent meteorological conditions on soil moisture flash drought initialization in Europe, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16260, https://doi.org/10.5194/egusphere-egu23-16260, 2023.