Flash drought is a critical sub-seasonal phenomenon characterized by a period of rapid drought intensification. It exhibits multifaceted challenges to agriculture, water resources, ecosystems, and the human environment.
Given the rapid land-surface desiccation associated with flash drought, the agricultural sector can experience substantial economic damage due to lower crop yields and curtailed livestock production. Addressing these challenges requires a fundamental understanding of flash drought occurrence.
Flash droughts have been defined in two ways, either as a short-lived yet severe event where soil moisture completely depletes or a multi-week period of rapid intensification toward drought. These are unique when compared to conventional drought development due to a lack of rainfall coupled with increased evapotranspiration. Evapotranspiration is the combination of evaporation from the land surface and transpiration from vegetation. Both of these processes act to transfer water from the land surface to the atmosphere.
An international team led by Dr. Jordan Christian has studied flash droughts recently and identified global hotspots from 1980–2015 via anomalies in evaporative stress and the standardized evaporative stress ratio. As per their paper Global distribution, trends, and drivers of flash drought occurrence published recently in Nature Communications, India is a hotspot for flash droughts and this could have major implications on the country’s crop production.
Flash drought development is driven by the simultaneous occurrence of precipitation deficits and above-average evaporative demand. Soil type and land cover type may increase the complexity of flash drought development.
Flash drought hotspots exist over Brazil, the Sahel, the Great Rift Valley, and India, with notable local hotspots over the central United States, southwestern Russia, and northeastern China.
Six of the fifteen study regions experienced a statistically significant increase in flash drought during 1980–2015. In contrast, three study regions witnessed a significant decline in flash drought frequency. 8 of these 15 regions were both a regional maximum in flash drought occurrence and regions of major agricultural production with croplands covering at least 20% of the total land area in a given domain.
A seasonality in flash drought frequency is also evident in regions located within the tropics and subtropics, with the phase of their pattern dependent upon the hemisphere in which they reside. For example, the four regions in the Northern Hemisphere tropics and subtropics (Mexico, the Sahel, India, and the Indochinese Peninsula) generally had their highest occurrence of flash drought in the boreal growing season.
Several factors contribute to the preferential occurrence of flash drought hotspot regions across the globe. The first of these is the role of land–atmosphere coupling in flash drought development. As such, many of the global hotspots for flash drought including India identified in this study are also located over regions with an enhanced signal of land–atmosphere coupling.
Anticyclones are also an important contributor to flash drought development. Through subsidence and the associated suppression of rainfall, upper-level ridges can limit the potential for soil moisture replenishment. Concurrently, less cloud coverage and warmer surface temperatures increase the evaporative demand of moisture from the land surface.
In addition to the contributions of sub-seasonal features on flash drought development (e.g., land–atmosphere coupling and blocking highs), climatic features can also influence the spatial distribution of flash drought events revealed from the composite analysis.
Regions with relatively high interannual variability in rainfall also have a tendency for increased flash drought risk. As discussed, a diverse set of meteorological and climatic drivers contribute to preferential regions for flash drought development. Similarly, various drivers will also contribute to the seasonality of flash drought occurrence. For example, the Asian-Australian monsoon provides extensive precipitation across India, eastern/southeast Asia, and northern Australia.
The Asian monsoon typically begins in June and continues throughout boreal summer, providing more than 57% of the total annual rainfall in these regions. Across India, a percentile-based methodology using soil moisture was used to identify flash droughts during the monsoon and non-monsoon seasons, with the majority of the flash drought events occurring during the monsoon season, especially across the central, northwest, and northeast regions of India.
A similar result was found using standardized evaporative stress ratio (SESR) over the India domain in this study, with flash droughts primarily initiating between May and September. From the analysis of seasonal flash drought occurrence, the region experiences the peak frequency at the beginning of the monsoon season. Thus, a delay, absence, or reduction of monsoon rainfall can significantly contribute to flash drought development, provided above-normal evaporative demand is also present to promote rapid land-surface desiccation.
The increased frequency of flash drought can be related to vegetation dynamics and atmospheric conditions, in which vegetation with greater photosynthetic capacity and increased solar radiation due to a lack of precipitation and clouds occurs in the dry season.
The risk for flash drought development may continue to increase in certain locations due to the effect of increased evaporative demand as increases in potential evapotranspiration are expected in a future warming climate. In contrast, locations with climatological increases in precipitation will have greater availability of soil moisture for evapotranspiration, which will mitigate the enhanced evaporative stress and reduce opportunities for rapid drought intensification.
Regions such as India and northern Australia may have decreased flash drought spatial coverage over the last several decades due to changes in the magnitude and timing of precipitation.
For example, decreases in the South Asian monsoon circulation have contributed to changes in mean precipitation and variability during the summer across India. Climate features such as these may have a critical role in reducing the likelihood of flash drought development over time.
While drought is primarily characterized by a lack of precipitation, flash drought development occurs due to a combination of below-average precipitation and enhanced evaporative demand. As such, the unique contribution of these features toward flash drought development involves not only the suppression of rainfall, but the additional influence of above-average evaporative demand to rapidly deplete moisture and lead to rapid land-surface desiccation.
The analysis presented in the study reveals 1) the preferential regions for flash drought across the globe, 2) the seasonality of flash drought occurrence for selected hotspots and agricultural regions, 3) notable trends in flash drought spatial coverage for the examined locations, and 4) the contribution of key drivers in flash drought development.
While flash drought frequency varies significantly across the globe, nearly every region experiences rapid drought development (excluding arid and cold regions). Furthermore, above-average evaporative demand and precipitation deficits contribute with similar frequency to flash drought development. Importantly, a majority of the regional hotspots of flash drought occurrence are regions with extensive agriculture production. In addition to flash drought frequency, seven out of the twelve hotspot regions had statistically significant trends and are also associated with major crop production.
A common theme associated with flash drought development is the impact on crop yields. Yield losses occur through rapid depletion of root zone soil moisture, which leads to limited moderation of surface temperatures and excessive evaporative stress on crops. Due to this direct impact, flash drought studies primarily focus on rapid drought development in the context of agricultural production.
However, research has also recently shown that flash droughts can initiate a sequence of cascading impacts, such as an increased risk for wildfires and heatwave development. Particularly in underdeveloped countries, flash drought that transitions into a long-term drought may lead to an increased risk of famine and destabilization of governments.
With such a diverse set of meteorological and climatological features having critical roles in the development of flash drought, multiple paths of future studies are needed to understand the drivers of rapid drought intensification across the globe. Furthermore, future research should focus on untangling the complex interactions between flash drought and socioeconomic impacts.
The results and flash drought events derived from this study provide a reference frame for improvements in flash drought predictability. Finally, the results illustrate that multiple pathways of research are needed to further our understanding of the regional drivers of flash drought and the complex interactions between flash drought and socioeconomic impacts.
As of India, there is an urgent need of an adaptation framework in place to manage the available water resources and also explore the use of drought-tolerant varieties.