India's reliance on agriculture, contributing significantly to its GDP, makes it highly vulnerable to weather extremes. Droughts, often associated with summer monsoon failures, severely impact natural resources, crop production, and the well-being of its vast population. While the causes and consequences of monsoon-related droughts are well-studied, the drivers and impacts of flash droughts remain less understood. Flash droughts, characterized by rapid soil moisture depletion due to precipitation deficits, increased air temperatures, or both, can drastically alter favorable crop conditions within weeks. This study addresses this knowledge gap by investigating flash droughts in India, emphasizing the urgent need to understand their implications for agricultural activities, irrigation demands, and groundwater abstraction, particularly considering projected extreme climatic conditions under climate change. The increased risk of flash droughts in a warming climate is less understood compared to traditional droughts. This study utilizes observations and large ensemble simulations from the Community Earth System Model (CESM-LENS) to reconstruct root-zone soil moisture for the observed (1951–2016) and future climates, examining the influence of anthropogenic warming on projected changes in flash drought frequency.

Existing literature extensively documents the causes and consequences of droughts resulting from summer monsoon failures in India. Studies link year-to-year monsoon variability to large-scale atmospheric and oceanic forcing, with El Niño events often leading to weaker monsoons and meteorological droughts. These meteorological droughts often translate into agricultural and hydrological droughts, causing devastating impacts. However, research on flash droughts in India is limited. While studies in other regions highlight the impact of rapid soil moisture depletion on agriculture and ecosystems, the specifics of flash droughts in India's context and their connection to climate change remain largely unexplored. This study aims to fill this critical gap in understanding the occurrence and future projections of flash droughts in India.

This study utilized gridded daily precipitation and temperature data from the India Meteorological Department (IMD) for the period 1951–2016. The Variable Infiltration Capacity (VIC) model, a land surface model, was employed to simulate root-zone soil moisture using the observed meteorological forcing. The VIC model's performance was carefully evaluated against in-situ and satellite-based observations. Flash droughts were identified based on all-India averaged VIC-simulated soil moisture, considering rapid soil moisture depletion from above 40th to below 20th percentile within a short period. The severity of flash droughts was quantified using a score combining duration, intensity, and areal extent. For future projections, the study employed a large ensemble (40 members) of the Community Earth System Model (CESM-LENS) simulations (1920–2100 under RCP 8.5). The VIC model was used to simulate soil moisture for each CESM-LENS ensemble member. Atmospheric conditions (geopotential height, mean sea level pressure, integrated water vapor, and wind) from ERA-5 reanalysis data were analyzed to understand flash drought drivers. Single-forcing experiments from CESM-LENS were used to assess the roles of greenhouse gas emissions (GHG), industrial aerosols (AER), and land-use/land-cover change (LULC) in driving the projected changes in flash drought frequency.

Analysis of the observed climate (1951-2016) revealed 15 flash droughts, all occurring during the summer monsoon season. The 1979 flash drought stands out as the most severe, affecting more than 40% of the country. Examination of atmospheric conditions during the 1979 drought revealed patterns consistent with monsoon breaks, characterized by positive geopotential height and mean sea level pressure anomalies, weaker westerly winds, and reduced integrated water vapor. Composite analysis of all flash droughts (excluding those before 1979 due to ERA-5 data availability) showed similar atmospheric signatures, indicating monsoon breaks as a key trigger. Future projections using CESM-LENS simulations revealed a substantial increase in the frequency of extreme hot and dry pentads. The frequency of extreme dry pentads is projected to increase 2.5-fold, while extreme hot pentads are projected to increase six-fold by the end of the 21st century. Consequently, concurrent extreme hot and dry pentads are expected to increase four-fold. The frequency of flash droughts with characteristics similar to the 1979 event is projected to increase seven to eight-fold. Intraseasonal variability in monsoon precipitation, with a projected decline in precipitation during the late monsoon season combined with increased temperatures, contributes significantly to the increased flash drought risk. Analysis of single-forcing experiments highlighted the dominant role of anthropogenic GHG emissions in increasing the risk of concurrent hot and dry extremes, with a risk ratio (ALL/XGHG) projected to reach 16-18 by the end of the 21st century. Industrial aerosols and land-use/land-cover changes exhibited less influence.

The findings demonstrate that flash droughts in India are intricately linked to monsoon breaks and amplified by anthropogenic warming. While projected increases in monsoon precipitation suggest generally wetter conditions, the intraseasonal variability in precipitation and the significant temperature increase lead to increased risk of flash droughts, particularly during the late monsoon season. The dominant role of GHG emissions in driving the projected increase highlights the crucial need for mitigation efforts. The combination of warming and the variability of monsoon rainfall creates conditions conducive to flash droughts, highlighting a complex interaction between climate change and the inherent variability of the monsoon system. These results have significant implications for agricultural planning and water resource management.

This study provides robust evidence of the increasing risk of flash droughts in India due to a combination of intraseasonal monsoon variability and anthropogenic warming. The projected seven-to eight-fold increase in severe flash droughts necessitates adaptation strategies focused on improving drought preparedness and water resource management. Future research could focus on refining the quantification of flash drought impacts on agriculture and exploring the potential of subseasonal-to-seasonal prediction to enhance early warning systems for flash droughts.

The study's reliance on the CESM-LENS model, while a high-resolution and well-validated model, means that the results are subject to the model's inherent uncertainties and limitations. The use of a single GCM, although carefully evaluated, needs to be considered when interpreting the findings. Further research involving multiple GCMs could enhance robustness. The study also focused on root-zone soil moisture, which might not fully capture all aspects of flash drought impacts on the entire soil profile. The assumption of a homogeneous response across India could also oversimplify the regional variations in flash drought vulnerability.