Flash drought

A flash drought is a type of drought characterized by its rapid onset, intensification, and severity over a relatively short timescale, usually within a few days or weeks. This concept has evolved during the last decade as researchers have become more interested in understanding and mitigating its impacts. Flash droughts differ from other drought categories such as meteorological, hydrological, agricultural, ecological, and socioeconomic droughts, in that they develop and intensify more quickly, posing unique challenges for monitoring, prediction, and mitigation.

History

Flash droughts have gained increasing attention from researchers and policymakers due to their significant impacts on agriculture and water resources. The term "flash drought" was coined to describe the rapid onset and intensification of drought conditions, which set it apart from other, more conventional drought types that develop over longer periods. Early research focused on understanding the unique characteristics and drivers of flash droughts, while more recent studies have explored their impacts on agriculture, ecosystems, and water resources.

Methodologies for Flash Drought Estimation

There are several methodologies used for flash drought estimation, including the application of remote sensing data, meteorological and hydrological models, and machine learning techniques. These methodologies aim to provide a comprehensive understanding of the spatiotemporal dynamics, drivers, and impacts of flash drought events.

Remote Sensing

Remote sensing data, such as satellite-derived measurements of soil moisture, evapotranspiration, and vegetation indices, can provide valuable information on the development and progression of flash droughts.Remote sensing data can be used to assess the rapid response of ecosystems to flash droughts by analysing multiple ecological metrics derived from satellite observations. These metrics, such as gross primary productivity (GPP), net primary productivity (NPP), and leaf area index (LAI), can provide insights into the reactions of ecosystems to flash droughts from both vegetation physiological and structural perspectives.

Using a methodology based on soil moisture measurements and specific criteria for flash drought events, researchers can identify flash drought occurrences and their impacts on ecosystems. To analyse the ecosystem response to flash droughts, standardized ecological metrics are used to investigate the relationship between flash drought and ecological impact, and response frequency is employed to measure the risk posed by flash droughts to ecosystems. Water use efficiency (WUE) and underlying water use efficiency (uWUE) are also considered to better understand the trade-offs between carbon assimilation and water loss through evapotranspiration (ET).

In some cases, it has been found that NPP is more sensitive than GPP and LAI to the occurrence of flash drought, with differences attributed to the vegetation respiratory process and physiological process of photosynthesis. The ecological response to flash drought can be quicker in certain regions with specific climate conditions. However, improvements in satellite observations of ecosystem productivity and modelling methods are still needed to better understand the mechanisms of drought impact on ecosystems.

The methodologies involving remote sensing data and ecological metrics can help researchers detect the ecological impacts of flash droughts and improve understanding of flash drought dynamics. This knowledge can aid in the development of effective monitoring and mitigation strategies for flash drought events.

General Flash Drought Concept

The flash drought concept is based on the rapid intensification of drought conditions, primarily focusing on soil moisture. A flash drought event begins when the 8-day mean soil moisture declines from above the 40th percentile down to the 20th percentile, with a decreasing rate of no less than 5% in percentile per 8-day period. This stage is known as the "onset" stage of flash drought. The evolution from the 40th to the 20th soil moisture percentiles represents the development of drought conditions. The recovery stage of flash droughts starts when the soil moisture percentile either increases or decreases more slowly. Once the soil moisture recovers to above the 20th percentile, the drought ends.

To analyse the ecosystem response to flash droughts, standardized ecological metrics are used. One such metric is the response time index, which can be calculated using the following equation:

$X_{i,j}={\frac {X_{i,j}-\mu _{j}}{\sigma _{j}}}$

In this equation, $X_{i,j}$ represents the ecological variable (such as GPP, NPP, or LAI) in year $i$ and at the timing $j$ in the $i$-th year. $\mu _{j}$ is the mean of $X$ at the $j$-th calendar 8-day, and $\sigma _{j}$ is the standard deviation of $X$ at the $j$-th calendar 8-day, both computed using data from a specific period (e.g., 2003 to 2018). This standardized metric allows researchers to investigate the relationship between flash drought and ecological impact, helping them understand the response of ecosystems to rapidly intensifying drought conditions.