Flash drought can develop fast, often in just a few weeks, and that makes it especially hard to monitor in the southeastern United States. A new Drought.gov article says the problem is particularly important in humid states such as Alabama, where annual rainfall may seem high, but hot weather, stronger evaporation, and quickly drying soils can still reduce the water available to plants in a short time.
The article highlights a new study funded in part by NOAA’s National Integrated Drought Information System that looked at whether the Evaporative Stress Index, or ESI, could help identify those rapid changes. According to Drought.gov, the researchers found that ESI aligns most strongly with soil moisture in the fall and can provide an early signal of developing flash drought in the Southeast when used alongside other indicators. For the original article, readers can review Drought.gov.
Evaporative stress may help spot flash drought earlier
The article explains that ESI is based on satellite estimates of evapotranspiration using the Atmosphere-Land Exchange Inverse model. In practical terms, it reflects how much water vegetation is actually using compared with how much the atmosphere is demanding. When plants start to experience moisture stress, evapotranspiration falls and ESI can move into negative territory.
That matters because the study found that changes in ESI often appeared alongside fast soil moisture declines. Drought.gov says those shifts can offer a useful and persistent early warning signal, especially in a region where flash drought may intensify before slower drought monitoring systems fully reflect the change. Readers can also explore the drought background page here What Is Flash Drought?.
Alabama was used as a test case for the wider Southeast
The study used Alabama as a testbed for the broader Southeast. According to Drought.gov, researchers compared weekly ESI values with soil moisture observations from 18 stations across the state over a 20 year period from 2001 through 2020. They then used a soil moisture based flash drought metric, called the Soil Moisture Volatility Index, to test whether ESI could corroborate flash drought events identified in the soil record.
The results showed a relationship between ESI and soil moisture, but one that changes with the season. Drought.gov says correlations were modest when all months were combined, then became much stronger from late spring through fall and were strongest from September through November. Winter showed the weakest relationship, and the article says mid summer correlations also dipped because of crop growth stage, wetter summer conditions, and mixed land cover in the satellite pixels.
Fall produced the clearest early warning signal
One of the most important findings in the article is that changes in ESI, rather than ESI by itself, often gave earlier warning of flash drought onset. Drought.gov says negative ESI change anomalies frequently appeared one to two weeks before soil moisture based flash drought was formally indicated.
On average, the article says this happened in about 60% of flash drought events. In the fall, agreement rose as high as 90%. The study also found that ESI change anomalies calculated about two weeks before flash drought onset were most often the strongest indicators. That makes the fall season especially important for drought watchers in the Southeast. For more about the indicator itself, readers can also review About the Evaporative Stress Index.
Researchers say the index works best with other drought tools
The Drought.gov article is careful not to present ESI as a complete answer by itself. It says the study did not evaluate ESI as a standalone drought metric, but instead focused on understanding how soil moisture and evapotranspiration relate during flash drought. That distinction matters because drought in the Southeast can be highly variable and shaped by complex land and atmosphere interactions.
Even so, the article says changes in ESI can capture lingering vegetation stress even when short term rainfall temporarily improves shallow soil moisture. That longer memory may help drought monitoring in situations where surface conditions look better for a short time, even though plants are still under stress. For broader climate and agriculture coverage in English, readers can also see Agroinformacion. The official report was published by Drought.gov on April 8, 2026.