Understanding Flash Drought Dynamics with Surface-Root Zone Soil Moisture and Vegetation Impacts

Flash droughts (FD) develop rapidly with severe agricultural and ecological impacts despite their shorter duration in comparison to prolonged droughts. While previous studies have monitored flash droughts using atmospheric or single-layer soil moisture (SM) indices with reanalysis datasets, few have compared both surface and root zone direct observations to understand the full vertical extent of soil moisture depletion during the event. While FDs are often characterized by rapid intensification, sufficient drying, and notable impacts, existing monitoring frameworks have primarily emphasized the first two criteria, with limited integration of impact assessment. This study addresses these gaps by utilizing SMAP (Soil Moisture Active Passive) L4 data to compare surface (0-5 cm) and root zone (0-100 cm) dynamics integrated with vegetation response (NDVI) during flash drought events across different FD prone U.S. regions. Findings reveal that although both layers typically begin declining simultaneously during flash drought, surface moisture drops more rapidly and recovers early compared to the root zone. This results in vertical decoupling during flash droughts and recoupling during recovery. Soil moisture may increase before FD onset due to precipitation, so rapid SM decline doesn’t always reflect gradual dry buildup, rather can be abrupt. Surface SM shows a stronger correlation with precipitation, especially during FD weeks where precipitation might not help root zones like the surface during recovery. Because FD is a sub-seasonal phenomenon, it can occur even in historically very wet years or rapidly evolve into more intense droughts during dry years. Although flash drought typically loses strength after the rapid decline, vegetation may worsen gradually thereafter. Vegetation health might decline rapidly, but impacts can also be delayed by 1–2 months and persist for 4–6 months depending on the regions. Consecutive FDs or those evolving into prolonged droughts might cause more severe and lasting impacts. The insights from this study support the development of early-warning systems, where the detection of rapid land surface desiccation can facilitate the protection of root zone and vegetation with timely interventions.