Impact Matters: Detection and Early Warning of Agriculturally Impactful Flash Droughts

Abstract Flash droughts are feared for their devastating impact on agricultural and other ecosystems, yet existing drought monitoring and detection metrics cannot directly convert to impact, limiting their usability for farmers and other stakeholders. To address this challenge, here we present an impact-centric framework based on the soil water deficit index (SWDI) that emphasizes the lack of plant available water and detects agriculturally impactful flash drought based on a rapid decrease of SWDI reaching a sustained level of plant water stress for most crops. This new approach picks up about 3 times as many events as a USDM-based detection in the western United States, labels many USDM-detected events as not impactful in the eastern United States, and identifies the central United States as a hotspot of impactful events. The flash droughts identified by this new approach are associated with strong negative anomalies of plant photosynthesis as approximated by remotely sensed solar-induced fluorescence (SIF), confirming the efficacy of the approach. Moreover, in the central U.S. hotspot, over 80% of the SWDI-detected flash drought occurrence can be predicted by the SIF rapid change index (SIF-RCI), another impact-based metric reflecting anomalies of the SIF subseasonal trajectory. Aside from the direct linkage to impact, the SWDI-based approach does not depend on climatological statistics and can therefore take advantage of new or short-record data such as SMAP. By establishing the efficacy of the proposed impact-centric framework, this study paves the way for global applications and enables the utilization of the latest Earth observations in addressing prominent societal challenges. Significance Statement Flash droughts wreak havoc on agricultural land worldwide. Useful and usable detection and monitoring of flash drought must be grounded on its impact on vegetation, especially on crops. Using the soil water deficit index (SWDI) as the indicator, here we define flash drought as a rapid loss of plant-available water leading to sustained soil water stress for most crops. This impact-centric approach enables early detection of flash droughts that are accompanied or followed by substantial negative anomalies of solar-induced fluorescence (SIF) (a photosynthesis proxy). Additional early warning of these agriculturally impactful flash droughts can be achieved through the SIF rapid change index (SIF-RCI). This study paves the way toward greater utilization of the latest Earth observations in safeguarding global food security.