Potential of sentinel 2-derived canopy water content as an indicator of flash drought: Case studies from European cereal crop areas

Flash droughts are concerning due to their rapid onset and intensification by heatwaves and rainfall deficit. This leads to rapid soil moisture depletion, causing crops to desiccate and die faster than in slow droughts, especially during critical crop growth stages, which affects the yield. The early detection of flash droughts is possible through the evaluation of the response of plant biophysical variables to these events. To assess that, this study analysed three crop biophysical variables and vegetation index derived from Sentinel-2 across distinct cereal-growing regions in Europe (ROI-1: Southern Spain; ROI-2: Northern Italy; ROI-3: Eastern Hungary) to evaluate their potential for detecting flash droughts. The Evaporative Stress Index (ESI) was used for detecting the drought onset, intensity, and duration, and the response of Normalized Difference Vegetation Index (NDVI), Leaf Area Index (LAI), Fraction of Absorbed Photosynthetically Active Radiation (fAPAR), and Canopy Water Content (CWC) were compared using spatio-temporal comparison and Pearson correlation for Wheat and Maize crops in Summer 2022 and Spring 2023 droughts. The findings revealed that CWC showed the earliest response to flash drought over irrigated areas of Spain and Italy compared to LAI and fAPAR. During drought, strong correlations between CWC and ESI (wheat and maize) (in ROI 1, r = 0.59 and ROI 2, r = 0.66) reflected a higher degree of conformity in capturing drought. However, the sensitivity of CWC to flash drought varied in the rainfed region, with weaker correlation observed in Eastern Hungary, where r = 0.4, ROI 3. These results show that there is potential in Sentinel 2-based CWC for early detection of flash droughts, particularly in irrigated systems. It can provide reliable and traceable information about crop stress at the onset of flash drought.