A New Drought Fluorescence Monitoring Index Established for Detecting Drought Evolution Characteristics in the Middle and Lower Reaches of the Yangtze River, China During 2001–2020

Timely and accurate drought monitoring is essential for optimizing agricultural water resource management and ensuring food security. Solar-induced chlorophyll fluorescence (SIF) has achieved rapid response and early warning when plants suffer drought stress. However, more research needs to be done on how to use SIF in a systematic way to monitor and evaluate drought. In this study, we improved three drought fluorescence monitoring indexes (e.g., fluorescence condition index (FCI), fluorescence health index (FHI), and temperature fluorescence dryness index (TFDI)) and developed a new drought fluorescence monitoring index (DFMI) based on GOSIF by integrating FCI, soil moisture condition index (SMCI), temperature condition index (TCI), and precipitation condition index (PCI). Soil moisture (SM), standardized precipitation evapotranspiration index (SPEI), net primary productivity (NPP) and traditional remote sensing drought indexes were employed to evaluate their performance, and the drought evolution characteristics in the middle and lower reaches of the Yangtze River (MLRYR) during 2001-2020 were analyzed based on the best index and run theory eventually. The results show that DFMI has superior drought monitoring performance, with correlation coefficients between DFMI and SM, SPEI, and NPP being higher than those of FCI, FHI, TFDI, and other drought indices during the 2001-2020 and 2011, 2019 drought periods. Based on DFMI, the drought generally showed decreasing trends in both annual and seasonal time scales during 2001-2020, with the northern MLRYR in spring, summer and winter on the opposite. The drought risk in cropland was higher than that in grassland and forest, which has increased significantly over the past 20 years. Moreover, the proportion of areas affected by drought was high before 2013 and the proportion of drought at different levels generally exhibited decreasing trends. The area with high drought frequency also decreased gradually and shrank toward the north with the increased drought levels. Overall, 27 drought events were identified during 2001-2020, wherein three drought events with the most substantial drought severities occurred during June 2003-April 2004 (11 months), January-May 2011 (5 months), and August-December 2019 (5 months), with drought severities of 0.76, 0.65, and 0.43, and average drought-affected areas of 69.95%, 85.57%, and 75.31%, respectively.