Normalized Solar-Induced Fluorescence Responds Earlier Than Vegetation Indices to the 2019 North China Plain Drought

Recently, solar-induced chlorophyll fluorescence (SIF) from satellites has shown potential for evaluating vegetation status and stress responses. Fluorescence quantum yield (Φ_F) is essentially linked to vegetation stress. However, the complex physiological and structural responses of SIF and Φ_F to drought need further study. This study normalized SIF as SIF_n to account for angular variations and fluctuations in photosynthetically active radiation (PAR), aiming for more accurate drought monitoring. SIF_n anomalies were compared to historical baselines (2019–2021 averages) of vegetation indices (VIs), raw SIF, and Φ_F during a 2019 drought in the North China Plain (NCP). The results show SIF_n provides an effective method for drought monitoring, showing the earliest decline compared to raw SIF, VIs, and Φ_F. In the first two weeks of drought, SIF_n decreased by 8.2%, 7.0%, 12.5%, and 8.2% across the four NCP subdivisions. SIF_n outperformed other indicators, proving sensitive to early drought detection. SIF_n was also examined for tracking drought alleviation by rainfall. The uncertainty under different viewing geometries was quantified. SIF_n anomalies showed a strong correlation with rainfall anomalies (R: 0.45 ~ 0.52) and meteorological factors like PAR (R: 0.80 ~ 0.84) and relative humidity (R:0.52 ~ 0.54). The correlation of near-infrared reflectance (NIRv) and Φ_F anomalies with SIF was weak during drought onset (R: 0.16 ~ 0.32) but strong at the end (R: 0.83 ~ 0.87). These suggest both canopy structure (mainly characterized by NIRv) and vegetation chlorophyll (Φ_F) are impacted by drought and influence SIF at different stages.