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