Tracking drought dynamics on China's Loess Plateau with the rapid change index of solar-induced chlorophyll fluorescence

Although solar-induced chlorophyll fluorescence (SIF) has been proven to be a useful indicator for dynamic drought monitoring, accurately quantifying the trajectory of vegetation SIF change during drought events remains a research challenge. In this case study, we applied a rapid change index (RCI) for the 8-day global ‘Orbiting Carbon Observatory (OCO-2)’ SIF product (GOSIF) to calculate the deviation between GOSIF trajectories and climate norms across China’s Loess Plateau (LP) from 2001 to 2020. We compared the performances of SIF-RCI with GOSIF values and GOSIF anomalies during drought events and evaluated the drought warning potential with the widely used 8-day Standardized Precipitation Evapotranspiration Index (SPEI). Our results showed that SIF-RCI captured the rapid change areas in GOSIF caused by drought and the trajectory of rapid vegetation changes compared to GOSIF values and GOSIF anomalies. The SIF-RCI negative signal accurately identified the onset of drought almost simultaneously with SPEI. The SIF-RCI positive signal also predicted drought recovery with at least 4 8-day periods of lead time. Meteorological factors, especially precipitation, had a great impact on vegetation photosynthesis and this was detected with GOSIF. These impacts varied in different climate zones and vegetation types. Our study contributes to regional drought monitoring and assessment.