Satellite-Based Monitoring of Ecosystem Level Drought Using Vegetation Optical Depth and Sun-Induced Chlorophyll Fluorescence

Climate change increases the severity and frequency of drought events. Over the globe, different ecosystems show different reactions to drought stress. By using two state of the art remote sensing techniques; vegetation optical depth and chlorophyll fluorescence, we can get a hand on reactions to drought stress at the ecosystem scale. The vegetation optical depth serves as a proxy for the total amount of water in the vegetation, while the sun-induced chlorophyll fluorescence serves as a proxy for the photosynthetic activity. Plant drought reactions break down into two strategies; isohydric behaviour and anisohydric behaviour. Isohydric plants tend to regulate their stomata, allowing them to reduce water losses during drought events. Anisohydric plants tend to be less strict in their stomatal regulation, allowing them to keep up their vegetation growth during drought events. In ecosystems dominated by isohydric plants, such as tropical rainforests, sun-induced chlorophyll fluorescence is a good indicator for the vegetation water status. In anisohydric regions, such as croplands, the vegetation optical depth provides better information on the vegetation water status, as these plants tend to show only little reactivity in their photosynthetic activity to drought stress. Combining the information of both metrics is expected to provide a more complete estimate of the plant water status.