# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
## 2 Optical Remote Sensing
### Solar-Induced Chlorophyll Fluorescence as an Early Drought Indicator  
An obvious shortcoming of spectral indices is that they lack explicit physical meanings. To deal with this, several vegetation physiological or biochemical parameters (which can be estimated via optical remote sensing) have been considered as drought indicators, e.g., the Leaf Area Index (LAI) [121], the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) [122; 123; 124; 125], and the leaf chlorophyll concentration [126]. However, most of these parameters still cannot overcome the problem of time latency. When precipitation shortages and soil moisture deficits take place, they show no significant change within a temporal range of ten days to two months [127; 128; 129; 130] because they are reflections of accumulated vegetation growth rather than instant indicators.  
Solar-induced chlorophyll fluorescence (SIF) is a stimulated emission which occurs after the chlorophyll absorbs light [131]. The spectral emission spans approximately 650-800 nm, normally having two local maxima--one located in the red region around 685-690 nm (called F\({}_{685}\)), and the other located in the far-red region around 730-740 nm (called F\({}_{740}\)) [132]. A conceptual illustration of the distribution of absorbed radiation in plant leaves is given as Figure 6. There are four possible fates for an absorbed exciton: (i) it can be consumed during photochemical reactions via Photosystem I (PSI) or Photosystem II (PSII); (ii) it can undergo radiationless decay without emitting electromagnetic energy; (iii) it can be quenched by a non-photochemical trapping center, in which case the process is called non-photochemical quenching (NPQ); and (iv) it can be re-emitted as a fluorescence emission [132; 133].  
A detailed introduction to the physiological and biochemical processes related to the emission of SIF can be referred to in [PERSON] et al. [134]. SIF has a notable potential in agricultural drought monitoring, and is beneficial for drought early warning due to its response to early water stress, which can be a precursor to impending agricultural drought, large scale vegetation dieback and community shifts. At the early stage, crop water stress  
Figure 6: Conceptual illustration of the distribution of absorbed solar radiation in plant leaves, adapted from [PERSON] et al. [133].  
is not readily apparent, causing a failure to recognize impending vegetation dieback before the process has advanced beyond recovery. In contrast, early stress can be reflected via SIF since SIF is an immediate signal of plant photosynthetic status [132; 135; 136; 137]. Studies based on leaf- and canopy-level experiments have demonstrated the possibility of using SIF to track diurnal changes caused by heat and water stress [138; 139].  
To calculate solar-induced chlorophyll fluorescence, the Fraunhofer Line Depth method [131], leaf and canopy fluorescence simulations using Fluorescence Model (FluorMOD) [140] and Soil-Canopy Observation Photosynthesis and Energy fluxes (SCOPE) are used [141]. The amount of chlorophyll fluorescence emitted by a leaf under natural sunlight only makes up to 1-3% of the absorbed light in the visible part of the spectrum, making it hard to quantify [131]. At Fraunhofer lines, however, fluorescence can be quantified because there is no reflected light at those wavelengths. In recent decades, several space missions have been utilized for global SIF measurements, and a space mission targeted at SIF, the FLUorescence EXplorer (FLEX), is planned to be launched in 2024, which will offer a much higher spatial resolution. A summary of related missions can be seen in Table 2.