# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
## 2 Optical Remote Sensing
### Spectral Indices as Drought Indicators
The influence of the atmosphere on NDWI is weaker than on NDVI because there is less atmospheric aerosol scattering within 860-1240 nm. Similar to NDVI, NDWI also fails to remove the effect of the soil background completely. To reduce this effect, [PERSON] et al. [51] proposed the Visible and Shortwave infrared Drought Index (VSDI), which made use of the Blue, Red, and SWIR bands to monitor agricultural drought over different land-cover types during plant growing seasons. Since the NIR-SWIR reflectance space has similar features to the NIR-Red space, [PERSON] et al. [82] analyzed the potential of the NIR-SWIR space in drought monitoring. The established Shortwave infrared Perpendicular Water Stress Index (SPSI) has the same form of PDI, but unlike PDI, SPSI fits well in various vegetation conditions, from bare soil to full coverage. [PERSON] et al. [83] further developed the Vegetation Water Stress Index (VWSI) to quantify the vegetation water stress at the canopy level. The relationship between the VWSI and fuel moisture content was examined through leaf- (i.e., PROSPECT [46,84,85]) and canopy-level radiative transfer simulations [47,86] incorporating field measurements. Based on the fuel moisture content under different water stress conditions and corresponding VWSI values, the authors established threshold values for VWSI to help precision irrigation. Several follow-up studies have utilized the NIR-SWIR space for estimating vegetation water stress, aboveground dry biomass, and yield [87] and understanding drought impacts on forest ecosystems [88]. Later, [PERSON] et al. [89] proposed the Modified ShortwaveWhen hyperspectral data are available, the Photochemical Reflectance Index (PRI) should not be neglected. PRI is often used as an early indicator of plant health [90; 91; 92; 93]. It can reflect the epoxidation state of the xanthophyll cycle pigments and photosynthetic efficiency [58], thus providing scale-invariant signatures of early water stress for various vegetation types [94]. PRI is usually calculated using 531 nm and 570 nm as the reference bands [58]:  
Figure 3:2  
However, there are a number of candidates, e.g., 512 nm [95], 515 nm [90], 519 nm [96] and 586 nm [91]. The aforementioned spectral indices are elementary to calculate and use. Compared with traditional long-time drought indices, the indices and their anomalies can be a near-real-time indicator of the onset, extent, intensity, and duration of agricultural water stress and droughts. However, it should be noted that it is hard to exclude the effects of other environmental stress and adversities, e.g., diseases and pests [63], since the indices are merely mathematical results and do not have a definite physical or biophysical meaning. Another caveat is that their relationships with soil moisture or crop water content are often restricted to a particular area and cannot be represented by a unified formula. Therefore, these indices are only capable of rough estimations of drought conditions, while their model parameters must be recalibrated when applied in regions with different soil and crop types or different climatic characteristics. For the convenience of our readers, Table 1 gives an overview of the indices mentioned above, along with references for typical applications of these indices in agricultural drought monitoring. Figure 3:3 Sketch map of the NIR-Red reflectance space. The triangle ABC is the outline of the scatter points. A represents full vegetation cover. AB is the wet edge, representing the water-saturated status. AC is the dry edge, representing the extremely dry status. BC is the soil line, representing bare soil. E corresponds to the pixel value. Line L is the perpendicular line of BC that crosses the original. EF is the perpendicular line of L which crosses E. EF intersects with AB at P, while also intersecting AC at Q. ED is the perpendicular line of EF that crosses E and intersects with BC at D. The length of the segment EF is the PDI, and the length of the segment ED is the PVI.