# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
## 2 Optical Remote Sensing
### Spectral Indices as Drought Indicators  
Optical remote sensing is used to identify soil and vegetation water status based on the spectral reflectance of soil and vegetation in the VIS-NIR-SWIR regions. Due to the complicated relationship between soil moisture, crop water content, and remotely sensed reflectance, this representation is usually implemented by proxy of a spectral index, a mathematical formulation of two or more bands. In particular, spectral indices that reflect the growth of vegetation are called vegetation indices (VIs). The simplest form is a ratio of two bands (e.g., the Moisture Stress Index (MSI) [49], the Simple Ratio Water Index (SRWI) [50]), or a linear mixture of several bands (e.g., the  
Figure 4: Simulated spectra of plant canopy with different leaf equivalent water thickness (EWT) levels. Generated using the PROSAIL (PROSEPCT-D + 4 SAIL) model [46; 47], with leaf parameters remaining the same as Figure 3 and canopy and other parameters set as: LAI = 2.0, ALA = 60\({}^{\circ}\), hspot = 0.5, \(\theta_{\mathrm{s}}\) = 30\({}^{\circ}\), \(\theta_{\mathrm{v}}\) = 30\({}^{\circ}\), and \(\phi\) = 60\({}^{\circ}\). Figure 3: Simulated spectra of plant leaves with different equivalent water thickness (EWT) levels. Generated using the PROSPECT-D model [46], with leaf parameters set as: \(N=1.5\), \(c_{\mathrm{a+b}}=40\)\(\mathrm{\SIUnitSymbolMicro g}/\mathrm{cm}^{2}\), \(c_{\mathrm{car}}=1.0\)\(\mathrm{\SIUnitSymbolMicro g}/\mathrm{cm}^{2}\), \(c_{\mathrm{ant}}=1.0\)\(\mathrm{\SIUnitSymbolMicro g}/\mathrm{cm}^{2}\), \(c_{\mathrm{brown}}=0.05\), and \(c_{\mathrm{m}}=0.004\)\(\mathrm{\SIUnitSymbolMicro g}/\mathrm{cm}^{2}\). Visible and Shortwave Infrared Drought Index (VSDI) [51]). However, the most prominent form is without doubt [52] that of the Normalized Difference Vegetation Index (NDVI) proposed by [PERSON] et al. [53]. Through an equation as simple as  
\[\mathrm{NDVI}=\frac{\rho_{\mathrm{NIR}}-\rho_{\mathrm{Red}}}{\rho_{\mathrm{NIR}} +\rho_{\mathrm{Red}}} \tag{1}\]  
NDVI can effectively assess the status of vegetation, thus reflecting environmental changes [54], including water stress and drought [55; 56]. Beyond its own use, in the following decades, forms of NDVI have been adopted by numerous spectral indices (e.g., the Normalized Difference Water Index (NDWI)[57], the Photochemical Reflectance Index (PRI) [58], the Normalized Difference Infrared Index (NDII) [59] and the Normalized Multiband Drought Index (NDMI) [60]). Since NDVI is directly related to the fractional vegetation cover (FVC), a common practice is to measure the severity of water stress based on NDVI anomalies instead of NDVI itself. Based on historical data of NDVI, [PERSON] [61] established the vegetation condition index (VCI):  
\[\mathrm{VCI}=\frac{100(\mathrm{NDVI}-\mathrm{NDVI}_{\mathrm{min}})}{\mathrm{ NDVI}_{\mathrm{max}}-\mathrm{NDVI}_{\mathrm{min}}} \tag{2}\]  
where \(\mathrm{NDVI}_{\mathrm{min}}\) is the historical minimum NDVI value for a specific location, while \(\mathrm{NDVI}_{\mathrm{max}}\) is the historical maximum NDVI value for the same location. Since VCI is normalized by historical values, it allows one to compare the impact of drought in neighboring areas, even when they have different ecological and economic resources. Similarly, [PERSON] et al. [62] defined the Anomaly Vegetation Index (AVI):  
\[\mathrm{AVI}=\mathrm{NDVI}-\overline{\mathrm{NDVI}} \tag{3}\]  
where \(\overline{\mathrm{NDVI}}\) is the multi-year averaged NDVI value for a given location in a specific month, thus reflecting the average soil moisture. Based on the deviation from this average, \(\mathrm{AVI}\) can indicate drought severity. Additionally, [PERSON] et al. [63] proposed the Standardized Vegetation Index (SVI):  
\[z=\frac{\mathrm{NDVI}-\overline{\mathrm{NDVI}}}{\sigma} \tag{4}\] \[\mathrm{SVI}=\mathrm{P}(Z<z)\]  
Of all these methods, VCI is the most acknowledged and has been used to monitor or evaluate droughts in many countries and regions after being proposed [64; 65; 66; 67].