## 2 Materials and Methods
### Remote Sensing Measurements
#### 2.3.2 Spectral Indices to Predict Leaf Physiology  
Due to soil water stress, plants adjust their physiology, morphology, and biochemistry. For instance, on a general basis, under drought, leaves tend to close stomata limiting water vapor loss via transpiration, which reduces photosynthetic carbon uptake and increases leaf temperature. In addition, leaf turgor and chlorophyll content are reduced, affecting growth [13; 22]. Note that chlorophyll, carotenoids and anthocyanins provide vital details about plant physiology status, absorbing energy, harvesting light, and protecting leaves from excessive light [68]. Thus, observable changes in leaf temperature and vegetation indices (VI), related to structure, pigments, and photosynthetic efficiency can provide valuable information about how leaf physiology is affected by drought. In our study, we calculated leaf-air temperature difference (\(\Delta\)T) and a total of 13 narrow-band indices (Table 2), that have previously shown sensitivity to soil water stress.  
The normalized difference vegetation index (NDVI) is the most commonly used VI to predict LAI, green biomass, fraction of photosynthetic active radiation, or fractional cover. However, because NDVI saturates at high LAI, it is commonly replaced by the enhanced vegetation index (EVI) [42; 69]. Another similar VI to NDVI is the optimized soil adjusted vegetation index (OSAVI), which also reduces soil effects [70]. Because the red edge is close to the NIR region, it is sensitive to changes in cell structure [75]. We used the red edge normalized difference vegetation index (rNDVI) [72; 76] and the maximum in the red edge (REIP) calculated from the first derivative [71], which have been proved to provide information about structural changes due to soil drought.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Acronym** & **Indices** & **Equation** & **Reference** \\ \hline \multicolumn{4}{c}{**Structure**} \\ \hline NDVI & Normalized difference vegetation index & \((9800-\text{e}670)\)(/\(\text{e}800+\text{e}670)\) & [38] \\ \hline EVI & Enhanced vegetation index & \(2.5^{\text{\text{\  
Indirect changes on chlorophyll content, related to reduced soil water, can be assessed by calculating the ratio R\({}_{700/670}\). This VI includes a chlorophyll absorption feature at 670 nm [42; 43] and the 700 nm band located at the red-edge region, which is sensitive to chlorophyll concentration [71; 75]. VIs tracking chlorophyll content such as the transformed chlorophyll absorption in reflectance index (TCARI) and the ration between TCARI and OSAVI have been successfully used to estimate water stress [10; 37]. In addition, TCARI/OSAVI reduces the effect due to changes on LAI and soil reflectance under soil water stress [39; 43]. The modified anthocyanin reflectance index (mARI) combine blue, red, and NIR bands to predict changes on anthocyanin content [68]. At 470 nm (blue band), carotenoid absorption can be detected [40] and this wavelength is used in the pigment specific normalized difference index (PSNDc).  
The photochemical reflectance index (PRI) is used to estimate photosynthetic activity [41; 76]. Because PRI is sensitive to structural changes, pigment levels, soil background, illumination effects, and viewing angles [42; 43], other derivations have appeared such as the PRI\({}_{(570-515)}\) and the sPRI, which have been proved to be more effective to detect water stress [43; 76].