# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
## 2 Optical Remote Sensing  
### The Effect of Water Content on Soil and Crop Reflectance in the Solar Region (400-2500 nm)  
Optical sensors can measure the surface reflectance in visible (VIS), near-infrared (NIR), and short-wave infrared (SWIR) bands, within which solar radiation is the driving force. In this range, water has two major absorption peaks centered at 1470 nm and 1900 nm, and two minor ones centered at 970 nm and 1200 nm, while even smaller peaks and shoulders can be found at 514 nm, 606 nm, 739 nm and 836 nm [32; 33; 34]. These characteristics lay the foundation of soil moisture monitoring within the solar range. Many researchers observedthat wetted natural soils showed lower reflectance [35]. This rule generally holds; however, we can see from Figure 2 that when soil moisture is high, the change in soil reflectance becomes subtler. There can even be an inversion at higher moisture levels, as pointed out by [PERSON] [36] and [PERSON] et al. [37]. That is to say, after the soil moisture exceeds a threshold, the reflectance of soil begins to increase with soil moisture. This critical point generally appears within the 0.15-0.40 g/cm\({}^{3}\) soil moisture range, which depends on soil types [38]. Since the critical point is below the saturation point, the reversion phenomenon cannot be neglected in soil moisture retrieval. Nevertheless, within the field of drought monitoring, we can safely assume that the soil moisture level is below this turning point. Thus, soil reflectance is considered to decrease monotonically with increasing soil moisture. The exact relationship between soil reflectance and soil moisture can be very complex and is affected by many other factors, such as mineralogy, salinity, texture, organic matter content, or roughness. Several empirical or physical models have been proposed [34; 39; 40; 41; 42], which we will not discuss in detail here. Water content also affects the reflectance of crops, mainly in NIR and SWIR bands [43]. To identify the sensitive spectral bands of vegetation water content, [PERSON] et al. [44] obtained the spectra of leaves with different water content by gradually drying saturated leaves at room temperature. The results showed that the spectral reflectance of leaf samples increased with decreasing leaf water content, and the reflectance at 1450 and 1930 nm are significantly correlated with the relative water content of leaf blades. [PERSON] and [PERSON] [45] pointed out that the reflectance at 950-970, 1150-1260, and 1520-1540 nm are well correlated with canopy water content. Here, simple simulations using the leaf radiative transfer model PROSPECT-D [46] and the canopy radiative transfer model 4 SAIL [47] are carried out to illustrate the effects of water content on crop reflectance (see Figures 3 and 4). It can be seen from the simulations that leaf water content has a remarkable impact on crop reflectance at both the leaf and canopy level, indicating that remotely sensed reflectance can be a measure of crop water content and water stress. However, sensitivity analysis shows that the other factors, such as leaf area index (LAI), average leaf angle (ALA), and leaf chlorophyll concentration, make it difficult to accurately retrieve leaf water content directly from reflectance data, especially at the canopy level [48]. Besides the direct influences on crop reflectance, the water content can also affect crop reflectance via its effects on crop physiology. The water needed for growth, development, and reproduction is mainly absorbed from the soil during crop growth. When soil moisture falls below a certain threshold and cannot meet the water demand, crop cells start to lose water, causing decreases in swelling pressure and changes in morphological structure and canopy cover (e.g., the leaf area). Water stress can also cause changes in biochemical (such  
Figure 2: Measured spectra of a soil sample with different soil moisture, c.f. [PERSON] et al. [42]. as chlorophyll, carotenoids) concentrations, resulting in further changes in crop spectral reflectance [12].