# Optical and Thermal Remote Sensing for Monitoring Agricultural Drought
## 5 Multi-Source Data and Data Assimilation  
### Combination of Remote Sensing and Other Data Sources  
Meteorological indicators can reflect the cause and development of agricultural drought, while remote sensing indicators can directly describe the state and influence of agricultural drought. The combination of existing remote sensing and meteorological indicators has already proven to be beneficial for the comprehensive monitoring of agricultural drought [233; 234; 235]. Except for comprehensive utilization of existing indicators, a series of agricultural drought monitoring models and methods that combine remote sensing and meteorological data have been constructed. Soil moisture budgets affect vegetation transpiration directly, which in turn changes vegetation canopy temperature [236; 237]. [PERSON] et al. [238] found that the difference between the foliage temperature and the air temperature of the crop under potential evaporation conditions is linearly correlated with the vapor pressure deficits of the air. The crop water stress index (CWSI) is proposed as:  
\[\text{CWSI}=\frac{\Delta T-\Delta T_{\text{PE}}}{\Delta T_{\text{NE}}-\Delta T _{\text{PE}}} \tag{20}\]  
where \(\Delta T=T_{f}-T_{a}\) refers to the difference between crop foliage temperature and the air temperature [238]. \(\Delta T_{\text{PE}}\) is the difference between the foliage temperature and the air temperature in the potential evaporation state of the crop and is the lower limit of the temperature difference, while \(\Delta T_{\text{NE}}\) is the difference between the foliage temperature and the air temperature of a crop in the absence of transpiration and is the upper limit of the temperature difference. Since foliage temperature cannot be obtained from the space, [PERSON] et al. [239] replaced the foliage temperature in CWSI with the canopy temperature. With CWSI, thermal infrared (canopy temperature) and meteorological data (air temperature) are combined to invert soil moisture under crop cover conditions indirectly. This method is based on the principle of energy balance and has a clear physical meaning and high precision. The soil moisture inversion accuracy of the CWSI in vegetated areas is better than that of the thermal inertia method. However, CWSI also has its limitations. CWSI has  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Index** & **Expression** & **Notes** & **Year** \\  
a poor effect when the canopy is sparse, which is the actuality during the early stage of crop growth. Moreover, massive input data are required for model calculation, and the calculation is complicated. The errors introduced by the surface meteorological data in scale expansion also affect the accuracy of CWSI. CWSI can only be applied under the condition of full vegetation coverage. [PERSON] et al. [221] combined VI and LST to expand the application of the CWSI theory to partially-vegetated conditions and established the water deficit index (WDI). The WDI can be used to estimate the relative soil moisture based on the trapezoidal feature space characterized by the VI and the surface-air temperature difference. The WDI replaces \(\Delta T\) in the CWSI with \(\Delta T^{\prime}=T_{s}-T_{a}\), in which \(T_{s}\) is the surface composite temperature, which can be seen as a weighted average of crop leaf temperature and soil surface temperature. In contrast to CWSI, WDI is applicable in agricultural drought monitoring under various vegetation cover conditions. Like CWSI, WDI requires air temperature as its input; it cannot be calculated only using remote sensing data. [PERSON] et al. [240] proposed the atmosphere-land exchange (ALEXI) model, which uses thermal infrared data from the Geostationary Operational Environmental Satellite (GOES) and LAI products from MODIS to simulate daily ET at a 10-km spatial resolution. Besides the remote sensing data, the ALEXI model also requires shelter-level wind speed and air temperature data. Based on the ALEXI model, [PERSON] et al.