# Remote Sensing for Irrigation of Horticultural Crops
## 4 Approaches to Precision Irrigation
### Infrared Thermography  
#### 4.2.1 Cwsi  
Scientists tried to find indexes that describe the relationship between and thermal characteristics of plants and water stress [147]. [PERSON] et al. [148] set up the CWSI, [PERSON] [149] the Surface Temperature (ST), [PERSON] et al. [150] the Water Deficit Index (WDI), [PERSON] et al. [151] the Stress Index (SI), and [PERSON] et al. [152] the Stomatal conductance index. Plants showing symptoms of wilting emit more longwave infrared radiation, revealing canopy temperature. Unfortunately, canopy temperature is affected by meteorological conditions at the time of measurement, so it cannot be an absolute indicator of water stress. We recommend several papers related to theoretical approaches dealing with infrared thermography: [PERSON] and [PERSON] [40] provided a rigorous and detailed explanation of the theory and practical applications of remote sensing; [PERSON] and [PERSON] [153] wrote an exhaustive review on thermal remote sensing technique and field variation of plants and environmental parameters involved. Other key papers include those from [PERSON] [94], [PERSON] et al. [154], [PERSON] et al. [155], and [PERSON] [156]. Among the indexes cited above, the most studied is the CWSI, which was developed at the U.S. Water Conservation Laboratory in the 1970s and 1980s [148; 157; 158] and has been commercialized and widely used in irrigation management. [PERSON] et al. [148] found that CWSI was defined by the ratio of actual to potential ET calculated from the Penman-Monteith equation, and highlighted the usefulness and limitation of the index. Although it appeared to be a promising tool for quantifying crop water stress, it was unable to quantify crop water needs, or the time threshold for watering a crop. A long list of indices is available at the website IDB ([[http://www.indexdatabase.de/db/i.php?offset=1](http://www.indexdatabase.de/db/i.php?offset=1)]([http://www.indexdatabase.de/db/i.php?offset=1](http://www.indexdatabase.de/db/i.php?offset=1))), which reports the basic information for each index including the following: name,abbreviation, formula, variables, explanations of variables, wavelengths, source, description; sensors, visualization of required spectral range; visualization of Sensor-Bands, applications, references. Given defined environmental conditions (air humidity, air temperature), the CWSI index is based on the quantification of the behaviour of a crop both when it is fully transpiring and when it is not. [PERSON] et al. [135] expressed CWSI in terms of evapotranspiration:  
\[\text{CWSI}=1-\text{ET}_{\text{c}}/\text{ET}_{0}\]  
where, \(\text{ET}_{\text{c}}\) is the actual crop evapotranspiration and \(\text{ET}_{0}\) is the reference evapotranspiration. According to [PERSON] [135], the relation between \(\text{ET}_{\text{c}}\) and \(\text{ET}_{0}\) approaches the theoretical values proposed by [PERSON] [148], because it is decoupled from the wind velocity as in the classical form, according to [PERSON] et al. [158]. [PERSON] [159] suggested replacing the estimations of VPD from minimum temperature with air temperature measurements in direct radiation on irrigated plots, with the goal of increasing and partly correcting the influence of the superficial soil temperature on LAI. The accurate assessment of the ET by means of the CWSI is assumed by the relation between \(\text{ET}_{\text{c}}\) and the leaf's water potential (\(\text{ET}_{0}\)) at dawn, according to [PERSON] et al. [160]. [PERSON] [115] defined the CWSI as:  
\[\text{CWSI}=((\text{T}_{\text{c}}-\text{T}_{\text{a}})_{\text{m}}-(\text{T}_{ \text{c}}-\text{T}_{\text{a}})_{\text{l}})/((\text{T}_{\text{c}}-\text{T}_{ \text{a}})_{\text{u}}-(\text{T}_{\text{c}}-\text{T}_{\text{a}})_{\text{l}})\]  
It uses the crop canopy temperature (measured by an IR thermometer or thermal images) and environmental parameters (air temperature, air humidity, wind speed, solar radiation).