# Remote Sensing for Irrigation of Horticultural Crops
## 4 Approaches to Precision Irrigation
### Infrared Thermography
#### 4.2.1 Cwsi
The canopy-air temperature difference (\(\text{T}_{\text{c}}-\text{T}_{\text{a}}\)) is normalized to the lower and upper limits of canopy-air temperature differences, which represent non-water-stressed and completely water-stressed crops, respectively. The subscript \"m\" denotes measured data, while \"l\" denotes a lower baseline (non-water-stressed crop), \"u\" denotes an upper limit (completely water stressed crop), which is an estimated parameter. The (\(\text{T}_{\text{c}}-\text{T}_{\text{a}})_{\text{n}}\) and (\(\text{T}_{\text{c}}-\text{T}_{\text{a}})_{\text{u}}\) limits may be estimated either on an empirical or theoretical basis as described by [PERSON] et al. [158] and [PERSON] et al. [148]. Both approaches require readings of air humidity and air temperature but it works better with wind speed and solar radiation data, and estimates of bulk canopy resistances. Irrigation scheduling based on (\(\text{T}_{\text{c}}-\text{T}_{\text{a}}\)) has often been determined by the stress degree day method (SDD), originally suggested by [PERSON] et al. [157] and [PERSON] [161]:  
\[SDD=\sum_{i=l}^{n}(\text{T}_{\text{c}}-\text{T}_{\text{a}})_{\text{i}}\]  
with \(\text{T}_{\text{c}}\) and \(\text{T}_{\text{a}}\) measured 1-1.5 h after solar noon at day i, during a n-day period. Irrigation was initiated as soon as SDD exceeded 0. The formula was used successfully for different crops [162], and according to [PERSON] and [PERSON] [153] SDD worked well for wheat, and in semi-arid climates where weather conditions were expected to differ little between consecutive days. According to [PERSON] and [PERSON] [163], the CWSI is also defined as:  
\[\text{CWSI}=(\text{T}_{\text{canopy}}-\text{T}_{\text{nws}})/(\text{T}_{ \text{dry}}-\text{T}_{\text{nws}})\]  
where \(\text{T}_{\text{canopy}}\) is the temperature of fully sunlit canopy leaves (\({}^{\circ}\)C), \(\text{T}_{\text{nws}}\) is the temperature of fully sunlit canopy leaves (\({}^{\circ}\)C) when the crop is non-water-stressed (well-watered) and \(\text{T}_{\text{dry}}\) is the temperature of fully sunlit canopy leaves (\({}^{\circ}\)C) when the crop is severely water stressed due to low soil water availability. Temperatures \(\text{T}_{\text{nws}}\) and \(\text{T}_{\text{dry}}\) are the lower and upper baselines used to normalize CWSI for the effects of environmental conditions (air temperature, relative humidity, radiation, wind speed, etc.) on \(\text{T}_{\text{canopy}}\). [PERSON] and [PERSON] [153] emphasized the limited application of the CWSI by the difficulty of estimating without actually measuring \(\text{T}_{\text{nws}}\) and \(\text{T}_{\text{dry}}\). [PERSON] [164] proposed a stress index (SI\({}_{\text{CWSI}}\)) analogous to the Idso-Jackson CWSI, using wet surface conductance rather than the finite value for a well-watered crop. \[\text{SI}_{\text{cwsi}}=(\text{T}_{\text{leaf}}-\text{T}_{\text{wet}})/(\text{T }_{\text{dry}}-\text{T}_{\text{wet}})\]  
[PERSON] et al. [165] suggested a modified CWSI (MCWSI) in grape and almond using the continuous leaf monitor data, where the well-watered and dry conditions were measured using a well-watered tree and a simulated dry leaf. The CWSI and MCWSI indices were calculated and were capable of representing the stress level of the grape and almond crops, so variable rate irrigation was implemented. The CWSI is considered a sensitive tool for monitoring and assessing plant water stress for a wide range of crops. CWSI was inversely correlated to cotton yields [166], and positively correlated with soil water depletion for a fully developed canopy when no soil reflectance was present [167]. The CWSI was also able to foresee a yield of several crops under stress and to develop schemes for managing irrigation [168; 169]. Furthermore, the Water Deficit Index, which is a two-dimensional CWSI normalized for vegetation cover, was related to crop water stress [150; 170]. [PERSON] et al. [171] studied the upper and lower baselines, and CWSI values on watermelon were slightly different.