# Remote Sensing for Irrigation of Horticultural Crops
## 4 Approaches to Precision Irrigation
### Leaf Temperature Variability  
Both baselines account for fully sunlit canopy leaves, but it is evident that the average canopy temperature is a summary of attributes of different leaves, an extensive variety of leaf temperatures, with sunlit leaves showing much higher temperatures than shaded leaves [156; 196]. Both shade and sunlit leaves constantly move to adjust light interception to maximize their water use efficiency and protect leaves from excessive heat and radiation load. In both well-watered transpiring and water-stressed canopies, there will be a wide range of sunlit and shaded leaves, since it is known that that dry matter accumulation in crops is directly related to the interception of solar radiation, up to the critical LAI. [PERSON] et al. [196] reported that the fraction of light absorbed by leaves, which controls crop growth rate, plant development, crop biomass accumulation, is directly related to the interception of solar radiation, up to the critical LAI [197]. The light distribution within the canopy affects, inter alia, the photosynthetic efficiency. Under comparable environmental conditions, the most important are the light levels, since leaf photosynthetic rates decline with age after leaf full expansion. The complexity is made worse by the difficulty of estimating the true age of each leaf in a plant community. Furthermore, the head-on viewing of a crop may increase the number of leaves that appear shaded, even though they are sunlit. As reported by [PERSON] [156], _\"there are always a few sunlit leaves on the shaded side and some shaded leaves visible on the sunlit side\"_. [PERSON] et al. [175] reported that measurements of tall trees are particularly challenging, because (i) open sky is within the sensor field of view; (ii) a tree canopy has a vertical rather than horizontal structure; and (iii) there is a canopy temperature difference between the shaded and the sunlit sides. Crops with genetic modifications (reduced leaf and modified stipule morphology) may result in enhanced yield when water is a limiting factor, avoiding drought stress [198]. It is still critical to define a well-watered crop, which according to definition should transpire at a rate regulated by physical conditions (e.g., VPD), and not present any physical resistance to transpiration fluxes. However, even well-watered plants show a midday depression of stomatal conductance in case of high VPD [199]. Different indices and procedures have been proposed [108; 111; 153; 164; 200]. As correctly stated by [PERSON] and [PERSON] [163], the CWSI has not been widely used for irrigated crops since it is necessary to acquire the temperature difference of both transpiring and non-transpiring leaves under the same environmental conditions. Stated below [PERSON] and [PERSON] [163] obtained CWSI values ranging from 0 to 1, when adopting the \(\mathrm{T_{air}}+5\)\({}^{\circ}\mathrm{C}\) for estimating the temperature of non-transpiring leaves (\(\mathrm{T_{dry}}\)), as proposed by [PERSON] et al. [201] and [PERSON] et al. [108] for corn and grape, respectively. The authors found that the maximum temperature difference between the canopy of deficit-irrigated vines and \(\mathrm{T_{air}}\) was influenced by irrigation frequency. Greater maximum temperature differences were found for vines irrigated once a week compared to those irrigated three times per week. The cultivar response to deficit irrigation was different with 'Syrah' vines, which were 2-4 \({}^{\circ}\mathrm{C}\) cooler than 'Malbec' vines [202], and differences were also detected in leaf surface temperature among grapevine cultivars undergoing water stress. [PERSON] et al. [202] reported \(\mathrm{T_{nws}}-\mathrm{T_{air}}\) values more than 7.5 \({}^{\circ}\mathrm{C}\) for 'Pinot-noir' vines subject to water stress, when air temperature was 32.3 \({}^{\circ}\mathrm{C}\) and air humidity 2.37 kPa.