# Remote Sensing for Irrigation of Horticultural Crops
## 4 Approaches to Precision Irrigation
### ET Estimation
ETp values (mean seasonal daily ET) ranged between 4.2 and 4.1 mm\(\cdot\)d\({}^{-1}\), and from 0.88 to 0.93 in 2013 and 1.02 to 1.04 in 2014 for the midseason values, respectively. The results indicated that the values of Kc are site- and year-specific and depend on local ET\({}_{0}\) rates, rainfall frequency, cumulative thermal requirements, and effective canopy cover. The resulting crop water requirements were lower than the actual average irrigation volumes applied by farmers, thus suggesting the possibility of improving the water use efficiency of these crops by adopting the proposed methodology. We agree with the authors that, in the Mediterranean, few studies have investigated the satellite Kc estimation for vineyards. [PERSON] et al. [135] estimated ET starting from field weather data and Kc estimated with a function related to vegetation indices such as NDVI or index adjusted for reflectance from soil. [PERSON] et al. [136] studied how to estimate Kc directly from reflectance and VIs derived from a satellite. The authors found that NDVI-Kc quantified from remote sensing could be useful for estimating WUE, irrigation scheduling and performance, and agricultural water budgets. The NDVI-Kc may be also utilized for agriculture water conservation, starting from local to continental scales. Recently, [PERSON] et al. [137] published a paper with the aim of demonstrating how spatialized meteorological data and remote sensing techniques may allow one to estimate actual ET and the soil water content of a small plot (about 0.1 ha) of an olive orchard. As a rule, for Mediterranean areas, trees were covered by several herbaceous species. The authors found that the NDVI-Cws (coefficient of water stress) method is a suitable tool for estimating daily olive tree actual transpiration and soil water content in complex and multi-layer Mediterranean agricultural ecosystems. The novelty of the adopted method was to consider the spatially fragmented, two-layer structure of an olive orchard. For instance, the ETa of trees and grasses were combined with weather data (precipitation) to assess the site-specific water balance, which was assessed through comparison with daily measurements of soil water content. Moreover, this highlighted the importance of separating trees and grasses for estimation of the NDVI, adopting statistical operations to satellite imagery with different spatio-temporal properties. [PERSON] et al. [138], with the aim of partitioning ET into soil evaporation and crop transpiration, studied the ratio-based vegetation indices (NDVI, SAVI, EVI) retrieved from optical remote sensing, highlighting their limitations and how VIs can induce large model error. NDVI explained half of the variability in transpiration, the largest component of ET. However, to estimate total ET one should consider the Photochemical Reflectance Index (PRI). To estimate soil evaporation, the authors suggest two new ratio-based indices derived from NIR bands 743 and 953 nm and visible blue 428 nm and from SWIR1 bands 1518 nm. [PERSON] and [PERSON] [139] used an approach based on the conjecture that variations in the size of the crop have much stronger effects on its ET than variations in stomatal conductance. This assumption seems to be valid for irrigated crops, according to several studies [140; 141; 142]. The results showed the potential of this method for field application. Since there is little agreement on the nature of the relationships between VIs and Kc, the latter can be estimated from spectral measurements. The rationale is that both the component of the crop coefficient that represents transpiration (Kcb) and VIs are sensitive to crop biomass (LAI and ground cover fraction--fc). The relationships between Kcb and VIs could be linear or non-linear. The linearity of these relationships depends on the crop architecture [143]. [PERSON] et al. [142] used the same approach to derive Kcb values of wheat crops, analyzing the relationships between VIs, and both LAI and fc.