Remote imagery to assess water stress variability within the orchard.

Abstract This paper describes the current status of the measurement of the spatial variability of water status at tree scale in fruit crops through remote sensing, and discusses the limitations and opportunities of these technologies. Remotely sensed multispectral and thermal imagery can provide high precision water status maps in orchards through stress indices, which are a very useful tool for irrigation monitoring and deficit irrigation strategies especially in areas where water resources are limited. They are also a powerful tool for breeders working on water stress phenotyping. The data can be obtained from multispectral sensors onboard satellites, airplanes, or unmanned aerial vehicles (UAVs). The main limitations of remote sensing, when working at tree scale, can be summarized in the following two points: (i) the processing time required to obtain water stress maps when almost real-time information is required (i.e., for irrigation scheduling purposes) and technical knowledge to interpret them; and (ii) the large costs of the technology. Some possible solutions may include: offering a consulting service that provides technical support, agronomic knowledge and specific training courses, the development and implementation of uniform and cheap standards, and promoting new research on image upscaling methods (sharpening) that, through the fusion of images at different scales, are able to increase the resolution offered by satellites and allow access to data quickly and inexpensively as a complement to UAV. Despite numerous efforts, a powerful and flexible methodology for obtaining evapotranspiration and water stress maps remains the greatest challenge for this technology.