High-resolution UAV-based thermal imaging to estimate the instantaneous and seasonal variability of plant water status within a vineyard

Abstract

Thermal imaging can become a readily usable tool for crop agricultural water management, since it allows a quick determination of canopy surface temperature that, as linked to transpiration, can give an idea of crop water status. In the last years, the resolution of thermal imaging systems has increased and its weight decreased, fostering their implementation on Unmanned Aerial Vehicles (UAV) for civil and agricultural engineering purposes. This approach would overcome most of the limitations of on site thermal imaging, allowing mapping plant water status at either field or farm scale, taking thus into account the naturally existing or artificially induced variability at those scales. The aim of this work was to evaluate to which extent high-resolution thermal imaging allows evaluating the instantaneous and seasonal variability of water status within a vineyard. The novelty and significance of our approach is that the specifically designed and build unmanned aerial vehicle (UAV) provided very high-resolution imaging (pixel <9cm), and that it was used at a commercially relevant acreage (7.5ha). This set-up was used to obtain Crop Water Stress Index (CWSI) from thermal images in a clear-sky day. CWSI values were and compared to stem water potential (Ψs) and stomatal conductance (gs) measured at 14 sampling sites across the vineyard at the moment when images where acquired. In order to evaluate the potential of CWSI acquired in a single day to estimate within-vineyard patterns of variation in water status, a spatial modeling approach was used. CWSI correlated well with Ψs and gs at the moment of image acquisition, showing to have a great potential to monitor instantaneous variations in water status within a vineyard. The information provided by thermal images proved to be relevant at a seasonal scale as well, although it did not match seasonal trends in water status but mimicked other physiological processes occurring during ripening. Therefore, if a picture of variations in water status is required, it would be necessary to acquire thermal images at several dates along the summer.