Thermal and digital images as a tool for detecting water status in Grapevine

Viticulture in recent decades is particularly affected by reduction of water availability due to rising temperatures, drought and heat waves. Climate change projections towards global warming and drought set grape production at risk. Vineyards are largely located in semi-arid areas, such as Mediterranean regions, where the intensity of the drought season largely affect the final yield and quality of production. In order to complete its vegetative cycle, the vine needs large quantities of water. However, an optimal use of irrigation water is imperative for performing a sustainable cultivation, and this can only be achieved through a number of cultural practices, including regulated deficit irrigation and soil management. Moreover, robust techniques to accurately detect plant water stress are necessary.

The development of new technologies related to proximal sensing are assuming great importance in vineyard management. Among them, are non-invasive methodologies based on infrared thermography for assessing plant water status, and supporting tools for irrigation scheduling. Moreover, proximal sensing techniques based on digital images are becoming a valuable tool for detecting crop physiological status.

In this study, thermal and visible images of three varieties of grapevine, under two deficit irrigation regimes, were analysed and evaluated as a tool for supporting crop irrigation management. The experiment was conducted in two vineyards located in Sardinia, Italy, and consisted of two regulated deficit irrigation (RDI-1 moderate and RDI-2 severe) treatments and two reference treatments maintained under stress and well-watered conditions. Digital images were acquired daily, during the entire growing season, using Campbell CC5MPx digital cameras. Thermal images were acquired using the InfRec R500Pro thermal camera (Nippon Avionics Co., Ltd.). Artificial surfaces were used as target reference for wet and dry temperature. Vegetation indices from thermal and digital images, i.e. Crop water stress index (CWSI) and green and red chromatic indices (ExG, GRVI, REI), were then calculated for each observation day.

The analysis of thermal images gave an accurate estimation of the differences in the water status of the vineyard over the RDI treatments. This technique proves to be able to well-differentiate different regimes in water management, confirming its good performance. The differences in CWSI values between moderate or severe water deficit treatments (RDI-1 and RDI-2) were in almost all cases (sites and varieties) statistically significant. These results were also confirmed by the seasonal pattern of both green and red chromatic indices (RGBs indices: ExG, GRVI, REI).

The development of non-destructive, cost-effective and easy-to-use methods for continuous monitoring of grapevine water status is a challenge to be faced in the future. In this context, proximal sensing techniques tested in this study, can provide useful information to develop tools and models for irrigation management.