Plant water stress indicators and their applicability in precision irrigation

Irrigation management is a key sector for the competitiveness and resilience of the French agricultural sector. With the current climate crises, access to water is becoming increasingly difficult. Météo-France recently updated its seasonal normals (1991-2020), noting a general decline in spring rainfall across mainland France, and a decrease in northern and eastern France on a year-round basis. Current irrigation management methods are no longer efficient enough to cope with climatic, social and economic constraints, and this is becoming increasingly apparent each summer. This was particularly the case in 2022, and 2023 began with a historic winter drought (32 days without rain in February-March) and a 2-month delay in groundwater replenishment, according to the French Minister for Ecological Transition. This led some prefectures in the south of France to introduce water restriction measures at the start of spring. Unfortunately, such situations will recur in the future.

To be more competitive and demonstrate their good practices and efforts to preserve water resources, growers are applying for labels such as Plante Bleue and Haute Valeur Environnementale (HVE). Such labels require candidates to be as efficient as possible in managing water resources. The use of sensors, weather stations or any other measuring tool providing data for more precise and efficient irrigation is a prerequisite for obtaining these labels. At the Salon International de l'Agriculture 2023, French President Nicolas Sarkozy called for "a water sobriety plan" along the lines of "energy sobriety", referring to "the end of abundance". This plan includes an incentive to reduce our water consumption by 10%. Precision irrigation is a major lever that needs to be explored.

Understanding the water balance in the soil-plant-atmosphere continuum is essential, as plant water requirements are regulated by (1) soil water reserves and root water accessibility, and (2) the microclimatic conditions surrounding the plant. In the event of water stress, the plant adopts various strategies to ensure its survival and maintain a water status adapted to its environment. Today, plant water status can be assessed using a range of sensors for monitoring soil, plant and climate. Advances in metrology mean that water balance components can now be monitored 24 hours a day. Irrigation control using electronic probes is one of the most effective ways of doing this, but at a very high cost (several hundred euros). The current irrigation management market offers few sensor-controlled management solutions, and these do not reflect the water stress experienced by crops in real time.

What are the most relevant agronomic indicators? What are the levers to test for water savings? Does it make sense to...