2.9.7 How much water is needed?

Consequences of over-irrigation

Over-irrigation can result in reduced quality (and associated with a significant increase in yields), excessive vegetation, higher risks of fungal contamination (Botrytis), overconsumption of water, and increased leaching in the case of sprinkler systems, which can then result in increased use of phytosanitary products. Conversely, moderate water stress during véraison can favour the production of high-quality tannins.
If the irrigation system is carefully managed, it is unlikely that symptoms of over-irrigation will be observed.

To calculate the optimal amount of water to be applied during irrigation, it is necessary to take into account the current water stress that the winegrower wishes to correct and the level of stress desired post-irrigation. It is also possible to observe symptoms linked to under-irrigation when intervention occurs too late.

Choice of drip nozzle

Drip nozzles can be non-self-regulating, which means that the pressure can vary from one drip nozzle to another, the flow rate can therefore vary, as can the quantity of water supplied. Conversely, they can be self-regulating, in which case the pressure is the same over the entire surface to be irrigated, and the quantity of water supplied is the same.

The drip nozzles can also vary according to their flow rate and their frequency. They can be placed every metre or less (0.75 m) when plots are either very well drained (sandy soil, for example), or the soil is relatively shallow. Similarly, the flow rate can vary from one to two litres per hour.

Calculating the water supply

The calculation of the water supply will depend on the quantity of water desired, and the frequency and flow rate of the drip nozzles. As a reminder, 1 mm of water represents 1l/m2.

The water supply in mm/h is calculated by :

Quantity of water supplied = Flow of the drop nozzle (l/h) / frequency or lattice (m*m)

Where the lattice represents the distance between 2 drip nozzles * the distance between 2 rows of drip nozzles.

Example of calculation for a narrow vineyard (1m x 1m) with 2l/h drip nozzles:
Hourly water supply for the installation = 2/ (1*1) = 2mm/h.

The installation supplies 2 mm of water per hour.

How long will it take to supply 4 mm?
4/2 = 2h. Two hours of irrigation will be needed to replenish the groundwater reserves by 4 mm.

Whether the drip nozzles are self-regulating or not, it is advisable to check the installations regularly. Finally, there are online tools that allow you to manage localized drip irrigation. They generally inform the user of the volume of water to be applied per vine and/or at the plot level as well as the duration of irrigation required.