2.9.4 When is irrigation necessary? What criteria should be taken into account?

What is the climate?

Moderate water stress (starting at véraison) is a climatic indicator of grape quality. A water deficit will naturally regulate the vines with regard to several physiological parameters such as vigour and sugar load. This stress corresponds to a period when the daily climatic demand, or "potential evapotranspiration" exceeds the quantity of water that can be extracted from the soil by the roots. This is mainly determined by temperature, solar radiation, relative humidity and rainfall. It can also be impacted by other factors such as soil type, grape variety, altitude (air temperature generally decreases by 1°C for an elevation of 150m) and topography.

In winegrowing, irrigation is therefore developed in semi-arid areas where it is indispensable, i.e. when the water stress is severe and risks impairing the quality of the grapes and therefore the wines in the short term and the sustainability of the vineyard in the long term. This is considered to be the case when rainfall is less than 350 mm/year. Irrigation can also be implemented when the annual rainfall is high, for example, 900 mm/year, but is concentrated in the winter period, as at Eisele Vineyard in California, where less than 100 mm falls between April and September but 600 mm falls over the rest of the year. When the useful water reserves in the soil are consistently insufficient, a system that allows for rapid replenishment of water resources may be necessary.

The availability of water to vines is not the only quality criterion, the thermal amplitude, i.e. the difference between day and night temperatures, plays the role of a second climatic indicator of quality in the grapes. These two criteria must be met in order to express the vine's best quality potential, which must be guaranteed by judicious technical itineraries, both in the vineyard and in the cellar. Irrigation alone does not solve all climate issues.

What is the soil type?

The water stress suffered by vines depends on the rainfall but also on the soil type. The soil's capacity for water retention is not only conditioned by its structure and texture but also its depth and composition.

For example, deep soil (>1m) will have a greater capacity to hold water than shallow soil whose depth generally does not exceed 40 to 60 cm. This does not mean that deep soil will necessarily constitute a greater water reservoir than shallow soil, since this will also depend on the soil texture and its composition, but it will have more capacity. Regarding the texture, soil rich in clay will retain more water and will require little or no irrigation. If an irrigation system is necessary on this type of soil, the water supply should be less regular and more consequent, of the order of 15 to 20 mm. Conversely, soil that is rich in sand will have a greater draining capacity, and the irrigation should be more regular and lighter, of the order of 5 mm.

How deep are the roots?

Water constraint also depends on the interaction between the reserves of useful water in the soil that can be easily exploited by the roots, called root exploration, the volume and depth of the root system and type of roots (main or tap root and secondary roots).

The larger and more developed the root exploration, the greater the contact capacity of the roots with the soil and the greater the water retention capacity.
The volume and depth of the root system have an effect on the vegetative growth and the vines' water consumption. The greater the rooting depth, the greater the evapotranspiration. Root volume and depth may depend on:

The soil's natural depth and its useful water reserve
Cultivation techniques, especially repeated tractor use, which can affect the soil's macroporosity and create a compact layer that cannot be penetrated by the roots.

Generally speaking, irrigation may be needed on 40 to 80 cm roots when rainfall is insufficient to replenish the soil's water reserves during the vine's growth cycle. When the rooting is denser and/or more developed and between 80 and 120 cm, irrigation may not be necessary, even when water constraints are present. Good root exploration allows better access to water at depth. Whatever the case, it is important to take into account the type of soil. For example, vines in sandy soil may require irrigation, even when the root system exceeds 120 cm.

Monthly evapotranspiration over a year (southern hemisphere) in relation to root depth (Myburgh et al., 1996)

What is the grape variety?

When the soil dries out, vines may suffer a sharp drop in leaf water potential. However, each grape variety has its own physiological characteristics, and some have a higher stomatal sensitivity and are more resistant to drought than others.

This is the case, for example, with Cabernet Sauvignon or Grenache, also called iso-hydric grape varieties, which have "pessimistic" behaviour when exposed to water stress and close their stomata. This limits evapotranspiration but negatively impacts photosynthesis and sugar accumulation. Conversely, the so-called aniso-hydric or "optimistic" grape varieties, such as Syrah or Chardonnay, maintain their growth under water deficit conditions, and the accumulation of sugars continues, sometimes to the detriment of the vine, which loses its advantage when severe dehydration of the soil causes irreversible damage.

Finally, there are early and late-ripening grape varieties. The earliness of a variety can be defined by the amount of heat it needs to reach maturity. This is measured in "growing degree days", i.e. the time needed for the vines to develop taking into account the temperature. A classification established by Van Leeuwen et al (2008) lists all the grape varieties by growing degree days. This classification can be used to adapt the choice of plant material to climatic changes as well as to production methods, including irrigation management.

Environmental, political and social context

Irrigation system selection should ideally take into account water supply and availability. Certain countries are faced with increasing water scarcity. In the current environmental context, drip irrigation seems to be the most efficient and economical of all options, whatever the circumstances. Currently, in France, there are associations such as AIRMF (Association des Irrigants des Régions Méditerranéennes Françaises) whose aim is to promote, develop and preserve the conditions for the implementation of irrigation for crops, particularly vines.

Cost of installation and use

Irrigation installation costs and water usage costs must be taken into account. For example, an underground system is more expensive than an above-ground one. Some systems are also more water intensive and the cost of use will therefore be higher.