Effect of irrigation volume depends on vineyard spacing and rooting depth
- A volume of 6 gallons per plant may be large or small. Vine appreciates the size of water “drinks” differently depending upon its daily water use. In turn, vine water use is mainly a question of vineyard spacing, leaf area exposed and varietal affinity for water. Some vineyards like to drink a lot in order to produce good fruit quality, some like to drink as little as possible.
- For instance, if a plant transpires on average 2 gallons per day, a 6-gallon irrigation covers the vine water needs for 3 days. After day 3, vine transpiration is expected to decline. On the contrary, if a plant uses less than 1 gallon per day, which is often the case with small vines in tight spacing vineyard, a 6-gallon irrigation covers vine water needs for more than a week. After day 7, vine transpiration may show a decline.
- Depending upon the vineyard situation, the same “drink” is perceived as relatively large or relatively small. Consequently, irrigation volumes have to be different to account for vineyard specific properties.
Vines are smart: when soil gets moderately wet, they use less water.
- Imposing and maintaining a period of moderate or even low moisture content is actually a good thing, especially for quality viticulture. As you force the plant to regulate its transpiration to a lower level than it could “potentially” have, you are promoting a better fruit composition. That is why vineyard sites or vintages that impose a lower soil water content rather than a higher soil water content are often sought after by wine lovers.
- Application: should you favor short or large intervals between irrigations? Once you determine the irrigation volume necessary for vine transpiration to rise up to its maximal level – called potential transpiration – just wait as long as possible until the next irrigation. Why? There are numerous benefits that will be discussed later from a winemaker standpoint. From a “vine sustainability” standpoint, the vicious side effect of frequent irrigation is that a constant re-wetting will favor a shallow root system development. The vine ends up taking water from a smaller root reservoir, located just under the dripper and root growth is promoted near the soil surface. Then, because of soil evaporation, shallow soil surface gets always drier first after each wetting a episode. This further precipitates the need for the next irrigation as the root system is concentrated near the surface. Consequently vines use more water than they could under frequent irrigation regime. They simply never reduce their transpiration. Or when they do, it is because you waited too long between two irrigations. When that happens, transpiration decline is brutal and often leads to fruit shriveling.
- It is the same principle as drug addiction: frequent irrigations train the vine to become a junkie, waiting for its next “fix”.
In conclusion vine are smarter than we think when using water. The consequence is that irrigation should adapt to plant response. Using the plant as a sensor enables you to respect it more and to irrigate according to its needs and your production objectives. Additionally, it is the best way to optimize your practices and reveal the best of your terroir.