Vine water deficit: size matters !

water deficit

What does a vine really experiences when it is in a good “terroir” or during a “good vintage”?

To uncover the “terroir” or the “vintage” effect on vine activity, the best is simply to ask directly the plant. In other words, the best is to measure directly on the vine how it responds  to the environment.
One of the most fundamental expression of vine “response” can be characterized through its water use. For that reason water use is often the key to predict the potential of a terroir or of a vintage at producing good wines. But before discussing why water use is so critical for a vine and how it affects fruit and wine quality in the next articles, let’s explain how vine water use can be measured.
The purpose of any vine water use measurement is to appreciate a level of water deficit. Any index of vine water deficit aims at evaluating the amount of water effectively used by the vine and see if this amount  falls short of some reference amount.  Typically, the “reference” amount is the maximal amount of water a vine can use – or “transpire”. The maximal amount of vine water use is proportional to  the climatic conditions and vine size (ie. leaf area exposed to light). It is important to understand that soil moisture deficit plays no role in the maximal amount of water a vine can use. In fact, by definition, soil moisture is never limiting when vine water use is maximal 

 Take home

Figure 1 shows vines with a relatively small leaf area exposed, and consequenlty, 100% of their water needs can be satisfied with a small amount of water (2.6 liter) as measured by the sap flow sensors attached to their stem.
Figure 1: vines using a maximal volume of 0.7 gallons (2.6 Liters) of water for a typical day in July (Napa)

Figure 2 shows vines with a relatively large leaf area exposed, and consequently,  a large amount of water is needed to satisfy 100% of their water needs (11.5 liters) as measured by the sap flow sensors attached to their stem.

Figure 2: vines using a maximal volume of 3 gallons (11.5 Liters) of water per day for a typical day in July (Napa)

 

Conclusions

If you were a vine, spending water is a bit like spending money. Your leaf area exposed to sunlight drives your purchase power. By “spending” water away you get an extra comfort by cooling off your leaf temperature. More importantly, you get to “purchase and swallow” more or less atmospheric carbon and convert it into sugars and other chemical compounds. Your maximal water use would be the maximal amount of dollars you can spend in one day.. it drives in turn the maximal amount of chemical energy you can use for maintaining your organs alive and functionning.

Before we further talk about managing vine water deficit (here) and plant based crop coefficient (here), it is important to appreciate the range of variations in daily water volume that different vines can display.

 

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