How do you know when to irrigate your vines and how much? How do you know if your vines can become more drought-resilient and thrive with less irrigation?
At Fruition Sciences, we have produced a guideline for these types of questions. We measure two key quantities to determine vine water status: plant transpiration and evaporative demand. Plant transpiration comes directly from water used by vines (as measured by sap flow sensors). Evaporative demand can be calculated using data from weather stations.
Typical water use and evaporative demand patterns
The main concept in our guideline is to keep vine transpiration below evaporative demand. Irrigation is triggered only when vine transpiration drops below a certain threshold, often represented as a fraction of evaporative demand. Consequently, throughout the season, vines are expected to use less water than the evaporative demand. As you can see in the following charts, for select customers in France (figure 1a) and California (figure 1b), vine transpiration (the red line) was consistently below evaporative demand (the green line). This pattern is common and expected.
Figure 1a: Typical transpiration and evaporative demand patterns (France)
* Note: the pale blue line represents the irrigation amount, and the dark blue line represents rain.
Figure 1b: Typical transpiration and evaporative demand patterns (California)
What does over-irrigation look like?
For some of our customers in South America, we observed that over-irrigation induced a level of vine transpiration similar or even higher than evaporative demand! Figure 2 shows that transpiration was higher than evaporative demand and stayed continuously at an excess level throughout December, January and February (figure 2). As can be seen below, vine transpiration (the red line) was frequently at and occasionally above evaporative demand (the green line).
Figure 2: Transpiration and evaporative demand patterns with over-irrigation
The chart below displays the ratio of transpiration to evaporative demand for these customers. This ratio can be used to identify the period during which the vine is largely “over-transpiring” due to excess irrigation. In a normal situation, this ratio typically fluctuates between 10% (common with low vine spacing) and 80% (common with tight vine spacing). In an over-irrigation situation like this, however, the ratio went above 100% for a large part of the season. This suggests that water was “forced” out of the vine, which in turn made the berries more susceptible to dehydration later on.
Figure 3: Transpiration-to-evaporative demand ratio with over-irrigation
Can your vines become more drought-resilient?
In general, our customers report 40% to 80% water savings per year while maintaining yield levels with Sap Flow sensors. This is because vines develop a higher resistance to berry shriveling, when they were “programmed” to get less water earlier during the season.
We previously wrote about a successful experiment we did with six wineries using a grant from the Water Metropolitan District of Southern California. The wineries involved achieved on average a 65% reduction in water usage with no yield loss and improved fruit quality.
Jackson Family Wines, one of our customers, was able to achieve 40% to 80% water savings over a 3-year study in the same vineyard. They also managed to accomplish this level of savings with no yield loss and better wine quality.
So can your vines live with less water? The answer is yes; your vines can thrive with less water than you might think.
Our product Sap Flow helps winemakers enhance fruit and wine quality while saving significant amounts of water. We have also discussed water scarcity issues in California here and plan to provide an update and best practices surrounding this issue at the Napa Vintage Report conference on January 18, 2017. Tickets to the conference are available at https://www.vintagereport.com/en/napa-2016.