Crop Coefficient (5) : How heat affects maximal vine water needs and water deficit?
Waves of heat = Waves of greater water needs
As heat wave comes and goes, vine water use goes up and down as we discussed here. Consequently during heat, a greater amount of water is used to satisfy vine water needs. Just like a speeding car consumes more gasoline, even if it runs the same distance as a slower car.
Because the amount of water in the root reservoir is depleted at a faster rate, irrigation may be needed earlier once heat wave has passed. It is therefore important to monitor carefully the rate at which vine transpiration ratio declines during the days following the heat, even if the climate is cooler. THe vine may be in danger of reaching “ the bottom of the root reservoir”.
Case study #1: Heat occurs soon after the end of shoot growth.
Figure 1 shows how much Transpiration ratio increases during a heat wave and how quickly it relapses afterwards. In this example, vine experienced a moderate soil moisture supply restriction before entering the heat wave. A lot of water was still available in the root system. Thus, enough water supply could be provided to meet the extra vine water needs during heat.
Figure 1: Transpiration ratio variations measured in June 2017 during a 2 days heat event in California after shoot growth stop (source: Fruition sciences)
Transpiration ratio may exceed Kcb as in Figure 3 since an extreme heat can drive an extreme rise in Transpiration ratio…(and yes, it means that the Kcb theory momentarily fails during heat, but this is beyond the scope of this blog).
In practical terms:
- Vine growers should not worry if they see a profile similar to Figure 3. The important step is checking Transpiration ratio profile after heat wave is over. You want to anticipate a brutal drop in transpiration ratio caused by a sudden limitation in root water supply…and irrigate if needed.
- You should avoid getting into a heat wave with a low transpiration ratio value. Typically, Transpiration ratio value is considered low when it reaches ½ the value of Kcb (orange dotted line in Figure 3)
Case study #2: Heat occurs before the end of shoot growth
Kcb value is unknown since Kcb is still a moving target (as discussed in blog 3). If transpiration ratio increases, it means that vine physiology is adapting to heat by consuming more water in response to heat. After heat wave , if shoot growth has not stopped, Transpiration ratio keeps increasing as before and it typically looks like Figure 4.
Figure 2: Transpiration ratio variations measured in June 2017 during a 2 days heat event in California before shoot growth stop (source: Fruition sciences)
In practical terms:
- If transpiration profile keeps growing after heat wave, you know that the vine never experienced any lack of moisture supply and irrigation was not needed.
- If leaf burning symptoms are observed, they are not caused by a lack of water availability in the root system. Leaf damage in this situation reflect the physical damage caused by very warm temperature, regardless of vine water use. Irrigation is not a good remedy to apply. Other alternative may be considered such as limiting ground heating effect, reducing vapor pressure deficit with misting, reducing climatic demand with shade cloth over the canopy…
What does heat wave do to Kcb ?
- Heat wave increase the value of Kcb only while it lasts.
- Kcb decreases after heat wave is over
- If you were to set your Kcb during a heat wave, you would end up overestimating vine water needs and over irrigating.
How to adapt irrigation strategy to heat wave?
- Before: Check that Transpiration ratio value is not too low prior to entering heat wave
- During and After: Check if Transpiration ratio declines and trigger an irrigation if tr ratio declines to dangerously low value.
Start saving water
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