Crop coefficient (2) : Why using a plant based crop coefficient ?

Context

In the context of precision irrigation practices, a stratified sampling approach can be based on the diagnosis of single plant water needs. This approach requires the use of a plant based crop coefficient (Kcb), defined at the plant scale. In our previous blog we discussed the difference between vineyard and vine crop coefficients.  In this blog we will discuss how a plant based crop coefficient, Kcb, varies with time.

Why use a plant based crop coefficient ?

Water use analysis at a plant level

 Mathematically, the concept captures the relationship between vine’s maximal need for water and climatic demand. It’s a plant-based measurement, related to the total amount of vine leaf area that effectively sees the light, also called “leaf area exposed”.

Plant based coefficient is useful to calculate vine transpiration alone without confusing it with soil evaporation and soil transpiration . It removes “sources of noise” when estimating water use from a plant.

Analysis of  fruit ripening in the context  water use 

By focusing on the same group of vines we get a precise assessment of vine water deficit. Then, by tracking fruit maturation on the same group of vines, we get a precise assessment of fruit composition variations, the ultimate judge to assess if a practice is good or bad from a winemaker standpoint.

What is the time profile of a plant based crop coefficient?

In  a previous blog (Leaf Area Growth – Too little, too much, just right) we discussed how shoot length increases with thermal time. Intuitively, one can expect the larger the leaf area, the greater the vine water use. (You can read on that topic here.) Thus, it sounds logical that as leaf area increases, Kcb increases. But, what is exactly  the relationship between leaf area size and basal crop coefficient? Is it really as simple as “a big vine needs more water than a small vine”?

Historically, vine Kcb has been estimated according to how much light a vine could intercept at various stages of its development. In the late 80’s, Riou and his colleagues showed that Kcb  varied with thermal time. Then, Allen and his colleagues (1998 and 2009) recommended general values to estimate grapevine Kcb variations with thermal time. Those guidelines have been used to estimate irrigation needs in the context of the traditional method (ie. FAO-56). Time profile of recommended values for Kcb are reported in Figure 1 .

 

Figure 1: Time line of plant based crop coefficient obtained with sap flow vs. traditional method (adapted from Poblete Echevarria and Ortega Farias, 2013)

However, because the traditional method (FAO-56)  provides only general guidelines and also because Kcb is very sensitive to site specific conditions, the recommended values are an overestimate which leads to over irrigation most of the time. Thus, new approaches to precision irrigation have suggested to use sap flow measurements instead of the traditional FAO-56 guidelines to assess precisely what are vine water needs in a site specific context. Figure 1 shows the difference between measured Kcb obtained from sap flow measurements and recommended values. The use of sap flow (green line) confirms that Kcb is lower than we think (red line).\

Practical take home and conclusions:

Because Kcb is site specific, general values for Kcb recommended by the FAO-56 typically overestimate vine water needs, thus the amount of water recommended for irrigation. Characterizing vine water use variations at the plant scale directly contributes to:

    • reducing irrigation
    • confirming situations where  dry farming is possible (watch CNN video with Dominus Estate about dry farming here)
    • enhancing the vine’s natural ability to sustain water deficit
    • reflecting  unique effects of site specific conditions on fruit and wine compositions

Next we will discuss how we can apply this knowledge to improve irrigation decisions while leaf area is growing or during heat wave.

Stay tuned…

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Posted by Vintage Report

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