Crop coefficient (2) : stratified sampling and plant-based analytics
The purpose of stratified sampling is to remove sources of noise when collecting vineyard data. In turn, analyzing “cleaner” data improves the ability to monitor vineyard performances precisely. To implement a stratified sampling strategy, it is necessary to focus:
- over a small vineyard area so we can remove the effect of within vineyard variations on plant indexes
- on plant-based measurements so we can connect precisely fruit and vine performances along a season cycle
By analyzing vine and fruit responses from the same location, we can measure how environmental pressures, such as soil or climate variations, are really perceived by the plant. From there, we can highlight what are the site specific connections between vine health indexes and fruit composition and what makes a vineyard unique. A direct application of stratified sampling is the attunement of practices according to vineyard specific conditions. 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, instead of the Kc used for the whole vineyard 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 ?
To remove the effect of soil and vineyard spatial variations we use the basal crop coefficient (Kcb). Mathematically, the concept captures vine’s maximal need for water according to climatic demand. It’s a plant-based measurement, which means it gives an unbiased view of vine water needs only.
By eliminating the contribution of soil evaporation and soil transpiration as well as the effect of vineyard spatial variations to vineyard water use, we remove “sources of noise” when estimating the real water use from a single plant.
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 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 (below).
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 tend to overestimate vine water needs, thus the amount of water recommended for irrigation.
Characterizing vine water use variations with sap flow 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
In our next blog we will discuss how we can apply this knowledge to improve irrigation decisions while leaf area is growing or during heat wave.
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