Early in the growing season, particularly in June of 2017, Northern California experienced a series of heat waves. The combination of hotter and drier days, while daylength was at its greatest, rises the question: what does a heat wave mean for vine water use? To address this complex and challenging question we are going to discuss the notion of vine crop coefficient over the next blogs. But first, before exploring what those heat waves mean for your vine crop coefficient and how to handle it, let’s discuss a few basics about crop coefficient.
Crop coefficient and irrigation: a single coefficient or a dual coefficient approach?
The right amount of water at the appropriate time has a direct influence on wine quality as many authors have reported in the literature. Irrigation scheduling means addressing two key questions: when do we irrigate and how much water do we need to apply? To answer those questions the effect of irrigation must be analyzed jointly on plant and fruit responses.
The traditional method (also called the FAO-56 method) is based on the concepts of reference evapotranspiration (ETo) and single crop coefficient (Kc). The newer approach consists of considering 2 crop coefficients by separating Kc into basal crop coefficient (Kcb) and soil evaporation coefficient (Ke). As reported by a chilean team of researchers, Poblete-echeverria and Ortega Farias (2013), this approach is particularly relevant when the ground is partially covered or when irrigations are frequent. This situation prevails in most of the New world wine regions. Vineyards can grow under more arid condition (Chile, California, etc..) and may require irrigation at some point during the season; because of aridity most cover crops between vine row dry out during the season resulting in less than 50% of the vineyard is covered by grapevines. Hence, under such conditions, the application of dual crop coefficients in vineyards is more appropriate for estimating vine water needs alone.
Crop coefficients are site specific
Generic values for Kc and Kcb values for grapevines exist in the literature. However, those coefficients have been developed under different agro-climatic conditions and do not account for differences in canopy size, row orientation, training system or vine spacing, as reported by Ortega-Farias et al. (2009).
One of the main objectives of vine sap flow monitoring is to reveal the basal crop coefficient in the context of your specific vineyard conditions. Such approach to irrigation is challenging but offers rewards.
- By uncovering site specific vine water needs, you can better control the conditions of fruit maturation and adapt to seasonal conditions (as reflected through low vs. high shoot growth rate for instance- see previous blog here).
- By attuning irrigation practices to what the vine really needs, you can implement a more precise strategy considering its direct impact on fruit composition.
- By revealing what is really unique about your vineyard, you maintain its original footprint on wine composition..
Why using a plant-based crop coefficient (Kcb) instead of a vineyard based crop coefficient (Kc)?
There are 2 main reasons why irrigation strategy becomes more precise thanks to plant based measurements.
Reason #1: The most commonly used crop coefficient, Kc , is a mathematical concept designed to evaluate how much water the whole vineyard can use when soil moisture supply is non limiting. The term “non limiting soil moisture supply” means that vineyard water use will only be conditioned by vineyard architecture (row width, vineyard spacing, plant size, plant material, etc…) and climatic demand (ETref). Thus, by definition, the crop coefficient for an entire vineyard combines the water use of soil, cover crop -if it is alive- and vine into one single measurement. However, by tying together soil evaporation, soil transpiration and vine transpiration, the ability to evaluate the amount of water the vine only needs is lost. In other words, the crop coefficient Kc , defined at the vineyard scale, overestimates the amount of water needed to satisfy vine individual needs. The amount of water used according to Kc is greater than the amount of water the vine needs. This is precisely what is shown in figure 1.
The amount of water used at the whole vineyard scale (according to Kc) is measured using a technique called eddy covariance; the amount of water used at the plant scale (according to Kcb) is measured using sap flow technique.
In figure 1, the slope of the green line (vine water use) is 20% lower than the red line (total vineyard water use). This means that the amount of water use estimated by the Kc approach overestimates vine water use by 20%.
Reason #2: In response to spatial variations, fruit composition varies within the vineyard. For that reason, “stratified sampling strategy” and “selective fruit picking according to vineyard spatial variations” are implemented to support harvest decision and to improve wine composition. By using a same Kc for an entire vineyard, the effects of vineyard spatial variations on vine water use and fruit maturity are averaged, as if the vineyard were uniform. However, to improve irrigation and fine tune triggering thresholds for irrigation, you have to understand the effects of contrasted vine water deficit levels onto fruit composition. To gain this knowledge, you have to remove the effect of vineyard spatial heterogeneity, thus monitoring vine and fruit responses at the same location during the season. This strategy is the basis for stratified sampling. By focusing on a same group of plants all along the season, a stratified sampling strategy combined with Kcb will reveal the time variations of vine water deficit, and their effect on fruit composition during maturation.
We will discuss how to implement this approach in practice soon…Stay tuned!