MONDAY, JULY 22, 2024. BY STAN GRANT, VITICULTURIST.
Evapotranspiration or ET frequently comes up during discussions of vineyard irrigation and for good reason. ET is a dominant driving force for water movement within vineyards, as well as grapevine water uptake and use (Figure 1). At the same time, ET is a measurable atmospheric index useful for scheduling water applications to benefit grapevine growth, fruit yield and berry quality.
In spite of its importance, ET as a concept can be vague. Part of the ambiguity is due to the number of vineyard and atmospheric factors involved in ET and their relative contributions to it. The varying meanings of ET can also be confusing. In this article we will attempt to clarify ET as well as underscore its importance in vineyards.
Figure 1. Water keeps leaves turgid and is consumed during sunlight-driven photosynthesis. (Photo Source: Progressive Viticulture, LLC©).
The Meanings of Evapotranspiration
Water in the soil, vines and atmosphere form a continuum within a vineyard. And within the continuum, water moves from where it is in abundant supply to where it is in short supply due to an energy gradient (Figure 2). In this way, the relatively dry atmosphere draws soil water, which is especially abundant after winter rains or after irrigations, to the soil surface where it passes as water vapor. This well-known process is called evaporation.
Figure 2. The vineyard moisture continuum. (Diagram Source: Progressive Viticulture, LLC©).
Similarly, soil water taken up by roots is pulled through a vine. While vines retain and use some of the water taken up, they lose a sizeable portion (≈ 95% to 98%) to the atmosphere as it passes through tiny pores (stomata) on the underside of leaves as water vapor. Transpiration is the name of this type of evaporation. Substantial transpirational water loss is a necessary consequence of the stomata being open to allow carbon dioxide in the air to enter leaves to support of photosynthesis.
Evapotranspiration (ET) represents the combined evaporation and transpiration of water losses from plant-covered land surfaces, such as vineyards, to the atmosphere. Vineyard water loss is one meaning of ET. Agronomists refer to water loss from an agricultural field to the atmosphere as ET crop or ETc. Appropriately, ET grape or ETg is vineyard-specific ET.
While ET most commonly refers to water loss from a field, that is the draw on the moisture supply, it may also refer to the atmospheric water demand. The atmospheric demand, of course, being a principal cause of water movement within the vineyard water continuum, is greater than the vineyard water supply and consequently, grape ET. In practice, this second meaning of ET, which is known as the potential ET, is perhaps just as important for comparing vineyard climates as it is for vineyard water management.
There is one more meaning for ET that is significant to vineyard water management. While instruments are available to reliably estimate grape ET, estimates are more commonly calculated. These calculations involve the reference ET or ETo. The reference ET represents water lost from a uniform stand of unstressed, cool-season grass mowed to maintain a standard height of 4 to 7 inches. Water loss from grass, of course, is not the same as vineyard water loss and accordingly, the conversion of the reference ET to grape ET requires a vineyard-specific conversion factor, the crop coefficient or Kc. More about the vineyard crop coefficient appears later in this article.
To summarize, ET may apply to vineyard water loss, atmospheric water demand or reference crop water loss. Accordingly, we must be cognizant of context during discussions involving ET and be specific in our use of the terms grape ET, potential ET and reference ET.
Evapotranspiration – Atmospheric Factors
While water loss from a Class A Pan provided the reference ET for early ET-based irrigation scheduling, for practical reasons, weather station measurements of solar radiation, temperature, humidity and wind speed have replaced them. These, the main atmospheric elements contributing to ET, are factors in reference ET calculations (Figure 3A).
Figure 3. (A) A modified Penman equation for calculating reference ET (ETo) from environmental measurements and (B) the formula for calculating grape ET (ETg) from the reference ET, the crop coefficient, and the management factor.
The reference ET equation utilizes the vapor pressure deficit in place of relative humidity because it is a measure of atmospheric moisture independent of temperature. In practice, reference ET calculations exclude the very small portion of solar radiation used in photosynthesis (2% to 3%) and on a daily basis, they usually disregard the soil heat flux factor.
Of the atmospheric factors influencing ET, available solar radiation provides most of the energy for evapotranspiration, while temperature, wind speed and humidity affect the evapotranspiration rate. ET increases with higher temperatures, decreasing humidity and increasing wind speed.
Changes in Evapotranspiration Over Time
ET follows daily changes in atmospheric conditions, with ET being the highest during the day and lowest at night. These changes are evident in grapevine leaf water potential measurements made with a pressure chamber or bomb.
During warm and sunny days, while ET is high, the dry atmosphere pulls hard on free moisture within vines, creating strong negative pressure or tension. Under these conditions, the leaf water potential is comparatively low (i.e., more negative). In contrast, at night and especially just before dawn, ET is low and the leaf water potential is relatively high (i.e., less negative). Weather conditions that diminish ET, such as cool temperatures and cloudiness, are also evident in pressure chamber readings.
ET also follows seasonal changes in atmospheric conditions. We will use a Clements-Lodi AVA vineyard as an example. Grape ET was low early in the growing season but quickly increased during April and May as temperatures warmed, days became longer, shoots were growing and canopies were developing (Figure 4). In late June, grape ET began to level off and proceeded slowly to peak values during July. Towards the beginning of August, grape ET began to decrease as temperatures began to cool, days became shorter and leaves aged and became less active.
Figure 4. Monthly reference ET (ETo), grape crop coefficient, and grape ET (ETg) for a Cabernet Sauvignon vineyard on a vertical shoot positioned (VSP) trellis near Clements, California during the 2001 growing season. (Chart Source: Progressive Viticulture, LLC©).
Optimized irrigation scheduling shadows seasonal changes in ET, but during the ripening period, may purposely deviate from them during periods triple digit maximum temperatures to minimize grapevine heat stresses and related damage.
Evapotranspiration – Vineyard Factors
While the ability of the root zone soil to conduct water and the capacity of the vines to take up and translocate water is important, the rate of transpiration is mainly under the control of the stomata on the underside of leaves. Consequently, canopy characteristics that affect the number and position of leaves are critical grapevine and vineyard water use factors. Such characteristics include the quantity of leaves per unit of land, the percentage of leaves exposed to sunlight, the leaf area per unit crop (i.e., the crop load) and general leaf health and age (Figure 5). To some degree, variety-specific leaf attributes can contribute to transpiration rate, including leaf size, shape, surface characteristics and number of stomata per unit leaf surface.
Figure 5. Vineyard ET (ETg) depends on sun-exposed leaf area per unit of land. (Photo Source: Progressive Viticulture, LLC©).
The grape crop coefficient, mentioned above with regard to grape ET calculations, incorporates canopy characteristics and other vineyard factors affecting water use and loss to the atmosphere (Figures 3B and 4). Here are a few examples of how vineyard factors affect the grape crop coefficient and vineyard ET. Decreasing the distance between vine rows increases the leaf area per acre, and correspondingly, increases the crop coefficient and vineyard ET (Figure 6). Similarly, vineyards with divided canopies, such as quadrilateral cordon-trained vines, have more leaf area per acre and use more water than those with single canopies. Some practices, such as pruning and fertilization, influence the rate of canopy development early season crop coefficients and vineyard ET.
Figure 6. Crop coefficient in relation to growing degree days. Crop coefficient value depends on the distance between vine rows. (Data source: Williams 2001).
Cover crops increase the amount of foliage per unit land, increase vineyard water use and depending on the size of their canopies and the length of their presence during the growing season, can decrease the amount of moisture available for vines. In fact, cover crop competition for soil moisture is the primary reason vigorous cover crops can be useful for controlling grapevine canopy growth. To minimize competition and conserve soil moisture for vine use, till cover crops very early in the growing season to terminate their transpiration, while simultaneously creating a loose, low-density soil surface layer that impedes evaporation.
Typically, in California, the contribution of non-tilled cover crops to vineyard ET diminishes after grapevine bloom as vine canopy development nears completion and cover crop health and extensiveness decline. Still, the presence of vegetation, standing or mowed residues, on the vineyard floor can have small effects on vineyard atmospheres and indirectly influence vineyard water use.
Evapotranspiration and Vineyard Management
Of course, grapevines do not lose all of the water they take up to the atmosphere. Vine tissues retain a small volume of water, which keeps them turgid and contributes to cell expansion. At the same time, metabolic processes including photosynthesis consume water as a reactant. This is how vineyards make beneficial use of soil moisture.
In contrast, moisture lost to the environment, be it to the atmosphere in response to potential ET or to the soil below the root zone due to over-irrigation, represents a true loss. Careful regulated deficit irrigation scheduling based on grape ET is a very effective means of reducing water losses and controlling costs associated with irrigation. Additionally, moderate grapevine water stress maintained through ET-based regulated deficit irrigation schedules enhances the efficiency of water use within vines for growth and fruit development. In this effort, the management factor (Km) serves as the deficit irrigation factor in the vineyard ET formula (Figure 3B, above).
Conclusions
Several atmospheric factors work in concert to drive water movement in vineyards and grapevine water use. Evapotranspiration is the concept incorporating the most significant of them into a measurable and for vineyard managers, actionable quantity. Using ET to advantage in vineyard water management requires a clear understanding of the factors involved, including viticultural as well as atmospheric.
A version of this article was originally published in the Mid Valley Agricultural Services July 2003 newsletter and was updated for this blog post.
Further Reading
Goldhamer, DA, and Snyder, RL. 1989. Irrigation scheduling: a guide to efficient on-farm water management. University of California Division of Agriculture and Natural Resources Publication 21454, Oakland, California.
Connor, DJ, Loomis, RS, and Cassman, KG. Crop ecology: productivity and management in agricultural systems. 2nd Ed. Cambridge University Press. 2011.
Ekern, PC, Jr, Robins, JS, and Staple, WJ. Soil and cultural factors affecting evapotranspiration. In Hagan, RM, Haise, HR., and Edminster, TW (Ed.). Irrigation of agricultural lands. American Society of Agronomy, Madison, WI. 1967.
Gates, DM, and Hanks, RJ. Plant factors affecting evapotranspiration. In Hagan, RM, Haise, HR., and Edminster, TW (Ed.). Irrigation of agricultural lands. American Society of Agronomy, Madison, WI. 1967.
Grant, S. Five-step irrigation schedule: promoting fruit quality and vine health. Practical Winery and Vineyard. 21, 46-52 and 75. May/June 2000.
Grant, S. Regulated deficit irrigation, parts I and II. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). July 18 and August 04, 2014.
Grant, S. Comprehensive vineyard water management. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). August 18, 2015.
Grant, S. Comparative wine growing climatology. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). September 29, 2016.
Grant, S. Using competition to best advantage in vineyard management. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). June 8, 2020.
Grant, S. Ripening period vineyard management. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). July 07, 2020.
Grant, S. Till or No-Till. Lodi Winegrape Commission Coffee Shop. (www.lodigrowers.com). January 23, 2023.
Grimes, DW, and Williams, LE. Irrigation effects on plant water relations and productivity of Thompson Seedless grapevines. Crop Sci. 30, 255-260. 1990.
Hillel, D. Introduction to soil physics. Academic Press, New York. 1982.
Howell, TA. Relationship between crop production and transpiration, evapotranspiration, and irrigation. In Stewart, BA; Nielsen, DR (Ed.). Irrigation of Agricultural Crops. Madison, Wis: American Society of Agronomy. pp. 392-435. 1990.
Kramer, PJ. Plant water relations. Academic Press, San Diego. 1983.
Keller, M. The science of grapevines. Academic Press, Burlington, MA. 2010.
Levin, AD, Matthews, MA, and Williams, LE. Effect of preveraison water deficits on the yield components of 15 winegrape cultivars. Am. J. Enol. Vitic. 71, 208-221. 2020.
Mullins, MG, Bouquet, A., and Williams, LE. Biology of the grapevine. Cambridge University Press, Cambridge. 1992.
Richie, JT, and Johnson, BS. Soil and plant factors affecting evaporation. In Stewart, BA; Nielsen, DR (Ed.). Irrigation of Agricultural Crops. Madison, Wis: American Society of Agronomy. pp. 364-390. 1990.
Williams, LE, and Matthews, MA. Grapevine. In Stewart, BA; Nielsen, DR (Ed.). Irrigation of Agricultural Crops. Madison, Wis: American Society of Agronomy. pp. 1019-1055. 1990.
Williams, LE. Using crop coefficients to schedule irrigations in the San Joaquin Valley – practical applications. University of California Grape Day 1999.
Williams, LE. Irrigation of winegrapes in California. Practical Winery and Vineyard. 22, 42-54. Nov/Dec 2001.
Featured image provided by Alina Tyulyu Photography.
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