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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Viticulture Notes

Vol. 14 No.1, January - February 1999

Dr. Tony K. Wolf, Viticulture Extension Specialist

Table of Contents

I. Current situation

II. What is the probability of needing vineyard irrigation

III. Upcoming meetings

I. Current Situation:

The combined Virginia State Horticultural Society and Virginia Vineyards Association technical meeting in Roanoke (11-12 January 1999) was a considerable success. We had 134 registrants for the 2-day technical program, and 98 at the one-day "basic" grape production seminar on 13 January. That was a little over a month ago. It is not too early to give consideration to a similar meeting in January 2000. The straw poll taken at the Roanoke meeting suggested that attendees approved of the time and location, as well as the concept of combining with the other fruit producers meeting. Accordingly, we will conduct a similar program next January, although the location will likely be either at Charlottesville or Williamsburg. The viticulture program will again feature a mix of pest management discussions, research updates, out-of-state speakers, and probably some enological aspects, even if only wine tastings. The advantage of combining again with the Horticultural Society is to have the benefit of a larger trade show, the interaction with other fruit producers, and the consolidation of certain "general" aspects of horticulture and pest management.

I would greatly appreciate your input on next year's technical program. Enclosed with this newsletter is a single-page questionnaire that seeks your opinion as to program topic(s) of interest. Please take a few moments to provide some input, and return the questionnaire to the address provided by 12 March 1999.

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What is the Probability of Needing Vineyard Irrigation

The extended drought that occurred during the latter half of 1997 rekindled questions about the need for irrigation in non-irrigated vineyards. The drought also reinforced the convictions of some new vineyardists that irrigation was as necessary a vineyard input as was a trellis or a sprayer.

Like deer fencing, irrigation is still an option in Virginia vineyards; some years it will not be necessary, in others it will be used extensively. In an arid environment, the decision whether or not to irrigate is obvious; growing season precipitation will consistently and predictably fall short of vine water needs. In Virginia, and other eastern states, drought years are interspersed with wet years, and precipitation can be highly variable even within a season. The unpredictable nature of droughts in a humid environment perpetuates the question whether irrigation is economically justified. In Virginia, the need for irrigation is as predictable or unpredictable as the weather.

No fewer than 4 separate types of irrigation have been used in Virginia vineyards. These include the relatively inefficient use of overhead sprinkler irrigation, pulse-type micro-sprinkler, relatively short-lived drip tape, and the most commonly used, drip lines with pressure-compensating emitters. The systems vary in cost, maintenance, and performance, but they all share the common requirement for a source of relatively clean water, and all require some energy input to move water to where it's needed. Above-ground system costs will range from around $1,500 to $2,500 per acre for a 5- to 10-acre vineyard. This includes the cost of pump, filter system, distribution lines and emitters. The cost of a well, if needed, is around $7 or $8 per foot drilled. Specific system requirements are beyond the scope of this discussion, and beyond the expertise of your humble state viticulturist. For specific design advice, you are strongly encouraged to consult one or more commercial irrigation companies and have a system designed for your peculiar needs. The aim of this discussion, on the other hand, is to attempt to provide a basis for determining how often irrigation might be needed in established vineyards.

The subject of why water is important to vine performance is treated in a more in-depth fashion in the Mid-Atlantic Winegrape Growers Guide1. Briefly, optimal grapevine performance results when vines receive neither too much nor too little moisture. Soil moisture will be replenished during the growing season either by rainfall or irrigation. In the absence of irrigation, rainfall must be sufficient to maintain plant-available moisture at or above approximately 50% in order to prevent reductions in vine performance. When soil moisture becomes limiting, a range of drought effects can be observed. These effects include slowing of shoot growth, reductions in berry size and other components of crop yield, delayed crop maturity, leaf shedding and even vine death. Moderate to severe drought has both short-term and long-term consequences. The latter can be observed as decreased vine size and crop yields in year(s) after the drought.

Predicting drought frequency: To judge the future need of irrigation, it will be useful to review the history of droughts under Virginia, or more specific site, circumstances. A commonly used indicator of drought is the Palmer Drought Severity Index (PDSI). The PDSI is a monthly value that indicates the severity of a wet or dry period. The PDSI is simply a meteorological drought index; it does not relate directly to plant performance, but can be used in consideration of any future water needs, whether that need be urban water reservoir construction or vineyard irrigation needs. The PDSI generally ranges from ­6 to +6. Negative values indicate dry periods while positive values denote wet periods. PDSI values of 0 to -0.5 indicate normal precipitation patterns; -0.5 to -1.0 = incipient drought; -1.0 to -2.0 = mild drought; -2.0 to ­3.0 = moderate drought; -3.0 to ­4.0 = severe drought; and greater than ­4.0 = extreme drought. Similar adjectives qualify the positive deviations from the 0 baseline (wet spells).

Monthly PDSI data are available electronically from the National Climate Data Center and extend back to 1895. The composite data for Virginia, for the last 18 years are shown in Figure 1. Again, the data do not show when vineyards suffered. They do, however, provide a reasonable representation of the frequency of years that drought affects may have affected grapevines. The data of Figure 1 are consistent with my own recollection of years in which I observed drought- affected vineyards in Virginia. Those years were 1986, 1988, 1991, 1995 and 1997. The droughts in 1993 and 1998 occurred in the latter half of the year, and did not have as great an impact as that such as 1991, which was dry all year. The wet years, such as 1989, 1992, and 1996 were equally memorable. What¹s the point here? One argument for installing an irrigation system would be the observed occurrence of seven "moderate to severe" drought years out of 19 (37%). In addition to the state-wide data depicted in Figure 1, the National Climate Data Center has data for individual locations within Virginia (

Figure 1. Palmer Drought Severity Index (PDSI) data for Virginia during the period from 1980 through 1998. See text for explanation.

Even during "normal" precipitation summers, monthly precipitation falls short of that lost from the soil surface (evaporation) and through vegetation (transpiration). The combination of evaporation of water from soil and transpiration from plants is termed "potential evapotranspiration" or PET. It is a potential loss only because it's not actually measured ­ it's modeled on the basis of meteorological conditions and uses a standard vegetation type. Averaged across all of Virginia's National Weather Bureau stations, PET values exceed rainfall by an average of 1.5 inches during the month of July. In other words, if all of these stations averaged 3 inches of rainfall during July, they would also average 4.5 inches of water loss via evapotranspiration. PET values exceed rainfall input by at least 2 inches during July at Dulles Airport and Charlottesville. The authors of a 1957 long- term analysis of monthly precipitation and PET values concluded, ". . .although the probability of drought and the deficiency of water cannot be deduced quantitatively from such data, they are eloquent in pointing out the over-all deficiency of precipitation in Virginia during the late spring and early summer months" (Van Bavel and Lillard, 1957).

A final and very compelling argument for vineyard irrigation is provided by the authors of a more recent analysis of drought probability in Virginia (Vellidis et al., 1985). Vellidis et al. extended the earlier study of Van Bavel and Lillard (1957), and evaluated the probability of drought occurrence as a function of nearly any crop-soil combination encountered throughout the state. To understand the results or their analysis, some soil moisture terminology must be understood. Field capacity is the soil moisture content after the surplus water has drained from the soil by gravity. Permanent wilting point is the soil moisture content at which a plant can no longer extract water from the soil. When a soil dries to the permanent wilting point, plants will not recover from water stress, even with the addition of rain or irrigation. The amount of soil moisture between field capacity and the permanent wilting point is termed plant-available water. Grapevines and other plants will suffer yield and quality loss if soil moisture becomes limiting. Generally, the moisture content of the soil should be maintained between 100 percent of plant-available water (field capacity) and some lower value; 50% of plant-available water is often used for the lower level. The specific water needs of grapevines have not been examined within Virginia. We may, however, assume that vine performance will suffer if soil moisture is reduced below the 50% plant-available water threshold. Vellidis et al. (1985) used site-specific historical climatic data to determine the soil moisture status throughout the growing season in order to predict drought occurrences. Briefly the method consisted of making a daily balance of available soil moisture and summing the number of days on which the supply of soil moisture was exhausted. Such a day was termed a "drought-day." The calculations had three basic elements: (1) the value for the maximum moisture storage capacity of the root zone; (2) daily precipitation amounts; and (3) estimated values of daily evapotranspiration. The storage capacity of soils can be found in soil survey reports. For example, the Winchester AREC research vineyard is planted on a Frederick Loam. This soil holds approximately 0.17 inches of water for each of the first 24 inches of soil. Assuming that most root activity (and presence) is within 24² of the soil surface, we can expect that the Frederick Loam will supply approximately 4² of water at field capacity. We will assume that grapevines will suffer drought effects at 50% of water-holding capacity. Thus, drought effects will occur when only 2 inches of soil moisture remain in the top 24 inches of the Frederick Loam. We will call this a soil moisture depletion basis of 2 inches. Vellidis et al. (1985) constructed a series of tables that provided an estimate of the number of "drought-days" for a specified soil moisture depletion basis; this was done on a probabilistic basis. A portion on one such table is provided below in table 1, for the months of June, July and August, in the northern Piedmont of Virginia. To illustrate the table use, we will use our 2" soil depletion basis as the definition of a drought day at the Winchester vineyard. During the month of July, we can predict that a minimum of 20 drought-days will occur 1 year out of 10; a minimum of 12 drought days will occur every other year. July is typically the "driest" month, but similar drought days can be expected in June and August (Table 1).

The number of drought days that can be regularly expected in Virginia, combined with the PDSI data, and the deficit of summer rainfall relative to PET values, all suggest that droughts will continue to constrain vineyard performance in some future years. Unfortunately, we do not have a definitive relationship between the number of drought days, and the economic damage to grapes and grapevines. Lacking such data I can not say that irrigation will be economically justified in every situation. Besides climate, there are additional factors that must be weighed into an irrigation decision. Vineyards that are established concurrent with irrigation installation typically reach full production a year or more in advance of those without irrigation. With a high-valued crop, one might argue that a 2-ton/acre advance crop is worth the cost of the irrigation system. Vineyards planted on deep, coarse soils that allow extensive root penetration will not suffer short-term droughts as quickly as vineyards that are on shallow soils. Large, high-yielding vines will have a greater water requirement than will small, lightly cropped vines. And, despite your agreement to the benefit of irrigation, the cost of a deep well, power, or an extended water main distribution line may exceed your equity resources.


Van Bavel, C. and J. Lillard. 1957. Agricultural drought in Virginia. Va. Agric. Expt. Stat. Tech. Bull. 128., Blacksburg, VA.
Vellidis, G., B. Ross, and D. Taylor. Agricultural drought in Virginia 1930-1983. VPI and SU, Information Series 85-3, Blacksburg, VA.

Table 1. Minimum number of drought-days in Virginia for North-Central Piedmont of Virginia for different months, probabilities, and moisture depletion capacities (1930-1983) (from Villidis et al., 1985).

Month Probability Soil moisture depletion basis (inches)
1 2 3 4 5
June 1/10 21 18 16 13 10
2/10 19 15 12 9 4
3/10 17 13 10 7 2
5/10 14 10 5 2 0
July 1/10 22 20 19 17 14
2/10 20 17 15 13 11
3/10 18 15 14 11 8
5/10 15 12 10 6 4
August 1/10 21 19 18 17 16
2/10 16 15 14 13 12
3/10 14 13 10 10 9
5/10 10 8 5 5 4

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Upcoming meetings:

14-17 March 1999
Wineries Unlimited Seminars & Trade Show
Lancaster, PA. For more information call: 800-535-5670

22-24 March 1999
Vineyard Fruit School and 28th Annual New York Wine Industry Workshop
Locating, Financing, and Establishing New Vineyards and Wineries
Information for beginning and established grape growers and winemakers
Ramada Inn, Geneva, NY.
This workshop, featuring university and industry speakers, will cover all aspects of starting or expanding new vineyards and wineries. Day 1 is intended for beginning growers and will cover initial planning for vineyard/winery operations, including: overview of vineyard enterprises, developing a business plan, costs of vineyard establishment, economics of startup wineries, and financing new operations. Day 2 is intended for both new and continuing growers and will cover vineyard establishment, including: site selection, variety options, pre-plant preparation, vineyard layout and planting, and vine training. Day 3, the 28th Annual Wine Industry Workshop will cover starting or expanding wineries, including: legal requirements, siting a winery, winery design, equipment needs, and waste handling. For more information contact: Dept. of Food Science, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 315-787-2288 fax: 315-787-2284.

31 May - 2 June 1999
12th International Enology Symposium
3-12 June 1999, Post-conference tour through Quebec, Ontario and New York wine regions The association holds a meeting every 3 years in a different part of the world. The meetings bring together an excellent group of technical experts from around the world. It is a great way to hear what is going on in various parts of the world and to meet representatives from all these places. Part of the meeting is always a post-conference tour of the wine growing area. The site in Montreal and the post-conference tour were chosen to highlight the exciting developments in the wine industry in NE North America.
Contact: Thomas Henick-Kling, ph: 315-787-2277; fax: 315-787-2284; e-mail:

16-18 June 1999
ASEV Annual Meeting
Reno Hilton, Reno, NV
American Society for Enology and Viticulture PO Box 1855 Davis, CA 95617-1855
Phone: 530-753-3142; fax: 530-753-3318

14-18 July 1999
Oak Symposium and ASEV/ES Annual Meeting
St. Louis, MO
We, 14 JUL 99: Symposium tour, visit an oak forest and barrel making facilities Th+Fr, 15- 16 JUL 99: Oak Symposium ­ Science, practical experience, and tastings 16 JUL 99: Trade Show and Technical Presentations, Annual Meeting of ASEV/ES Sat, 17 JUL 99: Technical Presentations, Annual Meeting of ASEV/ES Contact: Ellen Harkness, Purdue University
Phone: 765-494-6704; fax: 765-494-7953; e-mail:

1Wolf, T.K. and E.B. Poling. 1996. The Mid-Atlantic Winegrape Grower's Guide was recently reprinted and is available from: Department of Agricultural Communications, Box 7603, North Carolina State University, Raleigh, NC 27695-7603. The price is $20, payable "NC State University." 126 pages.

"Viticulture Notes" is a bi-monthly newsletter issued by Dr. Tony K. Wolf, Viticulture Extension Specialist with Virginia Tech's Alson H. Smith Agricultural Research and Extension Center in Winchester, Virginia. If you'd like to receive "Viticulture Notes" as well as Dr. Bruce Zoecklein's "Vintner's Corner" by mail, contact Dr. Wolf at:

Dr. Tony K. Wolf
AHS Agricultural Research and Extension Center
595 Laurel Grove Rd.
Winchester VA 22602

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