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How irrigation management affects wine quality.

The aptness for winemaking of certain Napa and Sonoma vineyards is legendary, and Craig Rous thinks he might know one of the reasons why.

"A lot of those vineyards are dry-farmed. It's my belief that the soils and climate on those select sites are doing naturally what we achieve through our irrigation scheduling," says Rous, research manager at Robert Mondavi's Woodbridge winery.

The Woodbridge area is too warm and dry to go without irrigation. That's not a big deal, and the area has become a major player in the wine industry. And now, the ability to manipulate water deliveries has allowed growers in the area to hit on an irrigation regimen that mirrors a premium dry-farmed Napa site.

Rous is also manager of grower education, and most of Mondavi's Woodbridge growers now follow an irrigation regimen that replenishes water levels at 70% to 75% of ET (evapotranspiration) throughout the season.

Under that regimen, mild stress is allowed to occur just before veraison. That stress tells the vine to quit putting on vegetative growth, and begin preparing fruit for harvest. The result is concentrated color and flavors, says Rous.

The system mirrors nature's method, he explains.

"Wild grapes evolved along stream-banks. Winter runoff and spring rains provide a flush of water early, but by mid-summer streamflows are down and mild stress is occurring. That tells the vine that it needs to begin making the fruit attractive to birds, because the reason a vine produces fruit is not for the fruit itself--it's so birds will eat the fruit, and in so doing scatter grape seeds. That's how a vine reproduces in the wild." The more concentrated the colors, flavors, and sugars, the more attractive the grape is to birds, explains Rous.

Those same attributes are important to wine quality, he points out. "With the blacks especially, there is a definite correlation between color and quality, quality being defined as more intense flavors, higher total acidity and lower pH."

The system, known as the Volume Balance Approach (VBA), is used on most of the Woodbridge winery's grapes, except those used to make white Zinfandel. "With white Zinfandel, you harvest at a lower sugar, and color isn't as important," he explains.

Otherwise, the system is used on Cabernet, Chardonnay, Sauvignon blanc, and Zinfandel for red wine.

"Terry Prichard (U.C.-Davis Extension water management specialist) has done research demonstrating the system's effects on Cabernet and Chenin, and Larry Williams (U.C.-Davis professor of viticulture) is just starting a trial on Chardonnay. I wouldn't have any qualms about using this system on any varietal. The signal to stop growing leaves and start maturing fruit is the same for every grape."

There is one risk, however. "There's a fine line between a little stress and too much stress," cautions Rous. "If you drop your water deliveries too much, you will have a significant drop in yield."

Therefore, all Mondavi grapes grown under VBA are grown using precise measurements of soil moisture and ET.

Growers begin the season with a neutron probe to measure winter-stored soil moisture. Rous notes that there are a number of methods to measure soil moisture, but he prefers the neutron probe.

"In VBA, we talk about a wet point and a dry point. The wet point is your total soil water. It's measured before bud break. The dry point is the amount of water that is in the soil after harvest and before any further irrigation."

The dry point measurement is subtracted from the wet point measurement. "This is the available soil water that the vine can use. This amount, when added to the water supplied through rainfall and irrigation, is the total water used by the vine for the whole season," explains Rous.

Rous says the neutron probe is the best instrument for reading soil moisture at several depths, and can help distinguish between available water and unavailable water. In addition, "Terry (Prichard) did his work with a neutron probe, so his models are designed for the data a neutron probe generates."

Prichard has calculated the amount of water a vine needs, as opposed to the amount it will drink if given all the water it wants. He calls the two sufficient water use and full water use; their relationship is the crop's performance coefficient. In his work with Chenin blanc, Prichard showed that yield did not decline until water was reduced more than 31%.

The sufficient water use numbers for Chenin blanc and Cabernet will be published by U.C.'s Division of Agriculture and Natural Resources next year. Until then, they are available from Prichard at (209) 468-2085.

Mondavi growers also use weather station estimates of reference evapotranspiration (ETo). Those estimates are available through CIMIS, the California Irrigation Management Information System. For information, call (916) 653-9347.

ETo must be calibrated to a specific crop. That's done by multiplying it by the crop's crop coefficient. The result is an estimate of a specific crop's ET. Mondavi's Woodbridge growers take it a step further and multiply ET by Prichard's performance coefficient. That tells them how much water is sufficient for their vines in a typical year.

To begin the year, they determine the available water by subtracting dry point from wet point. Through the season, they follow the vine's water usage by accessing CIMIS weather stations. The growers let the water drop close to the crop's dry point, then begin replenishing water at about 70% of ET.

"Our goal is to reach mild water stress before veraison," says Rous.

Once the vines reach that stage, further irrigations are scheduled at about 70% of ET to insure that they continue to mature the grapes and concentrate the color and flavors.

Post-harvest, a full irrigation is needed so the vine can replenish carbohydrates, supplement winter hardiness, and prepare for bud break the following spring. "A lot of people assume that VBA would lead to lower water use, but it isn't a lot. There's some savings, but mostly you're giving the vine the same amount of water, but at different times. The benefit is in grape quality."

Rous says VBA can be used on any grape in any part of the state where irrigation is used except those with shallow water tables and those with fog for significant portions of the growing season.

"If you need to irrigate, you can adjust your deliveries to induce mild stress. That's why I look at those dry-farmed Napa vineyards. They can't manipulate their soil's water, so I think nature must be doing it for them."


Craig Rous, Robert Mondavi Woodbridge Winery, P.O. Box 1260, Woodbridge, Calif. 95258; (209) 369-5861, fax (209) 369-1461

Terry Prichard, Paul Verdegaal, UCCE, 420 South Wilson Way, Stockton, Calif. 95205; (209) 468-2085, fax (209) 462-5181
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Author:McMullin, Eric
Publication:Wines & Vines
Date:Jun 1, 1994
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