The inland areas of the Pacific Northwest, where rainfall averages only 4 to 12 inches per year, present growing challenges for vineyard owners and wine grape producers. The arid conditions in this part of the country have not been conducive for vineyard owners who produce and market high-quality wine grapes.
To promote healthy grape production when nature fails to deliver, vineyard managers in the area typically employ an irrigation practice known as "regulated deficit irrigation". More than 60% of the wine grapes in the state of Washington are grown using this drip irrigation method. Unfortunately, the current irrigation methods are replete with problems that can cause over-irrigation and compromised grape quality.
Recently, researchers at Washington State University's Irrigated Agriculture Research and Extension Center completed a study that should provide vineyard managers new techniques for producing healthy and long-lasting grape crops. Joan R. Davenport was the lead author of the study published in the February 2008 issue of HortScience. Explaining the impetus behind the research, Davenport said: "Most of these vineyards use drip irrigation to supply supplemental water. Soil moisture is often measured to determine when to apply irrigation. However, without knowing the pattern of moisture under these conditions, the best place to check soil moisture content to mimic what the plant root sees was not understood. Our objective was to establish the soil moisture zone in this system."
Researchers discovered that areas immediately below the drip line and the drip emitter were not appropriate places for monitoring soil moisture.They found that there can be a "dry zone" just below the emitter created by the repeated dropping of water, thus causing the soil to seal. Davenport explained that "right below the drip line the moisture pattern was variable due to distortions (warping) of the lines, and thus was not a good indicator of the patterns of plant available water." After extensive trials, data indicated that soil sampling within a 20- to 40-centimeter radius of the drip line emitter best reflected the amount of water available to the plants.
According to Davenport, the research will have multiple benefits. "Environmentally, the results have the potential to help growers prevent over-irrigation. In the industry, over-irrigation can have serious adverse affects on grape quality, which is equally important to yield for premium wine grape production. For the industry, knowing where to monitor soil moisture to best reflect plant-available water means that there is less chance of stressing the vines by under-watering. Not only can this adversely affect fruit yield, it also has the potential to reduce plant cold-hardiness, making the plants more likely to suffer from winter injury, which leads to vine death in the area on average every 10 years."
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