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Dripped or sprinkled? Grape growers are talking water

What does a severe drought — this year and over the coming years — mean for the Rogue Valley’s wine-grape growing industry?

It’s no secret that the western United States is in a drought. Eighty percent of California is experiencing a drought not seen in 500 years. Southern Oregon is officially classified as being in “extreme drought.”

This year, the Medford Irrigation District shut down its water-delivery system in early September. The Talent Irrigation District has predicted a shutoff some time between Sept. 15 and 29. No more water is available for irrigating pastures or crops.

Before the 1960s, most wine grapes in the world were “dry farmed.” It either rained or it didn’t. This worked in the temperate zones of Europe with spring and summer rainfall and, obviously, produced some excellent wines. All of the early Napa Valley vineyards were dry farmed until overhead irrigation, the familiar pulsing rainbirds, were introduced. Then, drip irrigation was installed in the 1970s and early 1980s. Today, most vineyards use drip irrigation. The goal in both cases was to control the timing and the amount of water to the vines.

Now there is a lively debate going on within the West Coast wine-grape community on the best way to irrigate to most efficiently utilize increasingly scarce water. The two systems each have their advantages and disadvantages.

With drip irrigation, thin tubing is strung along the bottom row of the trellises with valves at each plant aimed at the ground. It has the advantage of putting water right at the base of the plant, directly to the roots. Drip irrigation produces less evaporation than overhead irrigation, greater precision in the amount of water delivered to each plant and greater control of the shape of a vine’s growth. Drip irrigation promotes a root system that is compact. There are rootstocks that are deliberately developed with a smaller, shallower root system to create smaller plants that can be planted closer together with fruit that ripens earlier. The downside of drip irrigation is that the vines dry out quickly and need to be watered more frequently.

Overhead irrigation also has its advantages and disadvantages. It provides frost control in the early spring when the first shoots appear and are vulnerable to cold nights by providing a cap of ice over the plants that insulates the buds. It provides more water in a single watering period, saturating the ground, thus requiring less frequent watering. There is also an argument that the larger, deeper root system created with overhead irrigation is better at optimizing the existing soil moisture when irrigation is no longer available.

Overhead irrigation provides water to cover crops planted between grape rows in order to provide soil nutrients and dust control. Cover crops can also be used to assist in creating a natural ecosystem to create a biological balance of various insect populations. A fuller root system may also take greater advantage of a vineyard’s terroir — the characteristics of a particular vineyard’s soil — that gives wine produced from those grapes its own unique, nuanced signature.

Overhead irrigation, however, uses more water with less precision than drip irrigation. It also wets the leaves and fruit, which can create an environment for some diseases.

Grapes close to ripening are watered on a system of “deficit irrigation,” keeping the plants under a certain amount of stress. It is a controlled stress that determines vine size and concentrates the plant’s energy into the fruit to encourage ripening. Each successive week, the plants are given half the amount of water that was transpired through the leaves into the atmosphere the previous week. Obviously, this system works only if you continue to have water. Plants on deficit irrigation denied too much water begin to wilt.

Here is the crux of the argument: use drip irrigation for more efficient use of water versus overhead irrigation for a larger root system to encourage the plant’s use of residual soil moisture? Or is using a combination of the two systems the optimum way to go to maximize the available water?

This year’s early ripening — the first harvesting in the valley of sauvignon blanc was done on Aug. 26 — dodges the bullet of the early shutoff of irrigation water. But there is a lively conversation in the valley about what happens in the grape industry if the drought gets worse?

Roberta Kent is a freelance writer living in Ashland. Reach her at rbkent@mind.net.