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January 2015 Wines&Vines 45 grapegrowing signs of drying out more readily than other varieties, but recovers when soil moisture im- proves. Cabernet Sauvignon grapes shrivel before grapevines shed their basal leaves pos- sibly returning moisture to the vine Field notes Decker collects field data from several fixed weather stations located in AVAs north and south of the winery. They report leaf wetness, rainfall, relative humidity, temperature, wind direction and speed. Adcon weather stations located in the Rus- sian River and Alexander valleys are also equipped with Kipp & Zonen pyranometers for measuring solar radiation. These devices along with sensors for detecting wind speed, air tem- perature and humidity gather data for calculat- ing the evapotranspiration rate, the sum of water vapor transpiring from the vine and evaporating from the vineyard. An Adcon telemetry system transmits sen- sory data via a low-powered radio to a base station that dispatches a vineyard's vital signs to Decker's desktop computer. He reviews information about the health of the vines from a database that stores values and maps trends displaying information as numbers, graphs and images. But the weather is only half the story. He measures leaf water potential with a PMS Instrument Co. pressure chamber. A reading of approximately -12 bars for Pinot Noir or -14 bars for Cabernet Sauvignon, for example, alerts him that the vines are ready to irrigate. To keep tabs on the vines once irrigation begins, he determines the transpiration rate or stomatal conductance with a Decagon porom- eter three to 10 times a week for each vineyard block. And, he calculates how much shade the canopy casts at midday. Once a week, he measures soil moisture with an Aqua da Vinci capacitance probe to help determine how much to irrigate each vine- yard block. It measures the amount of water contained in the root zone by detecting how easily an electrical charge travels through the soil. It reports "inches of water" in half-foot increments from 6 inches below the surface to a depth of 5 feet. "I rely on weather data, evapotranspiration rates, leaf water potential, soil moisture read- ings, stomatal conductance and visual cues to determine when and how much we should irrigate a vineyard," Decker says. Every vineyard block requires its own irrigation sched- ule. The stakes are high. A miscalcula- tion could make the difference between fine wine and plonk. Less is more Like so many vine- yard managers across California, Decker has faced an especially dry year. Nineteen inches of rain fell near the winery from August 2013 to July 2014, compared to a historical average of 30 to 35 inches. But Decker insists that the vineyards can produce their highest quality grapes when he stresses his vines to the limit by rationing irriga- tion water. Vines reach an irrigation threshold when additional water does not increase yield. Sur- plus water is lost as run-off and evaporation or can lead to excessive canopy growth, loss of nutrients, disease and mediocre fruit. Grapevine roots draw moisture from the soil to fuel growth and refresh the vine. Water moves from the soil to the plant and then the atmosphere through stomata, small pores gen- erally found on the underside of the leaves that exchange water vapor and oxygen for carbon dioxide. As water vapor transpires from the stomata, the vine draws more moisture from the soil. If the water is not replaced through irrigation or rain, the vine wilts. When replenishing the vines, Decker pur- sues a strategy that falls somewhere between overwatering and not watering at all. He prac- tices regulated deficit irrigation (RDI) to de- liver less water than the vine demands during the early stages of the growing season. He irrigates established vines and then withholds water as grapes start to ripen. "After véraison, the vines need water as a solute for transporting sugar to the grapes," he explains. "We saturate the root zone, and then let it dry out." He watches the vines closely to determine when to irrigate. "But it's the technology that dials it in," he says. Once the weather stations transmit solar radiation, air temperature, wind speed and humidity to Decker's desktop computer, it computes potential demand by multiplying the evapotranspiration rate derived from sen- sory data and the crop coefficient for each vineyard block. The computer assigns a crop coefficient based on the percentage of area shaded by the vines. The coefficient varies throughout the growing season with a low value in the spring, high value by midsummer and a declining value as winter approaches. To determine the irrigation requirements for each vineyard block, the computer subtracts the soil moisture as reported by the capacitance probe from the potential demand. But as Decker has learned over the past decade, re- plenishing the vines with the amount of water lost to the atmosphere is wasteful and does not deliver the highest quality fruit. "We irrigate red grapes at 30%-50% of the vineyard's evapotranspiration rate, white grapes at 70%-75%," he says. The strategy conserves water, increases yield and improves quality. Chardonnay, for example, produces its maximum yield at around 70% of the evapotranspiration rate. While yield drops for Cabernet Sauvignon at a lesser rate, the quality of the fruit improves. For red varieties in general, smaller berries enrich color with a greater skin to pulp ratio. And a diminished canopy exposes fruit to sun- light enhancing production of anthocyanins and flavor compounds. Balancing a budget Given all the variables that Decker must con- sider, irrigating each vineyard at the same rate would sacrifice quality and yield in a region blessed with so much diversity. So, he calcu- lates a water budget for every vineyard block that he farms. "I make recommendations on a case-by-case basis," Decker says. "Our Rockaway Ranch, for instance, has parts of the vineyard that require A Decagon porometer determines transpiration rate and stomatal conductance. The PMS Instrument Co. pressure chamber alerts when it's time to irrigate.