Article Courtesy of our friends at Wines & Vines
January 2015 Issue of Wines & Vines Copyright © Wines & Vines
Growing More with Less WaterRodney Strong Wine Estates Uses Multiple Technologies to Closely Manage Irrigation
by Tom Ulrich
Ryan Decker, grower relations manager for Rodney Strong Wine Estates, admits that he cannot control the climate, but he can deliver prescribed amounts of irrigation water to a constellation of vineyards located in one of the most diverse grapegrowing regions in California. He tends Chardonnay growing from dark clay near the San Pablo Bay, Petite Sirah, Touriga Nacional and Zinfandel planted in alluvial soil not far from the Russian River and Cabernet Sauvignon growing from a rocky, inland hillside close to the Sonoma-Mendocino county line.
Near the confluence of the Alexander Valley, Chalk Hill, Russian River Valley, Sonoma Coast and Sonoma County AVAs, the winery overlooks a river basin that meanders 32 miles, or nearly a third of its length, to the sea. During the growing season, warm air rises from inland valleys drawing fog through the Petaluma Gap and along the Russian River Valley where it gathers near Healdsburg, Calif., before spilling into the Alexander Valley.
South of the winery, fog dampens daytime temperatures and helps drive a seemingly endless cycle of misty mornings, warm days, windy afternoons and cool nights. To the north, fog dissipates from the river basin early in the day giving way to warmer afternoon and nighttime temperatures.
Alexander’s Crown, a rolling hillside vineyard located at the southern end of the Alexander Valley, records an average temperature of nearly 60º F from bud break to harvest. Twelve miles upriver, Brothers Ridge, a steep hillside vineyard, averages more than 67º.
Variations in climate help explain the diversity of this growing region. “The farther you are from the (Petaluma) Gap,” Decker says, “the weaker the marine influence.” At the boundary between maritime and continental air masses, the Alexander Valley, alone, produces 66 varieties of wine grapes.
Following a budget Decker must consider several wide-ranging variables before he calculates a water budget for each of the 14 estate vineyards that he oversees. A carefully crafted budget allows him to efficiently irrigate a diverse landscape, conserving water, enhancing quality and increasing yield. Along with evaluating the weather, he keeps track of nutrients, soil moisture, transpiration rates and yield for 1,158 acres of vines or nearly 125 vineyard blocks.
The lay of the land shapes the character of each estate. The River East vineyard, for example, yields Pinot Noir and Chardonnay from clay loam 90 feet above sea level. Brothers Ridge bears Cabernet Sauvignon, Merlot and Malbec from loam underlain by sandstone, fractured shale and ancient greenstone at an elevation of 400 to 1,030 feet.
Soil type, depth and texture affect irrigation rates and growth. Typically, alluvial soils are more fertile and retain more moisture than hillsides. Clay contains micro-pores holding irrigation water and roots closer to the surface, while rocky soils form macro-pores permitting them to penetrate deeper into the earth.
Elevation matters. Wind, a higher concentration of UV light, temperature inversions, rocky and shallow soil can sap vigor from the vines.
Variety makes a difference, too. Red grapes require less water than white. Vines grown for light-bodied wines demand more water than vines harvested for full-bodied wines. And varieties handle stress differently. Syrah shows signs of drying out more readily than other varieties, but recovers when soil moisture improves. Cabernet Sauvignon grapes shrivel before grapevines shed their basal leaves possibly returning moisture to the vine.
Adcon weather stations located in the Russian 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 temperature and humidity gather data for calculating the evapotranspiration rate, the sum of water vapor transpiring from the vine and evaporating from the vineyard.
An Adcon telemetry system transmits sensory 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 porometer 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 vineyard 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 readings, stomatal conductance and visual cues to determine when and how much we should irrigate a vineyard," Deck er says.
Every vineyard block requires its own irrigation schedule. The stakes are high. A miscalculation could make the difference between fine wine and plonk.
Less is more
Vines reach an irrigation threshold when additional water does not increase yield. Surplus 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 generally 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 pursues a strategy that falls somewhere between overwatering and not watering at all. He practices regulated deficit irrigation (RDI) to deliver 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 sensory 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, replenishing 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 sunlight enhancing production of anthocyanins and flavor compounds.
Balancing a budget
"I make recommendations on a case-by-case basis," Decker says. "Our Rockaway Ranch, for instance, has parts of the vineyard that require irrigation early and often, and parts that rarely need water until after véraison."
Such flexibility requires him to keep track of several variables at once.
For the vine, he considers variety, transpiration, rootstock, root structure and depth. For the vineyard, he evaluates evaporation, soil field capacity, soil type and permanent wilting point.
The evapotranspiration rate helps him determine how much water to add to the root zone to replenish the vineyard block. Soil type and root depth tells him how many hours to irrigate. "It might take three hours at 1 gallon per hour to saturate coarse-textured soil or six hours to saturate heavy clay soil," he says.
Rocky soil could receive two sets of three hours each, while clay soil could absorb one six-hour set. "The same holds true for soil depth," he says. "I irrigate shallow soils at a higher frequency and lower volume, and vice versa for deeper soils."
Rootstocks affect the volume and irrigation rate, too. "Some rootstocks like 039-16 are very vigorous, but if you stress them too much they shut down and never recover," he says. "Drought-tolerant, high-vigor rootstocks like 140Ru and 1103P can handle stress, and are typically used on low-vigor sites because of their potential for vegetative growth where resources are plentiful."
His ability to gather data on each vineyard block and calculate a water budget from an evapotranspiration rate, RDI coefficient, vine spacing, rainfall and irrigation patterns that reflect the character of a vineyard allow him to prescribe an efficient irrigation rate for each estate.
This year, he irrigated Rockaway Ranch at 40% of its evapotranspiration rate saving 56 acre-feet or nearly 18 million gallons of water across the 130 acre hillside vineyard. Taking into account that the evapotranspiration rate and the RDI coefficient can vary widely from vineyard to vineyard, Rodney Strong Wine Estates saved approximately 430 acre-feet or 140 million gallons of water this season.
A mountaintop or hillside vineyard like Rockaway Ranch contains several soil types. Swales or low spots can collect more water and contain more clay. Soil on slopes and ridges can be rocky and shallow—holding less water.
"Instead of splitting these variable soils into hundreds of uniform mini-blocks," he says, "we manipulate the irrigation system to deal with changing conditions."
Once he's determined demand, Decker can add an emitter so that the weaker vines receive 50% more water than the rest of the vineyard block. Or, he can place shut-off valves on drip lines that run through the vigorous sections of the block. Only the vines that need water will receive it.
"The idea here is to make a highly variable vineyard uniform by the time harvest arrives," he says.
Field capacity and permanent wilting point aside, Decker checks the forecast frequently for changes in weather. With weather stations located north and south of the winery, he can evaluate conditions at vineyards that are a short distance from one another, but miles apart climatically. "The evapotranspiration numbers can only account for what has already happened in the vineyard," he says. "If there is a heat spell or cool-down on the horizon, I will change the irrigation schedule to accommodate it. I do not want the vines to be over-stressed in hot weather or over-irrigated in cool weather.
A broader perspective
Every year, he evaluates the health of the vines through NDVI (Normalized Difference Vegetation Index) imagery. "I use NDVI aerial images to identify areas of high or low vigor, which helps to refine data collection and management," Decker says.
The technology senses small differences in plant growth based on changes in soil, leaf area and irrigation patterns. It's a first step toward modeling the relationships between water-holding capacity of the soil, the evapotranspiration rate, crop coefficient, quality and yield.
"Technology is giving us new insights into which vines in a vineyard are too vigorous, or too stressed," says UC farm advisor Glenn McGourty. "The end game is to harvest fruit that is uniform in ripeness and flavor."
Changes in climate, elevation, soil type, rootstock and variety can accelerate or slow down the pace of ripening, and cause excessive vigor or stress.
"Being able to detect problems as they happen, and then reacting with more or less water to balance the vineyard," McGourty says, "will lead to higher quality wine."
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