From across the vineyard, Susan Hubbard appears to be vacuuming between rows of vines at Robert Mondavi Winery in Oakville, Calif. But she's not interested in what's on the ground. She wants to know what's in it.
Hubbard, a hydro-geophysicist at the University of California, Berkeley, and a staff scientist at Lawrence Berkeley National Laboratory, is using technology developed to detect underground bunkers and bomb factories for a more innocuous purpose: to grow better grapes.
Two California wineries -- Dehlinger and Robert Mondavi -- are participating in Hubbard's USDA-funded study which uses ground-penetrating radar (GPR) to map soil moisture content, an essential factor in viticulture. GPR sends an electromagnetic signal into the soil to a predetermined depth; the signal then bounces back and yields very accurate moisture measurements.
Knowing exactly how much water is underfoot enables winemakers to control how much stress, or water deprivation, a vine receives. Just the right amount of stress results in a better balance between sugar and acidity in the grapes. Too much water can produce flabby wine; too little and the grapes can dry up and become useless.
"People realize that the soil moisture is a really important component, but it's difficult to understand because we can't see it and it's difficult to measure noninvasively," Hubbard said.
Vineyard managers typically measure moisture by digging large holes in the ground or using what's called a neutron probe at specific locations. "Typically we've used backhoe pits to look at the soil, and our measurements have been educated guesses at best," said Marty Hedlund, vineyard manager at Dehlinger Winery in Sonoma County. "With [GPR] you can pinpoint any exact spot and find out what's there without hurting any vines."
More detailed data on soil moisture could help grapegrowers make several important decisions, including what to plant, where to plant it, and when to irrigate and harvest. "It will be a very valuable tool for vineyard development; you'd know the soil before there's even a plant in it," Hedlund said.
During the past three years, Hubbard has been testing the GPR technology at Dehlinger and Mondavi and comparing the results with those of established methods such as neutron probes. So far, the GPR has been spot-on. The next steps, she said, are determining the full potential of the technology and delivering it to growers.
Daniel Bosch, director of vineyard operations at Robert Mondavi Winery in Napa Valley, sees several benefits to the technology. GPR devices could be paired with GPS receivers and laptops and pulled along by a tractor to map out an entire vineyard's moisture, he said, allowing growers to continuously measure changes in large areas. "If you collect moisture data for a few years and take enough measurements, you will have a good idea when to start irrigating based on the patterns alone."
Mondavi has already been collecting information on its vineyards through aerial and satellite imagery, and Hubbard will compare that with the GPR-gleaned moisture data. "We can look at the top of the [grapevine] canopy and determine vegetation density, but there's not a lot of information about why it is more dense or weaker in some areas," she said. Future work will combine soil and canopy measurements for more detailed observations and predictions.
"It's the whole terroir concept," Hubbard said. "Through lots of practice, the Bordeaux and Burgundy winemakers have been able to manage small-scale spatial variability very well. But in New World wine areas, people develop large acreages of vineyards and they don't have the time and equipment to consider sub-block variety."
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