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Researcher pulls a radar unit through vineyard
Hydrogeophysicist Susan Hubbard conducts field research between rows of grapevines at the Robert Mondavi Winery. She is dragging a surface geophysical device, called a ground penetrating radar, to map soil moisture in the vineyard. (Photo courtesy of Mike Kowalsky)

Radar and Fine Wine: Innovative research uses radar to map soil moisture, create better wine grapes

– Winemakers know that soil moisture is key to growing quality wine grapes, but accurately monitoring the soil's water content is a difficult and expensive task. Now, innovative research led by University of California, Berkeley, scientists is lending a high-tech hand to the fine art of grape growing by using ground penetrating radar, or GPR, to map soil moisture in vineyards.

"By providing detailed information about soil moisture, we can help viticulturists refine their irrigation strategies to use water more efficiently," said Yoram Rubin, UC Berkeley professor of civil and environmental engineering and lead investigator of the project. "This has the potential to improve grape quality while reducing energy and water use, which is especially important in drought-prone areas."

Rubin added that the technology's ability to provide an unprecedented view into the shallow earth's geology can be a key component in efforts to reduce agricultural pollution. "Since irrigation can be planned to meet the vines' needs exactly, we can eliminate excessive water infiltrating deep into the aquifers," he said.

For the past several years, the researchers have been testing their technology at the vineyards of the Robert Mondavi Winery and the Dehlinger Winery in California's Napa and Sonoma counties. They use a vacuum cleaner-sized machine to skim the soil surface, sending electromagnetic pulses into the first few meters of soil. The GPR signals are analyzed to determine moisture levels in different layers of soil.

"It's well-recognized that moderate water stress has a positive impact on wine grape quality," said Susan Hubbard, a hydrogeophysicist at Lawrence Berkeley National Laboratory and a UC Berkeley research engineer in the Department of Civil and Environment Engineering. "Too little water, and the vine can get over-stressed to the point where the crop is lost. On the other hand, an oversupply of water tends to favor development of the leafy vegetation at the expense of berry ripening and fruit quality."

The delicate balance of soil moisture is important in creating smaller berries, which have a higher skin-to-juice ratio. With this higher ratio, the wine grape is more concentrated, ultimately leading to a finer wine.

Current techniques of measuring soil moisture typically involve sampling the soil at a few spot locations within a vineyard. Not only are such techniques costly and invasive, they may not always create an accurate representation of the vineyard soil moisture distribution since the soil at one location may be quite different from the soil a few meters away, said Hubbard.

In contrast, the GPR approach is non-invasive and produces dense, high-resolution data, said Rubin.

Hubbard also noted that the technology could be used to scope out optimum plots of land for new vineyards. Where the soil is spatially uniform, vineyard managers could further refine the practice of matching grape variety to soil conditions. Cabernet Sauvignon and other red wine grapes, for instance, are planted in drier soils, while Chardonnay and other white wine grapes do best in moister soils.

The researchers also "are working on correlating soil moisture with vine growth, which would help us better predict what kind of spacing to use for the vineyard," said Daniel Bosch, director of the Napa Valley Vineyard Operations at Robert Mondavi Winery. "How close together the vines are determines how many leaves there are per acre, which determines how much water is being used."

Bosch added that controlling the moisture in the soil helps produce vines that are more uniform. When grapes mature at the same time, harvesting is more efficient.

The soil moisture readings come from a GPR unit with two antennas that the researchers drag between the rows of grapes. While one antenna beams radar waves into the ground, the other picks up the signal after it travels through the soil layers. The speed of the transmitted and reflected signals varies with the water content of the soil; waves move slower in wetter soil and faster in drier soils.

The researchers noted that the GPR project could also complement an ongoing project in remote sensing that the NASA Ames Research Center has been conducting at Robert Mondavi Winery. NASA researchers are using aircraft to create spatial images of a vineyard that can be evaluated for canopy density, vine physiology and fruit characteristics.

"We're creating models to understand the transpiration rate of the vegetation," said Lee Johnson, a senior research scientist in the Earth Science Division of the NASA Ames Research Center. "The greatest unknown to our model is what's going on beneath the surface. The texture of the soil can be such a controlling variable, but right now, we have no good way of mapping that on a fine scale. The GPR technology being developed has the potential to improve not only our model but many other earth science models that rely upon soil information."

While vintners may be the primary beneficiaries of these technological improvements, consumers may see a difference, too. "At Robert Mondavi Winery, we are constantly looking for ways to improve our grape quality so that we can make even finer wines," said Bosch, director of vineyard operations.

Hubbard added that while GPR technology could potentially benefit other crops, wine grapes were particularly appealing, because they are a "high-cash" crop that is particularly sensitive to soil moisture.

Grants to Rubin from the National Science Foundation, the U.S. Department of Agriculture and the University of California Water Resources Center supported this research.

To read more about this project, see a story in the October issue of Lab Notes, the UC Berkeley College of Engineering's online research newsletter at http://www.coe.berkeley.edu/labnotes/1003/rubin.html.