Getting the most out of every drop of water is a high priority for grape growers in the southern Napa Valley, where summers are hot and dry and vines have to be irrigated to make it through the growing season. But Stanford researchers have found that a significant portion of the water applied to the vines zips right by the plants, hardly even pausing.
"We found that about 10 percent of the water that is applied is lost below the vine rooting zone and does not have contact with the soil and vine roots," said Eve Hinckley, who worked on the project for her PhD thesis in the department of geological and environmental sciences at Stanford. "This is a conservative estimate."
The problem lies in deep cracks that are a chronic feature in the clay-rich soils of the areaDue to the physical and chemical properties of these soils, they naturally swell when wet and shrink as they dry, producing cracks. Hinckley says that tendency is exacerbated by the weekly cycle of irrigating during the growing season, when vines are typically watered for 4 hours a week. Under a regular regimen of swelling and shrinking, the cracks become more pronounced and water speeds through them without interacting with the soil.
Hinckley is presenting her results at the fall meeting of the American Geophysical Union on Wednesday, Dec. 16.
She gathered her data by burying devices called lysimeters about 16 inches down in the soil just below the root zone of the vines. That is also the depth to which many of the deep cracks penetrate in the vineyard where she did her study. The lysimeters captured water flowing through the soil, giving her data on the volume, chemical composition, and residence time of water in the soil.
The speedy passage of so much water through the cracks in the soil affects more than just the job of getting enough water to the vines. There are significant consequences on either end of that rapid flow. Upstream, it means that more
|Contact: Louis Bergeron|