Terrain and Terroir Studies
The very character of any wine comes from the grapes that go into it. And the character of these grapes is firmly rooted in the place where they are grown.
Beginning in July 2001 and continuing through 2003, the NVV commissioned three studies. The first, completed in 2002, is The Foundations of Wine in the Napa Valley: Geology, Landscape and Climate of the Napa Valley AVA by EarthVision, Inc. This report provided the first comprehensive examination of the geologic history of the Napa Valley as it related to Napa Valley's grape-growing capabilities.
The following two studies, completed in 2003 by Terra Spase, Inc., are entitled Soils and Wine Grapes in the Napa Valley and Weather and Wine Grapes in the Napa Valley. Each of these studies provide an appellation-by-appellation profile of soils and climates within the Napa Valley American Viticultural Area (AVA).
The Geology Study - Key Findings
By creating a geologic tour of the creation of the Napa Valley appellation spanning 150 million years, the study uncovered exactly why and how the AVA is now home to such an incredibly diverse area of "parent material" - material from which soils are created -- and topography. It is this diversity of environments within the appellation that allows wine grape varieties to thrive.
Scientists Jonathan Swinchatt, EarthVision, and David Howell, U.S. Geological Survey, developed a new organizing concept - the Earth Process Unit, or EPU - to describe the sediments of the Napa Valley AVA. Basically, these EPUs allow the scientists to group and map the valley sediments based on one of the three processes by which they were created - weathering in place; deposited by streams draining the hills; or deposited by the Napa River. This new concept neatly "bridges" the gap between geology and soils science, which focuses strictly on describing and analyzing soil materials rather than determining their origin, evolutionary history, and detailed spatial variation - the goal of geology.
Swinchatt and Howell made a theoretical discovery that answers the mystery as to how the mid-valley hills in the Napa Valley were created. Gravitational force caused entire portions of the Vaca mountains on the east side of the Valley to break away as the range was raised by massive earth movement. The resulting "nested displacement" created a series of hillside plateaus that can be seen from above. When these areas broke away, they sent huge amounts of material down slope to the Valley floor below. The ancestral Napa River carried much of the loose debris down river into San Pablo Bay and beyond, leaving behind the large displaced blocks of intact rock that make up the mid-valley hills in the Yountville area. Swinchatt and Howell are currently conducting follow-up research to test this theory.
The Soils Study - Key Findings
There exist an amazing 33 different soil series in the Napa Valley representing six of the 12 soil orders that comprise modern soil taxonomy. In other words, in an area just 30 miles long and five miles wide, half of the soil orders that exist on the planet can be readily found.
Extensive soil diversity can also be found within individual appellations, such as the Spring Mountain AVA, which contains 22 different soils series. This amazing diversity within a single appellation is a major reason viticulturalists may not assume that any given vineyard will be represented by one soil type.
Soil scientist Paul Skinner of Terra Spase goes beyond a detailed description of soils to provide a helpful view of what is called "vigor potential," - a qualifier that assists growers assess what and how to grow in a given soil series. For example, vigor potential describes the effects of 20 different physical and chemical soil attributes on vine growth, water status and production parameters. The study identifies estimates of soil vigor potential in all 33 soil series in the appellation.
The Climate Study - Key Findings
The study reveals the very first appellation-by-appellation compilation of climate data in the Napa Valley. A unique method of data collection involved a series of nearly 90 monitored weather stations collecting measures of temperature, humidity and precipitation every 15 minutes from 1996 forward. (It should be noted that climate is the equivalent of the average weather of a period of five to 30 years, while weather is an accounting of similar conditions over a shorter period of time).
The study provides climatic detail within AVAs as well, by mapping "growing degree days" - the accumulation of hours that the temperature rises above 50° F. These variations, similar to the different soils series within a given appellation, enable growers to make critical vineyard decisions.
Skinner introduces readers to the concept of the "Mercedes Effect." The effect results when consistent hot, dry summer conditions are combined with a cooling marine fog layer over much of the Napa Valley, encouraging a cessation of vegetative growth and a shift to fruit ripening "as smoothly as a Mercedes changing gears." This effect, the study points out, "…may be one of the most important but overlooked climatic characteristics that set the Napa Valley AVA apart from other grape growing regions of the world."
For those who care to dig deeper into the world of wine, the findings of the science trilogy are eye-opening," said Abe Schoener, winemaker, Luna Vineyards. "Not only do we now have an understanding of the Napa Valley as a whole, but we are able to corroborate, on a scientific level, what both wine trade and consumers already perceive - that the Napa Valley is an ideal place to grow an amazingly diverse array of wine grapes. It appears our reputation for high quality wines is well-deserved," Schoener added.
In an effort to make the geology, soils and climate trilogy more approachable and accessible, the NVV sought the assistance of veteran wine writer Gerald Boyd. Read Gerald Boyd's essay, The Science Behind the Napa Valley Appellation.
A complete copy of the science trilogy is available for review in the offices of the Napa Valley Vintners in St. Helena, California.