Climate Change (and a Team of Scientists) Revise the Winkler Index

A team of UC Davis researchers led by Beth Forrestel, assistant professor in the Department of Viticulture and Enology, is working to modernize the Winkler Index, which classifies the climate of wine growing regions and helps growers determine which grape varieties are best suited for a given area. The seminal original work by UC Davis professors A.J. Winkler and Maynard Amerine was instrumental in providing a guide to help wine growers revive California’s wine industry in the 1940s following the repeal of Prohibition. It eventually became the standard climate tool used for winegrape growing worldwide.

Using thousands of index cards to record their field data, Drs. Winkler and Amerine calculated average daily temperatures throughout the growing season to identify five climate regions, from coolest (Region I) to warmest (Region V). Climate change, and the rising temperatures and erratic weather that come with it, has already limited the accuracy of these regional classifications. Napa Valley, for example, was considered Region II when the Winkler Index was developed. Now, most parts are a Region III or IV. UC Davis students are digitizing the cards and using them to create a visual tool to show the movement of California’s viticultural regions through the classifications over time.

“There is a renewed interest by industry and stakeholders to understand how to best mitigate climate change effects on existing vineyards and choose appropriate cultivars for the future,” Beth said. “The new methods and data sources we’re bringing to this project will help us do just that.”

“Because we are in a period of climate change, we need more refined and comprehensive ways of measuring the effect of heat on plant physiology and grape maturity,” said Napa Valley winemaker Warren Winiarski, founder of Stag’s Leap Wine Cellars who is helping to fund the research. “The development of new methods of measurement would be extraordinarily helpful. With better knowledge of changes in the compositional elements in the grapes in the vineyard, we’ll have better guidance on how to respond in the winery and create the wines we want to make.”

New technology, more accuracy
Sophisticated monitoring technology and remote sensing data will help move beyond average climate conditions to paint a fuller picture of the environmental factors that most significantly affect plant growth, berry chemistry and, ultimately, wine quality.

Even though smoke, wildfires and pandemic-induced restrictions presented formidable obstacles to field research in 2020, the initial year of the study, Beth reports the project is gaining momentum. Data this season will be collected from Napa Valley cabernet sauvignon vineyards to document conditions in its many microclimates. Information will be gathered on a wide range of factors, such as light intensity within the grapevine canopy, soil characteristics, field-specific irrigation practices, estimated water use, berry chemistry, temperature and physiology—all things that ultimately have a bearing on wine quality. Heat stress is a particular concern.

While the initial focus is on cabernet sauvignon, the most widely grown and economically important wine grape variety in the world, the project is expanding to other cultivars as well. A study analyzing the comparative biochemistry of wine grape berries in 24 commonly planted varieties is underway, as well.

Additionally, two study blocks of 60 wine grape cultivars were planted this spring at UC Davis and at the university’s Oakville field station. These and some commercial vineyards—a number of Napa growers and winemakers have raised their hands to serve as collaborators, providing data from their own weather stations—will be used to compare wine grape performance and to update models of plant development and biochemical responses to heat accumulation. This study, which was modeled on the VitAdapt long-term cultivar trial in Bordeaux, France, is planned to be replicated with the same rootstock and common cultivars Adelaide, Australia, and Lodi, CA, as part of an international climate adaptation initiative.

Data for today…and tomorrow
Beth says the project will give growers the knowledge they need to take appropriate management actions like modifying irrigation practices right now. But she’s working with an eye toward a hotter future. “The establishment of a new index and the development of new models of grapevine and berry development will provide guidance for the viticulture and wine industry to cope with current and future climate, manage existing vineyards, and make decisions on future plantings and winemaking choices,” she said.

This article was excerpted from these original publications from UC Davis: