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IVES 9 IVES Conference Series 9 Updating the Winkler index: An analysis of Cabernet sauvignon in Napa Valley’s varied and changing climate

Updating the Winkler index: An analysis of Cabernet sauvignon in Napa Valley’s varied and changing climate


This study aims to create an updated, agile viticultural climate index (similar to the Winkler Index) by performing in-depth analyses of current and historical data from industry partners in several major winegrowing regions. The Winkler Index was developed in the early twentieth century based on analysis of various grape-growing regions in California. The index uses heat accumulation (i.e. Growing Degree Days) throughout the growing season to determine which grape varieties are best suited to each region. As viticultural regions are increasingly subject to the complexity and uncertainty of a changing climate, a more rigorous, agile model is needed to aid grape growers in determining which cultivars to plant where. For the first phase of this study, 21 industry partners throughout Napa Valley shared historical phenology, harvest, viticultural practice, and weather data related to their Cabernet sauvignon vineyard blocks. To complement this data, berry samples were collected throughout the 2021 growing season from 50 vineyard blocks located throughout 16 American Viticultural Areas that were then analyzed for basic berry chemistry and phenolics. These blocks have been mapped using a Geographic Information System (GIS), enabling analysis of altitude, vineyard row orientation, slope, and remotely sensed climate data. Sampling sites were also chosen based on their proximity to a weather station. By analyzing historical data from industry partners and data specifically collected for this study, it is possible to identify key parameters for further analysis. Initial results indicate extreme variability at a high spatial resolution not currently accounted for in modern viticultural climate indices and suggest that viticultural practices play a major role. Using the structure of data collection and analyses developed for the first phase, this project will soon be expanded to other wine regions globally, while continuing data collection in Napa Valley.


Publication date: May 31, 2022

Issue: Terclim 2022

Type: Poster


Paige Breen, James Campbell, Kayla Elmendorf, Seth Frey and Elisabeth Forrestel 

Department of Viticulture and Enology, University of California, Davis, USA

Contact the author


climate index, climate science, Growing Degree Days, Napa Valley, viticulture


IVES Conference Series | Terclim 2022


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