Wines & Vines

Collector's Edition WINES&VINES 111 GRAPEGROWING A 1999 study by Jorge Tonietto and Alain Carbonneau suggests that a multicriteria system of calculating HI, a dryness index (DI), and a cool night index (CI) are needed to account for potential water balance in soil and nocturnal temperatures in grape development. 8 Gregory Jones and others used PRISM climate models to assess spatial climatic distribution across California, Idaho, Oregon and Washington based on the four most commonly used indices in viticulture. 3,4 The results suggested that GST and GDD had the greatest correlations, but HI and BEDD demonstrated better differentiation of climate types. A similar study by Rosalyn Francine MacCracken and Paul R. Houser used the PRISM model to predict how well these four indices, along with a modified version of the GST accounting for the common length of a growing season (Modified-GSTavg or Mod- GSTavg), performed in U.S. states east of Cali- fornia, Oregon and Idaho. 5 The MacCracken and Houser study found all common indices to be less than perfect at predicting the viability of successful viticulture when comparing the indi- ces' results with existing viticulture. Mod- GSTavg showed more promise as a predictor of successful viticulture by taking into account the length of the growing season. The Jones et al. study and the MacCracken and Houser study looked at viticulture on a super-regional scale, albeit using climatic models with 400m to 400m resolution. One could argue that the Winkler Index was never meant to be utilized on a super-regional scale, but rather as a statewide guide specific to California. The original 1944 paper looked only at California locations, with Winkler later introducing other U.S. and international loca- tions into the scale for comparison. Winker et al. are clear that the regional scale was not meant to be definitive at a subregional scale: "These divisions into climatic regions should be considered as general demarcations." 9 Winkler et. al. suggest more extensive work be conducted on more local areas and specifi- cally note the work of Robert Sisson, who, dur- ing his 35-year career as viticulture farm advisor to Sonoma County from the 1950s to the 1980s, created a climate classification system unique to Sonoma County. Sisson's model broke the county into Marine, Coastal Cool and Coastal Warm climate types, based loosely on heat summations of hours spent between 70° F and 90° F. While much work has been done to delineate subre- gions of viticultural areas, both formally through the creation of new, smaller AVAs and informally through the identification of subregional dis- tricts by winegrowing associations, work to create climatic classification systems at more local levels unfortunately is scarce. Use of the Regions I-V designations of the Winkler Index presents several limitations and errors. GDD calculations, on which the Winkler Index is based, can be a valuable tool for com- paring growing areas, but GDD is just one aspect of climatic analysis that alone gives an incom- plete picture (as are other indices designed to summarize heat summation). The methodology, daily climatic patterns, period assessed and length of growing season can all tell a different picture of mesoclimates that might otherwise appear to be similar by Winkler designation. Ideally, each viticultural region would find a climatic classification system that best suits the unique characteristics that define it, while re- serving simpler indices like the Winkler Index for broader generalizations. Alternatively, GDD makes a good comparative tool when consis- tency in methodology and comparison are made within regional locations. Patrick Shabram is a geography professor at Front Range Community College in Colorado and an expert on American Viticultural Areas (AVAs). He conducts geographic research on wine-grape growing regions and is a consultant for growers interested in establishing new AVAs or modifying boundaries to existing AVAs. The references for this article are available online at winesandvines.com aacwine.com All American Containers YOUR RIGID PACKAGING SOURCE All American Containers, a Veritiv company Your wine. Our bottles. What an excellent pairing. WINDSOR, CA • SAN LEANDRO, CA • SIMI VALLEY, CA KALAMA, WA • PLANO, TX • BELLE VERNON, PA • TAMPA, FL MIAMI, FL • ATLANTA, GA • BRANCHBURG, NJ

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