Degree-day heat summations: The concept of characterizing grape growing regions by degree-day heat summations was applied in California by Professors Winkler and Amerine of the Department of Enology and Viticulture at U. C. Davis. Their method consists of summing the average daily temperature over 50° F during the growing season. Five climatic zones or regions were defined, as follows, and these differ in character and distribution from the plant communities:
• Region 1: less than 2,500 degree days
• Region II: 2,500 to 3,000 degree days
• Region III: 3,000 to 3,500 degree days
• Region IV: 3,500 4,000 degree days
• Region V: over 4,000 degree days
It is now generally agreed that Region III is optimum for most varieties of red wine, including Cabernet sauvignon, Merlot, Zinfandel, Syrah, and Petite Sirah; excellent wine is also produced in some areas of Region IV, including Cabernet sauvignon, Barbera, Zinfandel, and Sangiovese. Only Pinot noir seems to do well in Region II. For white wines, excellent White Riesling, Gewurztraminer, and Chardon-nay are produced in Regions II and III, and excellent Sauvignon blanc is grown in many areas of Region III and even Region IV.
El Dorado lies mainly in Region III, and is most similar in heat summation to the famous Oakville district of Napa Valley ( see fig.2, p. 6) for average degree day heat summations for well-known growing regions of California). The average El Dorado degree day summation for the past 6 years is 3238; for Oakville, 3237. Saint Helena (north of Oakville in Napa Valley) is much hotter than El Dorado, with a 6-year average of 3672 (Region IV). Local variation within the complex topography of El Dorado, however, can lead to dramatic differences in degree day summations even within a single vineyard. One carefully documented vineyard in Apple Hill at an elevation of 2,900 exhibits a range from 2,600 degree days (Region II) to 3,600 degree days (Region IV). Under such extremely variable conditions, detailed weather data are essential for proper placement of suitable grape varieties.
The range of variation in degree day heat summation throughout the fruit-growing region of El Dorado County is not yet well documented because of a paucity of temperature recorders. The best data now available is provided by the CIMIS recorder in Camino at an elevation of 2770' and the CIMIS recorder in the Shenandoah Valley of Amador county at an elevation of 1560'. Amador averages about 170 degree days/yr higher that Camino, but the average daily maximum temperatures in Amador are significantly higher and the average minimum temperatures there are lower. The lower minimum temperatures of Amador are a function of topography - the El Dorado CIMIS recorder is situated on top of a ridge, whereas the Amador recorder is in a low part of the Shenandoah Valley close to Indian Creek. This difference fortifies the observation that ridges tend to be warmer at night than near-by valleys. Some cooler valley sites in El Dorado have heat summations that are nearly 1,000 degree-days lower than Amador, or comparable to the Cameros district of Napa and Sonoma.