Monday, May 28, 2018

Growing degree-days maps of Australia and the western USA

In a previous post (Wine is sunlight held together by water) I discussed the use of both the Winkler Index and the Köppen-Geiger Climate Classification for determining suitable locations for high-quality wine-making vineyards. At the global scale the Climate Classification is probably the most suitable tool, but at more local scales the Winkler Index comes into its own, which is what I will discuss here.

Albert J. Winkler

This Index is based on Maynard Amerine and Albert Winkler's idea of Growing Degree-Days (GDD) — the sum of the daily temperatures above 10 °C (50 °F) during the grapes' growing season. There are several ways that the data necessary for the calculations could be collected for any one spot on Earth, but a standardized version was developed some time ago by Gregory V. Jones and his colleagues. They have published several papers about various regions of the world:
The basic idea is to take digital maps (latitude, longitude, and altitude) and digital weather records (monthly maximum and minimum temperatures, plus precipitation), and combine the two using a computerized climate model. The output is a grid map, with an estimation of the GDD value for each cell in the grid. The cells can be of any chosen size, but a common choice is 1 km square (100 hectares or 250 acres). Obviously, this is not a fine enough scale to tell anyone where to plant their vines, but it is good enough to tell them whether they are looking in the right area or not.

Of interest to us here is the fact that the above publications each contains a gridded map showing the Winkler Index zones for a different part of the globe. I have reproduced here a version of the maps for the western USA (California, Oregon, Washington, and Idaho) and for Australia.

Growing degree-days map of vineyard land in the western USA

The map of the western USA is based on 400x400 m cells = 16 ha = 40 acres. The colors refer to the five Winkler Index zones (I-V with increasing heat summation). Most of the northern areas of the map are unsuitable for viticulture because the heat summation is too low (<850 degree-days), while the south-eastern parts have a heat summation that is too high (>2700 degree-days). The Central Valley of California is clearly indicated in red, being the hottest place with grapevines in the western USA.

This map can be compared to the boundaries of the American Viticultural Areas (AVA), which would indicate which grape varieties might be most suitable for each AVA. However, the spatial variability of the GDD values within any one AVA can be large. For example, the best-known AVA, Napa Valley, mostly lies within Region III, but it actually ranges from Region I to the lower part of Region V. The most important cause of this effect throughout the western USA is variation in altitude within the AVAs. Consequently, parts of most AVAs have never been planted with grapes, and probably never will be.

Growing degree-days map of vineyard land in Australia

The grid cells of the Australian map are 25 each, since the map covers the whole continent. Note, however, that very little of Australia is actually suitable for viticulture, unlike North America or Europe, because the heat summation is way too high. Only in the south-eastern part are vines absent because the heat summation is too low.

Most of the suitable vine-land area of Australia already has formal Geographic Indication (GI) viticultural areas, which are outlined in red on the map. Those areas with suitable GDD values but that do not (yet) have vines are, in fact, almost all too dry for viticulture.

For Australia, the biggest influences on the differences in GDD values between GIs are latitude and continentality. For example, note that the GDD values generally decrease from north to south down the east coast, with the coolest climates being on the island of Tasmania. Furthermore, the inland GIs are more likely to have greater index values than are GIs closer to the ocean. However, the latter effect is interrupted by the Great Dividing Range, which runs all the way down the east side of Australia. The upland areas are much cooler than are the coastal areas. Indeed, the northern GI areas are all at high altitude.

As noted above for the western USA, even within each of the GI areas, parts have never had grapes and probably never will have, due to variation in altitude. This is particularly important in those GIs located in the Great Dividing Range.

Both Australia and the western USA have areas within all of the Winkler Index zones. For example, Jones et al. list some comparable Australian GIs and western US AVAs, based on their median GDD values:
  • Region I – Southern Tasmania (GI) with Puget Sound (AVA) and numerous Willamette Valley sub-AVAs
  • Region II – Coonawarra (GI) with Red Mountain (AVA); Upper Goulburn (GI) with the Sonoma Coast (AVA)
  • Region III – Bendigo (GI) with Alexander Valley (AVA) and the Dry Creek Valley (AVA); Margaret River (GI) with the Napa Valley (AVA)
  • Region IV – Adelaide Plains (GI) with Lodi (AVA)
  • Region V – Swan District (GI) with Madera (AVA).
Finally, it is worth noting (as Jones et al. do) that viticultural areas with similar grape varieties and wine-making reputations do not always have the same GDD values. For example, Burgundy (France), the Willamette Valley (USA), and the Yarra Valley (Australia) each excel with Pinot noir and Chardonnay, but the Yarra Valley (median 1558 degree-days) is significantly warmer than is either Burgundy (1118) or the Willamette Valley (1081). Similarly, Bordeaux (France), the Napa Valley (USA), and Coonawarra (Australia) are often compared based on the quality of their Bordeaux-mix of red grapes, but the Napa Valley (1883) is substantially warmer than is either Bordeaux (1410) or Coonawarra (1457). Intriguingly, Margaret River, a GI with a rapidly rising reputation for producing Australia's best Bordeaux-mix wines, is remarkably similar to the Napa Valley (ie. median 1844 degree-days).


There are a lot of limitations of using Growing Degree-Days to assess areas for their climatic suitability for growing high-quality wine grapes, but nevertheless they can be very informative, especially for indicating unsuitable locations.

PS. This week marks the second anniversary of this blog.


  1. From "UC Davis Heat Summation Scale"


    "The UC Davis heat summation scale is organized into five tiers. Region I is the coolest and Region V is the warmest. The following is a list of the recommended varietals to grow in each region:

    "Region I: Below 2,500 degree days; Chardonnay, Pinot Noir, Gewurztraminer, Riesling

    "Region II: 2,500-3,000 degree days; Cabernet Sauvignon, Merlot, Sauvignon Blanc

    "Region III: 3,000-3,500 degree days; Zinfandel, Barbera, Gamay

    "Region IV: 3,500-4,000 degree days; Malvasia, Thompson Seedless

    "Region V: Over 4,000 degree days; Thompson Seedless, other table grapes"

  2. Hi David,

    Thanks for the summary of this work and the overall issues surrounding GDD and other climate indices. One of the issues that I have run into for years is that most published and or personally shared GDD values are not comparable because of issues of data source, the time period of the data, averaging period, formula base and cut-off use, etc. Even our very good data today compared to how Winkler initially specified it is not ideal because they used a short period of record and there is no specified data averaging period in their record.

    This is detailed in the Discussion section of the paper:
    Jones, G.V, Duff, A.A., Hall, A., and J. Myers (2010). Spatial analysis of climate in winegrape growing regions in the western United States. American Journal of Enology and Viticulture, 61:313-326.

    Also all of the work we have done on GDD, as classified in the Winkler Index, has shown that the structure is as given below (in degrees C units). Namely Amerine and Winkler did not specify either a lower or upper bound on the WI. We found through both work in California and across nearly every major wine country in the world that Region I should actually be a Region Ia for the absolute coolest vinifera and many hybrids, and Region Ib for more traditional cool climate vinifera.

    Too Cool < 850
    (Region Ia) 850-1111
    (Region Ib) 1111-1389
    (Region II) 1389-1667
    (Region III) 1667-1944
    (Region IV) 1944-2222
    (Region V) 2222-2700
    Too Hot > 2700


    1. Thanks for your comment. The two issue that you raise are ones I deliberately avoided, so as not to complicate the post. However, they are both obviously important.

      Data comparability seemed to me to be the biggest issue when I first looked at GDD values, and I was therefore very pleased when I realized that you were trying to produce a standardized set of values. An online global map, comparable to the one for the Köppen-Geiger Climate Classification, would be ideal.

      As for the upper and lower limits, they are clearly relevant in many areas outside the ones that Winkler studied. Indeed, as you point out in your papers, GDD may not actually be the best scale to use for global comparisons.