Wines & Vines

Collector's Edition WINES&VINES 109 GRAPEGROWING commercial weather stations in- stalled in vineyards, are assumed to be placed to limit variables that may impact climatic data. The duration of the data (i.e., number of years of data), however, can have a significant impact on num- bers. Typically, meteorological normals (i.e., what is considered to be the average weather for a particular location) are based on 30-year averages, updated every new decade (e.g., 2010-1981, 2000-1971, etc.). Finding com- plete data sets covering 30 years in a given area can be difficult, so GDD averages used in viticulture are most commonly given in some significantly shorter duration. Further complicating data sets is what happens around a given weather station during that 30- year period. Urban development can create warmer conditions that, combined with a changing climate, lead to migration from one climatic classification to an- other over the course of historical data sets. Further, academic stud- ies are often done in Celsius, with the Winkler climatic regions con- verted to Celsius equivalents. For example, Region II is converted to 1,389-1,667° C (from 2,501- 3,000° F), giving the impression that the climate-region break- downs are based on something more accurate than the conve- nience of even 500 degree day intervals. (Since the original re- gion breakdown is in Fahrenheit, and this article is a discussion of those regions, Fahrenheit is used throughout this article unless oth- erwise noted.) To demonstrate how varied GDD numbers can be for a given location, data were assessed from a California Irrigation Manage- ment Information System (CIMIS) station at Camino in El Dorado County, Calif. This station was chosen because it is one of the longest continuously operating CIMIS stations, having been in- stalled Oct. 19, 1982. Data from this station are also nearly com- plete, especially after 1989. The station is not near a growing urban center. Camino is listed in "General Viticulture" (Region III with 3,400 GDD). This station is not the same weather station used in the Winkler calculations but offers some historical context for the Camino area in general. Thirty total years of data were assessed from 2017 to 1987, with 1988 removed from the analysis, as data were incomplete. The results in Figure 1 show that daily accu- mulations led to higher GDD to- tals than using monthly means. Variations in averages Both the daily accumulations and monthly accumulations to- tals at the Camino station are relatively consistent with the total found in Winkler et al., although the numbers are 139 degree days different, based on methodology. If a grower had been given a single year's data, however, and that year hap- pened to be either the year with the lowest GDD (1998), or the y e a r w i t h t h e h i g h e s t G D D (2017), very different conclu- sions could have been reached about the climate of Camino. The averages for the five most recent years show both a warmer climate than the 30-year average would suggest and greater vari- ance between methodologies. Keeping with standards of clima- tology and meteorology would discredit the five-year average as insufficient, as weather varies sea- son to season, and climate is a long-term averaging of weather. Yet getting five years of data is often much more achievable than obtaining 30 years of data. The climate is also warming. Figure 1 demonstrates the long-term tem- perature trend in GDD for Camino, with every academic indication that this trend will continue. The prospects are very likely that on- going climatic conditions at Camino will more likely reflect GDD above the 30-year average. Which Winkler Index climate re- gion is most appropriately applied to Camino? Any interested party could have a choice between Re- gion III or Region IV, or if they were really looking to cherry-pick, Region II or Region V. Comparative analysis is where a more appropriate usage of GDD may come into play. How does Camino compare to other nearby areas? A CIMIS station is also located at Diamond Springs, ap- proximately nine miles southwest of Camino. Diamond Springs is a much newer station, installed Sept. 20, 2010, so a 30-year aver- age is not possible. The five-year average at Diamond Springs based on daily accumulations is 3,997 degree days compared to 3,821 at Camino. The seven-year d a i l y a c c u m u l a t i o n a v e r a g e (2011-2017) at Diamond Springs is 3,913 compared to 3,679 at Camino. Every year with the ex- ception of 2017 shows higher GDD at Diamond Springs than at the Camino weather station (2017 shows Camino 30 degree d a y s h i g h e r t h a n D i a m o n d Springs). Even if you could not give a good long-term average GDD for Diamond Springs, it would be safe to say that the cli- mate at the Diamond Springs station is warmer than at the Camino station. As viticultural growing districts become more defined at subregional levels and as we test new environs for com- mercial viticulture, local com- parisons are valuable. Duration of high temperatures Another consideration with Win- kler climatic regions is the dura- tion of high and low temperatures. GDD calculations based on mean daily temperatures treat high and low temperatures equally, even if the high temperature is reached for only for a few minutes, but the low temperature persists for sev- eral hours. Areas impacted by afternoon marine inversions may be especially susceptible to briefly maintained high temperatures. Figure 3, created for a study done in the Livermore Valley AVA, shows the cumulative number of Hourly Readings Adopted from Shabram, 2017. All data collected from California Irrigation Manage- ment Information System. Temperature reading within 0.3°C or 0.5°F of the daily maximum temperature were counted from July 10-16, 2016. FIGURE 3: HOURLY READINGS AT HIGH TEMPERATURE 16 14 12 10 8 6 4 2 0 Pleasanton Point San Pedro Santa Rosa Oakville Brentwood Manteca Modesto n 7/16/16 n 7/15/16 n 7/14/16 n 7/13/16 n 7/12/16 n 7/11/16 n 7/10/16 Weather Stations Using daily accumulation method based on daily mean temperature. Data for 1986 and 1988 are incomplete and are not included in the graph. 1980 1985 1990 1995 2000 2005 2010 2015 2020 4500 4000 3500 3000 2500 FIGURE 2: GDD TREND APRIL 1-OCT. 31 AT CIMIS 13 CAMINO 1984-2017

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