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GROWING DEGREE DAY MODELS
Forecasting the future of viticulture
Julia Lenhardt and Elvis Takow, Esri
Observed climate change is impacting food security because of rising temperatures, changing precipitation patterns, and more frequent extreme weather events. The vulnerability of crop health to climatological variables requires adequate tools for forecasting those variables so that our planet can support food systems and the livelihoods that depend on them. The increasing use of growing degree day (GDD) models helps agricultural experts understand more about biological processes in plants and their response to rising temperatures.
GDD models
GDD models are an integral tool in understanding plant phenology. All GDD models implicitly assume that plant development directly relates to time and temperature. As a result, numerous bioclimatic indices can measure crop suitability and are mostly developed based on climatic variables. GDD is historically and currently the most used measure of climatic suitability for viticulture, the study of grape cultivation.
GDD is calculated as the average of the daily minimum and maximum temperatures compared to a base temperature, which is assumed to be the minimum temperature at which plant growth occurs. In the case of grapevines, the base temperature is typically set to 50oF/10oC:
GDD = Tmax+Tmin –10 2
Grapevine suitability regions
The idea of a heat summation above a base temperature defining grapevine growth and grape maturation was first observed by Swiss botanist Augustin Pyramus de Candolle and elaborated on by Amerine and Winkler (1944). As such, an index of heat summation for California was developed that is now widely used as a guide for selecting appropriate grape varieties and for determining a given area’s suitability to produce quality wine grapes.
The index is calculated by summing the GDDs for the period of April 1 through October 31 in a year. This time frame represents the growing season of grapevines in the Northern Hemisphere. Amerine and Winkler used this index to define five climatic zones, or Winkler regions, for California, with recommended grape varieties suitable for each region.
Forecasting GDD
Daymet minimum and maximum daily temperature data, produced by the Oak Ridge National Laboratory, extends from January 1, 1980, to the current calendar year for North America. Once stored in a multidimensional raster, temperature data can be used in trend and predictive analysis tools in ArcGIS Pro. Linear trend analysis of the existing temperature data is used to forecast minimum and maximum daily temperatures for the year 2050. A heat summation index is calculated for the period of April 1 through October 31 using the GDD formula. With the ability to forecast future GDD distribution, the predictive analysis tools can help categorize the data into the five regions defined by Amerine and Winkler to generate a map of Winkler regions in 2050.
Comparing the past to the future
A comparison of Winkler regions from 1980 to the forecast regions in 2050 shows a significant loss of Region I, particularly in Northern California. Meanwhile, the warmest region is expected to grow northward and toward the coast. Regions II, III, and IV also show small increases. Therefore, grape varieties suitable to Region I are less likely to be healthy or profitable in California in 2050.
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