Spatial Analysis of Climate in Winegrape Growing Regions in the Western United States

Abstract

Knowledge of the spatial variation in temperature in wine regions provides the basis for evaluating the general suitability for viticulture, allows for comparisons between wine regions, and offers growers a measure of assessing appropriate cultivars and sites. However, while tremendous advances have occurred in spatial climate data products, these have not been used to examine climate and suitability for viticulture in the western United States. This research spatially maps the climate in American Viticultural Areas (AVAs) throughout California, Oregon, Washington, and Idaho using the 1971–2000 PRISM 400 m resolution climate grids, assessing the statistical properties of four climate indices used to characterize suitability for viticulture: growing degree-days (GDD, or Winkler index, WI), the Huglin index (HI), the biologically effective degree-day index (BEDD), and average growing season temperatures (GST). The results show that the spatial variability of climate within AVAs can be significant, with some regions representing as many as five climate classes suitable for viticulture. Compared to static climate station data, documenting the spatial distribution of climate provides a more holistic measure of understanding the range of cultivar suitability within AVAs. Furthermore, results reveal that GST and GDD are functionally identical but that GST is easier to calculate and overcomes many methodological issues that occur with GDD. The HI and BEDD indices capture the known AVA-wide suitability but need to be further validated in the western U.S. Additionally, the research underscores the necessity for researchers, software developers, and others to clearly communicate the data time period and method of calculating GDD so that results can be correctly interpreted and compared.