Differences in observing and reporting practices for daily maximum and minimum temperatures between U.S. and Canadian stations, as well as differences in observing times among observation sites in the U.S., introduce small differences in reported daily temperature extremes. When used in growing degree day (GDD) calculations, these small differences accumulate over time, resulting in significant inconsistencies in weekly or longer period GDD totals among stations. When used in near-real time, GDD accumulations affected by these differences may lead to delayed or premature scheduling of climate sensitive agricultural operations.
Several studies have documented problems when comparing GDD accumulations at different sites and have developed adjustment procedures for the systematic bias resulting from observation time differences. Baker (1975) focused his adjustment scheme on seasonal, May-September, base 50 GDD at St. Paul, MN. Schaal and Dale (1977) calculated monthly standardization factors for 86-50 method GDD in Indiana. However, their work was limited to adjusting 0700 and 1900 observations to midnight values. [For convenience in graphical and tabular presentations, twenty-four hour time is used throughout this publication.] Blackburn (1983) also developed a method by which monthly average temperatures could be adjusted for observation time bias. Implementation of his adjustment scheme, however requires access to hourly temperature data which are available at only a few sites.
Due to the limitations associated with the previous standardization methods, DeGaetano and Knapp (1993) developed a more flexible method to standardize weekly GDD totals, based on any observation time or the Canadian observation method to a common observation hour. This adjustment procedure can be successfully applied to data from any station in the northeastern United States or southeastern Canada and requires only daily temperature data for the week of interest. In addition, GDD totals calculated using each of the three most common methods, base 50 (GDD50), 86-50 (GDD86-50) and Canadian Corn Heat Units (CHU), can be standardized using the adjustment procedure.
10 pp.
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