Standardization of weekly growing degree day accumulations based on differences in temperature observation time and method

Abstract

Efforts to develop comparable growing degree day (GDD) accumulations across the US-Canadian border have revealed significant anomalies resulting from differences in observing times among US stations and reporting practices between US and Canadian stations. Simulations using hourly temperature data indicate that the period required to reach a given GDD threshold value during a growing season often varies by 2 weeks or more solely because of observation time differences. Previous work on methods to adjust such biases has concentrated on seasonal totals and long-term averages. In this study, empirical methods are developed to standardize weekly GDD accumulations to a common observing time. Hourly data for a 10 year period from five northeastern United States stations are used in the development of the adjustment procedure. However, only daily maximum and minimum temperatures are needed to implement the adjustment scheme. Different weekly adjustment factors are used for each month from March through November. In addition, the weekly adjustment factors are decreased in magnitude during individual spring and autumn weeks in which fewer than 25 degree days (13.9°C GDD) accumulate. Correction factors are also reduced during summer weeks which exhibit minimal day-to-day variation in temperature. An additional adjustment is applied to observations taken in the late afternoon and evening to ensure that the maximum temperature observed at these stations has occurred on the same day as that reported by morning observing sites. Validation trials using data from six independent stations indicate that average weekly differences of as much as 10 GDD (5.6°C GDD) are reduced to less than 1 GDD (0.6°C GDD). After applying the standardization procedures, GDD values based on different observation times and the Canadian observation practice accumulate at similar rates.