Growing degree-days: one equation, two interpretations

Authors

Gregory S. McMaster, USDA-ARSFollow
Wallace Wilhelm, University of Nebraska-LincolnFollow

Date of this Version

11-1-1997

Comments

Published in Agricultural and Forest Meteorology 87 (1997) 291-300.

Abstract

Heat units, expressed in growing degree-days (GDD), are frequently used to describe the timing of biological processes. The basic equation used is GDD = [(T_MAX + T_MIN)/2] - T_BASE where T_MAX and T_MIN are daily maximum and minimum air temperature, respectively, and T_BASE is the base temperature. Two methods of interpreting this equation for calculating GDD are: (1) if the daily mean temperature is less than the base, it is set equal to the base temperature, or (2) if T_MAX or T_MIN < T_BASE they are reset equal to T_BASE. The objective of this paper is to show the differences which can result from using these two methods to estimate GDD, and make researchers and practitioners aware of the need to report clearly which method was used in the calculations. Although percent difference between methods of calculation are dependent on T_MAX and T_MIN data used to compute GDD, our comparisons have produced differences up to 83% when using a 0°C base for wheat (Triticum aestivum L.). Greater differences were found for corn (Zea mays L.) when using a base temperature of 10°C. Differences between the methods occur if only T_MIN is less than T_BASE and then Method 1 accumulates fewer GDD than Method 2. When incorporating an upper threshold, as commonly done with corn, there was a greater difference between the two methods. Not recognizing the discrepancy between methods can result in confusion and add error in quantifying relationships between heat unit accumulation and timing of events in crop development and growth, particularly in crop simulation models. This paper demonstrates the need for authors to clearly communicate the method of calculating GDD so others can correctly interpret and apply reported results.