Growing degree-days: one equation, two interpretations

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 degrees C base for wheat (Triticum aestivum L.). Greater differences were found for corn (Zen mays L.) when using a base temperature of 10 degrees 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. (C) 1997 Published by Elsevier Science B.V.