GROWING DEGREE-DAYS - ONE EQUATION, 2 INTERPRETATIONS

Heat units, expressed in growing degree-days (GDD), are frequently use d to describe the timing of biological processes. The basic equation u sed is GDD = [(T-MAX + T-MIN)/2] -T-BASE, where T-MAX and T-MIN are da ily maximum and minimum air temperature, respectively, and T-BASE is t he base temperature. Two methods of interpreting this equation for cal culating GDD are: (1) if the daily mean temperature is less than the b ase, 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 metho ds to estimate GDD, and make researchers and practitioners aware of th e need to report clearly which method was used in the calculations. Al though percent difference between methods of calculation are dependent on T-MAX and T-MIN data used to compute GDD, our comparisons have pro duced differences up to 83% when using a 0 degrees C base for wheat (T riticum aestivum L.). Greater differences were found for corn (Zen may s L.) when using a base temperature of 10 degrees C. Differences betwe en the methods occur if only T-MIN is less than T-BASE, and then Metho d 1 accumulates fewer GDD than Method 2. When incorporating an upper t hreshold, as commonly done with corn, there was a greater difference b etween the two methods. Not recognizing the discrepancy between method s can result in confusion and add error in quantifying relationships b etween heat unit accumulation and timing of events in crop development and growth, particularly in crop simulation models. This paper demons trates the need for authors to clearly communicate the method of calcu lating GDD so others can correctly interpret and apply reported result s. (C) 1997 Published by Elsevier Science B.V.