Abstract
The concept of thermal time, measured in degree-days, is widely used among the agricultural community in Nebraska to make decisions in corn (Zea mays L.) production. Instead of the real-time temperatures that are experienced by corn plants, most of the widely available temperature data are limited to daily timescale observations from standard meteorological stations. And a variety of equations are used by different agricultural groups (e.g., researchers, advisors, farmers, and seed companies) to estimate thermal time for corn. Two problems could arise: (a) the estimation method is lacking in accuracy; and (b) different estimation methods are used for the same purpose by different groups. Consequently, citing these inaccurate and maybe inherently different thermal time results could lead to biased decisions in corn production. The goal of this study is to evaluate six commonly used estimation methods by comparing the estimated thermal time with the hourly temperature approximated thermal time. We analyzed the root mean square error and mean absolute error for six metrics of total growing season (from May through September) degree-days based on the temperature data from a total of 14 long-term observing locations in Nebraska. In particular, we selected four location-extreme year cases to demonstrate the six methods’ estimation performance on a daily timescale. We found that the most commonly used adjusted Tmax and Tmin rectangle method provided poor estimation in the study area. Instead, single-sine, double-sine, or Tavg-based method was more superior depending on the metric of degree-days.
Highlights
The concept of thermal time, measured in degree-days, is widely used in crop research and field management to track crop phenological development (Cross and Zuber, 1972; Gilmore and Rogers, 1958; Russelle et al, 1984)
We analyzed the root mean square error and mean absolute error for six metrics of total growing season degree-days based on the temperature data from a total of 14 long-term observing locations in Nebraska
Most of the widely available temperature data are restricted to observations from meteorological stations that are in the vicinity of crop fields, usually including daily maximum and minimum temperatures
Summary
The concept of thermal time (or heat units), measured in degree-days, is widely used in crop research and field management to track crop phenological development (Cross and Zuber, 1972; Gilmore and Rogers, 1958; Russelle et al, 1984). Thermal time is commonly estimated based on these two daily temperatures, with three types of methods as follows: (1) averaging (or so called rectangle) methods, such as Tavgbased method (Tavg is the arithmetic mean of daily maximum and minimum temperatures) and adjusted Tmax and Tmin method (Arnold, 1960; McMaster and Wilhelm, 1997); (2) sine-wave methods, such as single-sine method (Baskerville and Emin, 1969), and double-sine method (Allen, 1976); and (3) triangulation methods, such as single-triangulation method (Lindsey and Newman, 1956; Neild, 1967), and double-triangulation method (Sevacherian et al, 1977). Double-sine and double-triangulation methods account for the fact that minimum temperature at the beginning and the end of a specific 24-hour period may not necessarily be the same; they use the day’s minimum temperature. Double-sine and double-triangulation methods divide each day into two 12-hour periods and represent the first 12-hour period by daily minimum and maximum temperatures of that day while representing the second 12-hour period by daily maximum temperature of that day and daily minimum temperature of the following day (Allen, 1976; Sevacherian, et al, 1977)
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