Evaluation of several degree-day estimation methods in California climates

Procedures for estimating degree-day accumulations are frequently employed instead of the more accurate method of calculating degree-days from hourly temperature data because on-site temperature data are commonly restricted to daily minimum and maximum temperature records. Data from seven methods of estimating degree-days at each of nine locations during 2 years in California were compared by month to degree-day values calculated by hourly summation. Methods included three sine-wave approaches, three triangulation approaches and the averaging (i.e., rectangle) method. Results of the double-sine and corrected-sine (i.e., corrected for day length) methods were nearly identical to those of the single-sine method. The double triangulation and corrected triangulation methods produced very similar results to the single triangulation method. The averaging method and sine-wave methods deviated to a greater extent from degree-day accumulations calculated from hourly temperatures from November through February than did the triangulation methods. Degree-day estimations from the late spring and summer months were more similar to one another for all estimation methods than during the cooler months of the year. Since no advantages were noted in the more complicated double and corrected methods, the single triangulation method or the sine-wave method is preferred as they are less complicated procedures. Of the various temperature threshold cut-off methods evaluated, error levels were unaffected when estimating degree-days using the sine-wave method. The employment of a horizontal cut-off with the triangulation method did not significantly increase the amount of error in the estimation of degree-days. However, an increase in error was observed when employing the intermediate cut-off and vertical threshold cut-off techniques with the triangulation method for computing degree-days.