Growing degree hours - a simple, accurate, and precise protocol to approximate growing heat summation for grapevines

Despite its low accuracy and consistency, growing degree days (GDD) has been widely used to approximate growing heat summation (GHS) for regional classification and phenological prediction. GDD is usually calculated from the mean of daily minimum and maximum temperatures (GDD(mm)) above a growing base temperature (T (gb)). To determine approximation errors and accuracy, daily and cumulative GDD(mm) was compared to GDD based on daily average temperature (GDD(avg)), growing degree hours (GDH) based on hourly temperatures, and growing degree minutes (GDM) based on minute-by-minute temperatures. Finite error, due to the difference between measured and true temperatures above T (gb) is large in GDD(mm) but is negligible in GDD(avg), GDH, and GDM, depending only upon the number of measured temperatures used for daily approximation. Hidden negative error, due to the temperatures below T (gb) when being averaged for approximation intervals larger than measuring interval, is large in GDD(mm) and GDD(avg) but is negligible in GDH and GDM. Both GDH and GDM improve GHS approximation accuracy over GDD(mm) or GDD(avg) by summation of multiple integration rectangles to reduce both finite and hidden negative errors. GDH is proposed as the standardized GHS approximation protocol, providing adequate accuracy and high precision independent upon T (gb) while requiring simple data recording and processing.