Comparison of thermal units derived from daily and hourly temperatures

Abstract

Crop development as a function of thermal time has important applications in crop management and crop modeling. Several reports have indicated that calculation of thermal units on an hourly basis, and averaging these values over 24 h, was superior to calculations of thermal units using the average of maximum (Tmax) and minimum (Tmin) temperatures. In this report, daily and hourly calculations of thermal-unit accumulation were compared for two sets of cardinal temperatures for representative warm-season crops. Daily (DTU) and hourly thermal units (HTU) agreed closely except when average temperatures were above 34°C or when close to the base temperature (Tb). Simulations across 43 yr of weather data comparing the time required to cumulate 200°C d were nearly identical for calculations based on DTU and HTU for two sets of cardinal temperatures. For warm-season crops with similar cardinal temperatures, there is no advantage in hourly calculations of thermal units over daily values.