Daytime temperature sum — a new thermal variable describing growing season characteristics and explaining evapotranspiration

The effective temperature sum, the sum of the positive differences between diurnal mean temperatures and 5 °C, is used in ecology, agriculture, forestry, and hydrology. It does not differentiate between day and night. However, most assimilation and evapotranspiration occurs in the daytime. A daytime temperature sum is presented that is better related to assimilation. First, temperatures above 7 °C were integrated during the period, starting when the sun rises five degrees above the horizon, and ending when it descends to two degrees. The sums of these values were then reduced to be, on average, equal to the traditional ones. The new variable appeared to smooth out much of the spatial variation caused by night-time temperatures. It also gives a more distinct start and end to the thermal vegetation period. When using the daytime temperature sum instead of the traditional sum in explaining evapotranspiration, a better result was obtained.