Conservation and variability of the evaporative fraction during the daytime

Abstract

Several papers have recently used the tendency for the evaporative fraction (the ratio of latent heat flux to available energy, denoted EF) to be nearly constant during the daytime, in other words, the conservation of EF, to estimate daytime evaporation given only one or two estimates of EF during the middle of the day. However, the physical basis for this conservation, and the conditions under which it might be expected to occur, have not been systematically investigated. To do this, data from the First ISLSCP Field Experiment (FIFE) are used, along with other observations from the literature and some simple modeling. It is concluded that a complicated combination of weather conditions, soil moisture, topography, and biophysical conditions contribute to the variability or conservation of EF on individual days. However, two important factors in determining the amount of variability of EF on a given day are related to cloudiness and the advection of moisture and temperature.