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Experimental studies indicate that evaporative fraction (EF), the ratio between the latent heat flux and the available energy at the land surface, is a normalized diagnostic that is nearly constant during daytime under fair weather conditions (so-called daytime self preservation). This study examines this observation and investigates contributions to the variability of EF due to environmental factors (air temperature, solar incoming radiation, wind velocity, soil water content or leaf area index). It is shown here that the phase difference between soil heat flux and net radiation needs to be characterized fully in application models that invoke EF daytime self-preservation. Further conditions under which the diurnally constant EF assumption can hold are also discussed