Sun-synchronous optical and thermal remote sensing is a promising techniqueto provide instantaneous ET (evapotranspiration) estimates during satelliteoverpass. The common approach to extrapolate the instantaneous estimates tovalues for daily or longer periods relies on the assumption that the EF(evaporative fraction, defined as the ratio of latent heat flux to surfaceavailable energy) remains nearly constant during daytime. However, there isstill no consensus on the validity of the self-preservation of the EF. Weuse FLUXNET (a global network of eddy covariance stations) measurements toexamine this self-preservation, and the conditions under which it can hold.It is found that the instantaneous EF could represent daytime EF under clearsky conditions, especially between 11:00 and 14:00 LT (local time) forall stations. However, the results show that the EF is more variable duringcloudy sky conditions, so that an increase in cloud cover results in anincrease in the variability of the EF during daytime.
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