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Abstract
Abstract. Sun-synchronous optical and thermal remote sensing is a promising technique to provide instantaneous ET (evapotranspiration) estimates during satellite overpass. The common approach to extrapolate the instantaneous estimates to values for daily or longer periods relies on the assumption that the EF (evaporative fraction, defined as the ratio of latent heat flux to surface available energy) remains nearly constant during daytime. However, there is still no consensus on the validity of the self-preservation of the EF. We use FLUXNET (a global network of eddy covariance stations) measurements to examine this self-preservation, and the conditions under which it can hold. It is found that the instantaneous EF could represent daytime EF under clear sky conditions, especially between 11:00 and 14:00 LT (local time) for all stations. However, the results show that the EF is more variable during cloudy sky conditions, so that an increase in cloud cover results in an increase in the variability of the EF during daytime.
Highlights
Estimates of land surface ET are crucial for better understanding climate and hydrological interactions (Jung et al, 2010; Oki and Kanae, 2006)
The midday (12:00 to 13:00 LT) EF is closest to daytime EF with R2 = 0.920 ± 0.053, RMSD = 0.050 ± 0.013, RE = −4.47 % ± 2.48 %
Since the analysis is based on long-term FLUXNET measurements under a wide range of surfaces and environmental and climate conditions, we conclude that the instantaneous EF can generally represent daytime EF under clear sky conditions, especially from 11:00 to 14:00 LT
Summary
Estimates of land surface ET (evapotranspiration) are crucial for better understanding climate and hydrological interactions (Jung et al, 2010; Oki and Kanae, 2006). The EF is supposed to isolate vegetation and soil control from other factors in the determination of surface energy balance components. It can remove the daily sinusoidal-like variations of the latent heat flux and sensible heat flux at the land surface, and it remains almost constant during daytime under clear sky conditions (Gentine et al, 2007, 2011; Li et al, 2008) (Fig. 1). Shuttleworth et al (1989), Nichols and Cuenca (1993) and Crago and Brutsaert (1996) used in situ measurements of surface energy balance components and showed that the EF is nearly constant during daytime under clear sky days. Since the daytime constant EF assumption is the basis for extrapolating instantaneous ET estimates to daily values, whether it holds or not is a fundamental issue
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