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| Titel |
How representative are instantaneous evaporative fraction measurements for daytime fluxes? |
| VerfasserIn |
Jian Peng, Michael Borsche, Alexander Loew |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078440
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| Zusammenfassung |
| Sun synchronous optical 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 EF. To address
this question, long term time series of data from a global network of EC (Eddy
Covariance) stations (FLUXNET) were analyzed across a wide range of ecosystems and
climates. It is found that the EF in different time periods of daytime under clear
skies are in good agreement with daytime EF except the period of 8:00-9:00h and
16:00-17:00h. In 11:00-14:00h, the minimum R2 value is higher than 0.75, and the
maximum RMSD is less than 0.087. These statistics indicate that EF during these time
periods is closer to daytime EF. The best correlation between instantaneous EF and
daytime EF appears at midday (12:00-13:00h). The possible reason for such result
is that energy fluxes change at a slower rate compared to early morning and late
afternoon. However, the EF exhibited more unstable under partly cloudy situations
compared with clear skies. The variability of EF increased with the increase in
cloudiness. For total cloud cover the R2 values between instantaneous EF in different
time periods and daytime EF obviously went down as compared to clear skies.
Poorer RMSD were also obtained at the same time. This is because cloudiness could
induce a decrease in the available energy and the latent heat flux, which further
causes the increase in both instantaneous EF and daytime EF. But these increases are
probably in different degrees. Thus the EF constant hypothesis might only be true for
clear skies. Nonetheless, the above results provide a basis for remote sensing-based
estimation of EF based on sun synchronous satellite observations. The midday overpass
satellites (e.g. MODIS and AVHRR) are supposed to give better results than other
overpass time platforms. The important conclusion from the present study is that the
EF constant assumption is valid over a wide range of ecosystems and climates. |
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