The actual evapotranspiration (LE) in energy units represents the latent
heat flux exchanged between the land surface and the atmospheric
boundary layer. The potential evapotranspiration (LEp) is
assessed as 80% of the net radiation available at the surface. The
relative evapotranspiration is the ratio LE/LEp. It is an
indicator of soil water availability and crop growth rate.
ONC is
interested in testing the use of LE in assimilation experiment in a carbon
cycle model.
OFM already
uses these parameters in crop yield modelling.
Actual (left) and potential (right)
evapotranspiration from METEOSAT for the first 10-day
period of August 2004
EARS provides
the actual evapotranspiration calculated by the
EWBMS system as
the difference
between the daily net radiation and the daily sensible heat flux, under
the assumption that the soil heat flux can be neglected. An additional
correction for photosynthetic light use is carried out. The net
radiation is estimated from the DSR
flux, the surface albedo, the
down-welling and up-welling longwave radiation flux. The sensible
heat flux is retrieved from the
LST and the air temperature. This method can only be
applied for clear sky conditions. As the LST is not estimated for
cloudy pixels, the sensible heat flux and, consequently, the latent
heat flux cannot be calculated in this way. In that case, it is assumed
that the Bowen ratio (sensible heat flux versus latent heat flux) is
constant during the period of cloud cover, so it is the same as on the
last cloudfree day (Rosema 1993; Rosema et al., 1998, Oroda et al.,
1998). Then, under the cloud cover, the actual evapotranspiration is
derived from the net radiation and the Bowen ratio.
References
Oroda, A., L. Isavna, A. Rosema, and R. Roebeling, Remote
Sensing Early Warning System: An Indicator of Biological Productivity,
in Sustainable Development of Dryland Areas of East Africa, Proceedings
of the International Workshop, Addis Ababa, November 9-12th, 1998, EU
DGXII.
Rosema, A., Using METEOSAT for Operational Evapotranspiration
and Biomass Monitoring in the Sahel region, Remote Sensing of
Environment, 45:1-25, 1993.
Rosema, A., R. Roebeling and D. Kashasha, Using Meteosat for
Water Budget Monitoring and Crop Early Warning, Agrometeorological
Applications for Regional Crop Monitoring and Production Assessment, D.
Rijks, J.M. Terres and P.Vossen eds, Accounts of the EU Support Group
on Agrometeorology (SUGRAM), EUR.17735 EN, 1998.
