ON THE USE OF SATELLITE THERMAL DATA FOR DETERMINING EVAPOTRANSPIRATION IN PARTIALLY VEGETATED AREAS

The use of satellite data for determining daily evapotranspiration over completely vegetated areas has been analysed by different authors. However, if the soil surface is partially covered by vegetation the problem becomes more complicated, because in this case the value of the temperature measured by the satellite varies with the viewing geometry. In this work we have studied the influence of the satellite observation angle for crops with different cover degree, field geometry and ground emissivity. For this purpose we have used the simplified relation proposed by Jackson et al. LET(d)=R(nd)-B(T(s)-T(a))i where ET(d) and R(nd) are the daily values of evapotranspiration and net radiation respectively, (T(s)-T(a))i is the difference between crop surface and air temperatures measured at midday, B is a semi-empirical parameter, and L is the latent heat of vaporization of water. And we have concluded that the imposition of a single value of B is unacceptable and specific values must be chosen according to certain broad categories of the viewing geometry, whose number will depend on the geometry of each crop.