FEASIBILITY OF USING ATSR DATA TO ASSESS EVAPORATION 
FROM DEFORESTED AREAS IN AMAZONIA 
John B. Stewart 
C.T. Mutlow 
C.A. Nobre 
LR. Wright 
Institute of Hydrology 
Remote Sensing Applications 
MacLean Building, Crowmarsh Gifford 
Wallingford, Oxfordshire, OX10 8BB 
UNITED KINGDOM 
ISPRS Commission VII / Working Group 3 
ABSTRACT 
A major influence on terrestrial ecosystems is the evaporation from the vegetation and soil. Measurements 
and estimates of evapotranspiration have traditionally been carried out using ground-based data. Thought this 
data can be very accurate for the small area represents, there is considerable uncertainty introduced when the 
data is extrapolated to larger areas. Remotely sensed data has the potential to extrapolate point measurements 
to mesoscale or even regions. Theoretically the evaporation can be determined from remotely sensed 
measurements of the sensible heat flux and energy input. The sensible heat flux is quantitatively related to the 
difference between the radiometric surface temperature and the near-surface air temperature times a transfer 
coefficient which depends on wind speed and vegetation roughness. When there is not a closed cover of 
vegetation the transfer coefficient also depends on the sparseness of the vegetation. Our research over sparse 
semi-arid vegetation has shown that radiometric surface temperature can be successfully used to estimate 
sensible heat flux after the introduction of a correction to the transfer coefficient By combining 
measurements of the energy budget with the sensible heat flux the evaporation can be determine. The current 
study assessed the feasibility of using a similar procedure to estimate the evaporation from Amazon, where 
the rain forest had been partly removed, so that there were areas of grass and cultivation as well as forest. 
Ground-based micrometeorological data obtained as part of the ABRACOS (Anglo Brazilian Amazonian 
Climate Observation Study) project in Rondonia will be used to calibrate the fluxes. The sensitivity of the 
estimated evaporation to the errors in the data and the theoretical assumptions were assessed. The main 
sources of error were found to be the variable emissivity, particularly of the soil, and the variability of the 
transfer coefficient. 
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