SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

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gradients, respectively, above the surface within a distance of the
order of l m. This approach, here called the Bowen ratio (BR) method,
has of ten been used for reference because it is physically sound and
the calculation doesn't include empirical factors. One must, however,
be aware of the assumption of equality of heat and water vapour
transfer coefficients when interpreting the data obtained by the BR
method (Tanner, 1960). The difficulty in accurately measuring the
temperature and water vapour gradients limits the use of this method
for research purposes mainly.
3. Experiments
Experiments were carried at two sites, an energy peat production
area at Höystösensuo (61°55'N, 26°07'E) during years 1988 and 1989 and
an oats field at Jokioinen Observatory (60°49', 23°30') during 1989.
Nearly continuous data recordings we re obtained over the summer months
in 1989.
To measure skin surface temperature we used IR-thermometers with
two viewing angles, 15° and 60°. Emissivity was assumed to be
constant at 0.98 for the vegetation covered and the bare ground.
Deviations from this value during the different phases of crop
development were expected to be small. For a parti al crop cover (oats
crop at an early vegetative phase) we further assumed that the IR­
thermometer will see plants and bare soil approximatively in the same
proportion as the two surfaces contribute to the gross
evapotranspiration. For a complete green canopy the surface seen by
the thermometer, placed a few meters height above ground, will depend
on the zenith angle. It was assumed that pointing the thermometer at
an 45° angle would give a representative estimate of the evaporating
top leaf layer. No significant dependence on the azimuth angle of the
IR-thermometer was detected.
Net radiation was measured with a rigid dome net radiometer.
Approximative values of Rn may also be obtained, for instance, by
measuring short wave global radiation and reflected radiation with
pyranometers and applying empirical correction factors. Pyranometers
suit better for unattended remote sites.