SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

to calculate h Hz are well documented in the litterature (see ego
Brutsaert, 1982 or Launiainen, 1983).
Then latent heat flux, using eq. (1) and (2) will now be
LE (3)
The difference in temperature between the air and the evaporating
surface is strongly dependent on R n , but it also reflects the moisture
conditions at the surface. For a bare soil: the dryer the surface the
less cooling due to evaporation. For avegetated surface: when stomata
close e.g. due to increase in soil moisture deficit transpiration is
reduced resulting in increase of the leaf temperature and vice versa.
The calculation procedure introduced by eq. (3) will here be
designated as the aerodynamic surface temperature (AST) method.
Application of eq. (3) for automated monitoring of evaporation may
look straight forward since the quantities Rn , G, To and Tz are
directly measurable and the theory for sol ving h Hz
exists.
Complications will, however, arise, because calculation of h Hz is based
on empirical formulae which are specific e.g. for the type of crop and
the phase of crop development. Empirical adjustments to correct for
the surface types have been reported for open water (Launiainen,
1983), arid terrain with sparse vegetational cover and homogeneous
fully crown wheat crop (Kustas et. al., 1989)
For this study we have initially tested eq. (3) with the standard
formulae by (Launiainen, 1983) to calculate h Hz ' At further stages of
the study different stages of crop development will be analyzed to
better account for the evapotranspiration from a multilayered source
for water vapour.
2.2. The Bowen ratio method
Another way to solve the latent heat flux from eq. (1) is to use
the so called Bowen ratio p = H/LE and thus
LE (Rn - G)/(1 + P) (4)
It can be shown that P = y I:.T/I:.e, where y is the psychrometric
constant, I:.Tandl:.e the vertical temperature and water vapour pressure
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