Geoscience Reference
In-Depth Information
Fig. 4.3
Variation of Bowen
ratio
Bo
pe
for moist
surfaces as given by
Equation (4.32);
the solid curve
represents an
α
e
value of 1.26 and the
two dashed curves
represent values of
1.20 and 1.30. The
data points (for daily
values) were
collected by Davies
and Allen (1973)
from different
sources.
0.5
0.4
α
e
=1.20
Bo
pe
0.3
1.26
1.30
0.2
0.1
0
10
15
20
25
30
Temperature (
°
C)
Related empirical equations, but with
R
s
instead of
Q
n
as in(4.31), have been pro-
posed by Makkink (1957), Jensen and Haise (1963), and Stephens and Stewart (1963).
The short-wave radiation is often well correlated with the net radiation, which is the main
component of
Q
n
over daily periods or longer. Such equations, which provide a good
alternative to (4.31) when only short-wave radiation and temperature are available, have
been used to determine irrigation requirements and as climatological indices of potential
evaporation. However, the physical significance of such indices is not always clear, as is
shown next.
Potential evaporation
Because several of the simple energy budget-type methods for wet surfaces are often used
as measures of potential evapotranspiration, a few comments are in order on this concept.
The term potential evapotranspiration appears to have been introduced by Thornthwaite
(1948) in the context of the classification of climate. It is now generally understood
to refer to the maximal rate of evaporation from a large area covered completely and
uniformly by an actively growing vegetation with adequate moisture at all times. The
area is specified as large to avoid the possible effects of advection. Although the concept
iswidely used, it has also caused confusion, because it does not encompass all possible
conditions and it involves several ambiguities. The concept requires closer specification
if it is to serve as an unequivocal parameter.
Transpiration, even at the potential rate, involves such biological effects as stomatal
impedance to the diffusion of water vapor, and the stage in the growth cycle of the
vegetation. For this reason, the term potential evaporation is probably preferable. It can
be defined to refer to the evaporation from any large uniform surface that is sufficiently
moist or wet, so that the air in contact with it is fully saturated. Note that a wet or moist
surface is not the same as one that has an adequate moisture supply for the roots of an
actively growing vegetation; over short non-wet vegetation with adequate moisture the
