Geoscience Reference
In-Depth Information
Vapor removal by
turbulent air
Condensation
Vaporization by
kinetic energy
∇
Surface
Liquid water
Fig. 4.1
Liquid water molecules with sufficient kinetic energy escape from the liquid surface by vaporization.
A removal mechanism is needed to prevent the establishment of an equilibrium state in which
vaporization would become balanced by condensation.
have priority. The formulations in class (ii) are indirect since they make use of quantities
that are physically quite different from water vapor transport, as such; however, since
none of these quantities is part of the hydrologic cycle, energy budget methods still allow
independent estimates of the water vapor flux. This is not the case for the formulations
in class (iii). Although conceptually the most obvious and appealing among the three,
water budget formulations require a knowledge of all the other components in the water
budget; thus they are unsuitable whenever the objective is closure of the hydrologic cycle
and independent estimates of all its components are required.
4 . 2
MASS TRANSFER FORMULATIONS
4.2.1
In terms of turbulent fluctuations
Direct or eddy-correlation method
Equation (2.29) constitutes the basis for the direct measurement of the rate of evaporation
[ML
−
2
T
−
1
], or
E
=
ρ
q
w
(4.1)
w
and
q
and then computing
the cross-correlation over a suitable averaging period, which is usually taken on the
order of 15-30 min, sometimesupto1hatmost. While the theoretical basisofthis
method is straightforward, the requirements on the instrumentation are quite stringent.
For instance, for measurements at a few meters above the ground, the upper frequency
response limit should be at least of the order of 5-10 Hz. Therefore, it is only in recent
years that sufficient progress has been made in the development of suitable instrumen-
tation, that is commercially available. However, because the instrumentation is costly
and requires special skill to operate, at present this method is a realistic option only in
special experimental settings.
In practice
E
is determined by measuring the fluctuations
