Geoscience Reference
In-Depth Information
4
EVAPORATION
In terms of the water quantities transported on a global basis, evaporation is the sec-
ond most important component of the hydrologic cycle, after precipitation. The general
climatology of the hydrologic cycle reviewed in Chapter 1, indicates that over the land-
surfaces of the Earth evaporation amounts on average to approximately 60% to 65% of
the average precipitation. But this estimate provides only an idea of the order of mag-
nitude to be expected; the actual evaporation rate at any given time and place islikely
to be quite different from the climatological mean, and more thorough analysis is often
called for.
4 . 1
EVAPORATION MECHANISMS
As a physical phenomenon, evaporation is the transition of water from the liquid phase
to the vapor phase. This transition requires first, an energy supply to provide water
molecules the necessary kinetic energy to escape from the liquid surface; and second,
some mechanism to remove the escaped molecules from the immediate vicinity of the
liquid surface thus preventing that they would return to condense (see Figure 4.1). These
two requirements have traditionally given rise to two classes of methods to describe
evaporation, namely
(i)
mass transfer or aerodynamic formulations, which consist primarily of the descrip-
tion of the water vapor transport mechanisms in the near-surface air of the atmo-
sphere, and
(ii)
energy budget formulations, inwhich the main focus is on the energy supply
aspects of the phenomenon.
Actually, this classification scheme is somewhat unsatisfactory, because it is almost
never possible to consider mass transfer and energy aspects of evaporation in isolation
from each other; as will become clear below, energy budget methods usually cannot
avoid mass transfer considerations in their application, and vice versa. Nevertheless, this
classification will be used in what follows, mainly for historic reasons. In addition, a
third class of methods is considered, namely
(iii)
water budget formulations, inwhich evaporation is treated as the unknown rest
term in the continuity equation (1.7) or (1.8) for various types of control volumes
that include the landsurface-atmosphere interface as a boundary.
Among these three, the formulations in class (i) are based on the most direct descrip-
tion of the water vapor transport mechanisms, so that whenever possible they should
Search WWH ::




Custom Search