Geoscience Reference
In-Depth Information
regularly 'irrigated' so that the vegetation cover is
always yielding the maximum possible evapotran-
spiration, the water loss is called
potential evapo-
transpiration
(or PE). More generally, PE may be
defined as the water loss corresponding to the
available energy. Potential evapotranspiration
forms the basis for the climate classification dev-
eloped by C. W. Thornthwaite (see Appendix 1).
In regions where snow cover is long-lasting,
evaporation/sublimation from the snow pack
can be estimated by lysimeters (pans or box
containers) sunk into the snow that are weighed
regularly.
A meteorological solution to the calculation
of evaporation uses sensitive instruments to
measure the net effect of eddies of air transporting
moisture upward and downward near the surface.
In this 'eddy correlation' (or eddy covariance)
technique, the vertical component of wind and the
atmospheric moisture content are measured
simultaneously at the same level (say, 1.5m) every
10
-1
s (10Hz). The product of each pair of mea-
surements is then averaged over a time interval of
15-60 minutes to determine the evaporation (or
condensation). This method requires delicate
rapid-response instruments, so it cannot be used
in very windy conditions. Sonic anemometers are
used to measure the vertical and horizontal wind
components. These use sound pulses to measure
the difference in time that it takes for sound to
travel with and against the wind, allowing the
wind speed to be calculated. The humidity is
determined by measuring the absorption of
infrared radiation by water vapor in the air.
Theoretical methods for determining evapora-
tion rates have followed two lines of approach.
The first relates average monthly evaporation (
E
)
from large water bodies to the mean wind speed
(
u
) and the mean vapor pressure difference
between the water surface and the air (
e
w
-
e
d
) in
the form:
takes account of the factors responsible for
removing vapor from the water surface. The
second method is based on the energy budget.
The
net balance
of solar and terrestrial radiation
at the surface (
R
n
) is used for evaporation (
E
) and
the transfer of heat to the atmosphere (
H
). A small
proportion also heats the soil by day, but since
nearly all of this is lost at night it can be
disregarded. Thus:
R
n
=
LE
+
H
where
L
is the latent heat of evaporation
(2.5
10
6
J kg
-1
).
Rn
can be measured with a net
radiometer and the ratio
H
/
LE
= β, referred to as
Bowen's ratio, can be estimated from measure-
ments of air temperature and vapor content (dew-
point) at two levels near the surface; the levels are
typically at about 0.5 and 2m.
×
ranges from <0.1
for water to
10 for a desert surface. The use of
this ratio assumes that the vertical transfers
of heat and water vapor by turbulence take
place with equal efficiency. Evaporation is then
determined from an expression of the form:
R
n
β
E
= ---------
L
(1 +
)
The conversion of evaporation to energy units
is 1mm evaporation = 2.5 × 10
6
J m
-2
.
The most satisfactory climatological method
so far devised combines the energy budget and
aerodynamic approaches. In this way, H. L.
Penman succeeded in expressing evaporation
losses in terms of four meteorological elements
that are regularly measured, at least in Europe and
North America. These are net radiation (or an
estimate based on sunshine duration), mean air
temperature, mean air humidity and mean wind
speed (which limit the losses of heat and vapor
from the surface).
The relative roles of these factors are illustrated
by the global pattern of evaporation (see
Figure
4.6
). Losses decrease sharply in high latitudes,
where there is little available energy. In middle and
lower latitudes, there are appreciable differences
between land and sea. Rates are naturally high
β
E
=
Ku
(
e
w
-
e
d
)
where
K
is an empirical constant. This is termed
the aerodynamic (or bulk) approach because it
