Geoscience Reference
In-Depth Information
Again, because
β
is a rapidly changing ratio, the value of
b
is not reliably given by
taking the time average of the instantaneous values of
β
. The average value of
β
must be calculated from the two time-average fluxes.
Energy budget of open water
Measuring the energy balance components for expanses of water is difficult partly
because of the practical problems involved in mounting and maintaining relevant
equipment, but also more fundamentally because the fluid to fluid interface
problem is poorly specified. But often it is only the surface heat fluxes (especially
the latent heat flux) exchanges that are needed and attempts have been made to
measure these using micrometeorological methods or, in the case of latent heat,
using a water balance approach to determine the net evaporation.
The evaporation flux from open water is very strongly related to wind speed and
to the difference between the (saturated) vapor pressure at the water surface and
the vapor pressure at some level (usually 2 m) above the surface. Given the
comparative simplicity of the physics describing the exchange in the atmosphere
between the water surface and air, and the difficulty involved in making
measurements, evaporation rates are sometimes estimated from semi-empirical
equations that were derived by calibration against prior careful measurements.
Because near surface air is progressively modified as it moves across a water
surface to an extent that depends on the distance traveled, these semi-empirical
equations are also expressed in terms of the surface area, A w , of the evaporating
water.
If the measured wind speed at 2 m is U 2 , and the surface temperature of the
evaporating water is T s , for small water areas such that 0.5 m
<
A w 0.5
<
5 m (includ-
ing evaporating pans), an estimate of the evaporation in mm d −1 is:
()
E
=
3.623
A
0.066
e
T
e U
(4.8)
w
sat
s
2
where e is the vapor pressure at 2 m and e sat ( T s ) is the saturated vapor pressure at
temperature T s . For larger areas where 50 m
<
A w 0.5
<
100 km, such as lakes, an
estimate of the evaporation in mm d −1 is:
()
E
A
0.05
e
T
e U
=
2.909
(4.9)
w
sat
s
2
Important points in this chapter
Ideal surfaces : in many applications, including hydrological and
meteorological models, the land surface is assumed to be made up of a
patchwork, each patch being homogeneous in terms of surface characteristics
that influence surface energy fluxes.
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