Geoscience Reference
In-Depth Information
If RH is considered to be an all day average value,
e
s
should be used instead of
e
sat
(
T
dry
′
) in Equation (23.4).
If
T
dew
, (in
°
C) is available,
e
is calculated from:
eeT
=
(
)
(23.5)
sat
dew
The value of the daily average vapor pressure deficit,
D
, required in many of the
equations used to estimate evaporation then follows from:
Dee
=−
(23.6)
s
It may also be necessary to adjust the value of wind speed,
u
z
, if this is measured at
a height, z
u
, other than 2 m. To do this, it is necessary to assume the logarithmic
wind speed profile given in Equation (19.22) applies, and to prescribe values for
the aerodynamic roughness length,
z
0
, and zero plane displacement,
d
, at the loca-
tion where wind speed is measured. When this is done, the wind speed,
u
2
, that
would have been measured at 2 m is given by:
⎡
(2
−
d
)
⎤
ln
⎢
⎥
z
uu
zd
z
⎣
⎦
0
=
(23.7)
2
z
⎡
(
−
)
⎤
ln
u
⎢
⎥
⎣
⎦
0
In practice, when making this correction it is often assumed that the required
values of
z
0
and
d
are those relevant for a 'reference crop' of short grass (described
later), in which case
z
0
0.08 m are used.
Table 23.1 gives examples of the above calculations at three example sites, at
all of which albedo is assumed to be 0.23, but where humidity is measured in
different ways, as follows:
=
0. 0148 m and
d
=
SITE A
A site near Oxford, England, at latitude 51.7
N, elevation 129 m, on
July 15 (i.e., day of the year 196), where humidity is measured using
an aspirated wet and dry bulb thermometer, where cloud cover
(but not bright sunshine hours) is measured, and where wind speed
is measured at 10 m. At this site, the general climate is designated as
humid and measured pan evaporation is 5.5 mm per day.
°
SITE B
A site near Tucson, USA, at latitude 32.2
N, elevation 720 m on
May 15 (i.e., day of the year 135), where humidity is measured as
relative humidity, where the number of bright sunshine hours (but
not cloud cover) is measured, and where wind speed is measured
at 2 m. At this site, the general climate is designated as arid and
measured pan evaporation is 14 mm per day.
°
SITE C
A site near Manaus, Brazil at latitude 3.1
S, elevation 80 m, on
February 15 (i.e., day of the year 46), where humidity is measured by
°
