Geoscience Reference
In-Depth Information
estimate evaporation are themselves the estimates of daily average or daily total
values that can be derived from the limited measurements taken at such climate
stations, as discussed in the next section. In many cases the recommended
approach used to estimate daily values is obvious, or versions of the equations used
have been defined earlier; to reduce the scope for misunderstanding these methods
are also given explicitly below.
Daily average values of weather variables
Temperature, humidity, and wind speed
Daily maximum and minimum air temperature, T max and T min , respectively, are
usually measured at climate stations. These can be used to estimate the daily
average air temperature, T , and the daily average saturated vapor pressure, e s , from:
TT
+
T
max
min
=
(23.1)
2
and:
eT eT
(
)
+
(
)
e
=
sat
max
sat
min
(23.2)
s
2
where, for example, e sat ( T max ) represents the function of T max given as Equation
(2.17). Because the relationship between saturated vapor pressure and tempera-
ture is not linear, using Equation (23.2) is preferable to estimating e s from e sat ( T ).
A measurement of humidity may be available at least once (but often only once)
each day, perhaps in the form of a measurement of the wet bulb and dry bulb tem-
perature, T dry and T wet , respectively, or as a measurement of relative humidity, RH ,
or as a measurement of dew point temperature, T dew . If T dry and T wet (in
C) are
available, e is calculated from Equation (2.24) which is here re-written in the form:
°
eeT T T
=
(
)
g
(
-
)
sat
wet
dry
wet
(23.3)
to emphasize that the effective value of the psychrometric constant required may well
not be
γ
=
( c p P )/(0.6622
λ
) as used elsewhere, but rather a different empirically deter-
mined value,
*. This is very likely the case if the wet and dry bulb thermometers are
not aspirated and the wet bulb depression is then smaller than the true value.
If RH (%) is available at a particular time (e.g., 09:00 local time) when the dry
bulb temperature is T dry
γ
, the daily average value of e is best calculated from:
RH
e
e
T
=
(
¢
)
(23.4)
sat
dry
100
 
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