Geoscience Reference
In-Depth Information
Table 23.1
Demonstration of example calculations of the daily average values of air
temperature, saturated vapor pressure, vapor pressure, vapor pressure deficit, and wind
speed at 2 m at the three sites A, B, C specified in the text where humidity is measured
in different ways.
Origin
Variable
Units
Site A
Site B
Site C
(Data)
Maximum air temperature
(
)
22.00
37.00
30.00
°
C
(Data)
Minimum air temperature.
(
°
C
)
13.00
17.00
23.00
Equ. (23.1)
Average temperature
(
)
17.50
27.00
26.50
°
C
Equ. (2.17)
Sat. vapor pressure (Max. temp)
(kPa)
2.644
6.275
4.243
Equ. (2.17)
Sat. vapor pressure (Min. temp)
(kPa)
1.498
1.938
2.809
Equ. (23.2)
Average sat. vapor pressure
(kPa)
2.071
4.106
3.526
Assumed
Wet bulb psychrometric constant
(kPa
−1
)
0.066
-
-
°
C
(Data)
Dry bulb temperature
(
°
C
)
17.50
-
-
(Data)
Wet bulb temperature
(
)
15.00
-
-
°
C
Equ. (23.3)
Vapor pressure
(kPa)
1.538
-
-
(Data)
Relative humidity
(%)
-
20.00
-
Equ. (23.4)
Vapor pressure
(kPa)
-
0.821
-
(Data)
Dew point
(
)
-
-
23.00
°
C
Equ. (23.5)
Vapor pressure
(kPa)
-
-
2.809
Equ. (23.6)
Vapor pressure deficit
(kPa)
0.533
3.285
0.717
(Data)
Wind measurement height
(m)
10.00
2.00
5.00
(Data)
Wind speed
(m s
−1
)
7.00
4.00
5.00
Equ. (23.7)
Modified wind speed
(m s
−1
)
5.23
4.00
4.19
a dew point hygrometer, where cloud cover (but not bright sunshine
hours) is measured, and where wind speed is measured at 5 m. At
this site, the general climate is designated as humid and measured
pan evaporation is 5.1 mm per day.
Net radiation
When making daily estimates of evaporation the availability of a measured
(or more likely) estimated value for daily average net radiation is arguably the
most valuable meteorological variable because (providing water is not limit-
ing) available energy is the major control on evaporation rate. Also, as a daily
average value, net radiation estimates tend to be more 'transferable' from one
location to another within a region than other weather variables. Thus, as a
general rule, it is generally best to base an estimate of daily evaporation on
measured or estimated daily average net radiation if this is available or can
be obtained.
The equations needed to make an estimate of daily average net radiation
from measurements of sunshine hours or cloud cover, daily average temperature,
