Geoscience Reference
In-Depth Information
Table 23.3
Demonstration of the sequence of steps undertaken to calculate daily average net radiation as described in the
text applied in the three cases A, B, C specified previously. In two cases (A and C) cloud cover is measured and in the third
case (B) the number of bright sunshine hours is measured.
Origin
Variable
Units
Site A
Site B
Site C
Table 23.1
Average temperature
(
°
C
)
17.50
27.00
26.50
Table 23.1
Vapor pressure deficit
(kPa)
0.533
3.285
0.717
Table 23.1
Modified wind speed
(m s
−1
)
5.23
4.00
4.19
Table 23.2
Solar at ground (cloudy sky)
(mm day
−1
)
8.23
11.89
6.24
Table 23.2
Net longwave
(mm day
−1
)
−1.58
−3.30
−0.90
(Data)
Elevation
(m)
129.00
720.00
80.00
Equ. (23.11)
Air pressure
(kPa)
99.79
93.08
100.36
Equ. (2.1)
Latent heat
(MJ kg
−1
)
2.460
2.437
2.438
Equ. (2.18)
Delta
(kPa
°
C
−1
)
0.1260
0.2086
0.2033
Equ. (2.25)
Psychrometric constant
(kPa
°
C
−1
)
0.0659
0.0622
0.0670
(Data)
Selected value for Albedo (water)
(none)
0.08
0.08
0.08
Equ. (5.18)
Net solar radiation
(mm day
−1
)
7.57
10.94
5.74
Equ. (5.26)
Net radiation
(mm day
−1
)
5.99
7.64
4.84
Equ. (23.9)
Open water evaporation
(mm day
−1
)
5.76
12.14
5.16
5.256
⎛
293
−
0.0065
E
⎞
l
P
=
101.3
⎜
⎟
⎝
⎠
293
(23.11)
This equation is derived from Equation (3.13) assuming the pressure and temperature
at sea level are 101.3 kPa and 293 K, respectively, and that there is an environmental
lapse rate equal to that of the US Standard Atmosphere (6.5 K km
−1
).
Table 23.3 demonstrates examples of the calculation of open water evaporation
for the meteorological conditions in the three example cases A, B, C. In these
calculations the albedo of the water surface is assumed to be 0.08 and the advected
energy
A
h
and stored energy S
h
is assumed negligible.
Reference crop evapotranspiration
Early researchers correctly believed that the rate of evaporation from terrestrial
surfaces was primarily meteorologically determined. This led to the hypothetical
concept of 'potential' rates of evaporation, and empirical relationships were
sought to estimate these rates from available weather data (e.g., Thornthwaite,
1948; Blaney and Criddle, 1950; Hargreaves, 1975). Penman (1948) (see previous
section) was the first to formulate the basic physics of evaporation using two
terms, an energy term related to radiation and an aerodynamic term related to the
vapor pressure deficit of the air and wind speed. At that stage, Penman suggested
