Geoscience Reference
In-Depth Information
Table 23.4
Demonstration of the calculation of reference crop evaporation using the FAO, radiation-based, and
temperature-based methods as described in the text for the three cases A, B, C specified previously.
Origin
Variable
Units
Site A
Site B
Site C
Table 23.1
Maximum air temperature
(
)
22.00
37.00
30.00
°
C
Table 23.1
Minimum air temperature
(
°
C
)
13.00
17.00
23.00
Table 23.1
Average temperature
(
)
17.50
27.00
26.50
°
C
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
Extraterrestrial solar radiation
(mm day
−1
)
16.45
16.36
15.61
Table 23.2
Net radiation
(mm day
−1
)
4.76
5.86
3.90
Table 23.2
Assigned site humidity
(none)
Humid
Arid
Humid
Table 23.3
Latent heat
(MJ kg
−1
)
2.460
2.437
2.438
Table 23.3
Delta
(kPa
°
C
−1
)
0.1260
0.2086
0.2033
Table 23.3
Psychrometric constant
(kPa
−1
)
0.0659
0.0622
0.0670
°
C
Data
Measured pan evaportion
(mm)
5.5
14
5.1
Assumed
Value of C
p
in Equ. (23.24)
(s m
−1
)
224
224
224
Assumed
Value of
(A
pan
/
A
rc
)
(none)
1.15
1.15
1.15
Equ. (23.16)
Modified psychrometric constant
(kPa
−1
)
0.1797
0.1443
0.1597
°
C
Equ. (23.27)
r
clim
assigned in Equ. (23.26)
(s m
−1
)
44
70
37
Selected
Default pan coefficient
(none)
0.88
0.82
0.88
Equ (23.26)
Wind corrected pan factor
(none)
0.71
0.75
0.77
Equ. (23.15)
Ref. crop evap. (FAO)
(mm day
−1
)
3.81
10.36
3.84
Equ. (23.21)
Ref. crop evap. (radiation based)
(mm day
−1
)
3.94
7.85
3.70
Equ. (23.22)
Ref. crop evap. (temperature based)
(mm day
−1
)
4.01
7.54
4.21
Equ. (23.23)
Ref. crop evap. (pan: default
K
p
)
(mm day
−1
)
4.84
11.48
4.49
Equ. (23.23)
Ref. crop evap. (pan: wind corr.
K
p
)
(mm day
−1
)
3.91
10.53
3.95
Because the FAO tables provide estimates of evaporation for irrigated crops, they
describe the seasonal evolution in the value of
K
C
in terms of four growth stages
with crop-specific duration as shown for a hypothetical crop in Fig. 23.2.
The reluctance of the agricultural irrigation community to change practice and
adopt estimates based on using the Penman-Monteith equation with crop-specific
surface resistances may partly be due to a lack of appreciation of the basic flaws in
using Equation (23.12). However, a more fundamental inhibition on change is that
specified values of aerodynamic resistance and surface resistance are required for
non-stressed, well-watered, irrigated crops. Hitherto these have not been readily
available. Shuttleworth (2006) addressed this need by combining modern thinking
in surface energy exchange and boundary layer meteorology to derive a means of:
specifying aerodynamic resistance of any crop from readily available (2 m)
climate station data; and
●
converting existing
K
c
values to their equivalent values of surface resistance.
●
