Geoscience Reference
In-Depth Information
Table 23.6
Example calculations of daily average evaporation from unstressed crops calculated using the Matt-Shuttleworth
approach and the FAO crop factor method at the three example sites A, B, and C specified previously with values of
K
c
, (rs)
c
and
R
50
for cereal crops, an alfalfa crop, and small vegetables.
Origin
Variable
Units
Site A
Site B
Site C
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
Air pressure
(kPa)
99.79
93.08
100.36
Table 23.3
Latent heat
(MJ kg
−1
)
2.460
2.437
2.438
Table 23.3
Delta
(kPa
−1
)
0.1260
0.2086
0.2033
°
C
Table 23.3
Psychrometric constant
(kPa
°
C
−1
)
0.0659
0.0622
0.0670
Table 23.4
Modified psychrometric constant
(kPa
−1
)
0.1797
0.1443
0.1597
°
C
Table 23.4
Ref. crop evaporation (FAO)
(mm day
−1
)
3.81
10.36
3.84
Equ. (23.18)
r
clim
(s m
−1
)
37
104
38
Equ. (23.31)
(
D
50
/
D
2
)
(none)
1.10
1.29
1.18
Cereal crops
Table 23.5
Crop factor
(none)
1.15
1.15
1.15
Table 23.5
R
c
(none)
177
177
177
50
Table 23.5
(
r
s
)
c
(s m
−1
)
60
60
60
Equ. (23.38)
Re-modified psychrometric constant
(kPa
−1
)
0.1828
0.1465
0.1622
°
C
Equ. (23.37)
Matt-Shuttleworth estimate
(mm day
−1
)
4.31
13.85
4.45
Equ. (23.12)
FAO estimate
(mm day
−1
)
4.38
11.92
4.42
Alfalfa crop
Table 23.5
Crop factor
(none)
0.95
0.95
0.95
Table 23.5
R
c
50
(none)
196
196
196
Table 23.5
(
r
s
)
(s/m)
127
127
127
c
Equ. (23.38)
Re-modified psychrometric constant
(kPa
°
C
−1
)
0.2893
0.2234
0.2490
Equ. (23.37)
Matt-Shuttleworth estimate
(mm day
−1
)
3.04
10.56
3.42
Equ. (23.12)
FAO estimate
(mm day
−1
)
3.62
9.85
3.65
Small vegetables
Table 23.5
Crop factor
(none)
1.05
1.05
1.05
Table 23.5
R
c
(none)
230
230
230
50
Table 23.5
(
r
s
)
c
(s/m)
72
72
72
Equ. (23.38)
Re-modified psychrometric constant
(kPa
−1
)
0.1738
0.1401
0.1549
°
C
Equ. (23.37)
Matt-Shuttleworth estimate
(mm day
−1
)
3.88
11.66
4.01
Equ. (23.12)
FAO stimate
(mm day
−1
)
4.00
10.88
4.03
evaporation. In practice, there may also be a modeled drainage loss to depth from
this soil sample, but for crops that are irrigated with just enough water to replace
evaporative loss, this term may be negligible. The maximum volumetric water
holding capacity of the sample of soil is specified in terms of the depth and nature
of the soil, and any excess water above this is assumed to drain.
