Environmental Engineering Reference
In-Depth Information
6.3.10 Calculation of Saturation Vapor Pressure
Several empirical equations have been proposed relating
temperature to saturation vapor pressure. Only two equations
are presented below: the Tetens (1930) equation and the
Lowe (1977) equation. The slope of the saturation vapor
pressure versus temperature curve is also computed since
it is required as the
variable when solving the Penman
(1948) equation.
The Tetens (1930) equation for saturation vapor pressure
as a function of temperature is as follows:
The Lowe (1977) equation for calculating saturation vapor
pressure as a function of temperature is written as follows:
u
air
v
0
a
2
T
a
a
3
T
a
a
4
T
a
a
5
T
a
=
a
0
+
a
1
T
a
+
+
+
+
(6.27)
where:
u
air
v
0
=
saturation vapor pressure, kPa,
air temperature,
◦
C
T
a
=
a
0
=
0.6183580754,
a
1
=
0.0411427320,
0
.
6108 exp
17
.
27
T
a
T
a
+
a
2
=
0.0017217473,
a
3
=
0.0000174108,
u
air
v
0
=
(6.26)
237
.
3
a
4
=
0.0000003985, and
a
5
=
0.0000000022.
where:
The saturation vapor pressures calculated using the above
equations are shown in Table 6.4 and graphically presented
in Fig. 6.29.
u
air
v
0
=
saturation vapor pressure, kPa, and
air temperature,
◦
C.
T
a
=
Table 6.4 Values of Saturated Vapor Pressure and Slope of Vapor Pressure versus Temperature
Tetens (1930)
Lowe (1977)
Saturation Vapor
Slope Vapor
Saturation Vapor
Slope Vapor
Temperature
◦
C
Pressure
, kPa/
◦
C
Pressure
, kPa/
◦
C
Pressure, kPa
Pressure, kPa
0
0.6108
0.04445
0.6184
0.04114
4
0.8133
0.05724
0.8117
0.05586
8
1.0728
0.07306
1.0683
0.07289
12
1.4026
0.09248
1.3989
0.09297
16
1.8183
0.11614
1.8171
0.11686
20
2.3383
0.14474
2.3400
0.14542
24
2.9839
0.17909
2.9879
0.17956
28
3.7799
0.22008
3.7852
0.22026
32
4.7548
0.26868
4.7602
0.26859
36
5.9410
0.32595
5.9456
0.32565
40
7.3756
0.39307
7.3786
0.39263
44
9.1005
0.47130
9.1015
0.47079
48
11.1628
0.56201
11.1616
0.56145
52
13.6153
0.66666
13.6116
0.66600
56
16.5170
0.78683
16.5100
0.78589
60
19.9331
0.92418
19.9212
0.92265
64
23.9360
1.08050
23.9159
1.07788
68
28.6053
1.25766
28.5711
1.25322
72
34.0282
1.45764
33.9708
1.45042
76
40.3001
1.68251
40.2059
1.67125
80
47.5249
1.93443
47.3748
1.91760
84
55.8152
2.21566
55.5833
2.19138
88
65.2931
2.52855
64.9451
2.49459
92
76.0899
2.87551
75.5820
2.82930
96
88.3470
3.25906
87.6243
3.19764
100
102.2157
3.68177
101.2109
3.60182
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