Environmental Engineering Reference
In-Depth Information
Table 6.6 Comparison of Calculated PE over a 14-Year Period at Site in Northern Canada
Penman (1948)
Thornthwaite (1948)
Year
Cumulative PE (mm) Percent Variation Cumulative PE (mm) Percent Variation
1994
707.5
8.54
597.4
3.60
1995
635.0
2.59
556.4
3.51
1996
644.9
1.06
545.9
5.32
1997
666.6
2.27
565.2
1.98
1998
739.6
13.46
638.8
10.79
1999
624.2
4.25
591.2
2.54
2000
677.0
3.86
551.8
4.30
2001
709.1
8.78
593.1
2.87
2002
666.3
2.21
522.2
9.44
2003
688.3
5.59
583.6
1.22
2004
583.0
10.57
536.3
6.98
2005
591.5
9.26
582.6
1.04
2006
602.7
7.53
622.3
7.93
2007
590.2
9.45
585.5
1.55
Average
651.9
576.6
Standard deviation
49.7
32.5
using the Penman equation was about 12% higher than the
results obtained using the Thornthwaite (1948) equation.
There was only one year in the 14-year record when the
Thornthwaite equation gave PE values that were higher
than those calculated by the Penman (1948) equation.
Table 6.6 also shows the standard deviation of the cal-
culated cumulative PE. The standard deviation was 50mm
for the Penman method and 33mm for the Thornthwaite
method. It would appear that the Penman method better
captures the effect of the individual components of weather
and would be the preferred calculation procedure for use in
engineering practice.
The engineer requires the AE to quantify water balance
at ground surface (i.e., in addition to potential evaporation).
Actual evaporation can be calculated by modifying the Pen-
man (1948) equation and taking the underlying soil condi-
tions into consideration. Wilson (1990) made modifications
to the Penman equation and developed the Wilson-Penman
method (Wilson, 1990) for calculating actual evaporation.
Two other procedures were also proposed for the calcula-
tion of actual evaporation: the limiting function procedure
(Wilson et al., 1997a) and the experimental-based procedure
(Wilson et al., 1997a).
The concepts associated with AE can be described as fol-
lows. The net radiation effect of the sun and the mixing
effect of wind form the basis for PE from a wet soil surface.
At the same time as the sun and the wind are removing water
from the soil, the soil begins to dry and hold onto the water
more strongly. In other words, the soil suction increases as
the soil dries. The net result is a struggle between the climate
and the soil and the consequence is a reduced evaporation
from the ground surface (i.e., AE). If the vapor pressure in
the air above ground surface and the vapor pressure in the
soil at ground surface are the same, then evaporation from
the ground surface will cease due to lack of a vapor pressure
gradient. It is the “total suction” in the soil at the ground
surface (i.e., total suction
osmotic suc-
tion) that creates the equilibrium vapor pressure (or relative
humidity) in the soil at the ground surface.
The AE from a soil surface might be considerably less
than the PE. The AE equations are primarily the outcome
of research by Wilson (1990), who used evaporation from
thin soil layers and sand column drying tests to verify the
fundamental physical relationships required to extend the
Penman (1948) equation to the calculation of the AE from
drying soil surfaces.
Several methodologies have been proposed for calculating
AE. Two of the proposed methods have a thermodynamic
basis while one has an experimental basis. The equations for
AE along with different temperatures above the soil surface
and at the soil surface give rise to at least six possible method-
ologies for the calculation of AE from the soil surface.
=
matric suction
+
6.3.19 Actual Evaporative Flux
The AE from a soil surface can be considerably less than
PE. The quantification of AE at a particular site allows the
geotechnical engineer to better establish water balance con-
ditions at the ground surface.
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