Environmental Engineering Reference
In-Depth Information
and Anderson (1943) provided the total suction in the soil
is known:
because the salt concentration increases. The relative humid-
ity at the soil surface approaches 100% when total suction
in the soil is less than about 3000 kPa (see Fig. 6.43). Under
these conditions the AE is approximately equal to the PE
(i.e., AE
exp
u
soil
v
u
soil
v
0
−
ψgω
v
γ
w
R
273
.
15
T
soil
h
s
=
=
(6.68)
+
PE).
As total suction in the soil increases, the rate of evapora-
tion from the soil surface decreases. Consequently, the ratio
of AE to PE (i.e., AE/PE), bears a similar shape to that of
relative humidity (see Fig. 6.44). It is noteworthy that the
line describing the ratio of AE/PE falls slightly below the
relative humidity line. The equation for the relative humid-
ity line can be used to describe the ratio AE/PE; however,
it is necessary to apply an adjustment factor to the suction
values, as shown in the following equation:
=
where:
ψ
=
total suction, kPa (i.e., total suction is equal
to matric suction plus osmotic suction,
ψ
=
u
a
−
u
w
+
π
),
ω
v
=
molecular weight of water vapor, 0.018016
kg/mol,
soil surface temperature,
◦
C, and
T
soil
=
π
=
osmotic suction, kPa.
Osmotic suction in a soil is related to the salt content in
the soil. For typical field water content conditions, osmotic
suction may range from less than 100 to 1000 kPa or more.
The osmotic component of suction increases as a soil dries
exp
10
δ
ψgω
v
−
γ
w
R
273
.
15
T
soil
h
s
=
(6.69)
+
100
3000 kPa
80
60
40
20
0
10
6
1
10
100
1000
10,000
100,000
Total suction, kPa
Figure 6.43
Relationship between relative humidity and total suction.
1.2
Thermodynamic equilibrium
1.0
0.8
AE / PE Wilson (1990) data
Wilson fit (1990)
Modified residual suction
0.6
0.4
0.2
0.0
10
6
1
10
100
1000
10,000
100,000
Total suction, kPa
Figure 6.44
Comparison of predicted values of AE/PE using different suggested equations.
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