Environmental Engineering Reference
In-Depth Information
R
h
=
sensible heat transferring from ground surface to
air, J
/
m
2
/
day, but can be converted to mm/day
(i.e.,
Q
h
=
Air-entry value of
cracks and fissures
generated by weathering
1000
R
h
/L
v
,
where
Q
h
=
sensible heat,
Air-entry value for
intact cover soil
mm/day),
R
l
=
latent heat associated with the water phase change
including evaporation or freezing, J
/
m
2
/
day, but
can be converted to mm/day (i.e.,
Q
l
=
Bimodal soil-water characteristic
curve for weathered cover soil
1000
R
l
/
Desorption curve
L
v
,
where
Q
l
=
latent heat, mm/day), and
ground heat flux, J
/
m
2
/
day, but can be converted
to mm/day (i.e.,
Q
g
=
R
g
=
1000
R
g
/L
v
, where
Q
g
=
100,000 10
6
1
10
100 1000 10,000
Soil suction (kPa)
ground heat flux, mm/day).
Net radiation
Q
n
can either be obtained from weather
station records or it can be approximated using equations
such as those suggested by Penman (1948) and others (e.g.,
Irmak et al., 2003). The latent heat component,
Q
l
, can be
estimated using AE or the formation of ice near the ground
surface during freezing. The sensible heat component,
Q
h
,
reflected from the ground surface to the air was described by
Penman (1948) and has been incorporated into geotechnical
engineering practice (Wilson, 1990):
C
f
ηf (u)
T
soil
−
T
a
Q
h
=
(6.9)
10,000 100,000 10
6
1
10
100 1000
Soil suction (kPa)
where:
Figure 6.12
Effect of cracking that may occur as a result of
weathering of near-surface soils.
Q
h
=
sensible heat, mm/day,
C
f
=
conversion factor (i.e., 1 kPa
=
0.0075 mHg),
psychometric constant, 0.06733 kPa/
◦
Cat20
◦
C,
η
=
temperature of the soil surface,
◦
C
,
T
soil
=
6.3.4 Atmospheric Flux Conditions
An atmospheric moisture flux balance and a thermal flux
balance exist at the ground surface. Both moisture and tem-
perature flux conditions are involved in the calculation of
AE. Moisture can fall to the ground surface in the form of
rain or snow and is referred to as precipitation. The pre-
cipitation either infiltrates the soil or runs off. Precipitation
information can be obtained from weather station records
and is usually provided on a daily basis. Moisture flow
in saturated and unsaturated soils can be described using
Darcy's law.
Moisture also rises to the sky through the process of
evaporation. The evaporative flux of primary interest for
unsaturated soil modeling is AE. The ground surface mois-
ture flux balance equation is shown in Eq. 6.7.
The thermal flux balance equation can be written as
temperature of the air above the soil,
◦
C
,
T
a
=
f(u)
=
function depending on wind speed, mm/day/kPa,
f(u)
=
0
.
35
(
1
+
0
.
146
W
w
)
, and
W
w
=
wind speed, km/h.
The AE is difficult to measure directly but can be cal-
culated using thermodynamic considerations. Equations 6.7
and 6.8 are fundamental to describing the coupling of mois-
ture and heat flow processes. The AE depends on the water
content and temperature of the soil at ground surface. The
rate of evaporation of water also depends on the air temper-
ature. The air temperature above the ground surface and the
soil temperature at the ground surface are generally not the
same but are interrelated by the net radiation
Q
n
, latent heat
Q
l
, and sensible heat
Q
h
. The availability of surface water
is controlled by total precipitation, actual evaporation, and
runoff. These variables play an important role in partition-
ing convective heat flux into sensible heat and latent heat
(Wetzel and Boone, 1995).
R
n
=
R
h
+
R
l
+
R
g
(6.8)
where:
6.3.4.1 Some Procedures for Combining and Coupling
Heat and Moisture Boundary Conditions
The coupling process between moisture flow and heat flow
is quite complex. There are several procedures that can be
used to satisfy the conditions of coupling between heat and
net radiation, J
/
m
2
/
day
,
but can also be converted
to units of mm/day (i.e.,
Q
n
=
R
n
=
1000
R
n
/L
v
, where
Q
n
volumetric
latent heart of vaporization,
J/m
3
, and 1000 is the
conversion between mm and m),
=
net radiation, mm/day;
L
v
=
Search WWH ::
Custom Search