Environmental Engineering Reference
In-Depth Information
model (Wilson et al., 1994). The uncoupled limiting
function and experimental-based models assume that the
soil surface temperature is equal to the air temperature.
A large amount of heat (i.e., latent heat of vaporization,
which is about 2 . 5
movement of water within plants and the subsequent loss
of water as vapor through stomata in its leaves (Fig. 6.49).
Transpiration is part of the overall water cycle and therefore
part of the ground surface water balance boundary condition.
Trees, shrubs, and grass contribute to transpiration and are
called transpirators.
Plants can remove significant amounts of water through
transpiration. Computer simulation of a vegetated ground
surface is usually handled in two steps: first, the assumption
is made that there is no vegetation on the ground surface and,
second, further simulations are performed while considering
the presence of vegetation.
Numerous assumptions must be made when considering
the simulation of vegetation on evaporative flux. Plants can
be visualized as small pumps (i.e., sinks) that remove water
from a zone of soil below ground surface (Tratch et al.,
1995) (Fig. 6.49). The effect of vegetation has proven to
be quite complex. Transpiration is a function of the soil
zone from which water is extracted as well as the leaf area
characteristics of the plants. The growing season for the
vegetation must be assumed and there must be sufficient
nutrients available in the soil to sustain plant growth. The
long-term nutrient sustainability for plant growth can be a
significant factor when evaluating transpiration.
The simulation of plant-covered ground surfaces requires
a number of assumptions related to the vegetation. The
simulation of transpiration has been a topic of consider-
able interest in agriculture-related disciplines. Much of the
agricultural research has direct interest and application to
problems encountered in geotechnical engineering. Tratch
et al. (1995) gave consideration to the research in agricul-
ture and proposed protocols for geotechnical engineering
practice.
10 9 J/m 3 ), is absorbed during evapo-
ration from the soil surface. The result is a significant drop
in temperature at the soil surface in the first few days. The
temperature at the soil surface subsequently increases and
becomes close to the air temperature when evaporation is
reduced.
It has been demonstrated that there are a number of dif-
ferent possible ways in which AE from a soil surface can be
calculated. A total of six calculation procedures have been
described. The solution procedures were first separated into
coupled and uncoupled solutions. The uncoupled procedures
require considerably less computational effort. All six cal-
culation procedures appear to give quite similar results. It is
difficult to say whether uncoupled solutions are satisfactory
in all situations for computing AE from a soil surface. There
is need for further comparative studies to be undertaken on
the computational procedures.
×
6.3.21 Transpiration Flux
Transpiration T is the term used to describe evaporation
from vegetated surfaces. The term transpiration is usually
separated from the terms actual evaporation and potential
evaporation in geotechnical engineering. Actual evapora-
tion plus transpiration from plants gives rise to the term
evapotranspiration , as illustrated in Fig. 6.48. There are
advantages in separating the components of evaporation in
this manner because of the independent calculations required
for each component.
Evaporation accounts for the movement of water from
the surface of the earth to the air from sources such as the
soil and vegetated surfaces. Transpiration accounts for the
(50% RH at 22 °C)
Atmosphere = 1000,000 kPa
Evapotranspiration =
Transpiration + evaporation
Leaf = 1500 kPa
Transpiration
Trees
Grass
Evaporation
Stem = 500 kPa
Root = 200 kPa
Groundwater
recharge
Soil = 20 kPa
Figure 6.48 Illustration of evaporative components contributing
to evapotranspiration.
Figure 6.49 Movement of moisture through vegetation.
 
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