Environmental Engineering Reference
In-Depth Information
The transpiration process can be applied as a sink term
below a specified evaporation boundary. There are a num-
ber of factors that control the amount of water that can be
transpired by the vegetation, including bare soil evapora-
tion, leaf area index (LAI), plant limiting function (PLF) as
it relates to soil suction, and the root zone profile (Saxton,
1982). These are the key factors that need to be taken into
consideration when simulating transpiration for geotechnical
engineering applications.
The lack of available water for plant growth and high
evaporative demands may cause plants to biologically react
by closing stoma, reducing transpiration, and reducing
metabolic reactions (Saxton, 1982). The plant will reach its
wilting point if subjected to continuous stress. The wilting
point results in the dropping of leaves and tissue death.
Tratch et al. (1995) suggested a four-step methodology
for the prediction of moisture uptake via transpiration from
plants:
1. Determine PE, which is a measure of the maximum
evapotranspiration rate possible under the specified
atmospheric conditions.
2. Determine the potential transpiration (PT) rate based
on PE and the characteristics of the plant population
at the site. This is a measure of the maximum tran-
spiration rate possible under the specified atmospheric
conditions.
3. Distribute the PT, which is a surface flux, into a poten-
tial root uptake profile within the active root zone.
4. Modify the potential root uptake based upon the avail-
ability of moisture within the actual root uptake profile.
5.0
4.0
3.0
2.0
1.0
0.0
0 0 0 0 0 0 0 0 0 0 0
Time, days
Figure 6.50
LAIasfunctionofdaysofyear.
Ritchie (1972) observed that the transpiration component
of evaporation was dependent upon the LAI. The follow-
ing controls were suggested for the calculation of potential
transpiration PT:
PT
=
0 when LAI
<
0
.
1
(6.76)
PE
−
0
.
70 LAI
0
.
5
PT
=
0
.
21
+
0
.
1
<
=
LAI
<
2
.
7
(6.77)
PT
=
PE when 2
.
7
<
=
LAI
(6.78)
where:
PT
=
potential transpiration rate per unit time, mm/day,
LAI
=
leaf area index (unitless), and
PE
=
potential evapotranspiration rate per unit time,
mm/day.
The PE can be applied as a boundary flux that is depen-
dent on the atmospheric conditions. Potential evaporation
is the cumulative sum of bare soil evaporation and plant
transpiration (Granger, 1989).
Determination of the PT constitutes the second step in
evaluating the transpiration flux. Potential transpiration is
the PE modified by the LAI.
Tratch et al. (1995) suggested the use of one of three
plant quality covers: excellent cover, good cover, and poor
cover. Figure 6.51 shows these curves for a one-year period.
The mass flux due to transpiration can first be defined as a
ground surface flux. Then the ground surface moisture flux
is distributed through the soil profile. Two shape functions
were suggested by Tratch et al. (1995) for describing root
distribution: a triangular shape and a rectangular shape.
6.3.21.1 Leaf Area Index
The LAI represents the effect of the vegetation cover on the
energy available to extract water from the ground surface. A
plant cover with a large LAI has a larger potential to extract
water. There is an adjustment in the evaporation rate as the
plants absorb more energy.
The LAI
,
by definition, is the surface area of the leaves
divided by the surface area covered by the soil. The LAI
can be specified as a function of time (e.g., days of each
year) (Fig. 6.50). There is a growing season and there
is an inactive season when no growth takes place. For
example, the growing season may be assumed to start on
April 1 and end on August 15, in which case the length
of the growing season is 223 days. The LAI will control
the role that vegetation plays on overall evaporation from
the ground surface.
4.0
3.5
3.0
Excellent
2.5
2.0
Good
1.5
1.0
Poor
0.5
0
0
50
100
150
200
250
300
350
Time, days
Figure 6.51
Quality of plant growth in terms of LAI.
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