Environmental Engineering Reference
In-Depth Information
The seminal reaction of photosynthesis described by
Ingenhousz requires the simultaneous presence of chloro-
phyll, CO
2
, and water. If these reactants are simply placed
together, however, they do not spontaneously interact,
but require a plant and the input of electromagnetic
radiation from the sun. Although often ignored, it is hum-
bling to remember that the sun's energy is transmitted
through space as waves or electromagnetic energy a dis-
tance of at least 93 million miles (148 million kilometers), a
trip of roughly 8 min. It is this electromagnetic energy that
causes chlorophyll in chloroplasts to enter an excited state
and start the whole process of chemical synthesis of plant
food.
Radiation is only one form of energy transmission and is
defined as the emission of electromagnetic energy trans-
ferred as photon packets or waves. Other forms of energy
transmission include convection and conduction. Convec-
tion is the transfer of energy by a mass that carries heat,
typically a fluid such as air or water. Finally, conduction is
the transfer of heat by the movement of molecules.
The effect of the sun on all life forms on earth generally is
overlooked. Like water, this energy is something that we
cannot do without. The sun is a location of a thermonuclear
reaction, where the fusion of hydrogen (H) to helium (He)
occurs to release some of the mass of H as radiant energy.
About 50% of the radiant energy is released in the form of
the visible spectrum of wavelengths. Total input of solar
energy to the earth is about 13
from the earth's surface and drives the redistribution of
water around the globe in the form of local precipitation.
The input and flow of energy can be stated mathemati-
cally using a mass-balance equation similar to that shown in
Chap. 2 for the water budget. Hillel (1998) presents a form of
the heat-balance equation that is listed here
J
n
¼
ð
J
s
þ
J
a
Þ
ð
1
a
Þþ
J
li
J
lo
(11.1)
Where
J
n
is the net solar radiation, where 'net' is the balance
that remains after the outgoing flux is subtracted from that
incoming;
J
s
is the energy in the form of short-waves coming
from the sun, or advective flow;
J
a
is the short-wave radia-
tion coming from the sky, or diffusive flow;
J
li
is the incom-
ing, long-wave energy from the sun; and
J
lo
is the outgoing,
long-wave energy emitted back to the sky by the soil, which
is especially obvious during the night. On a clear day, little
radiation is reflected back to space, but clouds increase
reflection to the extent that the amount of radiation that
reaches the soil is considerably decreased. The term
is a
coefficient that describes this reflectivity, or albedo, of the
earth's surface.
Much like incoming radiation is felt as your body is
warmed, this energy also is absorbed by the soil, transforms
to heat, and thus, warms the soil surface. As such, the above
equation can be rewritten as
a
10
23
cal/year (Kormondy
1976). And half of this is lost by reflection or absorption in the
atmosphere. The energy that makes it to the earth does so at
about 2 cal per square centimeter per minute (2 cal/cm
2
/min),
known as the solar constant, where 1 cal by definition is the
amount of heat required to raise 1 g of water 1
C. Interest-
ingly, the earth intercepts only about 1/12 billionths of the
sun's total emitted radiant energy.
As can be concluded from the units used, this represents a
flow of energy input to the earth. Most of this radiation
consists of wavelengths between 0.3 and 3
J
n
¼
S
þ
A
þ
LE
(11.2)
Where
S
is the soil heat flux;
A
is the heat flux transmitted to
the air;
LE
is the evaporative heat flux; and
E
is the rate times
the amount of water evaporated. The transfer of heat to
evaporate water is one of the greatest losses of heat. This
helps explain the importance of the hydrologic cycle and
why in most areas water losses by
ET
can account for up to
70% of an area's water budget.
One aspect of heat transfer to evaporate water by plant
transpiration is that plants use this evaporating water to
reduce heat gain. Plants cannot regulate their temperatures
like mammals that can change their basal metabolism.
Therefore, some mechanism must help control the tempera-
ture of a plant, because the amount of energy input to, say, a
leaf, could potentially raise its surface temperature to nearly
11,000
F! This does not occur for several reasons. First,
about 30% of the energy passes through the leaf or is
reflected from the leaf surface. Second, the air spaces in
leaves absorb some of the heat. Third, more than 50% of
the energy is used to evaporate water from the leaf meso-
phyll tissues.
With every gram of water evaporated from a leaf, 580 cal
of energy are removed with the evaporating water molecule.
The balance of heat gains and losses can present a dilemma
for a plant, because slightly increased temperatures result in
m. Based on
changes in cloud cover or the elevation of land above sea
level, the fraction of the solar constant that reaches the
surface to become available for plants is decreased, to
between 1.2 and 1.4 cal/cm
2
/min in temperate areas and to
1.6 cal/cm
2
/min in desert areas where cloud cover is scarce.
Values higher than the solar constant sometimes are
measured, however, in mountains where direct sunlight and
reflected light can reach a plant at the same time.
This solar energy is used by plants to drive photosynthe-
sis and store this kinetic energy as chemical bonds in sugars
that later can be released to drive the process of plant
life. This process is referred to as “primary production” by
ecologists interested in the transfer of this energy through
various ecological trophic levels. As we saw in Chap. 2, this
input of radiant energy also drives the evaporation of water
m
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