Environmental Engineering Reference
In-Depth Information
point, a situation in which soil moisture is present in the pore
spaces of subsurface sediment but is too strongly bound to
these sediments to be bioavailable for plant uptake even
though roots hairs may be in contact with the water. More
on this subject will be discussed in Chap. 3.
The meteorological and soil-water factors influence tran-
spiration, but so do plant physiological factors. A plant is not
simply a passive straw through which water moves from the
soil to the air. Plant-based factors include the total leaf area,
size, location, and shape. Finally, as would be expected for
plant and water interactions, biotic factors also play a role in
controlling transpiration, such as the growth stage of a plant,
its color and health, etc.
The upper limit of transpiration is determined by the total
amount of solar energy available to evaporate water. In most
cases, transpiration cannot exceed pan evaporation rates. For
example, even though the surface area available for evapo-
ration from the leaves of a tree can exceed that of unplanted
bare ground by a factor of 6, the evaporation rate cannot
exceed that of the total amount of energy available, regard-
less of whether the area is planted or not. However, if water
is not limiting and plant transpiration is not regulated, or if
winds move dry air continually past the leaf, plant transpira-
tion may exceed pan evaporation.
were investigated by Patric (1961). In Patric's study, 26
weighing lysimeters were constructed by placing soil in
concrete tanks that measured about 10 ft by 21 ft by 6 ft
deep (3.0 m by 6.4 m by 1.8 m). Initially, grass was grown in
the tanks; later, the grass in some of the tanks was replaced
with Coulter pine. Relative to the tanks that still contained
grass, all soil moisture was removed from the tanks that
contained the pines, and these tanks had higher ET rates as
well.
Other methods that can be used to measure transpiration
include those based on measuring the loss of water vapor or
water weight (Lee 1942). To measure water vapor loss, an
apparatus called the Freeman method can be used in which a
cylinder is placed around part of a plant and a chemical is
used to absorb the water vapor lost by transpiration. Another
method is to place a chemical indicator strip, typically made
of cobalt chloride, directly on a plant leaf to determine the
amount of water lost based on the degree of color change in
the chemical. To measure water-weight loss, plant-tissue
samples either are removed and the loss of weight of the
sample over time indicates the amount of water lost by
transpiration or placed in a chamber called a potometer.
The advantage of these methods is gained only if the period
of time of the experiment is shorter than a few days.
Probably the most widely accepted methods to measure
transpiration in plants is based on artificially applied
radioisotopes or pulses of heat, both of which are used as
tracers of water flow in individual trees. For example, triti-
ated water ( 1 H 3 HO) was added to the soil around a Douglas
fir tree ( Pseudotsugamenziesii spp. ) in the western part of the
United States (Kline et al. 1976). This technique, as well as
the heat-pulse method and others, are discussed in Chaps. 3
and 9.
2.3.6 Measurement of Transpiration
The first known attempt to quantify transpiration was
performed by Stephen Hales in 1727 (Kramer and Boyer
1995). He related the amount of water transpired by individ-
ual potted plants to the weight lost from the pot; the greater
the weight loss the higher the transpiration rate. His
approach of measuring individual plant transpiration is still
used today at many plant laboratories around the world,
including those that are interested in the effect of various
concentrations of groundwater contaminants on transpira-
tion of plants used in phytoremediation. Hales also made
field measurements and calculated that almost 4 tons of
water per acre per day, or nearly 1,000 gal (3,780 L), was
transpired from a field of cabbages.
The measurement of the use of water by larger stands of
plants also is of interest, especially considering the need to
group plants that have high transpiration rates close together
for many phytoremediation applications. Similar to measur-
ing the weight loss of a plant over time in the laboratory, a
weighing lysimeter is used in the field to measure the water
lost on a much larger scale. A weighing lysimeter is essen-
tially a large pot placed in the earth, filled with soil, plant(s),
and watered, and the changes in water content related to
transpiration are quantified by weighing (Fritschen et al.
1973). The application of weighing-lysimeter tests and the
transferability of results to large trees in natural conditions
2.3.7 Evapotranspiration
Because of the difficulties in measuring evaporation and the
lack of knowledge about measuring transpiration, lumping
them together as one term does not indicate that the com-
bined rate is more accurate. In fact, it reveals the opposite to
be true. This is because, in many instances, ET is not derived
from careful measurement of evaporation or transpiration
but represents the balance of flow that remains after other
components in Eq. 2.5 are measured or estimated.
Evapotranspiration had been defined by C.W. Thornth-
waite in order to estimate evaporative demands in large
regions. The focus on larger areas tended to reduce the
effects of large variations in climate, plant distribution, or
changes in local topography. If water quantity is unlimited in
a particular area, potential evapotranspiration, ET p , can be
determined. Potential evapotranspiration is essentially the
maximum power of evaporation, or demand, of
the
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