Environmental Engineering Reference
In-Depth Information
available to plants if the soil is extremely sandy and porous
and readily drains away. This is the case in the upper Coastal
Plain of many states, such as South Carolina, which receives
40-60 in./year (101-152 cm/year) of precipitation but
supports only slash pine, stunted turkey oaks, and cacti—
this area is essentially a desert in the rain. A similar condi-
tion also can be found in the Pine Barrens of New Jersey. In
both areas, only longleaf pine ( Pinus palustris ) can thrive as
they have a deep tap root.
Conversely, abundant precipitation does not always lead
to lush growth if air temperatures are low, as is exhibited by
conditions on the Aleutian Island chain of Alaska. There, the
vegetation is dominated by grasses; no native trees can be
found, although the Adak National Forest, which is part of
the Alaska Maritime National Wildlife Refuge, consists of
33 spruce trees. Actually, these trees were planted in 1944 to
boost the morale of the military personnel stationed in Adak,
which had no trees at that time. This forest is so small, that
the sign at the entrance states, “You are now entering and
leaving the Adak National Forest.”
Fig. 6.3 Average annual daily solar radiation per month, in kilowatt
hours per square meter for the 30-year period 1961-1990, for the
conterminous United States (Modified from Marion and Wilcox 1994).
much as 6-7 h/day of solar radiation in June compared to
4-5 h/day in December. In general, the minimum value of
solar radiation can be determined for an area that is a candi-
date for phytoremediation by measurements taken on the
shortest day of the year, December 21. The values for
solar-energy input are important to know for a particular
contaminated site, because all phytoremediation processes
are based on the establishment of plant photosynthesis,
growth, and transpiration, all of which are dependent on
solar radiation for energy or water transport.
6.4.4.2 Solar-Radiation Maps
All green plants require electromagnetic energy from the sun
for photosynthesis as was outlined in Chap. 3. Less light
over short periods results in a lower and slower rate of
photosynthesis and, therefore, growth and reproduction
potential. More light over long periods results in more food
production, growth, and reproduction. It follows, then, that
plant distribution is related to light conditions in a similar
manner as to precipitation and air temperature. Hence, any
phytoremediation effort is related to the light conditions of
an area.
Insolation maps provide data about the length of time a
particular location has solar radiation. In the United States,
the average daily solar radiation per month, in kilowatt hours
per square meter per day—essentially the average hours of
sunlight energy input per day—ranges from 4 to 5 h/day
(hours per day) for most of the northeastern and central
plains states, 3-4 h/day on the coast of Oregon and Washington,
and 5-6 h/day in the southeastern United States, including
Florida (Fig. 6.3 ). Areas of California and some states along
the front range of the Rocky Mountains down to Texas have
between 6 and 7 h/day.
The solar data presented on insolation maps are spatial
interpolations of solar radiation measurements taken
between 1961 and 1990 and stored in the National Solar
Radiation Data Base (NSRDB). The average values
described above for solar radiation duration are averages of
the 30 years of data from up to 239 sites that comprise the
NSRDB (Fig. 6.3 ). Because the value given is the average
value for each site, the number of hours of sunlight typically
is lower during the winter months and higher during the
summer months. For example, Columbia, SC, can have as
6.4.4.3 Growing Season Length and Potential
Evapotranspiration
Solar radiation and the fluctuation of air temperature are the
primary environmental factors that control the annual cycle
of growth and dormancy in plants used for phytore-
mediation. The annual cycle of growth determined by favor-
able solar radiation and air temperatures is called the
growing season. In the northern United States, in areas not
affected directly by the Great Lakes, the growing season
can approach about 140 days. Nearer the Great Lakes, the
growing season increases to almost 200 days, because of the
high heat capacity of water and its ability to moderate
air-temperature fluctuations. Even when these areas receive
snowfall, the snow acts as a good insulator to keep the soil
temperatures in the root zone warm enough to encourage
root growth. In the southern United States, the growing
season can range from about 190 days to as much as the
entire year, as the ground rarely is frozen.
The length of the growing season for a particular area can
be highly variable, however, based on the topographic ele-
vation and frequency of precipitation. For example, Atlanta,
Georgia, and Lubbock, Texas, have similar average maxi-
mum and minimum air temperatures, about 90 F (32 C) and
28 F(
2 C), respectively, but different precipitation
amounts. Atlanta is at a higher elevation and has 29 in./year
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