Environmental Encyclopedia

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Environmental Encyclopedia 3

Hydrocarbons

occasionally. To meet this need, Honda introduced a hybrid-

engine version of its popular Civic line in 2002. With four

doors and ample space for four adults plus a reasonable

amount of luggage. The 5-speed manual version of the Civic

hybrid gets 48 mpg in both city and highway driving. With

a history of durability and consumer satisfaction in other

Honda models, and a 10-year warranty on its battery and

drive train, the hybrid Civic appears to offer the security

that consumers will want in adopting this new technology.

Toyota also has introduced a hybrid vehicle called the

Prius. Similar in size to the Honda Civic, the Prius comes

in a four-door model with enough room for the average

American family. During most city driving, it depends only

on its quiet, emission-free, electric motor. The batteries

needed to drive the 40-hp are stacked up behind the back

seat providing a surprisingly large trunk for luggage. The

70-hp, 1.5 liter gas engine kicks in to help accelerate or

when the batteries need recharging. Getting about 52 mpg

(22 km/l) in city driving, the Prius is one of the most efficient

cars on the road and can travel more than 625 mi (1,000

km) without refueling. Some drivers are unnerved by the

noiseless electric motor. Sitting at a stoplight, it makes no

sound at all. You might think it was dead, but when the

light changes, you glide off silently and smoothly.

Introduced in Japan in 1997, the Prius sells in the

United States for about the same price as the Honda hybrids.

The

Sierra Club

estimates that in 100,000 mi (160,000 km),

a Prius will generate 27 tons of CO

, a Ford Taurus will

generate 64 tons, while the Ford Excursion SUV will pro-

duce 134 tons. In 1999, the Sierra Club awarded both the

Insight and the Prius an “excellence in engineering” award,

the first time this organization has ever endorsed commercial

products.

Both Ford and General Motors (GM) have announced

intentions to build hybrid engines for their popular sport

utility vehicles and light trucks. This program may be more

for public relations, however, than to save fuel or reduce

pollution

. The electrical generators coupled to engines of

these vehicles will produce only 12 volts of power. This is

far less than the 42 volts needed to provide drive the wheels.

Instead, the electricity generated by the gasoline-burning

engine will only be used to power accessories such as video

recorders, computers, on-board refrigerators, and the like.

Having this electrical power available will probably actually

increase fuel consumption rather than reduce it. For uncriti-

cal consumers, however, it provides a justification for contin-

uing to drive huge, inefficient vehicles.

In 2002, President G. W. Bush announced he was

abandoning the $1.5 billion government-subsidized project

to develop high-mileage gasoline-fueled vehicles started with

great fanfare eight years earlier by the Clinton/Gore admin-

istration. Instead, Bush was throwing his support behind a

737

plan to develop hydrogen-based

fuel cells

to power the

automobiles of the future. Fuel cells use a semi-permeable

film or electrolyte that allows the passage of charged atoms,

called ions, but is impermeable to electrons to generate an

electrical current between an anode and cathode.

A fuel cell run on pure oxygen and

hydrogen

produces

no waste products except drinkable water and radiant heat.

Fossil fuels

can be used as the source for the hydrogen,

but some pollutants are released (most commonly

carbon

dioxide

) in the process of hydrogen generation. Currently,

the fuel cells available need to be quite large to provide

enough energy for a vehicle. Fuel cell-powered buses and

vans that have space for a large power system are currently

being tested, but a practical, family vehicle appears to be

years away.

While they agree that fuel cells offer a wonderful op-

tion for cars of the future, many environmentalists regard

putting all our efforts into this one project to be misguided

at best. It probably will be at least a decade before a fuel-

cell vehicle is commercially available.

[William P. Cunningham Ph.D.]

ESOURCES

OOKS

Hodkinson, Ron, and John Fenton. Lightweight Electric/Hybrid Vehicle

Design. Warrendale, PA: Society of Automotive Engineers, 2001.

Jurgen., Ronald K., ed. Electric and Hybrid-electric Vehicles. Warrendale,

PA: Society of Automotive Engineers, 2002.

Koppel, Tom. Powering the Future: The Ballard Fuel Cell and the Race to

Change the World. New York: John Wiley & Sons, 1999.

ERIODICALS

“Dark Days For Detroit—The Big Three’s Gravy Train in Recent Years—

Fat Profits from Trucks—is Being Derailed by a New Breed of Hybrid

Vehicles from Europe and Japan.” Business Week, January 28, 2002, 61.

Ehsani, M., K. M. Rahman, and H. A. Toliyat. “Propulsion System Design

of Electric and Hybrid Vehicles.” IEEE Transactions on Industrial Electronics

44 (1997): 19.

Hermance, David, and Shoichi Sasaki. “Special Report on Electric Vehi-

cles—Hybrid Electric Vehicles take to the Streets.” IEEE Spectrum 35

(1998): 48.

Jones, M. “Hybrid Vehicles—The Best of Both Worlds?” Chemistry and

Industry 15 (1995): 589.

Maggetto, G. and J. Van Mierlo. “Fuel cells: Systems and applications—

Electric vehicles, hybrid vehicles and fuel cell electric vehicles: State of the

art and perspectives.” Annales de chimie—science des mate

riaux. 26 (2001): 9.

Hydrocarbons

Any compound composed of elemental

carbon

and

hydro-

gen

, hydrocarbons may also contain

chlorine

, oxygen,

nitro-

gen

, and other atoms. Hydrocarbons are classified according

to the arrangement of carbon atoms and the types of chemical

bonds. The major classes include aromatic or carbon ring

Environmental Encyclopedia 3

Hydrochlorofluorocarbons

compounds, alkanes (also called aliphatic or paraffin) com-

pounds with straight or branched chains and single bonds,

and alkenes and alkynes with double and triple bonds, respec-

tively. Most hydrocarbon fuels are a mixture of many com-

pounds.

Gasoline

, for example, includes several hundred

hydrocarbon compounds, including paraffins, olefins, and

aromatic compounds, and consequently exhibits a host of

possible environmental effects. All of the

fossil fuels

, includ-

ing crude oils and

petroleum

, as well as many other com-

pounds important to industries, are hydrocarbons. Hydro-

carbons are environmentally important for several reasons.

First, hydrocarbons give off

greenhouse gases

, especially

carbon dioxide

, when burned and are important contrib-

uters to

smog

. In addition, many aromatic hydrocarbons and

hydrocarbons containing halogens are toxic or carcinogenic.

Hydrochlorofluorocarbons

The term hydrochlorofluorocarbon (HCFC) refers to halo-

genated

hydrocarbons

that contain

chlorine

and/or fluo-

rine in place of some

hydrogen

atoms in the molecule.

They are chemical cousins of the

chlorofluorocarbons

(CFCs), but differ from them in that they have less chlorine.

A special subgroup of the HCFCs is the hydrofluorocarbons

(HFCs), which contain no chlorine at all.

A total of 53 HCFCs and HFCs are possible.

The HCFCs and HFCs have become commercially

and environmentally important since the 1980s. Their grow-

ing significance has resulted from increasing concerns about

the damage being done to stratospheric

ozone

by CFCs.

Significant production of the CFCs began in the late

1930s. At first, they were used almost exclusively as refriger-

ants. Gradually other applications—especially as

propel-

lants

and blowing agents—were developed. By 1970, the

production of CFCs was growing by more than 10% per

year, with a worldwide production of well over 662 million

lb (300 million kg) of one family member alone, CFC-11.

Environmental studies began to show, however, that

CFCs decompose in the upper

atmosphere

. Chlorine

atoms produced in this reaction attack ozone molecules (O

converting them to normal oxygen (O

). Since stratospheric

ozone provides protection for humans against solar

ultravio-

let radiation

, this finding was a source of great concern.

By 1987, 31 nations had signed the Montreal Protocol,

agreeing to cut back significantly on their production of

CFCs.

The question became how nations were to find substi-

tutes for the CFCs. The problem was especially severe in

developing nations where CFCs are widely used in refrigera-

tion and air-conditioning systems. Countries like China and

India refused to take part in the CFC-reduction plan unless

738

CHF

HFC-23

CHCl

HCFC-123

FCClF

HCFC-133b

CHClF HCFC-151a

developed nations helped them switch over to an equally

satisfactory substitute.

Scientists soon learned that HCFCs were a more be-

nign alternative to the CFCs. They discovered that com-

pounds with less chlorine than the amount present in tradi-

tional CFCs were less stable and often decomposed before

they reached the

stratosphere

. By mid 1992, the United

States

Environmental Protection Agency

(EPA) had se-

lected 11

chemicals

that they considered to be possible

replacements for CFCs. Nine of those compounds are HFCs

and two are HCFCs.

The HCFC-HFC solution is not totally satisfactory,

however. Computer models have shown that nearly all of

the proposed substitutes will have at least some slight effect

on the ozone layer and the

greenhouse effect

. In fact,

the British government considered banning one possible

substitute for CFCs, HCFC-22, almost as soon as the com-

pound was developed. In addition, one of the most promising

candidates, HCFC-123, was found to be carcinogenic in

rats.

Finally, the cost of replacing CFCs with HCFCs and

HFCs is expected to be high. One consulting firm, Metroe-

conomica, has estimated that CFC substitutes may be six

to 15 times as expensive as CFCs themselves. See also Aero-

sol; Air pollution; Air pollution control; Air quality; Carcin-

ogen; Ozone layer depletion; Pollution; Pollution control

[David E. Newton]

ESOURCES

ERIODICALS

Johnson, J. “CFC Substitutes Will Still Add to Global Warming.” New

Scientist 126 (April 14, 1990): 20.

MacKenzie, D. “Cheaper Alternatives for CFCs.” New Scientist 126 (June

30, 1990): 39–40.

Pool, R. “Red Flag on CFC Substitute.” Nature 352 (July 11, 1991): 352.

Stone, R. “Ozone Depletion: Warm Reception for Substitute Coolant.”

Science 256 (April 3, 1992): 22.

Hydrogen

The lightest of all chemical elements, hydrogen has a density

about one-fourteenth that of air. It has a number of special

chemical and physical properties. For example, hydrogen

Environmental Encyclopedia 3

Hydrologic cycle

has the second lowest boiling and freezing points of all

elements. The

combustion

of hydrogen produces large

quantities of heat, with water as the only waste product.

From an environmental standpoint, this fact makes hydrogen

a highly desirable fuel. Many scientists foresee the day when

hydrogen will replace

fossil fuels

as our most important

source of energy.

Hydrogeology

Sometimes called

groundwater hydrology

or geohydrol-

ogy, this branch of hydrology is concerned with the relation-

ship of subsurface water and geologic materials. Of primary

interest is the saturated zone of subsurface water, called

groundwater, which occurs in rock formations and in uncon-

solidated materials such as sands and gravels. Groundwater

is studied in terms of its occurrence, amount, flow, and

quality. Historically, much of the work in hydrogeology

centered on finding sources of groundwater to supply water

for drinking,

irrigation

, and municipal uses. More recently,

groundwater contamination by pesticides, chemical fertiliz-

ers, toxic wastes, and

petroleum

and

chemical spills

have

become new areas of concern for hydrogeologists.

Hydrologic cycle

The natural circulation of water on the earth is called the

hydrologic cycle. Water cycles from bodies of water, via

evaporation to the

atmosphere

, and eventually returns to

the oceans as precipitation,

runoff

from streams and rivers,

and

groundwater

flow. Water molecules are transformed

from liquid to vapor and back to liquid within this cycle.

On land, water evaporates from the

soil

or is taken up by

plant roots and eventually transpired into the atmosphere

through plant leaves; the sum of evaporation and

transpira-

tion

is called

evapotranspiration

Water is recycled continuously. The molecules of water

in a glass used to quench your thirst today, at some point

in time may have dissolved minerals deep in the earth as

groundwater flow, fallen as rain in a tropical typhoon, been

transpired by a tropical plant, been temporarily stored in a

mountain glacier, or quenched the thirst of people thousands

of years ago.

The hydrologic cycle has no real beginning or end but

is a circulation of water that is sustained by

solar energy

and influenced by the force of gravity. Because the supply

of water on the earth is fixed, there is no net gain or loss

of water over time. On an average annual basis, global evapo-

ration must equal global precipitation. Likewise, for any

739

body of land or water, changes in storage must equal the

total inflow minus the total outflow of water. This is the

hydrologic or water balance.

At any point in time, water on the earth is either in

active circulation or in storage. Water is stored in icecaps,

soil, groundwater, the oceans, and other bodies of water.

Much of this water is only temporarily stored. The

residence

time

of water storage in the atmosphere is several days and

is only about 0.04% of the total freshwater on the earth. For

rivers and streams, residence time is weeks; for lakes and

reservoirs, several years; for groundwater, hundreds to thou-

sands of years; for oceans, thousands of years; and for icecaps,

tens of thousands of years. As the driving force of the hydro-

logic cycle, solar radiation provides the energy necessary

to evaporate water from the earth’s surface, almost three-

quarters of which is covered by water. Nearly 86% of global

precipitation originates from ocean evaporation. Energy con-

sumed by the conversion of liquid water to vapor cools the

temperature of the evaporating surface. This same energy,

the latent heat of vaporization, is released when water vapor

changes back to liquid. In this way, the hydrologic cycle

globally redistributes heat energy as well as water.

Once in the atmosphere, water moves in response to

weather circulation patterns and is transported often great

distances from where it was evaporated. In this way, the

hydrologic cycle governs the distribution of precipitation

and hence, the availability of fresh water over the earth’s

surface. About 10% of atmospheric water falls as precipita-

tion each day and is simultaneously replaced by evaporation.

This 10% is unevenly distributed over the earth’s surface

and, to a large extent, determines the types of ecosystems

that exist at any location on the earth and likewise governs

much of the human activity that occurs on the land.

The earliest civilizations on the earth settled in close

proximity to fresh water. Subsequently, and for centuries,

humans have been striving to correct, or cope with, this

uneven distribution of water. Historically, we have extracted

stored water or developed new storages in areas of excess,

or during periods of excess precipitation, so that water could

be available where and when it is most needed.

Understanding processes of the hydrologic cycle can

help us develop solutions to water problems. For example,

we know that precipitation occurs unevenly over the earth’s

surface because of many complex factors that trigger precipi-

tation. For precipitation to occur, moisture must be available

and the atmosphere must become cooled to the

dew point

the temperature at which air becomes saturated with water

vapor. This cooling of the atmosphere occurs along storm

fronts or in areas where moist air masses move into mountain

ranges and are pushed up into colder air. However, atmo-

spheric particles must be present for the moisture to con-

Environmental Encyclopedia 3

Hydrologic cycle

dense upon, and water droplets must coalesce until they are

large enough to fall to the earth under the influence of

gravity.

Recognizing the factors that cause precipitation has

resulted in efforts to create conditions favorable for precipita-

tion over land surfaces via cloud seeding. Limited success

has been achieved by seeding clouds with particles, thus

promoting the condensation-coalescence process. Precipita-

tion has not always increased with cloud seeding and ques-

tions of whether cloud seeding limits precipitation in other

downwind areas is of both economic and environmental

concern.

Parts of the world have abundant moisture in the

atmosphere, but it occurs as fog because the mechanisms

needed to transform this moisture into precipitation do not

exist. In dry coastal areas, for example, some areas have no

measurable precipitation for years, but fog is prevalent. By

placing huge sheets of plastic mesh along coastal areas, fog is

intercepted, condenses on the sheets, and provides sufficient

drinking water to supply small villages.

Total rainfall alone does not necessarily indicate water

abundance or

scarcity

. The magnitude of evapotranspiration

compared to precipitation determines to some extent

whether water is abundant or in short supply. On a continent

basis, evapotranspiration represents from 56 to 80% of an-

nual precipitation. For individual watersheds within conti-

nents, these%ages are more extreme and point to the impor-

tance of evapotranspiration in the hydrologic cycle.

Weather circulation patterns responsible for water

shortages in some parts of the world are also responsible for

excessive precipitation, floods, and related catastrophes in

other parts of the world. Precipitation that falls on land, but

that is not stored, evaporated or transpired, becomes excess

water. This excess water eventually reaches groundwater,

streams, lakes, or the ocean by surface and subsurface flow.

If the soil surface is impervious or compacted, water flows

over the land surface and reaches stream channels quickly.

When surface flow exceeds a channel’s capacity, flash

flood-

ing

is the result. Excessive precipitation can saturate soils

and cause flooding no matter what the pathway of flow. For

example, in 1988 catastrophic flooding and mudslides in

Thailand caused over 500 fatalities or missing persons, nearly

700 people were injured, 4,952 homes were lost, and 221

roads and 69 bridges were destroyed. A three-day rainfall

of over nearly 40 in (1,000 mm) caused hillslopes to become

saturated. The effects of heavy rainfall were exacerbated by

the removal of natural forest cover and conversion to

rubber

plantations and agricultural crops.

Although floods and mudslides occur naturally, many

of the pathways of water flow that contribute to such occur-

rences can be influenced by human activity. Any time vegeta-

740

tive cover is severely reduced and soil exposed to direct

rainfall, surface water flow and soil

erosion

can degrade

watershed

systems and their aquatic ecosystems.

The implications of global warming or greenhouse

effects on the hydrologic cycle raise several questions. The

possible changes in frequency and occurrence of droughts

and floods are of major concern, particularly given projec-

tions of

population growth

. Global warming can result in

some areas becoming drier while others may experience

higher precipitation. Globally, increased temperature will

increase evaporation from oceans and ultimately result in

more precipitation. The pattern of precipitation changes

over the earth’s surface, however, cannot be predicted at the

present time.

The hydrologic cycle influences

nutrient

cycling of

ecosystems, processes of soil erosion and transport of

sedi-

ment

, and the transport of pollutants. Water is an excellent

liquid solvent; minerals, salts, and nutrients become dis-

solved and transported by water flow. The hydrologic cycle

is an important driving mechanism of nutrient cycling. As

a transporting agent, water moves minerals and nutrients to

plant roots. As plants die and decay, water leaches out nutri-

ents and carries them downstream. The physical action of

rainfall on soil surfaces and the forces of running water can

seriously erode soils and transport sediments downstream.

Any minerals, nutrients, and pollutants within the soil are

likewise transported by water flow into groundwater,

streams, lakes, or estuaries.

Atmospheric moisture transports and deposits

atmo-

spheric pollutants

, including those responsible for

acid

rain

. Sulfur and

nitrogen oxides

are added to the atmo-

sphere by the burning of

fossil fuels

. Being an excellent

solvent, water in the atmosphere forms acidic compounds

that become transported via the atmosphere and deposited

great distances from their original site. Atmospheric pollut-

ants and

acid

rain have damaged freshwater lakes in the

Scandinavian countries and terrestrial vegetation in eastern

Europe. In 1983, such

pollution

caused an estimated $1.2

billion loss of forests in the former West Germany alone.

Once pollutants enter the atmosphere and become subject

to the hydrologic cycle, problems of acid rain have little

chance for resolution. However, programs that reduce atmo-

spheric emissions in the first place provide some hope.

An improved understanding of the hydrologic cycle is

needed to better manage

water resources

and our

environ-

ment

. Opportunities exist to improve our global environ-

ment, but better knowledge of human impacts on the hydro-

logic cycle is needed to avoid unwanted environmental

effects. See also Estuary; Leaching

[Kenneth N. Brooks]

Environmental Encyclopedia 3

Hydroponics

The hydrologic or water cycle. (McGraw-Hill Inc. Reproduced by permission.)

ESOURCES

OOKS

Committee on Opportunities in the Hydrologic Sciences, Water Sciences

Technology Board. Opportunities in the Hydrologic Sciences. National Re-

search Council. Washington, DC: National Academy Press, 1991.

Lee, R. Forest Hydrology. New York: Columbia University Press, 1980.

Postel, S. “Air Pollution, Acid Rain, and the Future of Forests.” Worldwatch

Paper 58. Washington, DC: Worldwatch Institute, 1984.

Van der Leeden, F., F. L. Troise, and D. K. Todd. The Water Encyclopedia.

2nd ed. Chelsea, MI: Lewis Publishers, 1990.

ERIODICALS

Nash, N. C. “Chilean Engineers Find Water for Desert by Harvesting Fog

in Nets.” New York Times, July 14, 1992, B5.

THER

Rao, Y. S. “Flash Floods in Southern Thailand.” Tiger Paper 15 (1988): 1–

2. Regional Office for Asia and the Pacific (RAPA), Food and Agricultural

Organization of the United Nations. Bangkok.

Hydrology

The science and study of water, including its physical and

chemical properties and its occurrence on earth. Most com-

741

monly, hydrology encompasses the study of the amount,

distribution, circulation, timing, and quality of water. It

includes the study of rainfall, snow accumulation and melt,

water movement over and through the

soil

, the flow of water

in saturated, underground geologic materials (

ground-

water

), the flow of water in channels (called streamflow),

evaporation and

transpiration

, and the physical, chemical

and biological characteristics of water. Solving problems con-

cerned with water excesses,

flooding

, water shortages, and

water pollution

are in the domain of hydrologists. With

increasing concern about water

pollution

and its effects on

humans and on aquatic ecosystems, the practice of hydrology

has expanded into the study and management of chemical

and biological characteristics of water.

Hydroponics

Hydroponics is the practice of growing plants in water as

opposed to

soil

. It comes from the Greek hydro ("water")

and ponos ("labor"), implying “water working.” The essential

Environmental Encyclopedia 3

Hydroponics

macro- and micro- (trace) nutrients needed by the plants

are supplied in the water.

Hydroponic methods have been used for more than

2,000 years, dating back to the Hanging Gardens of Babylon.

More recently, it has been used by plant physiologists to

discover which nutrients are essential for plant growth. Un-

like soil, where

nutrient

levels are unknown and variable,

precise amounts and kinds of minerals can be added to

deionized water, and removed individually, to find out their

role in plant growth and development. During World War

II hydroponics was used to grow vegetable crops by U.S.

troops stationed on some Pacific islands.

Today, hydroponics is becoming a more popular alter-

native to conventional agriculture in locations with low or

inaccessible sources of water or where land available for

farming is scarce. For example, islands and

desert

areas like

the American Southwest and the Middle East are prime

regions for hydroponics. Plants are typically grown in green-

houses to prevent water loss. Even in temperate areas where

fresh water is readily available, hydroponics can be used to

grow crops in greenhouses during the winter months.

Two methods are traditionally used in hydroponics.

The original technique is the water method, where plants are

supported from a wire mesh or similar framework so that the

roots hang into troughs which receive continuous supplies of

nutrients. A recent modification is a nutrient-film technique

(NFT), also called the nutrient-flow method, where the

trough is lined with plastic. Water flows continuously over

the roots, decreasing the stagnant boundary layer surrounding

each root, and thus enhances nutrient uptake. This provides

a versatile, lightweight, and inexpensive system. In the second

method, plants are supported in a growing medium such as

sterile sand, gravel, crushed volcanic rock, vermiculite, perlite,

sawdust, peatmoss, or rice hulls. The nutrient solution is sup-

plied from overhead or underneath holding tanks either con-

tinuously or semi-continuously using a drip method. The nu-

trient solution is usually not reused.

On some Caribbean Islands like St. Croix, hydropon-

ics is being used in conjunction with intensive fish farms

(e.g., tilapia) which use recirculated water (a practice is more

recently known as aquaponics). This is a “win-win” situation

because the nitrogenous wastes, which are toxic to the fish,

are passed through large greenhouses with hydroponically-

grown plants like lettuce. The plants remove the nutrients

and the water is returned to the fish tanks. There is a sensitive

balance between stocking density of fish and lettuce produc-

tion. Too high a ratio of lettuce plants to fish results in

lower lettuce production due to nutrient limitation. Too low

a ratio also results in low vegetable production, but this time

as a result of the buildup of toxic

chemicals

. The optimum

yield came from a ratio of 1.9 lettuce plants to 1 fish. One

pound (0.45 kg) of feed per day was appropriate to feed 33

742

lb (15 kg) of tilapia fingerlings, which sustained 189 lettuce

plants and produced nearly 3,300 heads of lettuce annually.

When integrated systems (fish-hydroponic recirculating

units) are compared to separate production systems, the

results clearly favor the former. The combined costs and

chemical requirements of the separate production systems

was nearly two to three times greater than that of the recircu-

lating system to produce the same amount of lettuce and

fish. However, there are some drawbacks that must be con-

sidered—disease outbreaks in plants and/or fish; the need to

critically maintain proper nutrient (especially trace element),

plant, and fish levels; uncertainties in fish and market prices;

and the need for highly-skilled labor. The integrated method

can be adapted to grow other types of vegetables like straw-

berries, ornamental plants like roses, and other types of

animals such as shellfish. Some teachers have even incorpo-

rated this technique into their classrooms to illustrate ecolog-

ical as well as botanical and culture principles.

Some proponents of hydroponic gardening make fairly

optimistic claims and state that a sophisticated unit is no

more expensive than an equivalent parcel of farmed land.

They also argue that hydroponic units (commonly called

“hydroponicums") require less attention than terrestrial agri-

culture. Some examples of different types of “successful”

hydroponicums are: a person in the desert area of southern

California has used the NFT system for over 18 years and

grows his plants void of substate in water contained in open

cement troughs that cover 3 acres (7.5 ha); a hydroponicum

in Orlando, Florida, utilizes the Japanese system of planting

seedlings on styrofoam boards that float on the surface of a

nutrient bath which is constantly aerated; an outfit in

Queens, New York, uses the Israeli Ein-Gedi system which

allows plant roots to hang free inside a tube which is sprayed

regularly with a nutrient solution, yielding 150,000 lbs

(68,000 kg) of tomatoes, 100,000 lb (45,500 kg) of cucum-

bers, and one million heads of lettuce per acre (0.4 ha) each

year; and finally, a farmer in Blooming

Prairie

, Minnesota,

uses the NFT system in a greenhouse to grow Bibb and

leafy lettuce year-round so he can sell his produce to area

hospitals, some supermarkets, and a few produce warehouses.

Most people involved in hydroponics agree that the

main disadvantage is the high cost for labor, lighting, water,

and energy. Root fungal infections can also be easily spread.

Advantages include the ability to grow crops in

arid

regions

or where land is at a premium; more controlled conditions,

such as the ability to grow plants indoors, and thus minimize

pests and weeds; greater planting densities; and constant

supply of nutrients. Hydroponic gardening is becoming more

popular for home gardeners. It may also be a viable option

to growing crops in some developing countries. Overall, the

future looks bright for hydroponics.

[John Korstad]

Environmental Encyclopedia 3

Hydrothermal vents

ESOURCES

OOKS

Resh, H. M. Hydroponic Food Production: A Definitive Guidebook for the

Advanced Home Gardener and Commercial Hydroponci Grower, 5th ed. Santa

Barbara: Woodbridge Press, 1995.

Saffell, H. L. How to Start on a Shoestring and Make a Profit with Hydroponics.

Franklin, TN: Mayhill Press, 1994.

ERIODICALS

Nicol, E. “Hydroponics and Aquaculture in the High School Classroom.”

The American Biology Teacher 52 (1990): 182–4.

Rakocy, J. E. “Hydroponic Lettuce Production in a Recirculating Fish

Culture System.” Island Perspectives 3 (1988–89): 5–10.

Hydropower

see Aswan High Dam; Dams (environmental effects); Glen

Canyon Dam; James Bay hydropower project; Low-head

hydropower; Tellico Dam; Tennessee Valley Authority;

Three Gorges Dam

Hydrothermal vents

Hydrothermal vents are hot springs located on the ocean

floor. The vents spew out water heated by magma, molten

743

rock from below the earth’s crust. Water temperatures of

higher than 660°F. have been recorded at some vents.

Water flowing from vents contains minerals such as

iron,

copper

, and zinc. The minerals fall like rain and settle

on the ocean floor. Over time, the mineral deposits build

up and form a chimney around the vent.

The first hydrothermal vents were discovered in 1977

by scientists aboard the submersible Alvin. The scientists

found the vents near the

Gala

pagos Islands

in the eastern

Pacific Ocean. Other vents were discovered in the Pacific,

Atlantic, and Indian oceans.

In 2000, scientists discovered a field of hydrothermal

vents in the Atlantic Ocean. The area called the “Lost City”

contained 180-feet tall chimneys. These were the largest

known chimneys.

Hydrothermal vents are located at ocean depths of

8,200 to 10,000 feet. The area near a hydrothermal vent is

home to unique animals. They exist without sunlight and

live in mineral-levels that would poison animals living on

land. These unique animals include 10-foot-long tube

worms, 1-foot-long clams, and shrimp.

[Liz Swain]

Hypolimnion: Lakes

see

Great Lakes

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IAEA

see

International Atomic Energy Agency

Ice age

Ice age usually refers to the Pleistocene epoch, the most

recent occurrence of continental

glaciation

. Beginning sev-

eral million years ago in

Antarctica

, it is marked by at least

four major advances and retreats (excluding Antarctica). Ice

ages occur during times when more snow falls during the

winter than is lost by melting, evaporation, and loss of ice

chunks in water during the summer. Alternating glacial and

interglacial stages are best explained by a combination of

Earth’s orbital cycles and changes in

carbon dioxide

levels.

These cycles operate on time scales of tens of millennia. By

contrast, global warming projections involve decades, a far

more imminent concern for humankind.

Ice age refugia

The series of ice ages that occurred between 2.4 million and

10,000 years ago had a dramatic effect on the

climate

and

the life forms in the tropics. During each glacial period the

tropics became both cooler and drier, turning some areas of

tropical rain forest

into dry seasonal forest or

savanna

For reasons associated with local

topography

, geography,

and climate, some areas of forest escaped the dry periods, and

acted as refuges (refugia) for forest biota. During subsequent

interglacials, when humid conditions returned to the tropics,

the forests expanded and were repopulated by plants and

animals from the species-rich refugia.

Ice age

refugia today correspond to present day areas

of tropical forest that typically receive a high rainfall and

often contain unusually large numbers of

species

, including

a high proportion of

endemic species

. These species-rich

refugia are surrounded by relatively species-poor areas of

745

forest. Refugia are also centers of distribution for obligate

forest species (such as the gorilla [Gorilla gorilla]) with a

present day narrow and disjunct distribution best explained

by invoking past episodes of

deforestation

and reforesta-

tion. The location and extent of the forest refugia have been

mapped in both Africa and South America. In the African

rain forests there are three main centers of species richness

and endemism recognized for mammals, birds, reptiles, am-

phibians, butterflies, freshwater crabs, and flowering plants.

These centers are in Upper Guinea, Cameroon, and Gabon,

and the eastern rim of the Zaire basin. In the

Amazon

Basin

more than 20 refugia have been identified for different

groups of animals and plants in Peru, Columbia, Venezuela,

and Brazil.

The precise effect of the ice ages on

biodiversity

tropical rain forests is currently a matter of debate. Some

have argued that the repeated fluctuations between humid

and

arid

phases created opportunities for the rapid

evolu-

tion

of certain forest organisms. Others have argued the

opposite — that the climatic fluctuations resulted in a net

loss of species diversity through an increase in the

extinction

rate. It has also been suggested that refugia owe their species

richness not to past climate changes but to other underlying

causes such as a favorable local climate, or

soil

The discovery of centers of high biodiversity and ende-

mism within the tropical

rain forest biome

has profound

implications for

conservation biology

. A “refuge rationale”

has been proposed by conservationists, whereby ice age refu-

gia are given high priority for preservation, since this would

save the largest number of species, (including many un-

named, threatened, and

endangered species

), from ex-

tinction.

Since refugia survived the past dry-climate phases, they

have traditionally supplied the plants and animals for the

restocking of the new-growth forests when wet conditions

returned. Modern deforestation patterns, however, do not

take into account forest history or biodiversity, and both

forest refugia and more recent forests are being destroyed

equally. For the first time in millions of years, future tropical

Environmental Encyclopedia 3

Impervious material

forests which survive the present mass deforestation episode

could have no species-rich centers from which they can

be restocked. See also Biotic community; Deciduous forest;

Desertification; Ecosystem; Environment; Mass extinction

[Neil Cumberlidge Ph.D.]

ESOURCES

OOKS

Collins, Mark, ed. The Last Rain Forests. London: Mitchell Beazley Publish-

ers, 1990.

Kingdon, Jonathan. Island Africa: The Evolution of Africa’s Rare Animals

and Plants. Princeton: Princeton University Press, 1989.

Sayer, Jeffrey A., et al., eds. The Conservation Atlas of Tropical Forests. New

York: Simon and Schuster, 1992.

Whitmore, T. C. An Introduction to Tropical Rain Forests. Oxford, England:

Clarenden Press, 1990.

Wilson, E. O., ed. Biodiversity. Washington DC: National Academy

Press, 1988.

THER

“Biological Diversification in the Tropics.” Proceedings of the Fifth Interna-

tional Symposium of the Association for Tropical Biology, at Caracas, Venezuela,

February 8-13, 1979, edited by Ghillean T. Prance. New York: Columbia

University Press, 1982.

Impervious material

As used in

hydrology

, this term refers to rock and

soil

material that occurs at the earth’s surface or within the

subsurface which does not permit water to enter or move

through them in any perceptible amounts. These materials

normally have small-sized pores or have pores that have

become clogged (sealed) which severely restrict water entry

and movement. At the ground surface, rock outcrops, road

surfaces, or soil surfaces that have been severely compacted

would be considered impervious. These areas shed rainfall

easily, causing overland flow or surface

runoff

which pick

up and transport soil particles and cause excessive soil

ero-

sion

. Soils or geologic strata beneath the earth’s surface are

considered impervious, or impermeable, if the size of the

pores is small and/or if the pores are not connected.

Improvement cutting

Removal of crooked, forked, or diseased trees from a forest

in which tree diameters are 5 in (13 cm) or larger. In forests

where trees are smaller, the same process is called cleaning

or weeding. Both have the objective of improving

species

composition, stem quality and/or growth rate of the forest.

Straight, healthy, vigorous trees of the desired species are

favored. By discriminating against certain tree species and

eliminating trees with cavities or insect problems, improve-

ment cuts can reduce the variety of habitats and thereby

746

diminish

biodiversity

. An improvement cut is the initial

step to prepare a neglected or unmanaged stand for future

harvest. See also Clear-cutting; Forest management; Selec-

tion cutting

In situ mining

see

Bureau of Mines

Inbreeding

Inbreeding occurs when closely related individuals mate with

one another. Inbreeding may happen in a small population

or due to other isolating factors; the consequence is that

little new genetic information is added to the

gene pool

Thus recessive, deleterious alleles become more plentiful and

evident in the population. Manifestations of inbreeding are

known as inbreeding depression. A general loss of fitness often

results and may cause high infant

mortality

, and lower birth

weights,

fecundity

, and longevity. Inbreeding depression is a

major concern when attempting to protect small populations

from

extinction

Incidental catch

see

Bycatch

Incineration

As a method of

waste management

, incineration refers

to the burning of waste. It helps reduce the volume of

landfill

material and can render toxic substances non-hazardous,

provided certain strict guidelines are followed. There are

two basic types of incineration: municipal and

hazardous

waste

incineration.

Municipal waste incineration

The process of incineration involves the combination

of organic compounds in solid wastes with oxygen at high

temperature to convert them to ash and gaseous products.

A municipal incinerator consists of a series of unit operations

which include a loading area under slightly negative pressure

to avoid the escape of odors, a refuse bin which is loaded

by a grappling bucket, a charging hopper leading to an

inclined feeder and a furnace of varying type—usually of

a horizontal burning grate type—a

combustion

chamber

equipped with a bottom ash and clinker

discharge

, followed

by a gas flue system to an expansion chamber. If byproduct

stream is to be produced either for heating or power genera-

tion purposes, then the downstream flue system includes

heat exchanger tubing as well. After the heat has been ex-

changed, the

flue gas

proceeds to a series of gas cleanup