THE CYCLE IN ACTION: GEMINI VS CAMERA CATCHES AN ATLANTIC STORM SWEEPING TOWARD MOROCCO'S COAST (LEFT).
AMONG THE NINE PLANETS, the earth is uniquely endowed with large
quantities of water in its liquid state. The world supply of 326 million
cubic miles of this substance would, if poured upon the 50 United
States, submerge the country to a depth of 90 miles. Equal in impor-
tance to the quantity is the earth's ability to maintain it in all three of
the fundamental states of matter-i.e., as a liquid, solid and vapor.
Water is the only common material that exists naturally in all three
states on earth and the earth is apparently the only planet in the solar
system that sustains water in this way. This circumstance has not only
determined the course of life on earth but may have limited life, within
this solar system, to earth alone.
For thousands of years, men have recognized-sometimes dimly, some-
times clearly-the significance of water's role. So abundant, so unusual,
so essential is it that it has never failed to stimulate wonder. Man is
himself a porous sac of water; only one third of his body by weight is
composed of other compounds. Water provides the surging oceans, the
mist from a marsh, the creeping glacier, the volcano's explosive steam,
a snowball, the five billion tons or more of moisture that may be whirled
through the air by a small hurricane.
This bewildering variety tells something of water's restless nature.
It is never still. The apparently inert tumblerful that stands beside a
dinner plate may simultaneously convert ice cubes into liquid, release
tiny amounts of vapor into the air above it, and condense vapor into
droplets on its smooth glass sides. This is the fidgety world of water in
microcosm. Projected onto a grand global scale, all 326 million cubic
miles of this active substance are constantly responding to a complex
of mighty natural forces-the rotation of the earth, the radiant heat of
the sun, and the gravitational effects of the earth and its companions
in the solar system. Added to these forces are the effects of surface
irregularities-the mountains, valleys and plains on the continents and
the oceans' basins-plus the chemistry and texture of terrestrial matter.
Each contributes to a dynamic and perpetual metamorphosis-the
shifting, changing, fickle nature of gaseous, solid and liquid water.
In one vitally important respect, however, water's behavior is stead-
fast: the total supply neither grows nor diminishes. It is believed to be
almost precisely the same now as it was three billion years ago. End-
lessly recycled, water is used, disposed of, purified and used again. Last
night's potatoes may have boiled in what was, ages ago, the bath water
of Archimedes. And while the idea of using "used" water may at first
repel a hygienic civilization, the knowledge that the world supply of
this vital substance cannot be depleted should offer comfort.
The durability of water raises the question of whether it has existed
forever. What, indeed, was the source of all water in the shadowy be-
ginnings of the young and lifeless earth? Modern scientists see a direct
connection to a grander puzzle, the origin of the earth itself. The oc-
currence and nature of water are clearly related to the size of our
planet, to its location in the heavens, and its formation.
The most widely accepted theory of the origin of the earth was de-
veloped in 1944 by the German theoretician Carl F. von Weizsacker
and later modified by Gerard P. Kuiper of the University of Arizona.
It proposes that the sun evolved from a vast gaseous cloud of hydrogen
and helium. Scattered throughout this cloud in the form of fine dust,
and comprising about 1 per cent of the whole, were the elements and
compounds of which the planets are made. Water-in the form of vapor,
crystals and droplets-was one of these compounds.
As this cosmic cloud spun about in space, gravitation-the mutual
attraction of the particles for one another-caused a core area to form
and to contract. As the density of the core increased, its temperature
also increased, finally reaching that level, high almost beyond imagin-
ing (about 23,000,000 F.), at which hydrogen nuclei fuse into helium
and release energy. Thus began the thermonuclear reaction that is the
self-sustaining source of the sun's heat and light.
The earth's watery nature
The sun condensed into a separate body before it had drawn all of
the nearby cosmic cloud into itself. What remained continued to whirl
about as huge eddying disks of colliding particles that formed ever-
larger aggregates of matter. After several million years they became the
nine identifiable members of the family of planets. The water that was
part of the original cloud became part of each, in an amount and con-
dition that depended first upon the planet's mass (which determines
its gravitational attraction) and second upon its distance from the sun
(which determines the planet's surface temperature).
The earth's watery nature results entirely from its middling size and
middling position. It is massive enough so that its gravitational force
will hold an atmosphere of water vapor and other heavy gases. Its lo-
cation 93 million miles from the sun keeps it near the center of a narrow
zone where temperatures permit water to exist as liquid, solid and
vapor. This zone, as distances in the cosmos are reckoned, is not large;
its 75-million-mile width is about 2 per cent of the solar system's radius.
The best way to appreciate earth's good fortune is by examining its
nearest neighbors. Under the most generous estimate of the tempera-
ture zone that permits water to exist in all three states, both Venus,
nearer to the sun than earth, and Mars, farther away, might contain
water in these forms. The presence of very small amounts of water vapor
and ice particles on Mars has been detected. If the water vapor in the
thin Martian atmosphere were precipitated, the liquid layer covering the
planet's surface would be only 1/2,500 of an inch thick. And it would
freeze immediately. The polar caps, where temperatures range from
-100 F. to -150* F., consist of hoarfrost not more than an inch thick
Venus, though closest to earth, remains the most difficult planet to
investigate, for its opaque atmosphere prevents visual study of what
goes on beneath. Radio waves do penetrate the cloud cover, however,
and they indicate a surface temperature of more than 1,000* F., too high
for ice or liquid water there. Some water may exist as part of the atmos-
pheric cloud, which is mainly carbon dioxide. What little can be learned
by spectroscopic observation, and it is admittedly contradictory, shows
only a small amount of water vapor above the cloud layer.
Beyond Venus, toward the sun, only tiny Mercury orbits. Recent radio
observation of Mercury reveals that the planet rotates on its axis one
and a half times for every orbit around the sun. The blistering heat
(770" F.) of the sunny side exceeds the melting point of lead (621* F.),
while the temperature of the dark side is closer to that of the earth's.
Such water as Mercury has must be trapped in the cold crust of the
polar areas; there is no possibility of surface water. Nor can Mercury
have an atmosphere; its small mass does not produce enough gravita-
tional force to prevent the escape of gas into space.
From Mars out to the farthest reaches of the solar system, the tem-
perature drops below -300 F. as the distance from the sun increases.
All of the water on the five planets beyond Mars is therefore frozen all
the time. Pluto, the most distant, is small, but Jupiter, Saturn, Nep-
tune and Uranus are giant planets and likely to possess quantities of
water commensurate with their size.
Oceans, glaciers and air
Included in the cosmic dust when clumping formed the earth were
the constituents of the water we use today. How those molecules were
transformed into oceans of liquid, glaciers of ice and humid atmos-
phere is a subject of debate. Two theories have been suggested, com-
petitive parts of overall theories which attempt to explain the steps in
the evolution of the planet. The most widely accepted of them holds
that the formative earth heated up-from the impact of additional me-
teorlike matter from space; from radioactivity, then 15 times greater
than now; and from the increase in pressure as gravity compacted the
original material. According to this theory the earth grew so hot, in fact,
that it melted, first in its center and finally, after 100 million years or
so, throughout. The water supply vaporized and then, since water
molecules at very high temperatures decompose chemically into their
constituent hydrogen and oxygen atoms, it disappeared. While water as
such no longer existed, the possibility of water remained, as the gases
swirled within and above the turbulent surface of the molten planet.
When the radioactivity declined and the compaction process slowed
down, the temperature cycle reversed. The earth gradually cooled. Be-
fore its crust solidified, most of the atmosphere of hot gases was lost;
it simply boiled away into space.'This was replaced with exhalation
from inside the earth, including water vapor. As the vapor steamed up
from fissures and craters and bursting bubbles of viscous rock to con-
dense in the cold of the upper atmosphere, it curtained the earth with
a dense, thick cloud of water droplets and snow.
As the earth cooled further, according to the theory, the water in
.: .. ":, : "::.. ::, ":...
."' "..' ... ... ... ... '.. U
IN THE SOLAR SYSTEM water is found in
all three of its forms-ice, vapor and liquid-
only in a relatively narrow ring (blue above)
-the 75 million miles between Venus
and Mars. Close to the sun, water apparently
exists only as vapor; farther from the sun,
it turns to ice. This fact limits the possibility of
life in the solar system to the planets
Earth, Venus and Mars, since the presence of
liquid water is essential to the life process.
the air approached nearer the surface until, finally, the earth was cool
enough so that water could strike ground without immediately evaporat-
ing. Then began a deluge that continued for centuries. When the cloud
mantle thinned sufficiently, the sun lighted and warmed the surface wa-
ters, the primeval seas that were eventually to bear life.
This version of the earth's birth is disputed by some authorities, No-
bel laureate Harold C. Urey among them. Urey suggests that the earth
was never completely molten, but rather was born cold and has retained
a cool, solid crust ever since. The water, he believes, emerged gradually
from this crust instead of raining down in a great deluge.
This theory is based on the fact that the original cosmic dust, com-
pacting into the rocks of the planet, could have trapped water mole-
cules inside their crystalline structure. The crust of the earth is today
largely silicate rock-hydrated crystals which contain water molecules
integrated into their atomic arrangements. The water can be driven out
of such hydrated crystals rather easily by heat. If the earth had been
born cold, as the Urey theory maintains, this water of hydration existed
from the beginning; it was released later in local areas by intermittent
heating, which could have been caused by the collisions between large
chunks of matter and the growing earth. Still later, the heat from vol-
canic activity and meteorite impacts would have boiled additional wa-
ter of hydration from rocks inside the earth.
The slow growth of the seas
Thus, over long periods of time, water may have accumulated in pools
on the surface of the earth. A slow growth of the oceans would explain
why they are no more salty now than they apparently were hundreds of
millions of years ago, despite the fact that whole continents have been
washed into them. If the oceans have always been their present size,
the addition of so much other material should have tended to increase
their saltiness progressively.
The two theories agree about one thing: the oceans, whether they
were created suddenly by a deluge or gradually by the dehydration of
rocks, filled basins that already existed. These basins are themselves
remarkable-the Pacific deeper than any mountain is high, its floor gap-
ing with chasms; the Atlantic, shallower, seamed by a submerged moun-
tain chain, edged with shorelines that seem to meet.
The origin of the basins, however, is also in dispute. At least three
proposals have been advanced. Urey suggests that they were gouged
out of the solid earth by space materials crashing into the planet. Most
scientists, however, accept a molten beginning for the earth and believe
that the oceans' basins evolved quite differently. They maintain that
some parts of the surface congealed before others. Islands of solid mat-
ter floated about in the molten mass, moved by currents produced by
the earth's rapid rotation. In time, their movement stopped, the surface
hardened, and the continental heights and ocean hollows were arranged
roughly as they are today.
Recent studies of the ocean bottom have rekindled interest in a much
more dramatic-but scientifically implausible-idea: the possibility that
the Pacific basin is the hole left behind when the moon was torn free
from the earth and that the Atlantic is the crevice created when an orig-
inal single continent split in two. This theory assumes that the earth
evolved from a molten mass and that its seething viscous surface re-
sponded to gravitational forces-mainly the sun's-causing great waves
and tides to roll about the earth. Eventually, the oscillating currents of
the sun's gases came to be in step with the oscillations of the earth's
tides. Surging back and forth in cadence, the earth's waves grew to enor-
mous size and sloshed outward into nearby space, forming the moon
and leaving the Pacific basin behind. On the other side of the globe, the
solidifying granite and still-viscous lower layers were pulled apart like
warm tar, formed continents, and left the Atlantic basin where the parts
separated. More detailed mathematical studies show this theory to be
contrary to basic principles of dynamics: the laws of conservation of en-
ergy, mass and angular momentum.
However the ocean's hollows came to be, they caught nearly every
drop of the water that later became available. Today almost all the wa-
ter on earth is in the oceans-and "almost all" is close to being absolute-
ly all: 97.2 per cent of the total volume. The usable fresh water above
and in the ground accounts for less than two thirds of 1 per cent of the
total, as the illustration on page 38 shows.
Taken together, the oceans and the ice caps and glaciers comprise
99.35 per cent of the earth's total water. The remaining two thirds of
1 per cent are apportioned to all of many other manifestations of water
around the globe. Included in that small fraction are the waters of all
the great rivers and lakes of the world, the inland seas, the streams,
springs, brooks and ponds, the pools and puddles, swamps and bogs,
the rain, snow and vapor in the atmosphere, the water in pipes above
and below the ground, in sewers and reservoirs, the snow and ice on
mountain slopes, the moisture in the land and-most importantly-the
groundwater that supplies the wells and helps to feed the streams and
rivers. Groundwater, in part, accounts for about 97 per cent of that
small usable supply, the water which remains when the oceans, ice caps
and glaciers are subtracted from the world total.
A puzzle of distribution
The obvious disproportion in the distribution of water puzzled the
ancients. In their view, rain and snow could not account for the quan-
tity in lakes and streams, because not enough fell. Who, living in the
arid region beside the Nile, could imagine that the river's annual flood
derived from precipitation on mountains thousands of miles away? The
men of other centuries also found it impossible to conceive of the osten-
sibly solid ground as an absorber and conveyer of rain; after all, digging
into the earth produced water only in certain favored spots. Until the
17th Century, most men explained springs and deep well water in one
Saline lakes and inland
Rivers and streams
Groundwater within dept
of half a mile
ICECAPS AND GLACIER
of two ways: either it came from a vast underground reservoir, a fresh-
water ocean hidden beneath crustal rock; or it moved from the sea
through subterranean channels, was purified somehow, and then rose
to pour out as springs or to lie in underground pools waiting to be
tapped by wells. Of the two explanations, the former satisfied less; it
ignored the need for replenishment of the underground storage.
The idea of a complete cycle-that water evaporated from the sea
and land, was drawn into the atmosphere, fell as rain and snow, sank
into the earth to reappear in watercourses, and then drained back into
the sea-had attracted brilliant men over the years, but it could not be
proved at that time and therefore was not generally accepted. With the
development of modern science in the 16th and 17th Centuries, how-
ever, attention was directed again and again to what seemed to be the
a WATER SUPPLY cyclical pattern of all nature: Newton's law that for every action there
must be a reaction, the recirculating blood system demonstrated by
WATER CENTAGE Harvey, the planetary orbits postulated by Copernicus. These rules of
VOLUME OF TOTAL
(Cubic miles) WATER balance and repetition had been established by close observation and
careful measurement. It was only natural, then, to seek a similar bal-
30,000 .009 ance in the world's water supply and to seek it with similar techniques.
25,000 .008 In the mid-17th Century, two French scientists individually attacked
300 .0001 the puzzle of the rivers. Each-Pierre Perrault first and Edmb Mariotte
55,300 .017 a little later-measured the precipitation in the watershed of the Seine
and then measured the river's rate of discharge, i.e., the amount of wa-
h 16,000 .005 ter it poured into the ocean in a given time. Their measurements, al-
,000,000 .31 though crude, proved that, contrary to ancient belief, precipitation alone
2,016,000 .625 could account for the river's flow. Moreover, enough water would remain
to supply the springs and wells. Mariotte went a step further; he showed
ERS 7,000,000 2.15
that rain deeply infiltrated the ground wherever it fell, seeping down-
3,100 .001through porous soil until it reached impermeable material.
ward through porous soil until it reached impermeable material.
UUO ANSa 317,u000,000 9.
TOTALS (approximate) 326,000,000 100
WATER IS DISTRIBUTED in great or
small amounts to every part of the earth. All
but about 3 per cent of the water is held
in oceans: the remainder is found as much as
three miles under the earth's crust or (as vapor)
as high as seven miles above the surface.
The table above shows the quantity and
percentage of water in all its habitats.
Halley's ingenious experiment
Another essential factor in the distribution cycle-the origin of rain
and snow-remained to be proved. Shortly after Perrault and Mariotte
completed their investigations, the English astronomer Edmund Halley
showed that the earth's precipitation was of such magnitude that it
could be balanced by evaporation: the evaporation from a large body of
water was of an order of size equal to the amount it regained from the
rivers that flowed into it. The key to Halley's discovery was the deter-
mination of the rate of evaporation. He used an ingenious but simple
apparatus which he described in these words:
"We took a Pan of Water about 4 inches deep and 7 9/10 inches diam-
eter, in which we placed a Thermometer, and by means of a Pan of Coals,
we brought the Water to the same degree of heat which is observed to be
that of the Air in our hottest Summers; the Thermometer nicely showing
it. This done, we affixed the Pan of Water, with the Thermometer in it,
to one end of the Beam of the Scales, and exactly counterpoised it with
weights in the other Scale; and by the application or removal of the Pan
of Coals, we found it very easie to maintain the Water in the same degree
of Heat precisely."
From the loss in weight of his small pan of water, Halley could calcu-
late the rate at which water evaporated at the temperature of "our hot-
test Summers." He then applied this rate to the estimated quantity of
water in the Mediterranean and determined quite accurately how much
evaporated from the large body of water. This he equated to the amount
of water poured into the Mediterranean by rivers.
The concept of a hydrologic cycle unraveled the ancient riddle of wa-
ter. Man could now understand that the water going out from the sur-
face of the earth must come back in equal amount-a perpetual cycle
with no beginning, middle or end. Maintaining this cycle requires that
at any moment an average of 3,100 cubic miles of water must be distrib-
uted throughout the global atmosphere in the form of vapor or water
droplets. While this may seem substantial, the amount is actually slight
relative to the size of the atmosphere. If all of it abruptly fell as rain,
the 3,100 cubic miles of water would cover the earth with barely an
inch. The turnover is quite rapid; once every 12 days, on the average, all
the water in the air does fall and is subsequently replaced.
The statistics of circulation
About 95,000 cubic miles of water goes into the air annually. By far the
greater part-approximately 80,000 cubic miles-rises from the ocean.
But 15,000 cubic miles is drawn from the land, evaporated off lakes,
streams and moist soil, and, most importantly, transpired from the leaf
surfaces of living plants. The total process is called evapotranspiration.
Of the water that goes up into the atmosphere, most-71,000 cubic
miles-falls back directly into the oceans. Another 9,000 cubic miles
falls on land but runs off into rivers and streams and is returned to the
oceans within days or, at most, a few weeks. The remaining 15,000 cubic
miles soaks into the land and is available to participate in plant and ani-
mal life processes. In these processes too, water intake matches outgo as
animal and vegetable life exhales, excretes and perspires what was ear-
lier ingested through root and mouth.
While the hydrologic cycle balances what goes up with what comes
down over the earth, no such reciprocity holds for individual areas. Wide
differences occur in rates of both evaporation and precipitation.
Evaporation might be expected to be greatest at the equator, since the
most solar energy strikes that area. But heavy clouds are more frequent
over the equator than in most other regions; they reduce the radiation
reaching the surface. And to the north and south, strong winds sweep
up more moisture than the relatively calm winds of the equator. Winds
play a critical role, for dry winds absorb more moisture than the winds
of temperate areas. The highest evaporation rates on earth occur in the
Red Sea and Persian Gulf, lying between 15 and 30 north latitude. The
unmitigated ferocity with which sun and hot winds heat these bodies
of water drives no fewer than 111/2 feet a year from the Red Sea. One con-
sequence is its extreme salinity.
Evaporation rates are still more variable on land; there is less exposed
surface water, and the extremes of temperature and wind are greater.
Some deserts have an evaporation rate of zero, because there is nothing
to evaporate. In rain forests, on the other hand, the rate approximates
that of the open ocean under the same conditions of wind and sunlight.
Drenching and drought
Precipitation varies even more than evaporation. Dramatic contrasts
in rainfall and snowfall can be seen within a few miles on land-for
example, Mount Waialeale in Hawaii receives an annual average of 460
inches, while 15 miles away, a rain gauge records only 18 inches in a year.
Such variations result from topographic interference with the flow of
wind over the earth's surface. Mountains prevent what might otherwise
be a fairly uniform distribution, through precipitation, of the moisture
carried by the wind. When a prevailing wind blows across the sea, or
across land areas where the evaporation rate is high, it becomes loaded
with moisture. Moving against a mountainside, it is forced upward and
cooled so that its water vapor condenses to snow or rain. By the time it
descends on the other side of the mountain, the once moist air, its water
load squeezed from it, has become a desert wind. The effects of moun-
tain barriers on patterns of wind and rain are called orographicc," which
literally means "written by the mountains." Orographic effects, by defini-
tion, do not occur over the open sea, where precipitation is consequently
much more even than on land.
In the United States, except Alaska and Hawaii, the average annual
precipitation is 30 inches and this is far from evenly distributed. Along
the northwestern coast, moisture-laden winds from the Pacific Ocean
blow continuously against the western slope of the Cascade Mountains
and drop between 140 and 150 inches of rain annually upon the tall, thick
forests of conifers. But in barren Death Valley, 900 miles southeast, only
1.7 inches of rain falls in an average year.
The hydrologic cycle that averages out these local variations into a
long-term global balance depends on the sun. The sun's radiant energy
provides the power to raise water into the atmosphere so that it can fall
again. The cycle resembles a steam engine. In the hydrologic engine, the
sun is the firebox, the boiler is the ocean and the land, and the con-
denser is the cool upper atmosphere. This engine performs work on
a grand scale: it makes weather, establishes climate, directs ocean
currents, cuts valleys and builds deltas, and sustains life on land.
Much of this work involves, in one way or another, the transfer of heat
around the earth. The water evaporating into the atmosphere enters the
global system of prevailing winds. In the form of vapor and droplets, it
travels thousands of miles before returning to the surface. In doing so, it
carries heat in huge quantities around the globe and moderates the tem-
perature extremes that would otherwise prevail.
The transfer of heat
As an example, consider what happens to water which has surfaced
from the cold depths of the sea. If it rises in the tropics, it mixes with
surface water which has a temperature of about 80* F. First of all, it
helps to hold that temperature down. Evaporating from the surface, it
is transformed from liquid into vapor, a change that requires energy.
The water absorbs this energy as heat. The vapor can be considered a
heat-carrying vehicle ready to travel the air currents flowing north and
south from the equator. If the vapor enters the current high above the
equator, the earth's rotation forces it to move northeasterly until it
reaches a latitude of 30. There, cooling, it may sink and slant across
the North Temperate Zone to meet a cold air current from the Arctic.
The joining of the disparate air masses results in a turbulent storm. The
vapor abruptly condenses into liquid, and all of the heat energy ab-
sorbed in the tropics is released and warms the frigid air. If in this
process the liquid water freezes, it releases still more heat energy.
Water vapor is not the only means by which vast quantities of heat
are transported from lower latitudes to higher, north and south of the
equator. "Rivers" flow in the ocean; some are warm currents of water
from the tropics and others are cold currents from the polar regions.
These streams vary in width, volume and speed. The channels in which
they flow are generally more stable than those on land, and they exert
long-term influences on the environments of the earth's regions.
Scientists estimate that if the temperature difference between a warm
current and the adjacent ocean is 20 F., every cubic mile of the current
yields heat equal to that obtained by the efficient burning of seven mil-
lion tons of high-quality coal. It is as a result of one such vast heat ex-
change-that caused by the Atlantic's warm Gulf Stream-that the Brit-
ish Isles and parts of Norway enjoy a temperate climate while Labrador,
at the same latitude on the eastern side of the Atlantic Ocean, is frigid.
Man's increasing knowledge of the hydrologic engine has answered
many of the ancient riddles of water, but not all of them. We know where
the rain comes from, yet we still cannot order its coming. We can explain
why water fills the well, yet we still cannot always forecast correctly
how much water any particular well will furnish. Today's attempts to
control the great global engine are, like the incantations and dances of
other times, merely attempts. Only new understanding, rising from an
intensified study of water, may finally give man more effective influence
over the bountiful compound that is essential to life on earth.
THE HYDROLOGIC CYCLE, the endless
circulation of the earth's water, is pictured here,
beginning at left with precipitation of rain
from a cloud. The rain sinks into the earth, some
of the water eventually seeping into the ocean.
some of it running into channels and lakes.
Simultaneously it starts on the reverse stage
of the cycle, evaporation (dotted lines). Some
water actually evaporates during the rainfall;
most of it rises from wet ground, from rivers
and lakes, from the leaves of plants and above
all from the ocean. The evaporated water
collects in clouds; as these cool, precipitation
occurs-and the cycle repeats itself.
LLUUUDS LUUL ANDU AUS FHUIV I A I IUN
4 .... l...l.- .. .... .. ... .... ..,
The Long Voyage
from Sea to Sea
The earth's original supply of water is still in use: little has
been added or lost in the hundreds of millions of years
since the first clouds formed and the first rains fell. The
same water has been pumped time and again from the
oceans into the air, dropped upon the land and transferred
back to the sea. This process-the natural mechanism that
evaporates ocean water, distributes it to every part of the
earth, then returns it to the sea-is known as the hydro-
logic cycle. At any instant, only about 0.005 per cent of the
total water supply is moving through the cycle; most of the
water is stored in the oceans, frozen in glaciers, held in
lakes or detained underground. In the U.S. a drop of water
spends an average of only 12 days passing through the air,
then may remain in a glacier for 40 years, in a lake for 100
years, or in the ground from 200 to 10,000 years, depending
on how deep it goes. Eventually, however, every drop is
moved on through the cycle. The hydrologic cycle uses
more energy in a day than man has generated throughout
history. But the cycle's machinery, powered by a constant
input from the sun, has more energy than it can ever use.
DRAINING THE CYCLE
Flooding the Australian coastal flats, the Norman number of rivers is practically uncountable-in
River winds past mangrove trees and empties the United States alone there are some three and
into the South Pacific (background). Rivers are a quarter million miles of river channel-yet riv-
the drainpipes of the hydrologic cycle. The total ers hold only 1/10,000 of the world's water.
k^ ^^../* *>'
*k ^"r.yep- *~
K ..-< 4*'js* b
- c r- ** 4*
~I-_- "r .._
from the Oceans
Every year more than 83,700 cubic
miles of water is pumped out of the
oceans by the hydrologic cycle. If the
oceans were not constantly refilled,
this water loss would lower their lev-
el by 39 inches a year. The "pump"
of the cycle is the sun, which sup-
plies energy to evaporate ocean water
and release it into the air as vapor.
Actually, the hydrologic cycle is an
extraordinarily inefficient machine.
It utilizes only a small fraction of
the solar energy available to it. Simi-
larly, once a raindrop is deposited
on a mountaintop, only about 5 per
cent of its potential energy is used
- .r --
in the work of erosion-95 per cent
is converted to heat through friction
as the water runs back into the sea.
Yet the cycle, always supplied with
more energy than it can use, oper-
ates like a perpetual motion machine.
It is continually being reinvigorated
by the prodigious force of the sun.
STOREHOUSE FOR THE CYCLE
The oceans, holding 317 million cubic miles of
water, constitute 97.2 per cent of the earth's
total supply. At the surface, countless sun-
heated molecules are evaporated every minute,
while a molecule at the bottom of the ocean
may wait 2,000 years before it enters the cycle.
* rr s
111 i i j
Se -*r.- -< w!
""Ipfc- ^^ 1 ^^
W- T- -777W-72
RELEASING THE RAINS
The driving rain of a midsummer thunderstorm
pours tons of water onto the dusty plains of the
western United States. The amount of precipi-
tation varies along the cloudbank. In the mid-
dle of the squall line (bright area) the rainfall is
lighter. There heat radiated from the sunbaked
earth evaporates raindrops almost as soon as
they condense, and the ground remains dry.
SEQUEL TO A STORM
Sun shines through the scattering remnants of
rain clouds, warming the earth and trees of a
California mountainside. Through evaporation,
the vapor in the atmosphere is replenished, and
new clouds are formed. Thus the process is
repeated-as the vapor rises and is cooled, it
condenses, falls as rain on another mountain,
only to evaporate and rise as vapor once again.
CI I -------------- Y;' ~Li~I~L a-,
in the Air
The water that the hydrologic cycle
pumps into the air by evaporation is
soon recondensed and distributed as
precipitation. At any one time, only
about 3,100 cubic miles of water (less
than 1/100,000 of the total supply) is
held in the atmosphere. Even this
fraction, if suddenly released as rain,
would be enough to cover the earth
with an inch of water.
The hydrologic cycle often short-
circuits itself. Much water evapo-
rated from the ocean quickly returns
in sea squalls; on land, some rainwa-
ter returns to the air as soon as it falls.
But the overall amount of water in
the air remains constant. Every in-
crement of vapor is ultimately ex-
pended, in raindrops, snowflakes or
hailstones. Water held in the air is
more than potential precipitation. It
represents quantities of latent ener-
gy, amassed by evaporating mole-
cules. As this energy builds up, the
cycle releases it through storms. A
run-of-the-mill thunderstorm re-
leases more energy than a 120-kiloton
nuclear bomb-and there are more
than 10,000 thunderstorms every day.
S A Waterlogged
j, When rainwater soaks down into the
ground, it may not actively return to
the hydrologic cycle for decades, cen-
turies or even millennia. The earth
holds more than two million cubic
miles of water underground-about
S37 times the amount stored on the
S, t surface in lakes and rivers. About
half the groundwater, saturating the
Sf soil or seeping through rocky strata
Sat depths down to half a mile, is ef-
fectively taking part in the cycle.
Usually it does so at a reduced rate,
bubbling up in springs-though oc-
casionally the return is sudden, as in
steaming fumaroles (right) or spew-
ing geysers. The other half, stored
between half a mile and three miles
Down, is unable to return to circula-
tion at all until some interior con-
vulsion of the earth releases it. But
no water is permanently shut out of
3 the cycle-not even the stagnated
4 rains of 10.000 years ago, trapped
"- -three miles under the earth's crust.
STEAM VENTS IN THE EARTH
Superheated steam forces its way through fis-
sures in the earth's surface, releasing ground-
water as vapor. Such spouts, or fumaroles, are
formed where an underground oven of hot rock
converts water to steam. Steam pressure builds
up until II drives the vapor through vents in the
earth Fumaroles, which can appear anywhere,
often are found in regions of volcanic activity.
A WELL WITHOUT A PUMP
Soaked to the skin, a workman struggles to spurt indefinitely, as rainwater seeps down-
stanch the flow of a newly struck gusher of ward to replenish the underground store. More
water stored under pressure. Such naturally commonly, the well depletes the water supply.
pressurized, or artesian, wells may continue to lowering the pressure and shutting itself off.
In the ice age, half of North America was under ice; this composite map shows four waves at once. Present coastlines are outlined.
More than a million years ago the
temperature of the earth fell slightly.
The total heat loss may have amount-
ed to around three or four degrees.
But even that small a difference in
temperature was enough to upset the
delicate balance by which the hydro-
logic cycle had held the earth's cli-
mate relatively stable for hundreds
of millions of years. With the cycle
out of kilter, the world entered the
Pleistocene era, or ice age. Scientists
believe that it was at least the third
time such an event had occurred.
Increasing quantities of the earth's
precipitation were stored as ice, and
glaciers alternately extended toward
the equator and receded. There were
four major advances, during which
the glaciers wore down the Appala-
in the Cycle
N t .. ,
Lakes, which covered 20 per cent of North America as the glaciers retreated (light blue), cover only 7 per cent today (dark blue).
chian Mountains, gouged out beds
for the Great Lakes (and hundreds
of others), and bulldozed great heaps
of earth and rock to form the New
England hills. At their greatest ex-
tent, the ice sheets, often thousands
of feet thick, held more than double
the amount of water frozen today.
They blanketed almost half of North
America, reaching to within 650 miles
of the Gulf of Mexico (above left).
These changes in the cycle had in-
numerable side effects. As increasing
amounts of the earth's water turned
to ice, the ocean levels fell 300 to 400
feet. As a result, about 2.5 million
square miles of land were added to
the east and Gulf coasts. Alaska and
Asia were connected by a land bridge.
The British Isles and Europe were
similarly connected. The oceans have
since risen close to their former level,
engulfing most of the added land
again (above right).
The glaciers began their fourth re-
treat only about 10,000 years ago and
are still receding. The hydrologic cy-
cle may have now regained its former
equilibrium-but there is no way of
knowing whether the ice age is over.
Lakes are anomalies of the hydrologic
cycle, short-lived features of the ter-
rain which begin to die the moment
they are created. The hottest, driest
place in the United States, Death
Valley in California, was covered by
a lake 180 feet deep only about 20,-
000 years ago. Utah's Great Salt Lake,
covering 2,500 square miles, is the
last remnant of Lake Bonneville,
which once spread over 19,000 square
miles. With no outlet except evapo-
ration, Lake Bonneville choked on its
accumulated salts, leaving behind
extensive reaches of arid salt flats.
Many lakes die of an excess of sed-
iment. Sand and gravel settle on the
bottom, slowly turning the lake into
a mud flat or swamp, and finally fill-
ing the lake entirely. As some lakes
die, new ones replace them. The ap-
pearance of lakes is often associated
with changes in the hydrologic cycle.
Scientists calculate that early in the
Pleistocene, the semiarid New Mex-
ico country was changed into a re-
gion dotted with ponds and lakes
as the average temperature dropped
by 6' F., and the annual rainfall in-
creased by nine inches. When the
temperature rose again and precipi-
tation diminished, the lakes dried up.
THE LIFE-SPAN OF A LAKE
The near-freezing water of Lake Schrader in
Alaska. fed by melting snow from Mount Cham-
berlin, causes vapor in the warmer air to con-
dense into morning mist. The lake, one of the
northernmost in Alaska, was formed about 13,-
000 years ago, during the last glacial advance.
If it continues to silt up at its present rate. Lake
Schraderwill be gone 10,000 years from now.