II 5 I I
DEPARTMENT OF THE INTERIOR
DISTRIBUTION OF MAN'S LIQUID ASSETS
IS A CLUE TO FUTURE CONTROL
Most people know that water is unevenly distrib-
uted over the earth's surface in oceans, rivers, and
lakes, but few realize how very uneven the distri-
bution actually is. It is instructive to consider the
total inventory of water on the planet earth, the
areas where the water occurs, and the long-term
significance of the findings.
The world ocean-139,500,000 square miles of it-
contains 317,000,000 cubic miles of salt water. The
average depth of the ocean basins is about 12,500
feet. If the basins were shallow, seas would spread
far onto the continents, and dry land areas would
consist chiefly of a few major archipelagoes where
high mountain ranges projected above the sea.
Considered as a continuous body of fluid, the
atmosphere is another kind of ocean. Yet, in view
of the total amount of precipitation on land areas
in the course of a year, one of the most astonishing
world water facts is the very small amount of water
in the atmosphere at any given time. The volume
of the lower seven miles of the atmosphere-the
realm of weather phenomena-is roughly four times
the volume of the world ocean, but the atmosphere
contains only about 3,100 cubic miles of water,
chiefly in the form of invisible vapor, some of
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which is transported over land by air currents. If
all vapor were suddenly precipitated from the air
onto the earth's surface it would form a layer only
about one inch thick. A heavy rainstorm on a given
area may remove only a small percentage of the
water from the air mass that passes over. How,
then, can some land areas receive, as they do, more
than 400 inches of precipitation per year? How can
several inches of rain fall during a single storm in
a few minutes or hours? The answer is that rain-
yielding air masses are in motion, and as the water-
depleted air moves on, new moisture-laden air takes
its place above the area of precipitation.
The basic source of most atmospheric water is the
ocean, from which it is derived by evaporation.
Evaporation, vapor transport, and precipitation
constitute a major arc of the hydrologic cycle-
the continuous movement of water from ocean to
atmosphere to land and back to the sea. Rivers
return water to the sea along one chord of the arc.
In a subterranean arc of the cycle, underground
bodies of water discharge some water directly into
rivers and some directly to the sea.
Estimated average annual evaporation from the
world ocean is roughly 39 inches. The conterminous
United States receives an average of 30 inches of
precipitation every year, or about 1,430 cubic miles
in total volume. Evapotranspiration returns approx-
imately 21 inches of this water to the atmosphere
All water comes from the ocean and is returned to it in the
continuous Hydrologic Cycle.
(about 1,000 cubic miles). Obviously, some rain is
water that was vaporized from the land areas and
is being reprecipitated. Evidently the global hy-
drologic cycle, which sends water from sea-to-air-
to-land areas and back to the sea again, has short
circuits. These are called subcycles.
There are many complexities and variations in
the fate of water that falls as rain or snow. For
example, high in the central Rocky Mountains of
North America, the Yellowstone River heads in
Yellowstone National Park just east of the Conti-
nental Divide. The river water discharges through
the Missouri and Mississippi Rivers into the Gulf
of Mexico about 1,600 airline miles distant from
On the west side of the Continental Divide, not
far from the Yellowstone, rises the Snake River,
which flows across Idaho to join the Columbia near
Pasco, Washington, and its waters eventually reach
the Pacific Ocean about 700 airline miles from their
source and about 2,200 miles from the mouth of the
This is a good example of the continuous mixing
and transfer of water in the hydrologic cycle. An
air mass moving eastward across the Rocky Moun-
tains contains water evaporated from the Pacific
Ocean. Some of the water falls as rain or snow to
the west and some to the east of the Continental
Divide. Thus, two drops of rain falling side by side
along the continental backbone may end up, one in
the Pacific, the other in the Atlantic Ocean, although
both were derived from the Pacific.
No one knows how much water moves from the
Pacific to the Atlantic Ocean by vapor transfer,
precipitation, and runoff, but we do know a great
deal about runoff itself. Estimated total flow into
the sea from rivers in the 48 adjacent States takes
place at the rate of about 1,803,000 cubic feet per
second (a cubic foot is about 7.48 gallons), which
amounts to approximately 390 cubic miles per year.
Values for runoff (390 cubic miles) plus evaporation
(1,000 cubic miles) do not quite equal the precipita-
1,243,000 sq. mi.
2,368,000 sq. mi.
133 cubic miles
1,300 cubic miles
Amazon vs. Mississippi drainage
tion (1,430 cubic miles) because none of these values
is precise. Moreover, some water is discharged
into the sea directly from ground-water sources
without passing through streams. The missing 40
cubic miles of water, roughly 10 percent of the value
for streamflow, might represent direct ground-
Hydrologists have not generally considered that
direct ground-water outflow to the sea is so large,
but there is really no good basis that can be used
to dispute or support what the computations seem
to indicate. At any rate, the data are sufficiently
accurate for the present purpose, which is to show
the relative magnitude of water volumes involved
in the annual water cycle.
Some more specific data give a good idea of the
relative importance of large and small rivers in
maintaining continental water balances.
The Mississippi, North America's largest river,
has a drainage area of 1,243,000 square miles (about
40 percent of the total area of the 48 conterminous
States) and discharges at an average rate of 620,000
cubic feet per second. This amounts to some 133
cubic miles per year, or approximately 34 percent
of the total discharge from all the rivers of the
The Columbia, nearest American competitor of
the Mississippi, discharges less than 75 cubic miles
per year. Relatively speaking, the great Colorado
River is a dwarf, discharging only about five cubic
On the other hand, the Amazon, the largest river
in the world, is nearly ten times the size of the
Mississippi, and it discharges about four cubic miles
per day and some 1,300 cubic miles per year- about
three times the flow of all United States rivers.
Africa's great Congo River, with a discharge of
approximately 340 cubic miles per year, is the
world's second largest. The estimated annual dis-
charge of all African rivers is about 510 cubic miles.
Measurements of only the principal few streams
on a continent afford a basis for reasonably accurate
estimation of the total runoff item in a continental
water balance. The smaller streams are important
Grand Coulee Dam on the Columbia River. Franklin D.
Roosevelt Lake holds nearly 2.8 cubic miles of water.
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locally, but they contribute only minor amounts of
the total water discharged. Thus it is possible to
estimate the total runoff in all the rivers of the
world, even though many of them have not been
measured accurately. Sixty-six principal rivers of
the world discharge about 3,720 cubic miles of water
yearly. The estimated total from all rivers, large
and small, measured and unmeasured, is about 9,200
cubic miles yearly (25 cubic miles daily).
Crude estimates have indicated that the total
amount of water that is physically present in stream
channels throughout the world at a given moment
is about 300 cubic miles. Evidently, river channels,
on the average, contain only enough water to main-
tain their flow for about two weeks. Some have
much more water, others much less, but it seems to
be a fair average. How, then, do rivers maintain
a flow throughout the year, even during rainless
periods much longer than two weeks? The answer
to that question will appear later, in the discussion
of ground water.
After oceans and rivers come lakes, which can be
called wide places in rivers. This is certainly true
of the many small lakes that are impounded by
relatively minor and geologically temporary obstruc-
tions across river channels. But no single, over-
simplified metaphor accurately describes all lakes,
which are widely varied in their physical charac-
teristics and the geologic circumstances under which
they occur. The handsome little tarn occupying an
ice-scooped basin in a glaciated alpine area is
radically different from the deep and limpid Crater
Lake of Oregon, which fills the crater of a now-
extinct volcano. Lake Okeechobee in Florida is
totally different from any of the North American
Great Lakes, which occupy huge basins formed in
a complex manner by glacial excavation at some
places, moraine and outwash deposition at others,
isostatic subsidence of that whole region of the
earth's crust, and other factors. The Great Lakes
of North America, in turn, bear no resemblance to
Lake Tanganyika in the great Rift Valley of Africa.
Processes that are poorly understood created the
rift by literally pulling two sections of the earth's
crust apart, leaving a deep, open gash, part of which
is occupied by the lake. And these are only a few
examples of wide variations in the nature of lakes.
The earth's land areas are dotted with hundreds
of thousands of lakes. Wisconsin, Minnesota, and
Finland contain some tens of thousands each. But
these lakes, important though they may be locally,
hold only a minor amount of the world supply of
fresh surface water, most of which is contained in
a relatively few large lakes on three continents.
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Water volume of lakes in cubic miles
Whether a lake contains fresh or salt water
makes a considerable difference in its usefulness to
man, so the earth's greatest lakes are considered in
both of the categories, fresh and salt.
The volume of all the large fresh-water lakes in
the world aggregates nearly 30,000 cubic miles, and
their combined surface area is about 330,000 square
miles. "Large" is a relative term that requires
explanation. For this leaflet a lake is called large
Saline lakes and
SAverage in stream
Ground water within
depth of half a mile
Icecaps and glaciers
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if its contents are five cubic miles or more. Thus
the listing includes Dubawnt Lake, Canada (about
six cubic miles), but excludes the Ziirichsee of
Switzerland (about one cubic mile). The range of
volume among the large lakes is enormous; from a
lower limit of five cubic miles to an upper one of
6,300 cubic miles in Lake Baikal in Asiatic Russia,
the largest and deepest single body of fresh water
in existence. Some appreciation of its volume may
be gained from the realization that Lake Baikal
alone contains nearly 300 cubic miles more of water
than the combined content of the five North
American Great Lakes. The latter loom large on a
map, but their average depth is considerably less
than that of Baikal.
Nevertheless, North American lakes are a major
element in the earth's water balance. The Great
Lakes, plus other large lakes in North America
(chiefly in the 48 states and Canada) contain about
7,800 cubic miles of water-26 percent of all liquid
fresh surface water in existence.
Similarly, the large lakes of Africa contain 8,700
cubic miles, or nearly 29 percent of the total fresh-
water supply. Asia's large lakes contain about 6,400
cubic miles, or 21 percent of the total, nearly all of
which is in Lake Baikal.
Lakes on these three continents account for
roughly 75 percent of the world's fresh surface
water. Large lakes on other continents-Europe,
South America, and Australia-have only about 720
cubic miles, or roughly 2 percent of the total. All
that remains to fill the hundreds of thousands of
rivers and lesser lakes that are found throughout
the world is less than one-fourth of the total fresh
Saline lakes are equivalent in magnitude to fresh-
water lakes. Their total area is 270,000 square
miles and their total volume is about 25,000 cubic
miles. The distribution, however, is quite different.
About 19,240 cubic miles (75 percent of the total
saline volume) is in the Caspian Sea, and most of
the remainder is in Asia. North America's shallow
Great Salt Lake is comparatively insignificant with
seven cubic miles.
Laguna de Cotacatani, a morainal-dammed lake, in the
Andean region of northern Chile.
All these water sources we have discussed are the
obvious ones. There is another-soil moisture-
that may be the most significant segment of the
world's water supply because of the key role played
by plants in the food chain. Some plants grow
directly in water or marshy ground, but by far the
greater mass of vegetation on earth lives on "dry"
land. This is possible because the land is really dry
at just a few places, and often only temporarily.
How dry is dust? The dust of a dry dirt road may
contain up to 15 percent of water by weight.
However, plants cannot grow and flourish with so
little water because the soil holds small percentages
of moisture so tenaciously that plant roots cannot
extract it. Aside from desert plants, which store
water in their own tissues during infrequent wet
periods, land plants flourish only where there is
extractable water in the soil. Inasmuch as a quite
ordinary tree may withdraw and transpire about 50
gallons of water per day, frequent renewals of soil
moisture, either by rain or by irrigation, are essen-
tial. The average amount of water held as soil
moisture at any given time is on the order of 6,000
cubic miles for the world as a whole-an insignifi-
cant percentage of the earth's total water, but vital
to life. Relatively little vegetation receives artificial
irrigation, and practically all of it depends on
natural soil moisture, which, in turn, depends on
orderly and timely operation of the hydrologic
Another little-considered water reservoir has
been known to man for thousands of years. Scrip-
ture (Genesis 7:11) on the Noachian Deluge states
that "the fountains of the great deep [were]
broken up" (cleft open), and Exodus, among its
many references to water and to wells, refers (20:4)
to "water under the earth." Many other chronicles
show that man has known from ancient times that
there is much water underground. Only recently
has he begun to appreciate how much.
Beneath most land areas of the world there is a
zone where the pores of rocks and sediments are
completely saturated with water. Hydrologists call
this ground water, and the upper limit of the
saturated zone is called the water table. The water
table may be right at the land surface, as in a
marsh, or it may lie hundreds of feet below the land
surface, as in some arid areas. Water in the un-
saturated zone above the water table is called vadose
water and includes the belt of soil moisture. Water
in the intermediate part of this zone has passed
through the soil and is percolating downward
toward the water table.
The world volume of that part of the vadose
water below the belt of soil moisture is probably
somewhat more than that of soil moisture-say
10,000 cubic miles. It is highly important because,
although it is not extractable by man, it is potential
ground-water recharge, and ground water is ex-
tractable. Each new influx of water from precipi-
i Belt of soil moisture
Relation of the water table to saturated and unsaturated
station on the land surface followed by percolation
through the soil provides a new increment of re-
charge to the ground water.
Below the water table, to a depth of half a mile
in land areas of the earth's crust, there is about one
million cubic miles of ground water. An equal if
not greater amount is present at a greater depth,
down to some 10 to 15 thousand feet, but this deeper
water circulates sluggishly because the rocks are
only slightly permeable. Much of the deep-lying
water is not economically recoverable for human
use, and a good deal of it is strongly mineralized.
Ground water flows through moderately to highly
permeable strata, which are called aquifers, at rates
of a few inches to perhaps several hundred feet per
day; 40 to 50 feet per day would be a rather high
rate of flow.
Depending on how far the ground water must
travel to reach a surface discharge area, water in
shallow to moderately deep zones may remain
underground from a few hours to 100 years or
longer. Water at great depth may take tens or
hundreds of thousands of years to pass through an
aquifer, and some is completely stagnant.
The volume of ground water in the upper half-
mile of the continental crust probably is about 3,000
times greater than the volume of water in all rivers
at any one time, and nearly 20 times greater than
the combined volume of water in all rivers and lakes.
It is easy to see, therefore, that ground-water
reservoirs have tremendous importance as
equalizers of streamflow. ]
Under natural conditions,
most ground-water res-
ervoirs are full to over-
flowing, and the overflow we ...
water provides what is
called the base flow of ;
surface streams enabling
them to flow even during -
long, rainless periods and ,h
after winter snows have
According to calcula-
tions, the volume of
ground water in storage
in the United States to a
depth of half a mile is
equivalent to the total of
all recharge during about
the last 150 years. This estimate is crude, but it
helps to emphasize the important fact that ground-
water reserves, although immense, are not wholly
self-renewing annually. At places where they have
been depleted by pumpage they might take many
decades to recover even if pumping were stopped
Consider for example, a location in the dry south-
western United States where annual recharge to
an aquifer is on the order of only two-tenths of an
inch of water. In such areas it is not uncommon to
pump two feet or more of water per year for irriga-
tion or other uses. In this over-simplified example
if the entire aquifer were pumped at that rate,
yearly pumpage would be equivalent to 120 years'
recharge, and ten years of pumping would remove
a 1,200-year accumulation of water. New recharge
during the pumping period would be negligible.
Mechanical problems and economic factors would
prevent complete dewatering of an aquifer, but the
example is valid in principle.
The next big items on the water-balance sheet are
icecaps and glaciers. They may seem unimportant
in the water cycle because, although the ice masses
alternately shrink or grow a little from time to time,
new ice is added about as fast as old ice melts. The
polar ice masses, however, have a great influence
Vast expanse of the Antarctic ice sheet, shown in relief
model, represents 85 percent of all the ice in the world.
on weather, and everything that happens in the
polar regions indirectly affects everyone through-
out the world (NATURAL HISTORY, October,
1963). Moreover, if a shift in climate led to exten-
sive melting of icecaps, there would be a rise in sea
level with important effects in all low-lying coastal
Mountain glaciers, such as those of the Alps in
Europe (after which alpine glaciers are named), the
Himalayas of Asia, and the Cascades of North
America, are like average rivers in some respects.
They are important locally, but they contain only
an insignificant fraction of the world's water. The
total volume of all alpine glaciers and small icecaps
in the world is only about 50,000 cubic miles (com-
parable to the combined volume of large saline and
An alpine glacier is one that rises in mountainous
uplands and, by plastic deformation, flows along a
valley. A continental glacier, or icecap, is one that
is plastered over the landscape, mountain and valley
alike. Icecaps tend to flow radially outward from
their center of accumulation. Wastage occurs by
sublimation from the surface and by melting or
caving away around the periphery. Average ice-
caps, like those on Novaya Zemlya, Iceland, and
Ellesmere Land, are analogous to average lakes.
They are locally important, but hold only an insigni-
ficant share of the world's water and only a small
part of the total volume of perennial ice.
The Greenland icecap is an entirely different
matter. About 667,000 square miles in area and
averaging nearly 5,000 feet in thickness, its total
volume is about 630,000 cubic miles. If melted it
would yield enough water to maintain the Missis-
sippi River for somewhat more than 4,700 years.
Even so, this is less than 10 percent of the total
volume of icecaps and glaciers. The greatest single
item in the water budget of the world, aside from
the ocean itself, is the Antarctic ice sheet.
Since the advent of the International Geophysical
Year 1957, considerable information has been ac-
cumulated about Antarctica. Data on the thickness
of the ice sheet are relatively scarce, but there is
enough information to permit an approximate esti-
mate. The area of the ice sheet is about six million
square miles; the total volume therefore is between
six and seven million cubic miles, or some 85 per-
cent of all existing ice and about 64 percent of all
water outside the oceans.
The hydrologic importance of the continent and
its ice may be illustrated quite briefly. If the
Antarctic icecap were melted at a suitable uniform
rate it could feed:
O P -. _
,.nd nil~rii. ~~
1. The Mississippi River for more than 50,000 years.
2. All rivers in the United States for about 17,000
3. The Amazon River for approximately 5,000 years.
4. All the rivers in the world for about 750 years.
The statistics about water given here are rather
simple, but they are sufficiently important to tabu-
late in order to get them more clearly in mind. The
table on pages10 and 11 gives a comparative view of
the world's water.
About 97 percent of all water in the world is in
the oceans. Most of the remainder is frozen on
Antarctica and Greenland. Thus, man must get
along with the less than one percent of the world's
water that is directly available for fresh-water use.
Obviously, he must find much more effective ways
of management if he is to prosper.
Water is a global concern, and the water cycle
recognizes no national boundaries. Man has be-
come so numerous and his activities so extensive
that he has begun to affect the water cycle-
certainly on a regional scale and very likely on a
global scale. To learn more about the world's water
and how to use it, many countries have joined to-
gether in a program -the International Hydro-
logical Decade-aimed at overcoming the now
existing critical deficiency in hydrologic knowledge
on a global scale.
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