GROUNDWATER RISES TO THE SURFACE OF THE SAHARA AT THE OASIS OF OUNIANGA KEBIR, FORMING SPRINGS WHICH CREATE AN ISLAND OF LUSH GREENERY IN THE DESERT.
EVEN IN THIS AGE OF HOMAGE to rational science, the ancient rites of
the water witches survive. Several years ago, a group of dowsers, meet-
ing in the little Vermont town of Danville, demonstrated a search for
water. They tramped across several acres of meadow, divining rods ex-
tended, until a mysterious tug signified the location of a water "vein."
Eight feet down they dug, 10 feet, then 12 feet-the predicted depth.
The pit remained as dry as the scorched dust of summer. Sifting the
soil through his fingers, one dowser said, "We didn't dig deep enough."
He was absolutely right. In most parts of the world, any hole dug deep
enough will yield water-with or without the aid of the diviner's forked
hazel stick. Water is plentiful in New England, and at least two to five
gallons of water per minute can be produced from almost any hole deep-
er than 20 feet. Near Miami, a yield of 1,000 to 1,500 gallons per min-
ute is considered commonplace from wells averaging 50 feet in depth.
The sand underlying Tallulah, Louisiana, yields 7,000 gallons per min-
ute. Even around Tucson, drillers count on striking a dependable house-
hold supply at several hundred feet.
Potable water exists in the ground-in some quantity, in some form,
at some depth-nearly everywhere on earth. The Sahara itself, a syno-
nym for total aridity, is underlain by water: an estimated 150,000 cubic
miles spreading over 2.5 million square miles of land area. Indeed, al-
most all of the world's stock of fresh water-two million cubic miles, or
more than 97 per cent of the total available-is inside the earth. Half
of this huge supply is believed to be within a half mile of the surface
and is therefore reasonably accessible.
Most underground water is constantly in motion, ultimately to emerge
at the surface again, pulled by gravity from under sloping terrain into
springs and streams, lifted up by plants and pumped out by man. Its
vast bulk, traveling out of sight, supplies much of the water used for
drinking, washing and industrial processing. Water dissolves salts from
the earth; it creates caves and the stalactites and stalagmites that orna-
ment them; it produces bubbling mineral springs and showy geysers.
Since groundwater is hidden, the existence of superstition about its
location is almost inevitable, and descriptions of its movement under-
ground are not only capricious but often bizarre. Few untrained men,
particularly in humid areas where water is relatively abundant, know
much about reading the surface signs of it. In dry areas, however, weath-
er and water supply are subjects of absorbing interest, and many a farm-
er in Arizona and New Mexico can guess the volume of a stream, the
amount of water that a field of grain will require and the possible depth
of subsurface water.
Groundwater is no more mysterious in its nature and movement than
water on the surface and in the air. Whatever the environment, water
exhibits its usual remarkable properties and obeys the general laws of
physics and chemistry. Gravity attracts the water from the skies, pulls
it beneath the surface of the ground, distributes it among permeable
layers and influences the directions in which it will flow.
A WATER TABLE is formed through a process
similar to the one shown above: water
running into soil (top) sinks in and forms
an underlying layer of saturated earth (center).
A hole dug in the earth-in other words,
a well-will fill up to the level of the water
table (bottom). The water table varies in depth
according to precipitation: it is closest
to the surface in wet periods and farther down
during droughts, when wells sometimes go dry.
Wherever precipitation touches the earth, some of it soaks in. It seeps
downward toward the center of the earth until blocked at some depth
by nonporous rock; simultaneously it spreads out horizontally so that
vast volumes of earth become saturated with water. The water soaks
into the dirt and moves through the permeable earth from pore to pore.
Only rarely, in a few caverns, does groundwater exist in clearly defined
pools or flow in identifiable streams. There are no "veins" of water. A
well is simply a hole reaching down into the saturated region. Water
seeps from the saturated earth into the hole and any hole in the neigh-
borhood, dug to the same depth, will usually produce just as successfully.
A watery underworld
The proportion of water that sinks into the ground varies with the
character of the soil. If the soil is dry and porous, large amounts will
seep in. The worst condition for absorption is a sudden downpour of
rain on a sloping surface of less permeable material, such as clay; most
of that precipitation will quickly run off the surface.
The outermost surface of the earth is composed largely of porous, fair-
ly loose material, principally sand, gravel, silt and decayed vegetation.
Most of this surface is underlain by porous rock such as sandstone and
limestone. Beneath this everywhere is bedrock, so compact, as a result
of molten origin or of subsequent heat and pressure, that it is totally
impermeable. All layers above this impermeable base rock hold ground-
water. The layers are classified by water content into two regions: the
zone of aeration and the zone of saturation.
Seeping below the surface, water first enters the zone of aeration, a
transition level where the earth contains both water and air. Its depth
varies widely, from an inch or less near the edge of a swamp to hun-
dreds or thousands of feet elsewhere. In this zone, water shows its pow-
ers of adhesion by clinging to particles of soil and rock. The amount
held in the pore spaces by this molecular attraction fluctuates widely
and rapidly. Immediately after a rainstorm, the zone of aeration may
be surfeited with water; shortly after, it may contain little; during a pro-
longed drought, it may contain almost none at all. Some water that en-
ters this region sinks through to the layers beneath; some is absorbed
by plants or evaporates into the air. The zone of aeration ends in a moist
region called the capillary fringe. It contains water lifted from the still-
lower zone of saturation by capillary action. Its depth depends upon
the diameter of the soil's pores: if the pores are relatively large, little
water will be drawn up and the belt will be narrow; but if they are fine-
bored and continuous, water may climb as high as eight feet. Sometimes,
though not often, this "fringe" reaches all the way to the surface.
The lower moist layer, comprising the zone of saturated earth, forms
a principal water resource. Wells dip into it; springs, rivers and lakes
are its natural outcroppings on the surface of the globe. Water seeping
downward can go no farther; every pore, crack and interstice is filled.
The top of the saturation zone-the boundary between it and the
capillary fringe-is called the groundwater table, or simply the water
table. The water glinting at the bottom of a shallow well is an exposed
part of the water table. Around it and continuous with it, the same wa-
ter table extends-whether exposed or not, above the ground or in it.
The surfaces of lakes and rivers are also exposures of the water table
and, to a hydrologist's eye, blend with the water table into the landscape.
This relationship of earth to water table can be demonstrated by pour-
ing water into a sand-filled tub. The water sinks through the sand and
disappears, and soon the surface is completely dry. If holes are poked
deep into the sand to simulate wells, and if grooves and pockets are
scooped out to simulate river channels and lake basins, the water will
appear at the bottom of each depression, and will reach precisely the
same level in each. That level is the water table of the sand tub.
The tub of sand is an oversimplified model of an aquifer-a layer of
gravel, sand, porous rock or other coarse materials through which water
flows more freely than elsewhere in the earth. Few aquifers are as homo-
geneous as the sand in the tub; their contained water meets differing
degrees of flow resistance, due to differences in porosity and particle
size. As a result, the water flows at varying speeds in seeking its level,
and it almost never becomes truly level, as it does in the sand-tub model.
The rise and fall of water tables
The changing elevations in the earth's water table are revealed by its
surface waters. Some lakes are higher than others. Streams run down-
hill. The water table, which must connect them all, also slopes. Its con-
tours reflect in part the landscape above it; it is high under mountains
and dips toward river valleys. Occasionally, the surface contour drops
S more sharply than the water table beneath it. It cuts into the water
table and exposes saturated earth so that water issues forth: a spring.
If a wide swath of the land's surface dips beneath the water table, a
lake or swamp occurs. Across the lowest dip of a valley the water table
supplies a river. In fact, a river's channel is often a continuous spring
that sustains the river's flow under sunny skies when no rain falls.
One of the factors influencing the contour of the water table is the
contour of the land above it. This connection is best seen in an idealized
landscape: a low and gently sloping hill with a river valley on either
side, all underlain by homogeneous porous material. As rain falls and
seeps downward, water accumulates underground at the base of the
porous material. The water table rises uniformly, as in the sand-tub
model. It remains essentially flat until, as more rain falls, it rises so far
above the base that it reaches the lowest portions of the two valleys.
It will now seep out into the valleys and fill those channels.
Thereafter, groundwater feeds into the two rivers. As rain continues
to fall on the hills, it soaks into the earth, seeps down to the aquifer
and-since the aquifer is now higher than the valley-seeps out the sides
of the hill. If the amount seeping out of the hill and drained off by the
river channels precisely equaled the amount soaking into the ground
DECLINING PRESSURES that occur
in artesian water systems are explained by this
diagram. Artesian water, trapped deep
in the ground, is under great pressure near
its source. When tapped by a well in this area,
it surges up strongly. As it flows underground,
artesian water encounters friction, which
reduces its head of pressure. As wells are
drilled farther from the source, the water
in them rises to lower and lower levels.
to add to the aquifer, the water table would not change. But friction
intervenes. Friction occurs between the water and the walls of the in-
terstices through which it moves; to a lesser degree, friction also occurs
between sliding planes of molecules within the liquid itself. The water,
its movement thus retarded, piles up under the hill.
Now the water table is no longer level, but peaked. Its highest point
lies immediately below the hilltop, its lowest near the river surfaces.
The table's steepest slopes resemble the slopes of the hill but are con-
siderably less abrupt.
Nature's complex plumbing
The elevation of parts of the water table causes water to move un-
derground. The water above presses down on the water beneath, just
as it does in the pipes in a house. For the underground water system
operates like a vast natural plumbing system. The flow of water through
its pores and spaces is governed by the same factors that govern the
flow of water through man-made piping: the size of the pipes and the
pressure pushing water through.
The water-transporting pores in the ground connect to form extend-
ed passageways. If the pores are large, water meets little resistance and
easily flows through. If they are small, or if the passageway must carry
water over a long distance, flow is retarded. The flow halts completely
against nonporous material.
Unlike a municipal water installation, nature's underground plumb-
ing system cannot maintain an even flow. A change in any of the physi-
cal factors described above may reduce or increase the flow of ground-
water. The flow increases when water from rain or snow percolates down
faster than it flows out at the point of discharge; the water table ele-
vates, raising the pressure head. Conversely, the flow abates when in-
coming water does not equal the outflow, lowering the water table and
the pressure head. Should the water rise at the discharge point, the
pressure head again lessens; as the height difference between source
and discharge point is reduced, discharge and flow decline. They may
halt altogether or even reverse. For example, sudden floods can boost
the height of a stream's surface above the water table. In such a case,
water no longer runs from the aquifer into the stream; the flow reverses
and water runs out of the stream into the riverbanks.
The pressure moving underground water often is not great. Water
seeps into wells and trickles from springs. But occasionally a hole in
the ground taps water under great pressure. This condition, common
to many regions, is called artesian. The name comes from the province
of Artois in northern France, where a series of productive wells stirred
interest in the 12th Century, but artesian conditions have been recog-
nized for thousands of years. Many centuries before Christ, the Chinese
brought in artesian wells by driving bamboo casings down many hun-
dreds of feet; the job often required generations to complete, and the
wells were called "grandfather wells."
The pressure that powers artesian wells is created when groundwater
seeps between two layers of nonpermeable materials. These layers pre-
vent the release of water. They act like the walls of a pipe, and can hold
water under pressure. The pressure is generated as the aquifer slopes
down through the earth, the water at upper levels pressing on confined
water lower down. The amount of pressure released where a well pierces
the wall depends on the difference in elevation between that point and
the highest level of water in the aquifer. This difference may be great,
since water may fill an artesian aquifer nearly to the top of the "pipe."
Artesian aquifers can be found at any depth, but many plunge far
into the earth. In London, wells were dug in the 18th and 19th Cen-
turies to a depth of 300 to 500 feet into the water-saturated chalk
layer, sandwiched between clay strata, underlying a large area beneath
the city. They continue to supply millions of gallons daily.
Not until the mid-19th Century was artesian water reached by drill-
ing with efficient cable tools. One of the first such wells was completed
in the Paris suburb of Grenelle in 1841. It took seven years to drill,
stirred international interest, reached a depth of 1,798 feet and, when
it came in, stunned the drilling crew by shooting a parabolic fountain
high in the air. In nearby Passy, soon after, a well was sunk 1,900 feet
into the same saturated substratum. From the completed borehole a
continuous stream of water jetted 54 feet above ground at a rate of more
than five and a half million gallons a day.
A hot-water system
The same type of geological formation that supplies cold water from
artesian wells is also the source of many hot, or thermal, springs. Arte-
sian aquifers often dip deep into the earth, and the underground tem-
perature increases with depth-about 1"F. for every 60 feet. If the im-
permeable rock layers confining the water dip down far enough, they
may carry it to regions that will heat it to high temperatures.
One of the better-known thermal springs is that at Warm Springs,
Georgia, where many polio patients are treated. The source of its water
is rain falling on Pine Mountain, two miles south of the village. The rain
sifts into a permeable rock formation known locally as the Hollis, which
carries it northward for a mile at a depth of a few hundred feet. Its
average temperature at the start is about 62"F. The Hollis plunges
down to 3,000 feet, where it ends against impermeable rock. The water,
now hot and also under pressure, is turned back. Forced to the top, it
emerges at a temperature of 88"F.
Man-made connections to the underground plumbing system influ-
ence its operation, sometimes drastically. Withdrawing water from an
aquifer at any one place inevitably affects the entire aquifer. This is true
of artesian systems as well as water-table aquifers. Withdrawal may
decrease the flow, the pressure, the amount delivered, or all three. Un-
less the recharge of the aquifer equals the rate of withdrawal, the supply
will ultimately dwindle.
THE POROSITY OF SOIL, which strongly
affects the availability of groundwater, may
vary greatly in equal quantities of matter.
In the two examples diagramed here, for
example, the amount of substance is the same
in each case, but one is almost twice as
porous as the other. The rock above (perhaps
sandstone) is 47.6 per cent porous;
by contrast, the porosity of the soil below
(which might be clay) is only 25.9 per cent.
Even a single well has pronounced effects on the water table. The
well takes water from the saturated earth around it, literally drying
out part of the saturated zone and causing a dent to form in the water
table's smooth surface. When heavy pumping removes water from the
well more rapidly than the inflow replaces it, the dent in the water
table becomes an increasingly deep, wide cone. It becomes what hy-
drologists term a "cone of depression"-a conical section of the wa-
ter table depleted of water. Cones of depression form around every well
from which water is drawn. Each depression cone, by increasing the
pressure head locally, boosts the flow from the surrounding area of an
aquifer until the inflow to the well balances the rate of withdrawal. Arte-
sian water also must be replenished if the flow is to be maintained. Dis-
charge from the wells must be matched gallon for gallon by recharge into
the aquifer over a period of time. This balance rarely occurs, and many
wells that once flowed under their own pressure now require pumping.
In the midwestern United States, a classic example of artesian con-
dition occurs in a great sandstone layer, sandwiched between two im-
permeable rock layers, which comes to the surface in the Black Hills of
Wyoming and South Dakota. The aquifer receives rain and surface run-
off and transports the water underground, curving across several states.
Many richly flowing wells owe their productivity to this great aquifer.
But since the 1890s, while demands on the Black Hills supply have mul-
tiplied, the pressure and rate of flow of these artesian wells have de-
clined greatly. The level in a well near Pierre on the Missouri River
dropped 300 feet in 35 years.
The waters of antiquity
The continuing drain on water supplies already tapped has prompted
suggestions for probing deeper into the earth's stock of underground
water. There are many water-bearing layers below even those now
tapped by artesian wells and they contain far more water than the
sources now being used.
Some of this untouched water has been trapped deep in the earth for
millions of years, buried in the pores of sedimentary rock laid down
beneath ancient seas. It is called connate water, from a Latin word
meaning "born at the same time." Connate water has been absent from
the hydrological cycle for ages. But there is also water inside the
earth that has never taken part in the hydrologic cycle. It is part of the
original supply of water molecules that were drawn from cosmic dust to
form the earth billions of years ago. It may exist as steam or it may be
bound into crystalline rocks as water of hydration. Occasionally some of
this "juvenile" water escapes to the surface by volcanic action.
Unfortunately, neither connate nor juvenile water is usable for most
purposes. It has been held deep in the earth, where it is hot and under
great pressure. In intimate contact with mineral substances for many
hundreds of millions of years, this hot, pressurized water has become
loaded with dissolved salts.
Tapping deep sources of the earth's water is not only impractical, it
also seems unnecessary. The supplies of fresh groundwater fairly close
to the surface are more than ample to fill human needs if all of them
are explored and properly managed. And growing understanding of
the nature of water underground is making the search for usable water
less a matter of art and luck, and more a reliable, scientific procedure.
Nature teamed with technology
Simple facts have long served to guide the seeker after a water sup-
ply. Since the water table-the upper boundary of available water-
slopes less steeply than the ground, it is closer to the earth's surface at
the bottom of a hill. Such a location has been favored by well-diggers
for thousands of years. Surface water, in ponds, swamps, streams, offers
an even stronger hint; there the water table has actually emerged
from below ground. The flora of an area provides another of nature's sim-
ple clues, for plants indicate water's presence as well as its absence. In
arid lands, the xerophytes-literally, "dry plants"-attest to water's
scarcity. Cactus, for example, adapts to the lack of water and its pres-
ence denotes the absence of readily accessible water. The phreatophytes,
or "well plants," on the other hand, grow only where they can send roots
deep into the capillary fringe and, indeed, into the water table itself.
To the trained observer, they say, "Dig here." Several phreatophytes-
salt grass, mesquite and greasewood-betoken water's presence in the
arid regions of the United States; in other areas the willow tree, syca-
more, cottonwood and palm tree are equally important indicators.
While nature's hints are helpful, the finding of water rests heavily to-
day on growing technology. The supply for a modern town is so im-
portant that careful-and expensive-investigation is warranted. Stud-
ies of the geology of an area and of its existing wells enable an engineer
to eliminate large areas as unproductive and choose the most favorable
zones for water's occurrence. He may drill test holes to reveal what lies
beneath, or he may turn to indirect methods. Two commonly used tech-
niques depend on electrical resistivity and seismic refraction. The for-
mer measures the amount of resistance offered to an electrical current
by underground rock structures. The seismic-refraction method meas-
ures the speed with which a surface shock-usually from a dynamite
explosion-travels a known distance through the earth. The shock
wave's velocity tells whether rock structures several hundred feet under
the surface are porous or solid and, if porous, whether they hold water.
Water content increases the shock wave's velocity; porosity decreases it.
Aerial photographs of an area are also one of hydrology's valuable
tools. A trained hydrologist will read from them marks of erosion, vegeta-
tion, drainage patterns, gravel pits and other significant signs of ground-
water. And more elaborate research on a broad scale-including anal-
yses of groundwater movements over large areas with the aid of an
electronic computer-is providing the basic knowledge that will solve
the remaining mysteries of water-seeking.
A SPRING IS BORN usually in one of three
ways. Most often- groundwater filling all
available pore space above a layer of rock
simply surfaces as a spring (top). Sometimes
a layer of groundwater is exposed by a
geologic fault created when a shift occurs in the
earth's strata (center). The third type of spring
is found where a layer of artesian water,
usually under great pressure, broaches
the surface (shown at bottom, with a common
water-table spring above it for comparison).
Mapping a World
Spreading everywhere under the earth's surface, ground-
water constitutes a major source of usable water. It fills
the wells of towns and farms, and helps replenish the sup-
plies of large cities. Where surface water is scarce, ground-
water is so vital for irrigation that men have fought over the
right to tap it. It is distributed everywhere about the earth
-under the arid flats of Death Valley and the highest peaks
of the Himalayas alike. It slowly seeps through surface soil
or settles in deeply buried strata, where it is trapped at
great pressures. Only one half mile down, there are an esti-
mated 50,000 cubic miles of water beneath the United
To map, manage and conserve this resource, hundreds
of scientists are engaged in tracing its vague wanderings.
They perform a miracle of detection: although they rarely
see the groundwater-except when it seeps into a well-
they can estimate how much is stored under any county
across the continent, how fast it is moving, how soon pump-
ing of one well will affect another's yield, where water under
a farm came from and where it will be decades in the future.
CARVER OF EERIE CAVERNS
Lying 200 feet below the surface, an enormous, people to be a major source of groundwater, are
blue-lit cavern, laced with calcite, stretches for actually quite rare. The pools or rivers in these
miles beneath the Ozark National Forest in Ar- cavernous formations represent only about 5 per
kansas. Such limestone caves, assumed by many cent ofthe total groundwater reserves ofthe earth.
--ii ---- s_.--.,i _. .....i-_l:;i.ls~., ,.~;i.~*-~__~~ ,txiui~i;):l_.:G::Zixli~j-%i~FgT-~i~T~~
Falling as rain or flowing in rivers,
water seeps into the soil and begins
a massive underground migration.
Groundwater roves as restlessly as
any river, although its movement is
often extremely slow. It might take
135 years for water to travel lateral-
ly through a single mile of sand.
A considerable quantity of ground-
water soaks the soil near the surface.
Bounded at bottom by a rocky bar-
rier, water seeps downward until it
creates a zone of total saturation.
The top of this zone is known as the
water table. Not all water remains
near the surface, however. Impelled
by gravity, some groundwater finds
its way into deeper beds-artesian
strata-where it is caught between
impermeable layers of rock and held
under pressure by the weight of the
water above. It often spreads over
20-mile stretches, fed by a river or a
spring and runs out into distant sur-
face soil or into the sea. The height
to which this pressurized water will
rise if tapped (indicated by the dot-
ted line above) diminishes as the wa-
ter flows farther from its source.
To tap groundwater, a well can be
dug into the surface soil, or drilled
deeper down to an artesian stratum.
A surface well, dipping into the wa-
ter table, simply fills up like a straw
stuck into a glass. But an artesian
well taps water under pressure; it
comes surging upward, often higher
than the water table and sometimes
many feet higher than the land itself.
WATER TABLE -
CHANNELS AND BARRIERS
Shale, sand and gravel all store water, but they transmit it at varying
speeds. Water passes easily through the large and loosely packed grains
of gravel. It travels more slowly through sand, which has small pores,
and scarcely at all through shale, whose pores are not interconnected.
_ C T_
A TELLTALE PRESSURE TEST
On a capped artesian well near Phoenix, a are so sensitive to underground fluctuations
hydrologist reads a meter which shows changes that they also pick up shock waves of earth-
of water pressure in a buried stratum. Some quakes and underground nuclear explosions
instruments used to determine well pressure -and can even distinguish between the two.
A Vigil over
Indiscriminate tapping of groundwa-
ter has brought farmers to the brink
of disaster in some parts of the coun-
try. When too much water was drawn
from a stratum under California's
San Joaquin Valley, the land, rent by
deep fissures, sank as much as 23
feet. Overdraft of groundwater in
one part of Arizona lowered the wa-
ter table by 400 feet-and 320,000
acres of farmland were lost as the
cost of irrigation became prohibitive.
The threat of such catastrophes
keeps hundreds of hydrologists out
in the field checking wells and test-
ing artesian pressures. For almost
two thirds of all wells in the U.S.,
"biography" cards (with data on the
amount of water pumped and the
type of soil tapped) are on file with
the U.S. Geological Survey. This gov-
ernment agency, with accountants'
caution, keeps track of the ground-
water reserves of the entire nation.
A YARDSTICK FOR IRRIGATION
Above a small dam that backs up well water and estimating how much water will evaporate
pumped for a California irrigation project, a or be used by plants, engineers calculate how
gauge records on a drum how much water is much will return to the ground. Thus they can
flowing into the ditches. Knowing the flow, foretell changes in the level of the water table.
TAKING THE PULSE OF A WELL
With a stopwatch and flowmeter, a fieldworker
measures the pumping rate of a well every half
hour. He also checks how far the water table
drops. Combining these figures, experts calcu-
late the yield of the well. Whenever feasible, a
new well is given this pump test. Data from all
tests in a region, fed into a computer, enable
scientists to map future water-table contours.
As one U.S.G.S. scientist (standing) connects a voltmeter to the groundwater computer for Kansas and Nebraska, another (right) reads on an
Computing an End
to Water Wars
On May 27, 1927, a band of California
farmers dynamited an aqueduct car-
rying water to a nearby county. Oth-
er "water wars" have been fought
with pitchforks, shotguns and fists.
Recently, such a dispute arose be-
tween Kansas and Nebraska-and
was arbitrated by the analogue com-
puter above. It was feared that the
wells for a proposed Nebraska irriga-
tion project would lower the water
table and impair the flow of a river
that supplied Kansas farms. The
U.S.G.S. was able to indicate sites
oscilloscope how far the water table will fall in 50 years. From such spot-readings, the scientist at left is drawing a water-table contour map.
for the irrigation-project wells, and
also reassure the Kansas farmers that
their river would not run dry.
The brain behind this hydrological
soothsaying is a giant circuit that
electrically simulates the actual soil
conditions, the water usage, the di-
reactions of flow and the water-table
level in any groundwater system.
The computer can accurately predict
what changes will occur in water-
table contours in any part of the re-
gion, depending on how much water
is drawn out over a given period of
time. The components of this com-
puter (described in detail on the next
two pages) made it possible to reduce
the contested area of Kansas and Ne-
braska, a groundwater system cover-
ing 7,600 square miles, to a manage-
able 53 square feet of electric circuit.
A scientist works on the computer for a groundwater system. Its construction required 30,400 resistors. An average TV set has only 73.
SCALING DOWN THE LAND
A sample groundwater system gives the raw
data used in making a regional analysis From
pump tests of representative wells in each
square mile of terrain. average values are cal
culated for the land's storage capacity and re.
distance to water flow. A coarse grained sec.
tion of land, which can store and transmit
water readily Idiagram at leltt, is represented
foreground) by a capacitor of high electrical
capacitance and four resistors of low electrical
resistance. Dense soil (right) is represented by
elementsol low capacitance and high resistance
to Simulate Flow
GRAPHING THE WATER LEVEL
The drop in a water table where a well is in
use. shown schematically above. at left. can be
predicted on an electronic graph, as shown at
right This graph is produced by an oscillo-
The computer that arbitrated the wa-
ter dispute between Kansas and Ne-
braska took a month to build. It uses
electricity to imitate the storage and
flow of water in soil, through the use
of simple elements known as resis-
tors and capacitors: a resistor im-
pedes current much as soil impedes
the flow of water; a capacitor stores
electricity much as soil stores water.
After gathering data on the actual
terrain, scientists simulate soil char-
acteristics they have measured for
each square mile by combining a ca-
pacitor and four resistors that are ap-
propriate to that square mile. If the
soil is easily permeated by water, the
resistors chosen have low resistance
to electricity. If the soil can store
large quantities of water, a capacitor
is used that has a high rating for elec-
trical capacitance-i.e., that stores a
large quantity of electricity. Thus,
scope that is connected to a simulated ground.
water circuit The break in the upper bar indi
cates when a well pump. which delivers five
gallons per minute, is turned on and off The
the flow of current through the cir-
cuit corresponds with considerable
accuracy to the flow of water through
the actual terrain.
The water-table level at any geo-
graphical spot can be determined by
simply measuring the voltage at the
corresponding spot on the computer.
The circuit is checked out by simu-
lating an actual pumping period, and
then comparing the results with
farmers' records for the same period.
To study the effects of a still-unbuilt
irrigation project, experimenters
draw more electricity from the ap-
propriate site on the model. The fu-
ture drop in the water table will be
indicated by a voltage drop in the cir-
cuit. Thus, water-table contours can
be mapped decades ahead. This is
routine work for the computer, which
can "pump" a century's worth of "wa-
ter" in a few millionths of a second.
lower bar representing thi change in water
level, shows a steep drop when the pump is
turned on and a steep rise when it is turned
off. Thereafter the water level gradually rises.
Agents of Waste
Scientists, constantly confronted by
unusual groundwater problems, must
often use great ingenuity to study
them. The salt cedar tree, common
in the Southwest, has posed one such
problem. Extending its roots down
to the water table, this tree breathes
groundwater into the air through its
leaves, causing a loss of 20 trillion
gallons to the atmosphere each year
over 900,000 square miles of the west-
ern U.S. Hydrologists have studied
the salt cedar's wasteful habits by
flying moisture-sensitive instruments
over the trees and radioing data back
to a computer on the ground. Large-
scale attempts have been made to
uproot and poison these trees, but so
far the salt cedars have proved too
hardy to eradicate.
Groundwater is lost in countless
other ways, including contamination.
Today scientists are studying such
problems as seepage of acid out of
mine shafts; pollution of rain over ur-
ban areas; and contamination of soil
by detergents from suburban sinks.
A STAND OF PROFLIGATE TREES
Perched on a hilltop, high above a field of salt
cedar trees in Arizona (above at left), a van
serves as headquarters for hydrologists con-
ducting research on these water-wasting trees.
The van, also shown below, houses a computer
used to analyze data radioed back from a spe-
cial research plane. Atop the van is a radar
antenna, used to track the path of the plane.
MARKING MOISTURE FROM THE AIR
Collecting data. a scientist talks by radio to
the pilot of a research plane (background),
heading toward salt cedar trees. Flying back
and forth at low altitude over the field, experts
record increased air moisture on specially sen-
sitive infrared film. From data on the air's mois-
ture, temperature and pressure, a computer
can calculate how much water the trees waste.
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