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EVAPORATION CONTROL

BY

Richard D. Reidhead, Jr.

AN ABSTRACT

Submitted to the College of Agriculture of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Resource Development

1960

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ABSTRACT

Increasing demands for water have brought a need for wise
management of this resource. Evaporation represents one of the
greater losses of water stored in reservoirs. Evaporation controls
may, in certain circumstances, be valuable in increasing water supplies.

The purpose of this thesis was to attempt to bring together a
review of the literature on evaporation and the various methods of
control. Information regarding how evaporation is effected by the
different types of control is reported with a subsequent evaluation of
each. The strengths and limitations inherent in these techniques are
given along with a brief descriptive account of the control itself.

Effective control of evaporation calls for application of knowledge
concerning the basic principles of evaporation. Important among these
principles are an understanding of the general structure of water mole-
cules and also of the effects of temperature, wind, and water depth on
evaporation.

Various control measures were discussed and evaluated. These
included controlling evaporation by: (1) construction of reservoirs with
maximum average depth, (2) concentration of water into single reservoirs,
(3) the use of windbreaks, (4) elimination of shallow water areas,

(5) elimination of aquatic and riparian vegetation, (6) roofs and floating
covers, (7) storing water in ground water reservoirs, and (8) evaporation
control by monomolecular films.

The reduction of evaporation by methods which limit the amount
of water surface exPosed to the atmosphere offers considerable promise.
Considering evaporation losses in management procedures of reservoir

systems may in some instances be profitable. Some degree of

ii

evaporation control can be achieved by designing a reservoir having

a maximum average depth, thus having a smaller water surface area
per unit of volume.

The reduction of wind over reservoirs will help reduce evaporation
but little has been done along this line.

Reduction of aquatic and riparian vegetation will lower the amount
of water consumed and lost by relatively low value plants. Experimenta-
tion is now being carried on in this field but problems involving the
elimination or replacement of riparian vegetation at reasonable prices
must be solved before the process can be recommended.

Use of roofs and covers must be limited to small areas because
of high costs and construction characteristics.

Underground storage has possibilities where physically possible.
Many questions still are unsolved in recharging procedures, pollution
possibilities and legal ownership of stored waters.

Because of the limitations 'of these physical methods of control
more attention has been recently directed to the use of chemicals capable
of suppressing evaporation. The combination of as many of the physical
methods of control as are economically possible with chemical control

will undoubtedly be considered more often in the future.

iii

EVAPORATION CONTROL

BY

Richard D. Reidhead, Jr.

» A THESIS

Submitted to the College of Agriculture of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Resource Development

1960

ACKNOWLEDGMENTS

The author wishes to express his sincere appreciations
to Dr. C. R. Humphrys for his direction and suggestions which
aided greatly in the completion of this thesis.

He also wishes to thank Dr. R. Barlowe, Dr. W. McCall,
Dr. L. A. Wolfanger and Mr. Kent Christiansen for their helpful
suggestions pertaining to this problem.

The Writer is grateful for the prompt replies and articles
relating to this subject received from the various private and
governmental agencies.

I am deeply indebted to my wife Pat, who gave so willingly

of her time and effort to support the writer in this undertaking.
Her encouragement and assistance was a tremendous help.

**************

CONTENTS

 

Page
INTRODUCTION . . . . . . . . . . ................ 1
STRUCTURE OF WATER MOLECULE ............. 6
EVAPORATION OF WATER ................... 11
Molecular Approach to Evaporation ........... 11
Surface Tension of Water . . . . . . .......... 15
Effect of Temperature on Evaporation .......... 17
Effect of Wind on Evaporation. . . ............ - 18
The Influence of Water Depth on Evaporation ....... 19
_ EVAPORATION CONTROLS .......... . ....... . 22
Construction of Reservoirs with Maximum Average
Depth 0 O O O O O O O O O O O O O l O I O O O O O O . Z4
Concentration of Water into Single Reservoirs . ..... 26
I Windbreaks O O O O O O 0 O O O O O O O O O I O O O O O O O 28
Elimination of Shallow Water Areas . . . . . . . . . . . Z9
Elimination of Aquatic and Riparian Vegetation . . . . . 31
Roofs and Floating Covers . . . . . . . . . . . . . . . . 32
'Storing Water in Ground Water Reservoirs. . . . . . . . 32
Evaporation Control by Monomolecular Films . . . . . . 36
Introduction to Monomolecular Films . . . . . . . 36
Theory of Monomolecular Films. . . . . . . . . . 37
Qualifications of an Effective Monomolecular Film 42
Effectiveness of Monomolecular Films. . . . . . . 43
CONCLUSIONS.... ........ 47

BIBLIOGRAPHY......................... 50

Figure

10.

11.
12.

13.

Table

II.

ILLUSTRATIONS

 

Page

. Simple covalent bonding of hydrogen and oxygen atoms.

.- A dipole of hydrogen positive polarity and oxygen

negative 0 O ..... O O O I O O C I O O O O O O C O O O O

. Hydrogen bonding and the tetrahedral arrangement of

electron pairs around the oxygen atom -. . . . . . . . .

Individual water molecule. . . . . . . . . . . .
., An array of water molecules. . . . . ......... .
. Evaporation and condensation in a covered dish. . . . .

. Shallow reservoir and deep reservoir. . . . . . . . . .

. Effect of tree plantings on wind-velocity. . . . . . . .

Methodsofrecharge..................

Reservoirs with and without protection of monomole-
cular films. 0 O O O O O I O O O O O I O O O I O O O O

Straight-chain molecule. . . . . . . . . . . . . . . . .
Relationship of straight-chain molecules . . . . . . . .

Straight- chain primary compressed and uncompressed.

Maximum Moisture Possible at Any Given Temperature.

ValuesofSurfaceTension. . . . . . . . . . . . . . . . .

vii

6

10
10
13
24
28

34

36
4O
40

41

l4

l6

INTRODUCTION

Water-~both too much and too little-—is an age-old problem of
mankind. Obtaining the necessary quantity and quality of water at a
desired place and time has been a concern of civilizations both past
and present. There is ample record of the need for adequate water
supplies. Lack of water has caused mass migration and starvation as
far back as ancient Egypt and there is anxiety today throughout our
nation over the adequacy of water supplies. In the Western States
this is an acute problem where the present use of water approaches
the total available supply. Increased use of water is, however,
essential for the growth of population and economy.

Modern civilization has imposed a heavy demand upon water
supplies. Of the total liquid water available in the world, only 0. 01%
is fresh water. The other 99. 99% is saline and unusable without
complex and expensive purification or conversion. In ancient villages,
with pastoral or simple societies, the total daily water requirements
were perhaps 3 to 5 gallons per person. Today, in homes with running
water, the average daily requirements for household and landscape
maintenance is in the order of 60 gallons per day.l More water is
also needed for use by agriculture, municipalities, industry, and
recreation.

The increased use of water for agricultural and industrial
purposes has been spectacular over the past two decades. By 1975

in the United States, demands for water are expected to be nearly

 

lWater, The 1955 Yearbook of Agriculture. United States
Department of Agriculture, p. 4.

double those of 1950--400 billion gallons per day. The extent of the
water requirement may be judged when it is realized that an acre-
foot of water (325, 851 gallons, or the amount of water necessary to
cover an acre of ground to the depth of 1 foot) is required to produce
2 tons of dried vegetables, 435 bushels of wheat, 1357 pounds of
sugar, 5 tons of oranges, 200 pounds of cotton linters, or 200 pounds
of beef. Industrially, an acre-foot of water is necessary to produce
4—;- tons of paper, 5 tons of steel, 40 tons of sulfur, 15 tons of
synthetic rubber, 1635 pounds of rayon, or 650 yards of wool cloth.
One butadiene plant in Texas utilizes 308, 000, 000 gallons, or 94. 5
acre-feet of water per day for cooling purposes only. 1

Perhaps the most significant aspect of the situation at the
present time is the trend which is indicated. This much is quite
clear--demands are increasing rapidly and as of today, water shortages
threaten in certain areas.

In several locations in Arizona, for example, demands have
increased above the supply. Surface water provides one-third of the
supply and ground water the other two-thirds. In some areas, ground
water is being used eighteen times as fast as it is replaced. Such a
process can have only one result, that of lowering the water table
and increasing pumping costs. Unless a method of conservation of
existing water supplies is used, or new sources found, the present
economy cannot develop without changing to a less consumptive use of
water. 6

Man has engaged in the control of flowing water since history
began. For irrigation, municipal supply, power, or flood control--

man has recognized the desirability of adequate and dependable

 

1Robert R. Cruse, Final Report on the Southwest Cooperative
Prgiect on Reservoir Evaporation Control. ' Southwest Research
Institute, San Antonio, Texas, March 18, 1958, p. 2.

 

 

water supplies. Man cannot schedule the occurrence and amount of
precipitation effectively, but he has long tried to overcome the
variation in the supply of water by storage. Water storage has
contributed materially to the development of the United States. The
successful agricultural economy in the semiarid sections of the West
depends largely on storage of a part of the annual water supply for
use during the growing season. The growth of our vast industrial
centers has been made possible only by adequate year—round water
supplies for municipal and industrial use. Industrial development
in many sections has paralleled the increase in available power
supplies. The retention of flood flows by flood-control reservoirs
has contributed materially to the protection of downstream develop-
ments. 1 I

The rapid development of the arid We st near the turn of the ‘
century was made possible largely by construction of storage
reservoirs. In regions where precipitation was insufficient for
agriculture, reservoirs were required to hold the spring floods from
melting snow and the quick runoff from short, intense summer stOrms.

Evaporation represents one of the greater losses of water stored
in reservoirs. Water and its wise management and use are vital to
the economic stability and growth of the western United States.
- Evaporation from reservoirs, soil surfaces, and plants does not
produce income and it should be investigated to determine ways of
preventing this loss.

The Lake Hefner Research2 reports that over 30 million acre-

feet, or more than twice the flow of the Colorado River, is lost by

 

1N. 0. Thomas, and G. E. Harbeck, Jr., Reservoirs in the
United States, U.- S.Geol. Survey Water-Supply Paper, 1360A (1956), p. 2.

 

 

2U. S. Geological Survey, Water-Loss Investigations, Vol. 1,
Lake Hefner Studies: U. S. Geol. Survey Prof. Paper 269, 1954, p. 1.

evaporation and by transpiration from vegetation along water courses
in the 17 western states. This would be enough water to irrigate
15, 000 square miles of desert, an area nearly half that of the state
of New York or Pennsylvania. Evaporation from reservoirs is
approximately 15 million acre-feet. Evaporation uses enough water
in the western United States to fill several major reservoirs. This
amounts to about the combined storage of such major reclamation
storage reservoirs as Roosevelt Lake in Arizona, Lake Shasta in
California, Elephant Butte in New Mexico, Hungry Horse in Montana,
Seminoe in Wyoming, Lake Mohave (Davis) and Havasu Lake (Parker)
on the Colorado River. This is about equal to the total usable storage
in California according to figures reported in a recent survey. 1
Evaporation takes a large share of all water supplies even before
they arrive at the locations where they are used. Water supplies
reach the use areas through systems of natural and cultural storage
basins and channels, and along these passages evaporation takes a
heavy toll. This lost water would otherwise be available for human
consumption, municipalities, industries, or for agriculture.
Evaporation is gradual and often escapes notice; yet over a
year's time it takes an important part of the supply. For example,
the water lost by evaporation throughout the United States exceeds the
amount taken for use in cities and towns. 2
In the humid portions of the country, evaporation loss is signifi-
cant although annual precipitation tends to affect it and it is not so
noticeable. This evaporation loss ordinarily is not recognized except

during rainless periods such as a drought.

 

lThomas and Harbeck, 9i- cit., pp. 17, 26, 53, 59, and 94.

2E. D. Eaton, Control of Evaporation Losses-Interim Report.
Senate Committee on Interior and Insular Affairs, Print No. 1, 85th
Cong., 2d Session, April 14, 1958, p. 7.

 

Since evaporation from bodies of usable fresh water constitutes
a severe water loss to western states, where summer rain storms are
few, the figures emphasize the importance of research on possible
control of this water loss.

This paper is not an original experiment in the true sense of the
word, but is an attempt to bring together a review of the literature
available on this problem. It is designed to review the (material on the
structure of the watermolecule, the evaporation process andxmethods
of control. Conclusions are drawn from various sources and are included

in the research. A bibliography is also attached.

STRUCTURE OF THE WATER MOLECULE

Evaporation is the process by which water is converted to vapor.
The exact laws governing this process are not clearly understood. To
obtain the best conception of this phenomenon and the laws governing it,
it may be desirable first to review the structure of water.

The water molecules are formed of three elemental particles,
or atoms, two of hydrogen (H) and one of oxygen (O)--or,
expressed as a symbol, HzO, which is one of the simplest
compounds. The three atoms are held together by two chemi-
cal bonds, thus: H-O-H. Atoms consist of negative charges,
or electrons (e), moving around a central positive nucleus.

The number of electrons in an atom is the atomic number of the
element in the periodic system of all the elements. Hydrogen
is the first of the elements, and thus has one electron. Oxygen
has eight. Chemical bonds are formed by pairs of electrons.
The chemical bonds in H-O-H are formed by completion of two
pairs, the electron of each'H atom associating with each of two
unpaired electrons from the oxygen atom. Of the remaining

six electrons of oxygen, four are much farther from the nucleus
than the other two. The eight outer electrons in H30 tend to
form four pairs of electrons that are as far apart as they can be
while still attracted to the oxygen nucleus. Thus they are near
the corners of a tetrahedron, a. solid bounded by four plane sides. ‘

The two hydrogen atoms are attached to the oxygen atom by simple

covalent bonding in the manner illustrated below.

H
. x o

H

000
O

0
X
o 0

Figure 1. Simple covalent bonding of hydrogen and
oxygen atoms.

 

1Water, The 1955 Yearbook of Agriculture, op. cit” p. 9.

 

‘ A further feature of a water molecule is that H attached toO is
asymmetrically surrounded by electrons, so that there is a separation
of charge or polar character. Because of the strongly electronegative
character of the oxygen atom, its nucleus exerts such a strong attractive
force on the hydrogen electrons that they are drawn out of their normal
position in relation to the hydrogen nucleus. Also, the two hydrogen
atoms are bonded to the oxygen atom. at an angle of 105° rather than
being set in a straight line. ' As a result of these two factors, a dipole
is formed in which the hydrogen end has a positive polarity, while the

oxygen end is negative, as illustrated below.1

Figure 2. - A dipole of hydrogen positive polarity and
oxygen negative.

Because the hydrogen electrons are drawn so far out of their normal
position, the hydrogen nucleus behaves somewhat like a proton; it may
share two electrons with the oxygen atom of an adjacent HzO molecule,
(forming what is referred to as a hydrogen bonding. - If other molecules
with non-binding outer electrons are-present, there is a tendency for

H to increase the. symmetry of its surroundings by approaching a pair of
electrons in line with its chemical bond to oxygen, or 2e-H-O in a line.

The resulting attraction, in case the 2e are on another water molecule,

 

1C. R.. Scrum, Fundamentals of General Chemistry, (New York:
Prentice-Hall, Inc., 1955), p. 192. T

 

is about 6 percent as great as that of the H-0 bond.
The properties of water arise from the hydrogen bonding and the

tetrahedral arrangement of electron pairs around the oxygen atom.

H

Figure 3. Hydrogen bonding and the tetrahedral arrange-
ment'of electron pairs around the oxygen atom.

- In the above diagram of the water-molecule, the dotted lines illustrate
the hydrogen-bonding tendency arising from the diffuse cloud of electrons
in the direction of the dotted lines. - Several molecules held together by
hydrogen bond, as in solid or liquid water, form an assembly of tetra-
hedral groups; each molecule is bonded to four other‘molecules. 1

The principles of bonding give the background required in explanation
for the properties of water. The prOperties can be divided into two classes,
depending on whether chemical bonds between the H and O atoms are broken,
leaving the H30 molecules intact.

Chemical changes like the rusting of iron are of the first class, in
which chemical bonds are broken. Physical changes such as evaporation

from a lake are of the second class.

. A simple property, a physical one, to consider as a first subject
is that water when heated evaporates very slowly as compared
with other liquids that have simple molecules. In other words,

 

1Water, The 1955 Yearbook of Agriculture, 22: .c_:_i_1_:.. p. 9.

 

the heat of vaporization is high. Vaporization is just the
separation of molecules from a liquid to the gas or vapor phase
by the attraction between molecules. This transition is the
inverse of condensation. The high heat of vaporization is a

direct result of the strength of the hydrogen bonding between
molecules. 1 '

The knowledge of the structure of water is important for a funda-
mental understanding of the process of evaporation. Only with this
basic understanding can we effectively approach the subject of evaporation
control. (See illustration 4 and 5.) The following figures are not

pictured according to scale but do illustrate certain basic relationships.

 

l11:51:11., p. 10.

 

Figure 4. INDIVIDUAL WATER MOLECULE

 

Figure 5. AN ARRAY OF WATER MOLECULES (SIDE VIEW)

11

EVAPORATION OF WATER

Molecular Approach to Evaporation

 

A great deal of light could be shed on the processes taking place
in evaporation by considering the question‘from a molecular approach.

- Suppose we could equip ourselves withsupermicroscopic eyes with which
to see the detailed structures of gases, liquids, and solids. - Itis

believed that there are trillions of individual molecules of H20 moving
about in an ounce of water. We assume that a very large number of mole-
cules are present in a small volume and that thesevlhave considerable
freedom of motion. If we could watch one particular-molecule we should
see it moving about in all directions, now sideways, now up, and now
down. The individual molecules do not all move with the same velocity.
The average velocity of all themolecules comprising any given mass
determines its temperature.

Looking first at a gas, a liter of steam1 for example, we would
observe some molecules moving a little faster than others and some moving
a little slower but the bulk of them would be-moving at about the same
speed. In the vapor or gas phase, the molecules are widely separated;
they are endowed with high velocities; and they experience frequent

collisions with their neighbors. 3

I

 

1Steam is the most common example of a vapor, but what one sees
issuing from the spout of a teakettle is not steam. . Steam is just as in-
visible as air. A glass teakettle shows nothing inside above the boiling
water. The visible cloud at the spout consists of small droplets of con-
densed steam vapor in which themolecules are no longer in the vapor
(or gaseous) phase but are in the liquid phase. The molecules of a vapor
are relatively far apart as in a gas, but steam vapor can be easily liquified
again, since it is near its condensation-point.

.zSorum, _p. 912.. p. 32.

12

If we were to heat the steam we would observe that the molecules
move faster, if we cool it they move slower. , As we cool the steam the
-molecules move slower and slower. Finally, at 100°C (212°F)‘ we observe
that groups of the slowest molecules get togerher and condense to form
drops of liquid water. Within the drop of liquid the H20 molecules move
about continuously. If the cooling is continued the steam is all condensed.
If we continue cooling the liquid water we observe that the motion of the
'HzO molecules become more sluggish. - Finally, at 0°C (32°F) groups of
H20 molecules are observed to orient themselves into definite, rigid
masses of ice crystals. , If the cooling is continued, the entire mass of
liquid water is changed into a solid mass of ice.1

The molecules that make up the water on the earth's surface and the
water vapor in the atmosphere are in ceaseless motion. Some of the
molecules of a liquid acquire sufficient momentum to break through the
surface and escape into the free air. At the same time some of the mole-
cules of water vapor in the air strike the water surface and are unable to
leave it. Strictly speaking, the movement of‘molecules from water
to air is evaporation and the reverse movement is condensation.

A volume is said to be saturated with vapor if no more vapor can
be introduced without producing some condensation. Consider a dish of
water covered by a large jar. This body of water is exposed in the
enclosed vessel. Only a small amount of liquid will change to vapor,
regardless of the water temperature, even though the space above the
water was originally completely dry. Although the molecules of water
vapor entering the space above the water surface are confined by the walls
of the vessel, at first there are so few that the‘. number returning to the

liquid are limited. Hence, the number of molecules in the enclosed space

 

’Ibid.

13

increases rapidly. With an increase in the concentration of water vapor,
there is an increase in the rate of return of the molecules to the liquid.
- Eventually, the vapor concentration becomes so great that as many mole-
cules return to the water as leave it. ~ At this time equilibrium between
the liquid and its vapor is attained, and the rate of evaporation is then
equal to the rate of condensation. '

If water is placed in an open dish out in the open air it will gradually
disappear. An examination will reveal that the molecules of liquid have

passed into the air where they exist in the form of gas molecules.

 

 

 

 

(b)

Figure 6. Water in open dish-(a) continues to evaporate until
all the water is lost. Water in covered dish‘(b)
evaporates until space above is saturated. Then
as many molecules fall back into water per second
as evaporate. ’

As the temperature increases, the rate of evaporation increases.
When the temperature of a saturated volume is lowered, sufficient
condensation takes place to establish a new equilibrium state. ' At the .
lower temperature, fewer molecules per cubic centimeter areirequired I
to produce saturation. On a cold winter day the atmosphere can hold
little moisture. On a hot summer day it can hold a great deal more.
This means that the higher the temperature, the greater is the vapor
dentisy required for saturation. Water continues to evaporate until

the atmosphere is saturated for that temperature.

 

. ’Ibid” p. 270.

14

The amount of water vapor in the air is an important consideration
for'many scientific purposes as well as for our own comfort. At 30°C
(86°F) a cubic meter of saturated water vapor weighs 30 gm. Other
values for the amount of water vapor at given temperatures are given

in the table1 below.

TABLE I

MAXIMUM MOISTURE POSSIBLE AT ANY GIVEN TEMPERATURE

 

 

°C °F I Grams per
Cubic Meter

 

-10 +14 2.1
0 32 4.8
10 50 9.3
20 68 19.1
30 86 30.0

 

A room of moderate size may easily contain 10 lbs. of water vapor
on a hot day. Whether saturated or not, the actual amount of water vapor
the air contains is called the absolute humidity. Absolute humidity is
defined as the mass of water vapor in the air per unit volume.

On the other hand, relative humidity is defined as the extent to
which atmospheric air is saturated with water vapor it could contain per
unit volume if saturated at that temperature. - Suppose the volume con-
tains one-half the amount of the water vapor that it could contain at a
given temperature if it were saturated. The relative humidity is then
50 percent. If it contains one-tenth what it could at saturation, the
relative humidity is 10 percent, which it may be in a desert if the air

is dry and readily takes up the moisture by the process of evaporation.

 

1Ibid., p. 271.

15-

If the humidity is 95 percent and the weather hot, the air is nearly
saturated with water and though the process of “evaporation is in effect
the air around does not absorb moisture as readily. In other words,
atmosphere with low humidity will absorb—more water by evaporation
than under similar conditions with high humidity. - Also, the rate of
evaporation from a free water surface is usually lower at night than
in daytime. The reduction is principally due to the lower night tempera-
ture of the air and the resulting inc rease' in relative humidity.

If the humidity is 100 percent the air is saturated with water vapor
and the vapor will condense at the slightest lowering of the temperature.
~ If the air is not saturated at a given temperature, a reduction of the
temperature will bring it to the point where" it is saturated. - Further
cooling will exceed this saturation point, and moisture will then condense
as fog, clouds, or dew on the surface of the earth. When air is saturated
with water vapor it is said to be at the dew point, this being the tempera-

ture below which further cooling will produce condensation of dew.

Surface Tension of Water

 

Water particles are attracted to each other. We call this property
cohesion. It also is‘attracted to some surfaces. This is adhesion.
The strengths of both cohesion and adhesion in water, of course, are
due chiefly to hydrogen bonding.

- First let us consider adhesion, the property of adherence to other
substances. The wetting of g1ass--irnportant in the action of a wind-
shield wiper--arises from the formation of hydrogen bonds between the
oxygen atoms, which are part of the structure surface of the glass, and
the hydrogen atoms of the H30 molecules.

The principle of cohesion is adequately explained in the following

passage.

16

The forces of cohesion--the tendency of water to stick to itself--
between the molecules of a liquid give rise to the phenomenon of
surface tension. Because of these forces, the surface of a liquid
behaves as if a thin skin were under a state of tension such as to
make it tend to contract. However, the tension does not depend
on the dietance the film is stretched. Themeasurement of sur-
face tension is defined as the force of contraction acting across
an imaginary line of unit length on the surface of a liquid. It is
also the force required to break'a surface film of unit width,

- Surface tension-is usually measured in dynes per centimeter. ‘

Different values of surface tension exist at different temperatures

and for various substances as indicated by the table below.

TABLE 112
VALUES or SURFACE TENSION

 

 

 

Liquid I T (dynes/cm)
Water 15°C 73. 5
Water at 80°C 62.6
Alcohol 24. 0

 

The existence of surface tension can easily be demonstrated. If
a needle coated with oil (so that the water does not creep up it's sides)
islaid gently on water, it will float in spite of the fact that steel is seven
times more dense than water. The surface is depressed enough so that
the resultant upward force is equal to the weight of the needle. If the

needle is heavy enough to break through the surface film, the needle sinks.

 

lRogers D.. Rusk, Introduction to College Physics, (New York:
Appleton-Century-Crafts, 1954), p. 197.

zlbid” p. 198.

17

In other words, a force is required to pull water apart and create
two new surfaces. The force, ina sense is opposed by a tension, and
this surface tension of water is much higher than that of other-liquids
because of the hydrogen bonding. A

The surface-molecules of a liquid are in a, state of equilibrium
different from those beneath the surface. The latter are surrounded by
neighbors on all sides and are pulled equally in all directions as they
move about.

-- For molecules on the surface, there are no upward forces of mole-
cular attraction above them. This causes them to crowd down upon their
neighbors below until equilibrium is established. The state of equilibrium
of the surface molecules is one in which they are under' a kind of pressure
which increases their energy in somewhat the same mannerithat the
energy of a spring is increased when it is compressed. To raise a mole-
cule from beneath the surface to a position in the surface film, work must

be done on it. - Hence, surface molecules are in a higher energy state. ‘

Effect of Temperature onwEvaporation

 

As the temperature ofa substance increases, themovement of the
molecules increases. In a liquid the molecules are darting about, .
colliding with their neighbors, and sometimes striking the surface of the
liquid. If they strike the free surface of the liquid, they may pass on
through and escape into outer space, provided they have sufficient speed
to overcome the surface attraction.

The rate at whichmolecules leave a water surface depends solely
on the characteristics of the water. - Since an increase in its molecular

energy indicates a rise, in the temperature of the water, the rate at

 

.’Ibid., p. 200.

18

which molecules break away from the water surface depends on its
temperature.

- Since the more rapidly moving molecules are more likely to escape,
those that are left are on the average slower moving, possessing less
energy and hence, represent a lower temperature. Thus, the remaining
liquid is cooler. The cooling due to evaporation can be illustrated in
several ways. One of the most common is the method of keeping water
cool by placing it in a canvas water bag. The evaporation which takes
place causes a reduction of temperature in the water remaining in the
bag.

The second result which may be deduced from the molecular
theory is that the change of state from liquid to vapor is associated with
a definite amount of heat--the so-called latent heat of evaporation. I
In order to prevent the cooling of the liquid and to maintain its temperature
constant, heat would have to be supplied in some way and the amount of

heat required would correspond to the amount of vapor formed.

Effect of Wind on Evaporation

 

Molecules move away from the vicinity of an evaporating surface by
the ordinary phenomenon of diffusion. Evaporation by molecular diffusion
is an extremely slow process and in the absence of air motion a thin
layer of air, the thickness of which is usually measured in millimeters,
becomes saturated with water vapor. If this is not removed the molecules
from this thin layer of air and the surface layer of the water will inter-
change with very little water vapor escaping to higher atmosphere. Wind
action greatly speeds up the evaporation process by removing this water
vapor immediately above the surface and replacing it with air which may

have a different temperature or moisture content, usually a dryer air.

19

The transfer of moisture is accomplished through the irregular or
turbulent mixing of the- lower air, by which bodies of the moister air
from near the surface of the lake are carried upward and replaced by
drier-bodies brought down from above in the process of turbulent mixing.
- Actually, the phenomenon of turbulent transfer in the atmosphereis by
far the most effective factor in carrying water molecules away from an
evaporating surface. Transfer by turbulent mixing is usually at least
25, 000 times as effective as molecular diffusion, and it may bacon-
sidered almost wholly responsible for any loss of moisture from- an
evaporating surface freely exposed to the air.1

The-movement of smoke demonstrates plainly the-mixing of the
air that takes place in the turbulent layer. Where a visible gas, having
approximately the same density as the air, is released the effect of
turbulent mixing is apparent.

The effect of wind velocity on the evaporation of moisture appears
to be primarily its effect in removing the vapor whichforms more rapidly

over the water surface than can diffuse through the atmosphere above.

The'Influence of Water Depth on Evaporation

Evaporation from a lake is affected by the surface water temperatures.
These are intimately related to the temperature of the entire body of
water, and this depends largely upon the lake depth. The depthvhas a very
pronounced influence upon the rate of evaporation from any large body of
water. The explanation of this phenomenon can be found in a comparison
of the cycle of events that occurs in a shallow lake with that which occurs

in a deep lake.

 

10- W. Thornthwaite and Benjamin Holzman, Measurement of
Evaporation from Land and Water Surfaces, U. S. Department of
Agriculture, Technical Bulletin No. 817, May 1942, p. 10.

 

20

Inlate winter or earlyspring the temperature of an entire body of
shallow water will be at or near 32°F. As heat received from the sun
increases the surface water temperature the weight of this water in-
creases and the warmer water sinks to the bottom. The colder but
lighter water rises to the surface where it' in turn is warmed to a higher
temperature and greater density, and the turnover process is repeated.
This continues until the temperature corresponding to the maximum
density of water (39. 29F) is reached. » From then on, as the surface
becomes warmer it also becomes lighter and therefore remains on the
surface. The water below‘the surface heats more slowly by direct
radiation and conduction, and by mechanical mixing produced by currents
and wave action. Before long the shallow water is soon heated to a
temperature closely resembling the temperature of the surrounding air.
Thismeans that during the warmerparts of the year a shallow lake
will be subject to high evaporation losses. ‘

In the fall, the overturning process is repeated in reverse. When
the average air temperatures become lower the surface water also
becomes cooler and heavier sinking to the bottom, with the warmer
water at. the bottom rising to the surface. This continues until the entire
body of water-has a temperature of 39. 2°F. ~After that, as the surface
water further cools it becomeslighter and remains on the surface. This
explains why, for a given reduction in air temperature, the surface
water temperature will drop much more quickly from 39°F to 32°F than

from 46°F to 39°F. 2

Consider now a deep body of water. Either in spring or in fall,
after the temperature of the entire-mass has reached 39. 2°,
further changes resulting from radiation, conduction, and mix-
ingare slow and extend only to a limited depth. _ Below 200 ft.
the temperature remains at or near 40° throughout the

 

‘C. O. Wisler and F. Brater, Hydrology, (New York: John Wiley
and~Sons, Inc., 1949), p. 145.

zlbid. , p. 146.

21

year. - However, the amount of heat that is required to raise
the temperature of a large mass of water from 39. 2° to the
temperature which it attains by autumn is enormous, and as a
result there is a considerable lag between the temperature of
the water and that of the air. The temperature of the water is
lower than the air temperature in summer and higher in winter,
whereas in' shallow bodies of water the mean daily temperatures
of the air and water do not differ so greatly. I

This lag in the temperature of the water in deep lakes behind the
temperature of the air above exerts an important influence on the rate
of evaporation from lakes. This reduces summer evaporation below
and increases thewinter evaporation above the observed rates on shallow

lakes.

 

llbid.

22

EVAPORATION CONTROLS

There are a wide variety of recommendations about the ways in
which evaporation could be controlled. This paper will cover several
techniquesor methods of reducing evaporation. I now will review
some of the elementary facts concerning water resources.

First, there is no evidence of any substantial change in the sum
total amount of water available on the average, over the years. We as
a nation are wasting vast amounts of water. - In a number of individual
areas throughout the‘United States, the supply of water-is most certainly
becoming scarce and the ground water table is being lowered. In such
cases further development in-municipal growth, in industrial growth, and
inupopulation growth will depend upon the economic supply of available
water.

There is ample evidence that our total use or consumption of water
is increasing steadily and will probably continue to increase. The
desirability of reducing evaporation losses in the western states has
received much attention in recent years because of the increasing demand
for water and the decrease in the supply available due to recent droughts.
The increase in demand for water is attributable tomany factors, among
which the most important are: (l) the rapid growthinpopulation in the
far western states since the beginning of World War II, (2) the upsurge
in industrial activity in the West, (3) the increase in the acreage of
irrigated lands, and (4) the increase in the per capita use of water result-
ing from a higher standard of living. ‘1 High evaporation loss in relation

to amount used is characteristic of the West. This is the result of the.

 

1G. Earl Harbeck, Jr. , Can Evaporation Losses be Reduced?
‘Proc. Am.- Soc. Civil Eng. , §_4, .IRI (1958), p. 1.

 

23

climate and the character of the streamflow. The great seasonal
variations in streamflow that are common in the West result from
extensive surface water being exposed.

Storage reservoirs are a principal means of governing the stream-
flow in order to increase usable water yield. Reservoirs, however,
expose extensive surfaces to evaporation, and thus they are a major
cause of water loss. This is true even though reservoirs may lessen
natural evaporation by confining floods in deep pools instead of being
spread over wide flood plains. The shallowness of some reservoirs to-
gether with the extended periods of holdover, increases the evaporation
losses of river systems. The net result of reservoir operation is, of
course, advantageous in conserving water that would otherwise not be
usable. l
The methods of evaporation control discussed in this paper are:‘2
. Construction of Reservoirs with Maximum Ave rage Depth
. Concentration of Water into Single Reservoirs
Windbreaks ' ‘

Elimination of Shallow Water Areas

Elimination of Aquatic and Riparian Vegetation

Roofs and FloatingCove rs

, Storing Water in) Ground Water Reservoirs

mammsswwr—a

Evaporation Control by Monomolecular Films

 

1Eaton, Control of Evaporation Losses—Interim Report, _p. 9113.,
p. 12.

 

.28. W. Freese, "Reservoir Evaporation Control by Other Techniques, "
First International Conference on Reservoir Evaporation Control,

(San Antonio, Texas: Southwest Research Institute, April 14, 1956), p. 45.

. S. W. Freese is probably one of the outstanding authorities on
reservoir evaporation control. Many thoughts and ideas utilized in this
paper on the various methods of evaporation control, with exception of
information about monomolecular films, comes from his above cited
report.

 

24

Construction of Reservoirs with Maximum
Average Depth

 

 

 

 

 

 

 

 

 

 

 

 

 

-SHALLOW RESERVOIR DEEP RESERVOIR

Figure 7. Shallow Reservoir and Deep Reservoir.

Many methods of reducing evaporation losses from reservoirs are
known. Project planners and designers are well aware of the desirability
of a reservoir having a minimum of surface area for a specified capacity.
- Evaporationlosses are held to a minimum by exposing the least possible
water surface area. This means that streams and reservoirs are kept
deep and limited in width. Familiar examples are stream channel improve-
ments that permit the water to move at relatively high velocity through
a confined channel instead of meandering slowly over a wide stream bed.

' An even more general practice is to use deep narrow canyons for
reservoirs.

- A small increase in height in a dam may result in a relatively large
increase in storage capacity, but if the surface area is increased dis-
proportionately thereby, a net loss in water may result because of the

greatly increased evaporation loss. Evaporation loss is only one of

25

many factors to be considered in designing a dam and reservoir, and
other considerations. may dictate‘a compromise. .-

There are limitations also on the opportunities for management
of surface reservoirs to hold storage at maximum depths, although this
.method is widely and successfully practiced. There are only relatively
few deep, narrow reservoir sites at suitable locations, with acceptable-
geology, and not pre-empted by railways.) highways, and other essential
developments. Another limitation of the canyon type sites is that it does
not provide adequate capacity in the canyon section. - In practice, reservoir
operation must contemplate a substantial amount of water to be stored at
_ relatively shallow depths with correspondingly large water exposure. 1

Often, there-is little choice as to available reservoir sites on a
particular stream. - Either there is only one site meeting the various
needs of the case or several possible sites having about the same area-
capacity characteristics. Where there is a choice of sites from the
‘ standpoint of average depth, the difference in evaporation, losses should
certainly be evaluated.

Freesez cites a comparison of two reservoirs in- West Texas, each
with-about 60, 000 acre-feet capacity, and located only 30 miles apart.
- At the 60, 000 acre-foot point, One reservoir has an area of 5300 acres
and an average depth of 11.3 feet whereas the other reservoir has an
area of only .3580 acres and an average depth of 16. 8 feet. - At an average
Operating capacity of 40, 000 acre-feet, the difference in surface areas
between the two lakes is 1275 acres. The average net evaporation~(that
is, after rainfall) in this particular area of West Texas is about four

feet perwyear. The difference in evaporation between the two lakes is,

 

.‘Eaton, pp. 5313, p. 15.
.zFreese, 22. c_it., pp. 45-46.

26

therefore, approximately 5, 100 acre-feet per year or 405 million
gallons per day.

Fortunately, the city which has a lake with less favorable area-
capacity characterics has ample water and there is no need within the
foreseeable future, for more water than is now available. Under these
circumstances, it is difficult to place a value on the water lost by
reason of the unfavorable average depth. At a cost of $. 05 per 1000
gallons, an acceptable figure for water from a lake in West Texas, the
loss amounts to $82, 000 per year.

It is possible to figure more accurately the saving to the city
which has a lake with the more favorable area-capacity characteristics.
In this case, the city is confronted with the necessity of securing an
additional water supply and the water will cost $. 10 per 1000 gallons
delivered into the lake, under consideration. The saving to the city by
reason of the more favorable evaporation characteristics, is $164, 000
per year. A

, A deep reservoir with a limited amount of surface area will thus
evaporate less than an equal amount of water stored in a shallow reservoir

which has a large water surface area.

Concentration of Water into Single Reservoirs

Water concentrated within a large reservoir usually has a great
advantage over storing in several smaller and shallower storage basins.
The advantage being that water from several storage areas may be,
gathered together increasing the depth and decreasing the total area
covered by the water.

More energy from the sun is required to evaporate water from a

deep reservoir as compared with a shallow storage reservoir.

27

The reduction in surface area exposed to the evaporation process also
reduces evaporation. - Reducing evaporation losses by this method may
be'accomplished by diverting water from one watershed into a single
reservoir onanother watershed or by operating a reservoir system on
the same drainage basin in order to concentrate the water into a lower
reservoir. V _

Operational procedures will in somemeasure reduce evaporation
losses from stored bodies of water. When a reservoir is kept empty at
all times losses due to evaporation are at an absolute-minimum. This,
of course, defeats the purpose of constructing the reservoir, which
was to store water for controlled release. An operational schedule
consisting of a full reservoir in the winter when evaporation is low and
empty reservoirs in the summer when losses are high would reduce
the amount of water evaporation from the-lake. Unfortunately, this type
of scheme is generally not practicable, especially in the West, where
~much of the runoff occurs in the spring as a result of themelting
mountain snowpack. Invmany instances, control of evaporation by this
process must first consider the water requirements of important water
users such as municipalities, irrigators, industries, and power
companies.‘ 9

, An example of diverting water from one watershed into another in
order to concentrate the amount storable in one reservoir is given by
, Freese. 2. He states that water in two watersheds could be combined
into one reservoir. The normal 2/ 3 capacity operating-level of two
Texas'lakes on different watersheds, have a combined water surface

area of 7, 450 acres. This water could be stored in one reservoir with

 

1Harbeck, gp. cit” p. 2.
.zFreese, 9p. git” p. 47.

28

a total water surface area of 5, 800 acres, with a reduction of 1,650

acres less of exposed water. The net evaporation that prevails in that
part of the country is approximately 4. 5 feet per year. Using the
figure already used of water valued at $. 05 per 1000 gallons the saving
resulting from the use of one reservoir instead of two, amounts to

$120, 000 per year.

Windbreaks

 

  

27 ,, ”/1 l j 5
~10 M.P.H. I I15 M.P.H.

 

Figure 8. Effect of tree plantings on wind-velocity.‘

The effect of air-movement on evaporation has been given a widely
different value by various writers. I feel that a reduction in wind
velocity over a reservoir will reduce evaporation, although wind above
some limiting velocity, perhaps 25 miles per hour. may have no
additional effect on evaporation. For winds sufficiently strong to remove
all vapor as rapidly as it may be formed, additional wind velocity may

produce no further effect. Of course, such a limiting velocity may vary

 

1Conservation Soil and Water for the Public Schools, State of North
Dakota 1948, Issued by Department of Public Instruction, Garfield B.
Nordrum, Superintendent, p. 70.

 

29

with other factors affecting evaporation. Below the limit where
additional wind cannot remove more water vapor, a correction factor
may be applied. In this range of wind velocity any reduction in wind
speed would also decrease the amount of evaporation. ‘ h

. Although many reservoirs already have windbreaks around them
it is very difficult to evaluate the benefits of wind protection. There
seems to be a lack of experimental data on the effect of windbreaks on
reservoir evaporation.

Windbreaks act as barriers to reduce wind currents. The tree
windbreak as shown in illustration (8) will reduce wind currents to a
practical degree for a distance of about 20 times the height of the trees.
Thus, a windbreak 30 feet high would slow down wind velocity to a
distance of 600 feet. This would result in some reduction of evaporation
losses. .

, A windbreak should be constructed to accommodate the water area
and wind direction. Windbreak plantings would also increase the amount
of moisture lost by transpiration. New experimental data would be
needed to determine the net saving of water.

' An additional advantage of a windbreak may be in improving the
multipurpose use of a reservoir in protecting small game or by creating

shade for rec reation.

 

Elimination of Shallow Water Areas

Reducing the surface area to a minimum is another method of
lowering reservoir evaporation. This is quite important especially in
the arid West, where losses may represent an appreciable part of the

available water supply. In the Southwest, evaporation losses may

 

IAdolph E. Meyer, Elements of Hydrology, (New York. John Wiley
and Sons, Inc., l928),p p. 195.

 

30

average as highas 72 inches per year, equivalent to 6 acre-feet of
water for each acre of pond area and generally about half of this loss
will occur in the four summer months of May to August, inclusive.

. For such regions, it is evident that reservoirs having the greatest
possible mean depthand least surface area will be the‘most efficient
for storage purposes.

In some places evaporation may set a limit on the extent to which
a proposed reservoir might be raised in capacity without an increase in
evaporation greater than the intended gain in water supply. Lake Worth
in Texas was built with substantial shallow water and swampy areas at
the upper end. ' A study was conducted of Lake Worth for the purpose of
eliminating, within economic limits, the shallow water areas and the
water vegetation in order to reduce evaporation and transpiration from
the lake.

It was found that by regulating the flow of water from Lake Eagle
Mountain which is above Lake Worth, a saving in evaporation could be
obtained. A reduction in the normal Lake Worth operating level by
4 feet reduced the water surface area approximately 700 acres. With a
net evaporation from the lake of 3 feet per year, at a cost of $. 05 per
* 1000 gallons the saving, due to the elimination of 700 acres of evaporation
surface, amounts to $34, 000 per year. 1

Freese also states that two to three feet is considered the minimum
depth-above the normal water level deemed necessary to control the
growth of willows and swamp land vegetation. With an evaporation rate
of 4 feet per year, the value of water at $. 05 per 1000 gallons and the
cost of fill being $. 25 per cubic yard, one could be able to economically

add earthen fill from a depth of . 72 feet below normal water level to

 

1Freese, op. sit” p. 48.

31

2%- feet above normal water level. The value of the water saved from
this method of control of evaporation would pay for the cost of fill in

20 years.

 

Elimination of Aquatic and Riparian1 Vegetation

Reservoir evaporation control is closely tied to the control of
transpiration by lake vegetation. ~ A short section on the reduction of
non-beneficial consumption of water by aquatic and riparian vegetation
is required. It is a well-known fact that water associated vegetation
such as water lilies, cattails, and riparian phreatophytes (salt cedar,
willows and cottonwoods) use and transpire large quantities of water
each year. The amount would depend on the type of plant and the con-
ditions under which it is grown. - A conservative estimate of three to
five feet of water is transpired by each acre of water-loving plants
and produces little economic value.

Techniques of clearing and removing vegetation from shallow water
and swamps are being perfected, and continuing research promises
further improvements. The high costs involved and the general need
for repeated treatment is still a major problem.

The elimination of shallow (areas as already discussed would
change the environment so that many water plants could not flourish,

thus saving valuable water.

Roofs and Floating Covers

 

City waterworks use concrete and steel roofed tanks throughout

the country. True, this is not for the prime purpose of preventing

 

lRiparian vegetation is plant growth on the shores of a lake or
stream.

32

evaporation losses but to protect the quality of the enclosed water.
' A roof, over water does prevent some evaporation because of lower
water temperatures and reduced action of wind.

- A concrete roof to cover a large tank is estimated to cost two
dollars per square foot or $87, 000 per acre. Using seven per cent
per year rate of interest, amortization, and maintenance, the annual
cost would be $6, 000 per acre of roof. This is beyond the most
_ marketable value of saved evaporation water. Assuming that a roof
would eliminate four acre feet of water per year, water being valued
at $. 15 per 1000 gallons due to its purity and possible domestic
market, the value of the water lost when compared with an uncovered
tank would be less than $200 per year per acre of water surface.
This amount compared with the annual cost of a roof is certainly
uneconomical for a large reservoir. 1

Floating covers could also limit the evaporation process. For
example, a plastic sheet may be stretched over a small area to reduce
the evaporation. Cost and maintenance would probably limit this

method of control to experimental stages for sometime.

Storing Water in Ground Water Reservoirs

 

The term ground water reservoir is commonly used as that area
of water bearing material or aquifer beneath the root zone of plants.
Water at this depth is affected very little if any, by the process of
evaporation. However, large amounts of ground water are being
removed by pumping especially in areas where surface waters do not

satisfy the needs of municipal, industrial, and irrigation users.

 

1Water, The 1955 Yearbook of Agriculture, 3p. §_i_t., p. 63.

 

33

The selection of ground water as a supply, rather than the -
surface water sources, has generally been on the basis of one
or more of the following advantages:

1. Ground water may be reached within a few hundred feet of
the place where it is to be used, and on the same property,
whereas surface water may require pipelines and right-of-
way over stretches of several miles.

2. Ground water may be available for use in areas where the
water in. streams and lakes has already been appropriated
by other users. ‘

3. Yield from wells and springs generally fluctuates less than
streamflow in alternating wet and dry periods.

4. Ground water is more uniform in temperature and soluble
mineral load than surface water, and is generally free of
turbidity and bacterial pollution. I

Pumping usually reduces the depth of the ground water much faster
than it is supplied by the natural process of nature. This leaves large
areas which are capable of storing surface water. These areas are
larger in many cases than existing surface reservoirs. There are also
other underground spaces where water'could be stored, as in certain
limestone formations.

Artificial recharge or depositing water in subsurface storage
basins has had various degrees of success. Failure to understand the
location of suitable storage formations and the movements of ground
water has led some to disappointment.

High quality water must be used for recharging ground water
aquifers in order to protect future users. There are also legal problems
concerning the withdrawal of water stored underground. New legislation
must be enacted or the underground storage basin confined to the use of

persons authorized to make withdrawals.

 

,lFreese, pp: _c_i_t., pp. 50-51.

34‘

, Recharging. Basins ,

 

 

 

 

/ on‘Porous Soil
‘Ic// //,// /’ Canal D 1 “7E S
("/l I], ’I / Natural _\ fl Channel
| __ ,
// /‘ 9 Canal /\(
// Dam to Store
Flood Flows D I K E S cess
EReturn
Recharging Basins Wells Down
onPorous Soil Valley

Figure 9. Methods of recharge.1

Methods which induce the water to infiltrate through the soil

into the waterbearing formations are the basin, furrow, and shaft or
pit process. The water is diverted to a place suitable for recharging
and allowed to infiltrate into the soil. - Exposing shallow water over a
large area allows (for considerable water loss by evaporation. But with
new technological methods it is certain that more water on the infil-
tration bed will reach the ground water aquifer. I

- Another method is the injection well method. This is a special
well designed and adapted for injecting water. This is very effective
if impermeable strata or layers exist between the surface of the land
and the water bearing formation. It is also useful if land is too valuable
to set aside-a large tract for, surfacemethods of recharging. The amount

of water injected into the aquifer is limited by the opening of the well

 

- lDean C. Muckel, Replanishing Underground Water Supplies on
the Farm, Soil and Water Conservation Research Division of Agricultural
- Research Service, U.- S. Department of Agriculture, Leaflet No. 452,
- September, 1959, p. 3.

 

35

and much care must be taken to prevent pollution and siltation. . As the
number of available sites for large surface reservoirs (decrease, more
attention will be given to the use of underground storage.
The‘cost of spreading water for recharging the ground water

varies. One rocky area absorbed'lo to 100 cubic feet per second of
base flow for about 85 cents per acre-foot. The San Gabriel River Basin
was recharged using a canal and basin system. The cost varied from
$3. 25 to $5. 25 per acre-foot for annual operation and maintenance plus
an average annual capital expense of $2. 0.0 per‘acre-foot. . Experience
in Los Angeles County indicates rates of infiltration may vary from 1. 5
to 10. O acre-feet per acre per day. This is cheap storage, because
frequently in SouthernCalifornia ground waterhas a value of $35 to $40
per acre-foot. 1 . 1

’ Freesez believes that a city could economically afford to spend
about $. 01 per 1000 gallons to filter, chlorinate, and recharge by gravity
underground aquifers. This would be feasible if a city were losing 25% to
33% of its stored water by evaporation and this evaporation water could

be saved by recharging ground water aquifers.

 

1George D.. Clyde, "Utilization of Natural Underground Water
Storage'Reservoirs, " Journal of Soil and Water Conservation, January,
1951, p. 15.

zFreese, _p. .c_it., p. 50.

36

Evaporation Control by Monomolecular Films

 

 

 

NO PROTECTION PROTECTIVE FILM

Figure 10. . Reservoirs with and without protection of
monomolecular films.

Introduction to Monomolecular Films

 

In. recent years the increasing demand on available water supplies
all over the world has focused attention on the possible use of mono-
molecular films to conserve this valuable and essential resource. The
method for chemical protection of water surfaces gives promise of
effective evaporation (control. In field tests as well as in the laboratory,
monomolecular films, such as that derived from hexadecanol, ‘ signifi-

cantly reduce evaporation losses. This method, while still in the stage

 

1Cetyl alcohol is another name for this material. Although in the
ordinary sense of the word it is not an alcohol but a wax. It is a white,
waxy, crystalline-like solid. The structure of the atom consists of a
long-chain primary hydrocarbon with 16 carbon atoms attached to an OH
group. This is generally available in flake or powder form. It is rela-
tively tasteless and odorless. Hexadecanol is derived from materials
such as tallow, sperm oil, or coconut oil.

37

of testing and development, appears to be susceptible to a practical
application at costs within economic limits. Investigations by a number
of governmental and private organizations support the expectation that
monomolecular films along with conventional techniques, can signifi-
cantly increase the usable water supply by reducing evaporation losses. ‘

1 . A realization of the tremendous effect which exceedingly thin
layers of certain substances might have upon the reduction of evaporation
of water is not new. There is extensive literature available describing
research on this subject. The early work of Rideal, 7' Langmuir and
Langmuir3 and other research indicates that certainmonomolecular
films were effective in suppressing evaporation, and that the substance
>most likely to be useful was hexadecanol. These experiments were
confined to a laboratory. The first field application was performed in
1952 by Mansfield,‘ an Australian» physical chemist.

Interest in this method of control has increased in the last few

years. For this reason more attention is devoted to this factor as an
evaporation control method than any of the others. This also holds

promise as an evaporation retardant.

Theory of Monomolecular Films

 

A simple manner to explain how a film will control evaporation

is to remind the reader of his last bowl of soup which contained a fatty

 

1Eaton, c_>p_. 113., p. 15.

2E. K. .Rideal, "The Influence of Thin Surface Films on-Evapora-
tion of Water, " J. Phys. Chem. , _2_2, 1585 (1925).

 

3Irving Langmuir and D. B. Langmuir, "The Effect of Monomole-
cular Films on the Evaporation of Ether Solutions, " J. Phys- Chem. ,
_3_l_._, 1719-1 (1927).

‘W. W. Mansfield, "Effect of Surface Film on the Evaporation of
Water, " Nature, 172, 1101 (1954), p. 16.

38

layer on the surface. - Under these conditions the soup will remain hot
and the water does not evaporate. . If one wants to cool the soup‘quickly
by evaporation, he sticks a spoon in it and breaks the film of oil by
agitation. ‘ All have seen the clouds of condensed steam come from hot
soup whentit is stirred. Also, if one blows on the soupit will cool
more rapidly than if it stands undisturbed. This simple experiment
illustrates how a fatty film slows the process of evaporation. ‘
In this case the film is sufficiently thick to-bei observed directly.

The extent and nature of any spreading on the water of monomolecular
film may be followed visually by. methods such as noting the movement
of talc powder dust on the film surface.

A Certain types of organic compounds possess the property of forming
a film one molecule in thickness when applied to a water surface. We
find that those organic chemical molecules that are good retardants of
evaporation also furnish a very interesting story about their molecular
structure. The retardation of evaporation by a film depends largely
on the molecular architecture. I

r We were taught in chemistry that the hydrocarbons could be very
long-chained molecules, one carbon atom following another in a long
chain with hydrogen atoms attached to each carbon. The short-chained
hydrocarbons of 1, 2, 3, or 4 carbon atoms carry the names of methane,
propane, ethane and butane. - For the long chains, the nomenclature
becomes very complicated. In general, we have given up the use of long
names and haveadopted the policy of saying for example,» C12, C13, or
C“, that is, the number of carbon atoms in the chain of hydrocarbon,

acid, alcohol, or. ester.

 

1V. K. LaMer, "The Physical Chemical Basis of Water Evaporation
Control by the Monomolecular Film Technique, " First International
Conference on-Reservoir Evaporation Control, (San Antonio, Texas:
Southwest Research Institute, April 14, 1956), p. 7.

 

 

39

A hydrocarbon forms a lens and does not spread on the water sur-
face. In order to make the hydrocarbons spread, we have to attach
an OH group at one end of the molecule. - Acids, alcohols, and esters
do spread because they contain an OH group in their molecule which is
attracted to the water molecules which also contain OH groups. I The
result is that a droplet of an acid, an alcohol, or an ester spreads out
and forms a monomolecular film on the surface of the water.

Certain classes of organic compounds known as polar compounds
possess the evaporation suppressing effect. These polar compounds
have a hydrOphylicz (water attracting) and a hydrophobic (water repelling)
portion comprising their molecular structures and possess the property
of being able to spread out on the water surface. The hydrophobic portion
of the molecule consists of a hydrocarbon structure and for the purpose
of retarding evaporation should contain 10 to 18 carbon atoms.3

The reduction of evaporation takes place when the monomolecular
layer is compressed. ~ When a monomolecular layer of certain polar
compounds is compressed, the molecules of water which are moving about
under the impulsive effects of heat within the body of water, are unable

to break through the compressed layer into the air but are retained by

the body of water. Therefore, an evaporation reducing effect is produced.

 

lSee Illustration.

zThe hydrophylic end of the molecule is a functional group, usually
characterized by the OH group. It has a great affinity for water, so that
when the substance is in contact with water, the molecules tend to align
themselves always with one end in the water and the remainder of the
molecule out of the water.

3The hydrophobic residue should contain at least 10 carbon atoms,
as a smaller residue implies a compound quite soluble in water. Those
chain lengths having over 18 carbon atoms generally form too brittle
a film and it will not reform well when ruptured by rain, boats, or other
causes.

HYDROPHOBIC
PORTION

HYDROPHYLIC
PORTION

 

Figure 11. STRAIGHT-CHAIN MOLECULE

Figure 12.

 

RELATIONSHIP OF STRAIGHT-CHAIN MOLECULES

4O

41

. For a water molecule to evaporate, it is necessary for this molecule
of water to break through these chains in the monolayer. It should be
evident that the higher the compression of the monolayer the more
difficult it would be for the molecules of water to thread their way
between the fatty acid molecules to escape and evaporate as vapor.

- The hydroxyl compounds, particularly the straight-chain primary
alkanols, were found to be the best compounds to use as evaporation
, retardants. - Comparison of the orientation of the molecules of various
types of alkanols (straight-chain, secondary and tertiary) in a mono-
molecular film, the primary alkanols by nature of their chemical
structure, will form the-most effective evaporation retardant film.
The straight-chainprimary structure is capable of forming a more highly
compressed film than the secondary, tertiary, or branched-chain

compounds . 1

\\

OH OH OH OH OOOOOOOO

UNCOMPRESSED H H H H H H H H
COMPRESSED

 

Figure 13. Straight-chain primary molecule.

- Illustration'Figure 13 shows a comparison of an uncompressed
and compressed straight-chain primary molecule. . As noted, the
uncompressed long chains do not stand erect on the surface but are

tilted to a certain extent depending upon the pressure to which the film

 

1Cruse, 2p. cit., p. 9.

42

is subjected. As the film is compressed, the angle of the tilt increases
until finally the molecules are packed very tightly together and are pre-
cisely perpendicular to the surface of the water. The molecules are

then compressed against each other like safety matches in their packet.

Thus, when packed together, the molecules stand on end, closely
packed, and form a film which helps resist evaporation of the
water thus covered. Chemical materials of this type applied to
the surface of water will, by their own special nature, spread
continuously unless confined by the physical barriers such as
shore lines or reservoir walls. . At any time a material is
present in excess of the amount necessary to form a compressed
film one molecule deep, the film-forming material may function
as an effective evaporation retardant. I

Qualifications of an- Effective Monomolecular Film

l.- A monolayer film will need to restrict the rate of evaporation
effectively by providing a greater resistance to the passage
of water vapor molecules from the water surface into the air
than are normally present.

2.- It must be a non-toxic, odorless, and an invisible substance.
a. It should not harm or otherwise affect fish and aquatic life.

b. The material used should be agreeable and safe for human
health.

3. The film should be lighter than water so that it can float but
not be overly soluble.

4. The film must spread easily and possess self-sealing
properties. This includes resisting the action of dust and
wind.

5. Only the compounds forming liquid films at normal tempera-
tures can be considered.

6. The length of the effectiveness of a monomolecular film
will determine its value as a suppressant. Therefore,
resistance to oxidation or degradation by fish and aquatic
life is desirable.

 

1B. W. Beadle and R. R. Cruse, "Water Conservation Through
Control of Evaporation, " Southwest Water Works J. , 38, No. 9, 1956,
p. 398.

 

43

7. The initial cost of the material and the cost of the appli-
cationrmust be economically feasible.

Effectiveness of Monomolecular Films

 

The type of material used for a monomolecular film will be a
determining factor in the amount of evaporation retarded. - Hexadecanol
was one of the first products used to produce a favorable monolayer to
control evaporation and many of the experiments are centered around
this product. Its value as a retardant, and the readily available supply
of cetyl alcohol, has caused it to be used almost entirely in projects
determining control of evaporation on large reservoirs.

‘ Hexadecanol has proved to be a good performer in reducing
evaporation. It has usually satisfied the requirements for effective mono-
molecular films. C", is a straight-chain fatty alkanol with theoretical
superiority. Despite the theoretical superiority, those straight-chain
alkanols having an even number of carbon atoms are susceptible to
bio-chemical oxidation and some consumption by aquatic life. Oxidation
not only takes place on the film after formation, but on the reserve
supply if it is stored on the water surface. Other fatty alcohol combi-
nations have shown in screening tests1 to be more effective at higher
temperatures in reducing evaporation.

. An evaporation reduction investigation conducted at Lake Hefner
in 1958 shows the importance that other products may have in relation
to cetyl alcohol. - Screening tests were conducted on 22 promising fatty
alcohol combinations. The material used in the large field test with a
composition of Cn-l. 0%; CM-4. 0%; C15-93%; and C,,-2% ranked fifteenth

in effectiveness out of 22 materials tested. Therefore, 14 other

 

,‘Screening tests are usually considered to be small laboratory
tests which determine if a product would be worthy of a more extensive
test on a large body of water. They are usually conducted out-of—doors
in 4 foot diameter U.- S. Weather Bureau class "A" evaporation pans.

44

materials have now been found superior to the hexadecanol product
used at Lake Hefner. This standing was based upon the evaporation
reduction factor at 82. 5° F, the highest temperature for the test.

The three materials at the highest temperature were about 1. 5 times
as effective as the material used at Lake Hefner. Higher temperatures
will cause larger amounts of evaporation and therefore, the use of
materials other than hexadecanol may produce a greater result in
evaporation savings.

Lake Hefner field investigations clearly demonstrated that it is
possible to apply a monomolecular film to a large lake. ‘ The studies
were for an 86 day period in 1958. Treatments weremade on only 55
days of this period because of high winds and other conditions making it
unfeasible to make applications on 31 days. The average coverage for
the 86 days is an indication of the degree of successful establishment
of a visible layer under the conditions that prevailed during the summer
of 1958 with the techniques used. The average coverage for the 86 days
of the investigation was 10 percent and the overall evaporation savings
for the same period was 9 percent or a total savings of 450 acre feet
of water. The maximum savings that could have been expected with
hexadecanol at the temperatures prevailing was 35%. 1

Under usual conditions where water saved from the process of
evaporation can be used and has value, the costs should not exceed
benefits.3 The Lake' Hefner studies were the first of its kind on a large
lake and techniques of application and the type of equipment best suited

 

‘ 1Lake'Hefner'has a surface area of approximately 2500 acres.

zWater-Loss Investigations; Lake Hefner 1958 Evaporation. Reduction
Investigations, Report by the Collaborators, June 1959, p. 43.

,3Conditions will be considered later where the value of the water
saved from evaporation may not be the most important item.

45

for the work were unknown. Therefore, the costs would probably be
higher than on a later test of the same nature.

Meticulous records were maintained on the costs of (l) hexadecanol
applied, (2) gasoline, oil, and repairs for operation of boats,

(3) salaries and wages of operators and laborers, (4) motor vehicle
operation, (5) rental of barge, (6) equipment depreciation, and

(7) miscellaneous expenses.1 ' '

The total cost of the Lake Hefner tests was $27, 542.42. The 9%

. reduction of water that would have otherwise evaporated amounted to

450 acre-feet. When this is divided into'the total cost, the average cost
was $61.21 for, every acre-foot of water saved. 2 The largest single cost
during this time was for 40, 040 pounds of hexadecanol. The cost that

was paid was $0. 515 per pound for a total of $20, 620. 60. This is
approximately 3/4 of the total field cost, and therefore any significant
reduction of material costs would have an important effect on the reduction
of the overall costs of the field operations.

, Reports based on discussions with manufacturers representatives
show that they anticipate that material costs may be reduced as muchas
50 percent of present costs. The reason for a reduction in cost will be
available when. a standardized material can be used in' large quantities.
Lake Hefner used hexadecanol in finely powdered form instead of the
granular or flake form. It required special processing by themanufacturer
and thereforean increase in cost to the consumer. Once a largemarket
for similarforms and types of material is available the price would seem
to be reduced. The development of snythetic materials also indicates a

, substantial saving. as compared with the use of the natural product. 3

 

,‘Water-Los s: Investigations; Lake Hefner 1958 ‘ Evaporation Reduction
Investigations, _p. git.) p. 89.

:zThis compares with the value of raw water in Lake Hefner which is
about $60 per acre-foot, as reported by the city of Oklahoma City.

 

’Water-Loss Investigations, Lake Hefner 1958 Evaporation Reduction
Investi ations, 2p. git” p.’ 91.

 

46

Improved techniques, both in the manufacture and field application could
be expected to further reduce these costs. Increased efficiency-of the
monolayer, whether it be an improved material or application, would
increase the evaporation savings and thereby effect a lower unit cost.
The large potential market for materials used as evaporation
retardants does not displace any other chemical or process now on the
market. , Robert R. Cruse states that in Texas alone there is the market
potential for 38, 400 tons per year of a qualifying evaporation retardant. 1
Upon consideration of the possibilities for the potential market of
evaporation retardants throughout the western states, as well as many
other localities throughout the world, one can see at once why many
chemical firms might be interested in the success of such a product.
Test results indicate that under certain conditions a successful

degree of evaporation reduction is possible for small reservoirs. 2

Many
problems still must be solved before the process can be recommended
for general use. - Among these are a simple effective method of application
and maintenance of the chemical film on the water surface in order to
reduce the amount of material required. The product used should satisfy
all the requirements of a monomolecular film. These should reduce
evaporation losses, cause no adverse effects, have a relatively long life,
and resist oxidation or consumption by aquatic life. Tests must be
conducted for a sufficient time and under varying conditions to gain data
that may be used in making an analysis of the economics of evaporation
retardation for particular localities. It is hoped that research will help

supply the needed information that will make evaporation control by films

a practical reality.

 

1Cruse, pp. cit., p. 6.

zCommonwealth Scientific and Industrial Research Organization,
, Saving Water in Dams, The Mansfield Process, C. 8.1. R. O. Leaflet 15,
Melbourne, Australia, 1956, p. l.

 

47

CONCLUSIONS

Water, a great natural resource, should be managed wisely,
especially when the supply of available water may be the limiting factor
of growth. - Economic growth whether in areas of irrigation, industry,
or cities may be hampered by water shortages. Many localities find it
necessary to place severe restrictions on water usage during long hot
dry periods. P0pulation needs and demands for dependable supplies of
high quality water are steadily growing while the supply of available
water is not increasing. Where the available supply of water is the
limiting factor of growth, research directed towards all phases of water
conservation is appropriate whenever possible.

Reduction in evaporation losses will benefit the entire nation.

The Seuthwestern- United States will benefit most, due to the present high
rate of evaporation losses and thelimitation of growth that a shortage

of Water has placed upon it. The humid portion of our country can

benefit also by retarding evaporation losses. Decreasing losses from
storage basins may be equivalent to increasing the size of the watershed
by a similar Mount. This increase would be welcomed by many water
beards in ’cities throughout the United States, especially those which
suffer from recurrent water shortages. Many individually owned supplies
could also profitably conserve water through evaporation control.

, Water in some reservoirs contains high amounts of dissolved solids,
especially in the Southwest. - Evaporation, in removing only the pure
water, serves to concentrate the dissolved materials already present.
The high salinity in certain reservoirs due to evaporation losses makes
the water unusable. Reduction of evaporation may not only save valuable

water but also render the water usable frmn the entire reservoir.

48

'Where there is only a slight difference in the amount of pure water that
can be removed from a lake before it is considered for rejection as a
water supply, evaporation retardation will generally be economically
feasible.

Our world has areas where rainfall and ground water are not
available throughout the year. . Here, people depend entirely upon the
reservoir storage supplies during the periods of low rainfall. - Evaporation
controls may extend the water supply in these reservoirs. ~ Economics at
critical low water levels may be a secondary consideration, especially
when needed for only a short period until additional precipitation becomes
available. 7

The reduction of evaporationby methods which limit the amount of
water surface exposed to the atmosphere offers considerable promise.

- Considering evaporation losses in management procedures of reservoir
systems may in some instances be profitable. . Some degree of evaporation
control can be achieved by designing a reservoir having a maximum average
depth, thus having a smaller water surface area per unit of volume.
. However, this approach is limited by such factors as topography,
accessibility, availability of dam sites and costs involved. Many of the
most desirable sites have already been used or are unavailable for
reservoir use.

. Reduction of aquatic and riparian vegetation will lower the amount
of water consumed and lost by relatively low value plants. - Experimentation
is now being carried on in this field but the development of replacing this
vegetation with less water consumptive plant users in a reasonable price
range will need to be solved before the process can be (recommended.

The reduction of wind over reservoirs will help reduce evaporation

(but to my knowledge, little has been done along this line).

49

‘Roofs and covers on which some research is now underway,
obviously must be limited to small areas because of costs and con-
struction characteristics.

Underground storage has possibilities where physically possible.
Many questions still are unsolved in recharging procedures, pollution
possibilities, and legal ownership of stored waters.-

Because of the limitations of these physical methods of control more
attention has been recently directed to, the use of chemicals capable of
suppressing evaporation. This affords considerable promise and has the
possibility of developing into a very profitable conservationpractice.

The combination of as many of the physical methods of control as are
economically possible with chemical control will undoubtedly be considered
more often in the future.

. Suppression of reservoir evaporation offers considerable promise
as means of conserving available water supplies. It is in the dry arid
portions where a monofilm technique will be of most value. . Where ample
water is available or high humidity prevails, evaporation retardants do

not appear to be economically feasible.

50

BIBLIOGRAPHY

This bibliography was compiled from the Survey of Literature
on Evaporation Suppression by George B. Magin, Jr. , Final
Report on the Southwest Cooperative Project on Reservoir
Evaporation Control by Robe rt R. Cruse, and many other
individual publications. ‘

 

 

 

 

 

Abbe, Cleveland, Prevention of Fog, Monthly Weather Rev. , U.-- S.
~Weather Bureau, 11:2, No. 2, 104, (1914).

Adam, N. - K. , Properties and Molecular Structure of Thin Films of
Palmitic Acid on Water, Part 1, Proc.. Roy,- Soc. London, 99A,
336-51 (1921).

- Adam, N. K. , Properties and Molecular Structures of Thin Films, II,
Condensed Films, Proc.- Roy.- Soc. London, 101A, 452-72 (1922).

Adam, N. K. , Properties and Molecular Structures of Thin Films, III,
- Expanded Films, Proc. Roy.- Soc. London, 101A, 516-31 (1922).

Adam, N. K., The Structure of Thin-Films, IV, Benzene Derivatives,
- A Condition of Stability in Monomolecular Films, Proc.. Roy. Soc. '
'London, 103A, 676-87 (1923).

Adam, N. K. , The Structure of Thin Films, Part V, Proc. Roy. Soc.
London, 103A, 687-95 (1923).

Adam, N. K. , The Evaporation of Water from Clean and Contaminated
’Surface, J. Phys. Chem. 32, 610-11 (1925).

Adam,- N. K., Unimolecular Surface Films, Kolloid-Z., _6_l_, 168-177
(1932).

- Adam, N. K. , Rapid Method for Determining the Lowering of Tension
of Exposed Water Surfaces with some Observations on the Surface
TenSion of the Sea and of Inland Waters, Proc. Roy. Soc. London,
_l_2_2,- Series B, 134-139 (1937).

Adam, . N. - K., The Physics and Chemistry of Surfaces, Third Edition
(see especially Chapter II), Oxford'Univ. Press (1941).

51

-Adam, N. K., and Dyer, J. W. W., The Molecular Structure of Thin
'Films, Proc.- Ry.- Soc. London, 106A, 694-709 (1924).

Adam,-N. K. , and Harding, J. B. ,- Surface Films, XVI,~ Surface
Potential Measurements on Fatty Acids on Dilute Hydrochloric
Acid, Proc. Roy.- Soc. London, A, 138, 411-30 (1932).

Adam, N. K. ,1 Askew, F. A. , and Pankhurst, K. G.- A. , Interaction
-Between Adsorbed Substances of Simple Constitution and Insoluble
Monolayers, Proc.. Roy.- Soc. London, 170A, 485-500 (1939).

- Addink, N. W. H. , Unimolecular Layers on Water, Chem- Weekblad
:12, 203-7 (1946).

- Albertson, M. L. ,- Evaporation in- Relation to Friction Velocity, C.- R.
-Acad.- S.ci., Paris, 239, 529-531 (1954).

Albertson, M. L. ,. Extension of the Reynold's Analogy to Evaporation,
. C.- R.-Acad.- Sci. , Paris, 239, 474-476 (1954).

Albertson, M. L. , Mechanics of Evaporation, La Houille Blanche,
10, No. 5, 704- 717 (1955), 11, No. 1, 36 (1956), _1_l__,_ No. 2, 282
(1956).

Allan,- A. J. G. , andAlexander, A.- E. , Monolayers at Low Surface
Pressures, Trans. Faraday Soc., :0, 863-73 (1954).

> Allan, A. J. G. , and Schulman, J. H. ,- Surface Films of Phenolic
Compounds, J.. Chem. Soc. , London, 1954, 1238-41.

- Allen, H. S. , A Text- Book of Heat, 1948 Macmillan and Co. , Limited
St.- Martin's Street, London.

Alty, T. ,- Reflection of Vapor Molecules at a Liquid Surface, Proc.- Roy.
. Soc. London, A131, 554-64 (1931).

Alty, T. ,1 Application of Knudsen's Law to the Evaporation of Water,
Nature, 130, 167-8 (1932).

Alty, T. , Maximum Rate of Evaporation of Water, Phil. Mag. , L5:
82-103 (1933).

- Alty, T. , and Nicoll,- F. H. , Interchange of Molecules Between a Liquid
and its Vapor, Can. J. Research, 4, 547-58 (1931).

52

American Society Civil Engineers, Standarchquipment for Evaporation
‘ Stations, Final Report of Subcommittee on- Evaporation of the
Special Committee on Irrigation Hydraulics, Trans.- Amy Soc.
~Civil Eng., 92, 716-718, Discussion, 719-747 (1934).

Anderson, P. J. , and Pethica, B.- A. , Thermodynamics of Monolayer
Penetration at Constant Area, Part 2, Trans.- Faraday Society,
_5_2_,- No. 8, 1080-1087 (1956). '

Anderson,- E- R. ,- Anderson, L. J. and Marciano, J. J. , Lake Mead
Water Loss lnvestigations--A Review of Evaporation Theory and

Development of Instrumentation, U.- S. Navy Electronics Lab
Report No. 159 (1950). A. '

- Anderson, E.- R. ,- Energy-Budget Studies, Water-loss Investigations,
Vol. 1, Lake Hefner Studies Technical Report, U.- S. Geol.~ Survey
Prof. Paper 269, 71-118 (1954) Circ. 229, 71-118 (1952).

- Angstrom,~ A- K. ,' Application of Heat Radiation Measurements to the
Problems of the Evaporation from Lakes and the Heat Convection
at their Surfaces, Geografiska Annaler, 2_, 237 (1920).

-Anon.,- Algae in Lakes, Texas Water Report, 4, No. 35, p. 3, June
20, 1957.

- Anon. ,- Annual Report, Division of Industrial Chemistry, C. S. I. R. O.,
p. 917, Jan. 1957.

-Anon., Chemical Engineering, Jan. 1948.

Anon. ,1 Chemical Engineering News, Feb. 13, 1956.

- Anon. ,- Special Investigations Memorandum No. 55-7, U.- S. Bureau of

' Reclamation, Division of Engineering Laboratories, Denver,
Colorado, Aug. 2, 1955. Processed.

- Archer, R.-J. , and LaMer, V. , K. , The Effect of Monolayers on the

Rate of Evaporation of Water, Ann. N. Y.-Acad.- Sci., §_8_, 807-29

(1954).

- Archer,. R. J., and LaMer, V. K. , The Rate of Evaporation of Water
Through Fatty Acid Monolayers, J. Phys. Chem. , 5_9_, 200-8 (1955).

53

Arthur, G.- B. , The Water We'Do Not Use, Publ. Wks. , §__8, No. 9,
134-136 and 176-185 (1957).

Baranaev, M. , The Effect of Surface Layers of Insoluble Substances
on the Rate of Evaporation of Water, J. Phys. Chem.- (U. S. S.R. ),
9, 69- 76 (1937).

Baranaev, M. , Kinetics of Evaporation, Uspekhi Khim. , '_l_, 1231 (1938).

Bartlett, Raymond, and Poulter, T.- C. , The Rate of Evaporation-Through
‘Surface'Films, Proc. Iowa Acad. Sci., 34, 214-5 (1927).

Beadle, B.- W. , andCruse, R.- R. ,- Water Conservation-Through Control
of Evaporation, J.- Am. Water Works Assoc. , a, 397-404 (1957).

\ . Beadle, B.-W. ,. and‘Cruse, R. ~ R. ,- Water Conservation Through Control
of Evaporation, Southwest Water Works J. , 38, No.. 9. PP« 16-28.
1956.. ”

- Betts, J. J.,. andPethica, B.-A. , The'Ionization Characteristics of
Monolayers of Weak Acids and Bases, Trans‘.- Faraday Soc. , _5_2,
No.. 12, 1581-1589 (1956).

Bigelow, F. H. , Studies on the Phenomena of the Evaporation of Water
Over Lakes and Reservoirs, Monthly Weather Rev. , _3_5, 311-16
(1907), L6, 24- 39, 437- 45 (1908), 38, 307- 13 (1910).

Bikerman, J. J. ,- Formation and Structure of Multilayers, Proc. Roy.
- Soc. London,- A 170, 130-44 (1939).

Blaney, H. F. ,- Evaporation from Free Water Surfaces at High Altitudes,
Am. Soc. Civil Eng. Proc., 8_2_ (IR 3, No. 1104), 1- 15 (1956),
discussion 8__3_, (,IRl No. 1257), 15 (1957).

Blaney,. H.v F. ,. and Corey, G. L. ,- Evaporation from Water Surfaces in
- California, Bull. 54-B,. Div. Water Resources,*California (1955).

Blank, Martin, and LaMer, V. - K. , Transfer of Monolayers Through
‘Surface- Channels, ‘11 Mechanism, J. Phys. Chem., 6_1_, 1611-14 (1957).

Bloodgood, D. W., Patterson, R. E., and Smith, R. L., Jr., Water
Evaporation Studies in Texas, Texas Agric. Expt. Sta. Bull. 787,
83p. (1954).

54

Boelter, L. M.-K. , Gordon, H.- S. , and Griffin, J.. R. ,. Free
Evaporation into Air of Water from a' Free Horizontal Quiet Surface,
Ind.- Eng.- Chem. , 3.8., 596-600 (1946).

Boelter, L.-M. K. , Gordon, H.. S. , and Griffin, J.- R... Free'Evaporation
into Air of- Water from a Free Horizontal Quiet Surface Correction,
- Ind.- Eng. Chem., '3_8, 1256 (1946).

Bonython, C. W. ,- Evaporation Studies Using Some South Australian Data,
Trans. Roy; Soc.» Sr Australia, 13, pt. 2, 198-219 (1950). r

Boon,- A.- G. ,. and Downing, A. L. , Observations on the Use of Cetyl
Alcohol for Conservation of Water, J.~ Inst. Water Engrs. ,, 11,
443-8 (1957). ‘ '

Bowen, 1. S. , The Ratio of Heat Losses by Conductionand by Evaporation
from any Water-Surface, Physical Rev. , _2_7_, 779-87 (1926).

Boyd, G.~ E. ,- Energy Relations in Monolayer Formation, The Spreading
of Long-Chain-Fatty Acids on Aqueous Surfaces. - Paper presented
at the 131st National ACS Meeting, New York,- Sept. 10, 1957,
during the Symposium on-Properties of Monolayers, Division of
Colloid Chemistry. '

Boyd, G.- E., and Schubert, J. ,- Energy Relations inUnimolecular Film
. Formation. , The Spreading of Cetyl Alcohol and Palmitic Acid on
- Aqueous Surfaces, J. of Phys. Chem., _6_l_, 1271-1275 (1957).

Burgess, B." F. , Jr. , Surface-Film Studies on DDT-Oil Larvicides,
- Soap Sanit. Chem.,. _2_5_, No. 8, 127-31 (1949).

- Carpenter, L.- G. .. Section of Meteorology and Irrigation-Engineering,
- Colo.- Agr. Exptr Sta.- Ann.« Rpt. 4, (1891).

Carrier, W.~H. , and Lindsay,» D.- C. , Temperatures of Evaporation of
Water intoAir, Mech. Eng., 41, 327-31 (1925).

- Carrier, W.~H. ,. and Lindsay, D.. C. , Temperatures of Evaporation of
Water into Air, Refrig.- Eng. , _1_1_, 241-57 (1925).

Cary,- A. , and Rideal,- E. K. , The Behavior of Crystals and Lenses of
, Fats on the Surface of Water, Part I. The Mechanism and Rate of
Spreading, Proc. Roy. Soc..London, 109A, 301-317 (1925).

55

Cary, A. ,4 and Rideal, E. - K. , The Behavior of Crystals and Lenses of
Fats on the Surface of Water, Part II. The Effect of Temperature
on the Equilibrium- Pressure, Proc, Roy.- Soc. London, 109A,
318-330 (1925). H

- Cary,- A. ,. and- Rideal, E. . K. , The Behavior of Crystals and Lenses of
Fats on the Surface of Water, Part III. The Effect of the‘ Polar Group
on the‘Equilibrium‘ Pressure, Proc.. Roy.~ Soc. London, 109A,
331-338 (1925).

Chakravorty, K.- R. ,- Evaporation from' Free Surfaces, J. Imp. Coll.
-Chem.-Eng.- Soc. London, _3_, 46-53 (1947).

- Chemical and Engineering News, Chemical Shields to Prevent Water
Evaporation, _3_5, No.. 23, 28 (1957). '

Chemical and Engineering News, Film~ Protects Climate's Prey, _3_6,
- No.. 26, 44-45 (1958). '

Chemical Week, Australians Have Developed a Trick to Save Water,
12" No. 4, 9 (1955).

Chemical Week, Control on- Water, L7, No. 9, 18 (1955).

Chemical Week, Process Plants Plan for Future Water Needs, 82,
No.. 26, 21 (1957).

Clyde, George D. , Utilization of Natural Underground Water" Storage
Reservoirs, Journal of Soil and Water Conservation, January,
1951s

(Commonwealth-Scientific and Industrial Research Organization, Saving
.Water in Dams--The Mansfield Process, C. S.I. R. O. » Leaflet 15,
4p. ,1 Melbourne, Australia (1956).

Commonwealth Scientific and Industrial ResearchOrganization, An
Instrument for Measuring Evaporation er' Natural Surfaces,
Nature, Vol. 181,. pp. 408-409, Feb. 8, 1958. ,

, Commonwealth Scientific and Industrial Research Organization, Div. of
Industrial Chemistry, Annual Report for Year Ending30th June
1957, p. 17, Melbourne, Australia.

56

Conservation Soil and Water for the Public Schools, State of North
Dakota 1948, Issued by Department of Public Instruction, Garfield
B. Nordrum, Superintendent, p. 70.

- Cowley, A.- W. ,- A Note on Diffusion Processes, J.- Imp.- Coll.- Chem.
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