ARID ZONE RESEARCH - XXVII
EVAPORATION REDUCTION
I
.
Titles in this series:
Reviews of research on arid zone hydrology
Proceedingsof the Ankara Symposium on Arid Zone Hydrology 1 Actes du colloque d'Ankara
sur l'hydrologie de la zone aride
III.
Directory of institutions engaged in arid zone research
Utilization of saline water. Reviews of research
IV.
V.
Plant ecology. Proceedings of the Montpellier Symposium Ecologie végétale. Actes du
colloque de Montpellier
Plant ecology. Reviews of research 1 8cologie végétale. Compte rendu de recherches
VI.
VII.
Wind and solar energy. Proceedings of the New Delhi Symposium Energie solaire et
éolienne. Actes du colloque de New Delhi Energía solar y eólica. Actas del coloquio celebrado en Nueva Delhi
VIII.
H u m a n and animal ecology. Reviews of research Ecologie humaine et animale. Compte
rendu de recherches
IX.
Guide book to research data on arid zone development
x.
Climatology. Reviews of research
XI.
Climatology and microclimatology. Proceedings of the Canberra Symposium Climatologie
et microclimatologie. Actes du colloque de Canberra
XII.
Arid zone hydrology. Recent developments
XIII.
Medicinal plants of the arid zones
XIV.
Salinity problems in the arid zones. Proceedings of the Teheran Symposium 1 Les problèmes
de la salinité dans les régions arides. Actes du colloque de Téhéran
xv.
Plant-water relationships in arid and semi-arid conditions. Reviews of research '
XVI.
Plant-water relationships in arid and semi-arid conditions. Proceedings of the Madrid
Symposium 1 gchanges hydriques des plantes en milieu aride ou semi-aride. Actes du
colloque de Madrid Intercambios hídricos de las plantas en medios áridosy semiáridos.
Actas del coloquio celebrado en Madrid
A history of land use in arid regions
XVII.
XVIII. The problems of the arid zone. Proceedings of the Paris symposium
Nomades et nomadisme au Sahara (in French only)
XIX.
Changes of climate. Proceedings of the Rome symposium organized by Unesco and WMO 1
xx.
Les changements de climat. Actes du colloque de Rome organisé par 1'Llnesco et l'OMM
Bioclimatic m a p of the Mediterranean zone. Explanatory notes
XXI.
Environmental physiology and psychology in arid conditions. Reviews of research
XXII.
Agricultural planning and village community in Israel
XXIII.
XXIV. Environmental physiology and psychology in arid conditions. Proceedings of the Lucknow
symposium Physiologie et psychologie en milieu aride. Actes du colloque de Lucknow
Methodology of plant eco-physiology. Proceedings of the Montpellier symposium 1 MéthoXXV.
dologie de I'éco physiologie végétale. Actes du colloque de Montpellier
XXVI. Land use in semi-arid Mediterranean climates
XXVII. Evaporation reduction
XXVIII. A geography of coastal deserts (in preparation)
I.
II.
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The reviews of research are published with a yellow cover; the proceedings of the symposia
with a grey cover.
EVAPORATION
REDUCTION
Physical and chemical principles and
review of experiments
by J. F R E N K I E L
Water Planning for Israel, Tel Aviv and
New York University
U
N
E
S
C
O
Published in 1965 by the United Nations
Educational, Scientific and Cultural Organization
Place de Fontenoy, Paris-7'
Printed òy Imprimeries Réunies de Chambéry
,
?
/-
i
0 Unescn 1965
Printed in France
NS. 6.l/III.32/A
F O R E W O R D
U
NESCO’S arid zone programme was started in
1951 and
became a ‘major project’
from 1957 to 1962. Its object was to promote research, particularly by providing
support for certain institutions in the arid regions. T h o u g h this period is now
over, arid zone research is nevertheless continuing to receive support as part of Unesco’s
programme in earth sciences. This includes the publication of further volumes in the
Arid Zone Research series. At present this series comprises twentysix volumes containing
both reviews of research and proceedings of symposia on arid zone hydrology, plant
ecology, climatology, utilization of saline water, h u m a n and animal ecology, etc.
Evaporation control is an outstanding problem in the arid zones and has been dealt
with in several of the previous volumes of this series. It should be mentioned in particular
that a symposium on the subject of water evaporation control w a s organized by Unesco,
. through its South Asia Science Co-operation O$ce
and in collaboration with the Indian
Council of Scientijc and Industrial Research, in 1862 at Poona. A survey of experiments
was conducted in 195911960 by the International Commission for Irrigation and Drainage
under a contract with Unesco. However, it was thought that particular attention should
be given to this subject and that a monograph dealing in detail with the reduction of
evaporation would complement the studies previously published. It need not be emphasized
that the points of view,selection of material and opinions expressed thereon in this volume
are those of the author. It is hoped that this work will be useful to researchers in the
various fields of science concerned with evaporation and with arid zone problems.
In presenting this volume Unesco wishes to thank the author for making this valuable
contribution to the Arid Zone Research series.
C O N T E N T S
Introduction
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Organizational framework of evaporation control research
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CHAPTER I. Physical and chemical principles of evaporation control :laboratory
experiments . . . . . . . . . . . . . . . .
1 Historical review .
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2 Evaporation resistance of monolayers .
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Film
pressure . .
Temperature
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Wind velocity . .
Purity of the.material
3 Spreading properties .
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experiments and
1 Evaporimeter experiments .
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Australian experiments . .
United States experiments .
Indian experiments .
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Experiments in the U.S.S.R.
Experiments in Israel .
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Experiments in Japan .
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Experiments in other countries
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field trials
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Spreading rates
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Spreading velocities
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Effect of chemical composition . .
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Effect of polymorphism
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Effect of physical characteristics
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Effect of wind
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Equilibrium spreading pressure . .
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Collapse pressure . . . . . . .
Influence of losses by evaporation and solution
CHAPTERII. Evaporimeter
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2 Field trials: general review
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Early Australian experiments
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East African experiments
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United States small-scale experimenta
United States large-scale experiments
Recent Australian experiments . .
Indian experiments
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Other field experiments
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3 Field trials: materials .
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Fatty alcohols .
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Mono-oxyethylene ethers
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5 Field trials: detection and evaluation of film coverage
6 Field trials: effect of wind
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7 Evaluation of evaporation savings . . . . .
P a n coefficientmethod . . . . . . .
Combined energy budget and mass transfer method
Simplified method . . . . . . . .
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8 Biological aspects
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Physical and chemical factors .
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Toxicity . . . . . . . . . .
Surface tension reduction . . . . . .
Changes in the plant and animal communities
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9 Economic evaluations
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4 Field trials: methods of application
Beads in raft method .
Solvent application
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Powder application
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Application as an emulsion
H o t spray application .
Aerial application . .
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10 Reduction of evaporation from soil and transpiration from plants b y
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11 Evaporation reduction by means other than monolayers .
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.I 66
means of fatty alcohols
Conclusions
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I
Bibliography
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69
INT-HODUCTION
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T h e rapid increase in both world population and per capita consumption of water
due to the rising standards of living and levels of economic activity, have greatly
intensified the d e m a n d for water all over the world. T o satisfy this demand, fresh
sources of water supply are being tapped and serious thought is being given to
methods of conservation of water. In the arid and semi-arid regions of the world,
in particular, where the lack of sufficient water resources m a y prove the limiting
factor in economic development, increased attention is being focused on the search
for n e w methods to conserve existing water supplies.
In any region where water supply from natural resources is not uniformly abundant,
its m a x i m u m use can be secured by storing water in times of surplus for use in times
of shortage. This shortage usually takes the form of surface storage although this
is by no means the only form available. For example, considerable research has been
done lately in connexion with sub-surface storage (see Shiff, 1961).
Water stored on the surface in lakes or reservoirs is subject to loss by seepage
and evaporation. While loss due to seepage is governed by the properties of soil
forming the íloor and the banks of the storage, and is not related to the climate
of the region, evaporation loss is a function of the climate characteristic of the location
of the storage. In general, the drier the region the higher the evaporation loss, and
therefore the evaporation losses are greatest where the water supply is most limited
and valuable.
Recognition of the magnitude of the loss of water by evaporation is not new.
More than a century and a half ago, Dalton related amount of evaporation from
water to the velocity and the saturation deficit of the air close to the water surface,
by a relationship which still appears in practically every mathematical expression
dealing with evaporation. W h a t is n e w is the realization that evaporation losses need
not necessarily b e written off in their entirety.
In 1925 it was shown that the application of monomolecular films or monolayers of
certain organic compounds to the surface of water decreases the rate of evaporation.
Following this discovery there were intensive laboratory studies of this phenomenon, but it w a s not until 1952, in Australia, that the first attempts were m a d e
to use this principle to reduce evaporation of water under natural conditions (Mansfield, 1953). In the wake of the initial Australian experiments, which aroused worldwide attention, other investigations have followed in m a n y countries including
Australia, East Africa, 'India, Israel, Japan, Union of Soviet Socialist Republics,
United Kingdom and the United States of America.
Evaporation reduction
These investigations have led to a rapidly increasing volume of publications
on the subject and several international conferences and symposia. These include:
(a) First International Conference on Reservoir Evaporation Control, sponsored
by Southwest Research Institute, held in San Antonio, Texas, in April 1956 (Southwest Research Institute, 1956); (b) Symposium on the Retardation of Evaporation
by Monolayers, sponsored by the Division of Colloid and Surface Chemistry of the
American Chemical Society, held in New York in September 1960 (La Mer, 1962); ,
(c) Symposium on. Water Evaporation Control, organized jointly by the Unesco
South Asia Science Co-operation Office and the Council of Scientific and Industrial
Research, held in Poona (India) in December 1962.
Evaporation control was also one of the subjects discussed at the second and
third International Congresses of Surface Activity held in London in 1957 (Schulman,
1957) and Cologne in 1960 (Deutscher Ausschuss für grenzflächenaktive Stoffe, 1960)
respectively as well as at the General Assembly of the International Association
of Scientific Hydrology held at Berkeley, California in August 1963 (International
Association of Scientific Hydrology, 1963).
Several comprehensive bibliographies have appeared dealing with the subject.
Perhaps the most extensive of these are the bibliographies published by Price’s
(Bromborough) Ltd. (1960) and Crossfield Ltd. (Stephens, 1962). A concise review
of literature with bibliography listing 322 references up to 1959 has been prepared
by Magin and Randall (1960). An annotated bibliography of non-American literature
(mainly Russian) was prepared by Klein and Wilkins (1961).1
Numerous short reviews of the present state of evaporation control, its problem
and potentialities have been published recently in the literature. These include
papers by D o m i n y (1962), Michel (1963a, 19636) and Bunker (1963).
All these manifestations of research activity point to the very considerable interest
both from the
that has been focused lately on the problems of evaporation control
,
theoretical and practical points of view.
The present publication will review both these aspects of evaporation reduction
up to mid-1964.
1.
All thew bibüogriphies list m a n y references to the literature in the varioui associated fields such as the theory of
evaporation. surface cherniaïry in general. hydrology, meteorology and even less directly connected subject#. In the
bihliograpby appended to the present work, only references bearing directly on the inbject of evaporation reduction
are included.
10
\
Organizational framework of evaporation
control research
~
T h e importance of the subject for national economies is reflect‘ed by the fact that
the relevant research work has been mainly carried out or supported by government
agencies.
In this chapter the organizations and institutions which have been active in
evaporation control research will be listed by country and the scope of their activities
briefly described.
in Australia, the Department of Physical Chemistry of the Commonwealth
Scientific and Industrial Research Organization (CSIRO), Melbourne, has been
actively engaged on both theoretical and practical aspects of evaporation control
since 1952. Other governmental scientific departments such as the Division of
Meteorological Physics, CSIRO, Aspendale, have co-operated at various stages
of the research.
Recently, extensive laboratory work on s o m e properties of monolayers bearing
on their water conservation ability has been carried out at the Department of
Physical Chemistry of the University of Sydney.
I n the Uniïed States of America, most of the work on evaporation control has been
directed by the Bureau of Reclamation at Denver, Colorado.
The Bureau of Reclamation, a n agency of the Department of the Interior, began
its extensive laboratory and field investigations in 1955. M u c h of the Bureau’s
work has been done in collaboration with other government agencies such as the
Robert A. Taft Sanitary Engineering Center, the United States Weather Bureau,
and the United States A r m y and Air Force. Other collaborators have included
educational institutions to w h o m specific projects under graduate study research
contracts have been given, and also several State and municipal water organizations.
T h e United States Geological Survey, the Southwest Research Institute, San
Antonio, Texas, the Texas Water Commission and the Illinois State Water Survey,
Urbana, Illinois, have also been active in the field.
T h e Department of Chemistry of Columbia University, New York, has been
engaged on fundamental laboratory work on the effect of monolayers on the rate
of evaporation since the early 1950s.
I n Great Britain, Price’s (Bromborough) Ltd., in co-operation with other firms,
has been active in laboratory and field studies in a n attempt to learn more about
the potentiality of the filming technique. S o m e biological aspects of evaporation
control has been studied by the Water Pollution Research Laboratory of the
Department of Scientific and Industrial Research.
11
Evaporation reduction
In East Africa, field experiments in evaporation control were conducted in the
years 1954-59 by the East African Meteorological Department in co-operationwith
other government agencies.
In India, evaporation pan and reservoir experiments have been in progress since
1956 under the sponsorship of a Special Government Committee. These experiments
have been carried out by the Central Water and Power Commission, N e w Delhi,
in collaboration with the Council of Scientific and Industrial Research. More
recently, extensive laboratory work has been in progress on new evaporation
retardants at the National Chemical Laboratory, Poona.
In Japan, the work on evaporation suppression has been mainly aimedatincreasing
the temperature of water o n flooded rice nurseries and rice fields. This work, which
started in 1952, has been centred at the National Institute of Agricultural Sciences,
Tokyo.
In Israel, laboratory and field work on evaporation retardants was done in the
years 1956-58 by the Weizmann Institute of Science, Rehovoth, in collaboration
with ,Water Planning for Israel Ltd., Tel Aviv.
In the Soviet Union, experiments in evaporation control have been in progress
since 1959, with the following Scientific Research Institutions (SRI)co-operating:
Ali-Union SRI of Hydrotechnics and Soil Improvement;
State Hydrological Institute;
Institute of Organic Chemistry of the Academy of Sciences of the U.S.S.R.;
Institute of Physical Chemistry of the A c a d e m y of Sciences of the U.S.S.R.;
All-Union SRI of Fats;
SRI of Synthetical Substitutes of Fats and Detergents of Bialogrod Sovnarkhov;
All-Union SRI of Oil Chemistry Processes.
In South Africa, screening tests on evaporation pans, and field tests o n natural
bodies of water were m a d e by the Hydrological Research Division of the Department of Water Affairs.
In Canada, evaporation pan and field tests have been carried out by the Lethbridge (Alberta) Research Station of the Canada Department of Agriculture.
In Spain, evaporation reduction field trials were carried out in 1957 and 1958
by Rio Tinto Mines in conjunction with Price’s (Bromborough) Ltd.
In Sweden, laboratory studies were conducted in 1958 at the Division of Hydraulics
of the Royal Institute of Technology, Stockholm.
J
J
C H A P T E R
I
Physical and chemical principles of
evaporation control :
laboratory experiments
~
1. Historical review
,
It has long been k n o w n that the rate of evaporation of water can be reduced by
applying oil to its surface, and it appears that this phenomenon has occasionally
been put to some useful purpose. Thus, A b b e (1914)reports on the work of Onofrio,
w h o used oil upon inland rivers and lakes in France to retard evaporation and thereby
reduce the formation of fog.
However, laboratory investigations of the effect of oil films on reducing evaporation from water do not appear to have begun until the 1920s. By that time the
early experimental work of Pockels (1891), Rayleigh (1899) and Devaux (1913)
had led to recognition of the existence of films of monomolecular thickness, and a
basic understanding of their structure. Miss Pockels developed the technique of
confining insoluble films on a water surface in a trough between movable barriers,
and consequently was able to measure the relation between the surface tension
and the area. Lord Rayleigh, using this technique, showed that the surface
tension feii steeply only when the surface was covered with a close-packed film one
molecule thick. Devaux showed that the movement of the film could be followed
when sprinkled with a fine powder, and that monomolecular films could even
become solid when compressed.
Hardy (1912, 1913) was the first to suggest that monolayers were formed from
polar molecules consisting of a hydrophobic (water-repelling)and a hydrophillic
(water-attracting)part, and hence that the monolayer molecules were orientated
with the hydrophillic part (a functional group such as a hydroxyl-OH or carboxylCOOH) buried in the water, and the hydrophobic part (a hydrocarbon structure)
tending to leave the water. Conclusive support for this hypothesis of orientation
was provided by Langmuir (1917).
T h e results of the first experiments on evaporation reduction by monolayers
were not promising. Devaux (1921)recognized the impermeability of multimolecular
layers of oil mixtures but failed to observe any reduction in evaporation produced
by monolayers. Hedestrand (1924)was unable to show any reduction of evaporation
reduction by the addition of monolayers of palmitic and oleic acids.
With the use of improved experimental techniques Rideal (1925) was able for
is
the first time to observe the reduction of evaporation caused b y monolayers. H
apparatus consisted of an inverted U-tube, one arm of which contained water at
13
~
Evaporation reduction
r o o m temperature upon which the monolayer was formed, the other a r m being
cooled in a n ice bath. The system was evacuated. By comparing the rates of condensation in the cold a r m with and without a surface film on the water, he could determine the effect of the film on evaporation and found that the presence of monolayers
could reduce the rate of evaporation by as m u c h as 50 per cent. In his experiments
Rideal used monolayers of fatty acids and he found that with these materials, the
evaporation reduction effect depended on the film pressure or surface concentration.
Langmuir and Langmuir (1927) extended the studies of Rideal to monolayers of
other substances and found that cetyl alcohol was superior to oleic, stearic and palmitic acids, as well as cetyl palmitate and myricil alcohol in respect of evaporation
reduction effect from water surfaces.
These investigators were the first to express the ability,of a given monolayer
to reduce evaporation in resistance units. They defined the total resistance to evaporation simply as the reciprocal of the measured mass transfer rate of water, with
units of cm.2 sec.-l gramme-1. T h e total resistance is composed of the gas-phase
resistance and interfacial resistance. By evacuating the system the gas-phase
resistance is reduced, and the relative effect of the monolayer on the total resistance
is increased and can be more easily measured.
Further observations on the evaporation reduction effect of various monolayerforming substances were m a d e b y Baranaev (1937),Sklyarenko and Baranaev (1938),
Glazov (1938), Kheinman (1940), Docking, H e y m a n , Kerley and Mortensen (1940).
All these investigators reported the superiority of cetyl alcohol (hexadecanol)
C,,H,,OH and possibly stearyl alcohol (octadecanol) C1,H,,OH over other evaporation
retardants, although quantitative results varied to some extent.
Sebba and Briscoe (1940)employed a different method for reducing the gas-phase
resistance. T h e y passed a stream of dried air across a water surface, and measured
the amount of moisture taken up. They emphasized the influence of the surface
pressure of the ñlm on the passage of water through the monolayer.
Langmuir and Shaefer (1943) developed a measuring technique which, with some
modifications, has since been used for laboratory studies by most investigators.1
In their experiments the evaporation from a water surface covered by mono- or
multimolecular films was measured b y the amount of water absorbed by a dessicant
placed a few millimetres above the treated water surface. They found that the rate
of escape of water molecules from the surface was decreased by a factor of about
10-4 by a compressed monomolecular film of cetyl alcohol. They emphasized the
importance of the purity of the monolayer-forming substance b y noting that the
contamination of 1 part in 1,800 of certain organic materials in the monolayer
could reduce the effectiveness of the film layer b y 60 per cent. Thesameauthors
contrasted the mechanism of evaporation reduction by a n oriented monolayer
on one hand and a layer of oil on the other. Whereas in the latter case the resistance
to evaporation depends on the thickness of the layer and follows the laws of molecular
diffusion, the evaporation through a monolayer is retarded by a finite energy barrier
which has to be overcome by the escaping water molecules.
Despite these encouraging laboratory results, no serious consideration appears
to have been given to the use of monomolecular films to control natural evaporation,
in fact during the 1940s increased attention w a s given to the use of multimolecular
films of oil mixtures for evaporation control.
H e y m a n n and Yoffe (1942, 1943) repo.rted that films 5 microns thick, consisting
of paraffin oil containing spreaders of high molecular weight, m a y reduce the evaporation of water to 15 per cent of the original value.
1. Recently, however,
14
new instrument which measure# evaporation wntinuouuly hai been deicribed by Walker (1963).
Physical and chemical principles
Powell (1943) experimented’with oil films having thicknesses ranging up to
about 2.5 cm.,and has shown that the reduction in evaporation becomes relatively
greater as the air velocity is increased, and that the order of magnitude of the effect
can be estimated from a knowledge of the diffusion coefficients of water vapour
through air and oil. He has also shown that for a given oil there is an optimum
thickness for which the rate of evaporation from the underlying water surface has
a m i n i m u m value. Gilby and H e y m a n n (1948)m a d e a study of evaporation through
duplex films 1-100 microns thick. A duplex film is a multimolecular film which is
thick enough for the film-forming substance to have the same physical properties
as in bulk and yet thin enough for the effect of gravity to be neglected. It m a y
be obtained by spreading a hydrocarbon oil with the aid of suitable spreaders. They
found that the efficiency of duplex films in reducing evaporation increased with
the wind velocity. With films m o r e than 10 microns thick, even a wind of 8 miles
per hour did not increase the rate of evaporation; the total evaporation resistance
was proportional to the thickness of the film and depended on the nature of
the spreader.
However, field tests using multimolecular films of oil (Rohwer, 1933; Docking
et al., 1940; H e y m a n n and Yoffe, 1943) were not successful. The films were easily
damaged by the action of wind, rain and dust; and once broken the films did not
reform. Another reason for failure was the fact that the film never persisted for
longer than a few days.
In 1953 Mansfield pointed out that although monomolecular films present in
general less resistance to water vapour transfer than multimolecular films, they
might still be more suitable for reservoir evaporation control because of their better
endurance under field conditions, and this realization gave a n e w impetus to further
laboratory research.
Archer and L a Mer (1954, 1955) made a study of long-chain fatty acids of the
series C,H,, + COOH. Using Langmuir and Schaefer’s method, they found that
liquid monolayers of these substances gave a resistance to evaporation which was
independent of film pressure over a large range of pressures. They also demonstrated
that the effectiveness of the film depended on the spreading technique adopted.
Rosano and L a Mer (1956), continuing Archer and L a Mer’s work, compared
the ability of monomolecular films of esters, acids and alcohols, as well as some
mixtures of these substances, to reduce evaporation. They found that, in general,
the compressible films were poor retardants, whereas the films exhibiting high
resistance to lateral compression retarded evaporation m o r e effectively. This conclusion has been verifiedrecently in respect to other compounds b y means of another
technique of measuring evaporation reduction, that of measuring the reduction
of surface temperatures (Jarvis, Timmons, and Zisman, 1962). Further laboratory
work on the influence of the spreading technique, the purity of the material, and the
film pressure, on the evaporation resistance of monolayers has been reported by
L a Mer and his co-workers (La Mer and Robbins, 1958; Robbins and L a Mer, 1959;
L a Mer and Barnes, 1959; Barnes and L a Mer, 1960, 1962a, 19626; L a Mer and
Aylmore, 1962; L a Mer, Aylmore and Healy, 1963).
T h e kinetics of spreading of monolayers were first studied by Cary and Rideal
(1925). They found that under m o s t conditions the rate of formation of a monolayer
from a solid was proportional to the perimeter of the air-water-solid boundary
and that the rate diminished with increasing film pressure, falling to zero when
equilibrium surface pressure was attained. Mansfield (1955a, 1956) has shown that
the initial spreading rate remained sensibly unchanged, even after a sufficiently
concentrated monomolecular layer has formed on the surface. He was thus able
to develop the 1/A criterion of dosage. This is the ratio of the perimeter of solid
in contact with the surface of water to the area of exposed water surface to be
15
-
Evaporation reduction
covered. By relating the film losses (see page 27) to this ratio, he calculated that for
pure crystalline cetyl alcohol, the 1/A value should exceed 2.5
10-3 cm;'
in order
to provide a sufficient rate of replenishment under normal wind conditions. H e was
also able to show that fatty acids, when applied to natural bodies of water, combined
with the chemicals in the water to form a rigid monolayer, which did not possess
the self-sealing properties necessary to withstand the ravages of dust, wind and
waves. This reqiiirement that monolayers be capable of self-sealinglimited the types
of compounds likely to form useful monolayers to those forming liquid films on
natural water at normal temperature. The only such compounds k n o w n were
long-chain alcohols. These alcohols, and in particular hexadecanol in pure form and
. mixed in various proportions with octadecanol, formed the subject of Mansfield's
extensive studies (1956, 1958). H e showed that the addition of small amounts of
octadecanol increased the evaporation resistance of a hexadecanol film. T h e optimal
proportion of the octadecanol was found to vary with the temperature and the
dosage from less than 10 per cent in mild climates (water temperature of 200 C.)
to a m a x i m u m of 35 per cent, under hot, arid conditions (water temperatures 260 C.
and more).
T h e question of thé optimal proportions in mixtures containing hexadecanol
and octadecanol has been examined extensively in screening tests which will be
described in Chapter II, Section 1. Here, it will be mentioned that, contrary to
Mansfield's findings, McArthur and D u r h a m (1957) have demonstrated that a
commercially available blend of cetyl alcohol (containing about 45 per cent of
hexadecanol and 40 per cent of octadecanol) exhibited greater evaporation resistance
than the film from relatively pure (90 per cent) cetyl alcohol at water temperature
200 C. They further showed ( D u r h a m and McArthur, 1957) that the efficiency of
the monolayer to reduce evaporation increased with dosage, as expressed in multiples
of the theoretical quantity needed to form a monolayer up to a m a x i m u m value
after which a constant efficiency was reached. T h e dosage required to reach this
m a x i m u m was less for the commercial cetyl alcohol than for 90 per cent cetyl alcohol.
T h e influence of dosage o n the performance of the monolayers in the laboratory
has been also observed by Genet and Rohmer (1961) as well as by Hellstrom and
Janson (1959).
It has been generally assumed, o n the basis of laboratory experiments, that the
resistance to evaporation of the monolayers of higher homologues of the hexadecanol
rises with the length of the hydrocarbon chain. The efficiency of these compounds
as evaporation retardants is, however, increasingly hampered b y their progressively
higher melting points which reduce their ability to spread o n the water surface.
T o overcome this difficulty, new compounds for evaporation reduction, denved
from long-chain alcohols, have recently been synthesized, first in Japan in 1956
(Mihara, 1961,1962;Mihara and Nakamura, 1961), and, later, in India (Deo, Sanjana,
Kulkarni, Gharpurey, Biswas, 1960) and the U.S.S.R. (Ogarev and Trapeznikov,
1963). These compounds, which incorporate a molecule of ethylene oxide (CH,CH,O)
at the hydroxyl end of the long-chain alcohol (thus forming glycol monoalkyl ethers'
(R-OCH,CH,OH),have been shown to be superior to the previous compounds
in the laboratory but so far have only had limited application in field trials.
x
7
1.
16
Or dtoxy-cthanols.
Physical and chemicai principles
2. Evaporation resistance of monolayers
T h e evaporation resistance of monolayers on a water surface has been found to
depend on m a n y factors, a m o n g them the film pressure, the temperature of the
water, the wind velocity and the purity of the film-forming substance. In this
section the influence of thefiedifferent factors will be reviewed.
FILM P R E S S U R E
.As mentioned in Section-l above, the resistance to
evaporation of the monolayers
of fatty acids has been found to be largely independent of surface pressure. T h e
monolayers of pure long-chain alcohols (CnH2n+i
OH) however, have been shown
to exhibit an increase of evaporation resistance with the surface pressure (Rosano
and L a Mer, 1956; Mansfield, 1956; L a Mer and Barnes, 1959, Barnes and L a Mer,
1962~;L a Mer, Aylmore and Healy, 1963). T h e functional form of this relationship
is that of a triple-sloped line, as can be seen from Fig:l,
reproduced from Barnes
ana L a Mer's paper.1 In this diagram the resistance to evaporation is given in units
of specific resistance (sec./cm.); for comparison it m a y be noted that the evaporation
resistance of clean water surface at 200 C. is 0.002 seclcm. (Davies and Rideal, 1961).
T h e higher resistance of octadecanol over the whole range of surface pressure is
clearly seen.
- 4
a-
)
3
2
c
$
e
u
c
YI
.-
YI
?
.U
.-
L
U
u
2 0
. o
'
10
Surface pressure (dynes/crn.)
20
30
40
'
FIG.1. Evaporation resistance as a function of surface pressure of octadecanol and hexadecano1 at 250 C. From Barnes and La Mer (1962a).
i.
However. in rn more recent paper (La Mer. Aylmore and Healy. 1963) this 'kink' in the reiistance-presnure curve at low
prcasures is aicribed to trace impurities. since it can be removed b y succensive purifications.
17
~
Evaporaiion reduction
4
9:l
3
'i
.-
E ,
O
10
20
30
i
40
Surface pressure (dynes/cm.)
FIG.2. Evaporation resistance as a function of surface pressure for mixed monolayers of
octadecanol and hexadecanol at 250 C. The ratios of octadecanol to hexadecanol are shown
in the figure. From Barnes and L a Mer (1962a).
T h e evaporation resistance of the mixture of pure octadecanol and hexadecanol
in different proportions as a function of surface pressure has also been investigated
by Barnes and L a M e r (1962a), and their results are given in Fig. 2. It is seen that
at lower pressures the resistances of each mixture was higher than that of either
pure component. No explanation of this phenomenon has been offered. Probably
the best over-all performance is exhibited by the 7 :3 mixture. It is pertinent to
note that alcohol mixtures of about these s a m e proportions were independently
chosen for recent large-scale field trials, on the basis of good results obtained in
outdoor screening tests (Chapter II, Section 1).
T h e evaporation resistances of long-chainalcohols containing u p to 22 carbon atoms
as well as glycol monoalkyl ethers, R-OCH,CH,OH (R being an n-alkyl chain containing 16-22 carbon atoms) have been studied by Deo, Kulkarni, Gharpurey and
Biswas (1961, 1962~).Their results are reproduced in Fig. 3. It is seen that the resistance values are in general lower for the glycol ethers (shown as C,OEtOH) than
for the corresponding alcohols (shown as C,OH) at the s a m e pressure. A t high film
pressures, however, the position is reversed and the glycol ethers exhibit higher
evaporation resistances.
TEMPERATURE
,
F r o m their studies on monolayers of fatty acids, Archer and L a M e r (1955) concluded
that the logarithm of the specific resistance of a given monolayer could be represented as a linear function of the reciprocal of the absolute temperature, in accordance
with the Arrhenius law for the velocity of a chemical reaction.
18
Physical and chemical principles
20
15
10
r:
5
- SI>
- 5
U
O
C
e
.mm-
e
U
..U
u
2 0
O
10
20
.30
40
50
Surface pressure (dyner/cm.)
FIG.3. Evaporation resistance as a function of surface pressure for monolayers of long-chain
alcohols (C,OH) and glycol monoalkyl ethers (C,OEtOH). From Deo, Kulkarni, Gharpurey
and Biswas (1961).
'
This relation has also been shown to hold approximately for monolayers of hexadecano1 and octadecanol (Barnes and L a Mer, 1962~)for surface pressures higher
than 15 dynes/cm.
Mansfield (1956,19586)has found that the evaporation resistance of a monolayer
generated directly from a fragment of solid highly purified hexadecanol placed
upon a water surface was approximately constant in the temperature range of 200 C.
to 300 C., but that the resistance fell rapidly at higher temperatures so that at
,500C. it is about one fourth of its value at 200 C. His technique of measurement
was similar in principle to that of Langmuir and Schaefer. However, the rates of
evaporation were measured not by weighing but by determining the drop in level
within a side tube of the evaporating vessel using a microcathetometer which could
be read to an accuracy of 1 micron.
R a m d a s and Narasimhan (1957)measured the effect of temperature on evaporation
from water surfaces covered with a film of pure cetyl alcohol, which had been spread
from a solution in hexane. Their measurements were m a d e with t w o glass petri
dishes exposed to a steady horizontal wind of 2 m.p.h., one of which served as a
control. T h e results which have been expressed as a percentage of the evaporation
from the clean water surface, show that there is a steady fall in the reduction of
evaporation with the rise of water temperature, from about 60 per cent at 200 C.
through about 35 per cent at 300 C. to about 15 per cent at 600 C.
Using a similar technique Deo, Sanjana, Kulkarni, Gharpurey and Biswas (1960)
19
Evaporation reduction
and Shukla, Deo, Sanjana and Kulkarni (1962) as well as Shukla, Kulkarni, Gharpurey and Biswas (1963) and Shukla, Deo, Katti, Kulkarni and Gharpurey (1963)
have measured reduction in' evaporation by monolayers of long-chain alcohols,
alkoxy ethanols, and their various mixtures, as a function of temperature in the
range 200 C. to 400 C. S o m e of the results are reproduced in Fig. 4. It is seen that
the reduction in evapor-ation generally increased with the chain length for both
n-alcohol and n-alkoxy-ethanol series and that for a given length of the alkyl chain
the alkoxy-ethanol showed higher evaporation reduction than the corresponding
alcohol, T h e efficiency of the monolayer increased with decreasing temperature
in which case the reduction
for all the alkoxy-ethanol films except for C,,-OC2H4OH
remained more or less constant in the temperature range examined. It m a y be noted
that a similar result with this compound was reported by Mihara and Nakamura
(1961). T h e anomaly is probably due to spreading difficulties at lower temperatures.
T h e efficacy of an equimolar mixture of Cl,-OEtOH and C,,-OEtOH at lower
temperatures is seen to be higher than that of either component alone, suggesting
-greaterrelative ease of spreading. This result, too, is confirmed by Mihara and Nakamura's observations with respect to OED-70 mixture (containing 55 per cent of
C,,H,,OC,H,OH and 45 per cent of CI8H,,OC2H4OH).
T h e flattening of the curve
1 O0
80
,
60
6p
Y
ó
z 40
gn
O
oc
.-
s
.-
+
U
a0
Bi
20
15
Temperature
'
20
(OC.)
FIG.4. Evaporation reduction
30
40
50
as a function of temperature for monolayers of long-chain
alcohols (C,OH) and alkoxy ethanols (C,OEtOH). From Deo, Sanjana, Kulkarni, Gharpurey
and Biswae (1960).
20
,
Physical and chemical principles
for the octadecanol (CISOH),
and the decrease in evaporation reduction for docosanol
(C,,OH) at lower temperatures was also ascribed to the difficulties of spreading at
these temperatures.
T h e influence of mechanically produced capillary waves (in the absence of wind)
on the evaporation of water through monolayers was the subject of a recent c o m munication by L a Mer and Healy (1964). They found that the rate of transfer of
water through the monolayer increased as the static surface w a s disturbed by waves
of amplitudes of 0.03-0.08 cm., the specific resistance to evaporation decreasing
roughly to 60 per cent of the static value. They ascribed this decrease to monolayer
molecules being submerged b y the wave rather than being collapsed into islands
of ‘duplex’,film.
2
I
W I N D VELOCITY
T h e influence of wind velocity on the evaporation reduction b y monolayers w a s
considered by Mansfield (19586). H
is conclusion was that the actual rate of evaporation from water covered by a monolayer becomes almost independent of wind
velocity beyond a limiting velocity (which is determined by atmospheric conditions).
This result is well illustrated in the measurements of Shukla and Kulkarni (1962),
in which the efficacy of the monolayers of alkoxy ethanols and the corresponding
alcohols as water evaporation retardants was studied in petri dishes over the wind
speed range of 1-13 m.p.h. These observations have also shown the alkoxy ethanols
to be better retardants as compared to the alcohols throughout this wind speed range.
T h e relative evaporation reduction for alkoxy ethanols plotted against wind speed
ie reproduced in Fig. 5.
PURITY OF THE MATERIAL
Evaporation resistances of commercial samples of hexadecanol and octadecanol,
and their mixtures in various proportions, have been investigated by Barnes and
L a Mer (19625) and L a M e r and Aylmore (1962). Their conclusion is that these
90
- 80
s
70
+ C,,OCH,CH,OH
C
.c
g
e
60
>
.-E-
C,,OCH,CH,OH
C,,OCH,CH,OH
C,,OCH,CH,OH
O
2
4
6
8
10
12
Wind speed (rn.p.h.1
FIG.5. Evaporation reduction as a function of wind speed for monolayers of alkoxy ethanols
(C,H,,+ ,OCH,CH,OH). From Chukla and Kuikarni (1962).
21 .
Evaporation reduction
materials are considerably inferior to pure materials, but that they can be improved
by maintaining the films under pressure. They recommend that commercial products
be tested and then further purified before field application.
Since commercial alcohols are mixtures of substances of different structure but
mainly of primary and to s o m e extent secondary alcohols, Trapeznikov and Ogarev
(1963) investigated the ability to reduce evaporation of mixtures of normal hexadecano1 and secondary hexadecanol CH,-(CH,),-CHOH-(CH,),-CH,.They found
that the evaporation resistance of mixtures of u p to 1 :1 proportions of the t w o
alcohols approximates to that of pure hexadecanol but that it falls rapidly to O
with further increase in the proportion of the secondary alcohol. By studying the
equilibrium pressure and spreading rates (Section 3 below) they concluded that the
evaporation resistance of these mixtures is determined by the monolayer of the
normal hexadecanol which displaces the less surface-active secondary alcohol
from the monolayer on to the microcrystals. For quantities of secondary alcohol
in excess of 50 per cent the normal hexadecanol is no longer able to displace all the
secondary alcohol from the monolayer.
Essentially analogous results were reported by the same authors (Ogarev and
Trapeznikov, 1963)in their study of mixtures of a n alkoxy ethanol (CI8H,,OC,H,OH)
and secondary alcohols of C,,-C,, chain length.
-3.Spreading properties
T h e ability to suppress evaporation, essential though it is, is not, b y itself, sufficient
to qualify a substance as a good retarder of natural evaporation. No less important
are its spreading characteristics. The evaporation retardant should form a monolayer
readily on the water surface, at the prevailing temperatures. After a monolayer has
been generated over a natural water surface, it will not remain intact but will suffer
continuous losses, due to various factors such as wind, waves, dust, biological
decomposition as well as evaporation and solution of the film (Mansfield, 1956).
Accordingly, to maintain its efficiency, n e w material has to be fed to the water
surface at a rate commensurate with the losses, which, at times, m a y be very considerable.
In recent years there has been considerable laboratory work to clarify the spreading
properties of different compounds used (or considered) for evaporation control.
Their spreading properties have been found to depend not only o n chemical composition but also on polymorphic structure and physical characteristics.
In this section laboratory work o n spreading properties such as spreading rates,
spreading velocities, equilibrium spreading pressure and collapse pressure, and
factors influencing t h e m will be reviewed.
SPREADING R A T E S
Several spreading rate measurements have been m a d e with pure cetyl alcohol on
water. Mansfield (1956, 1959c) determined the rate of generation of a monolayer
from a cylindrical casting of cetyl alcohol floating upon a water surface, at a temperature of 250 C., and found it to be 10.1 x 10la molecules per second per centimetre
of the solid in contact with the water surface, corresponding to a spreading coefficient
lo1, molecules dyne-l sec.-l. Roylance and Jones (1959), on the other hand,
of 2.4
have found the rate of spreading from the periphery of a single floating crystal
to be about 0.2
10la molecules cm;l sec.-l, equivalent to a spreading coefficient
x
x
22
Physical and chemical principles
'
of 5.1 x 10'0 molecules dyne-1 sec;'
or s o m e fifty times less than Mansfield's figure.
Stewart's (1960) measurements of spreading from single crystals revealed that
spreading from the faces of suspended crystals in vertical position proceeded appreciably faster than from the periphery of a floating crystal, the respective spreading
coefficients being 1.7-4.4 x 1OI2 and 0.11-0.21 x 10l2 molecules dyne-' sec.-'
at
24-250 C.-His extensive measurements with cast rods, which were m a d e over the
temperature range of 18-330 C. pointed to an even higher rate of spreading than
Mansfield's (spreading coefficient 6 x 10l2 molecules dyne-' sec.-l at 250 C.). Incidentally, the temperature coefficient of the spreading rate was found to be quite
large (1 x 1012 molecules dyne-' sec.? deg.-l) suggesting that the spreading rate
would tend to be affected by stray convection currents and small temperature
differences. Yet another figure for spreading rates of cetyl alcohol (2.8 x 10'3 molecules cm;l sec;')
was reported by Deo, Kulkarni, Gharpurey and Biswas (1962~).
It has been mentioned in Section 1 that Mansfield (1956, 1959c) found the initial
spreading rate to remain practically unchanged even after a coherent monolayer
had formed and had begun to move away from the spreading source. In the first
phase of spreading, the spreading coefficient was found to be 2.42 x 10l2 molecules
dyne-' sec.-l at 230 C., and in the second phase 2.31 x 10l2 molecules dyne-1
at 240 C. These results have been partially confirmed by Roylance and Jones (1961)
w h o found that the value of velocity constant is little affected by surface pressure
up to a value of about 30 dynes cm.-l For pressures higher than 30 dynes cm.-'
they found that the spreading coefficient diminished rapidly, falling to 1.7 x 10'0
molecules dyne-l cm;l by 36.5 dynes cm.3 or one third its value at or near zero
pressure.
Mansfield (1963)has shown, however, that this apparent variation of the spreading
coefficient resulted from too high a value adopted for the equilibrium spreading
pressure of the crystal edge (see page 26); when the appropriate valueis substituted,
the spreading coefficient becomes constant, within experimental error, u p to the
equilibrium spreading pressure. This result applied for a spreading both from the
edge of a crystal of hexadecanol and from powdered hexadecanol.
S P R E A D I N G VELOCITIES
Measurements of the spreading velocities of monolayer films have also been reported.
Mansfield (1956,1959c) found the spreading velocity of cetyl alcohol to be a linear
function of the ratio of the perimeter of the solid in contact with the water surface
to that of the perimeter of the front of the advancing film. His expenmental values
varied from a few cm./sec. u p to 12 cm./sec. at a temperature of 24 &lo C.
McArthur and D u r h a m (1957) compared initial spreading velocities for various
fatty. acids in the temperature range 10-300 C. and found that best results were
obtained with dry crystalline fatty acids containing 45 per cent of hexadecanol and
40 per cent of octadecanol with the addition of 10 per cent oleyl alcohol (C,,H,,OH).
This alcohol mixture spread at a higher velocity than a 92 per cent pure alcohol
over the entire temperature range tested. At 250 C. the spreading velocities were
32 and 27 cm./sec. respectively for the two mixtures. Similar measurements were
reported by Miller and Bavly (1959).
Data on the initial spreading velocities of solutions of fatty alcohols in various
solvents were published by McArthur (1960). He found that there was little difference
between solutions in various petroleum hydrocarbons but the value was considerably
less than that of a dry crystalline fatty alcohol. Where ethyl alcohol was used as
solvent, however, the initial spreading velocity was comparable with that of the
crystalline solid alcohol.
23
I
Evaporation reduction.
Ø
./
E F F E C T O F C H E M I C A L COMPOSITION
Ì
T h e effect on the spreading rate of adding stearyl alcohol to cetyl alcohol was examined by Stewart (1960). H e found that the rate was reduced by the addition
of stearyl alcohol but not in' a regular manner. Thus, a 5 per cent stearyl-cetyl
mixture spread almost as fast as pure cetyl alcohol whereas there was a marked
drop in the rate with a 16 per cent mixture. Above this stearyl concentration the
rate decreased more slowly. For pure stearyl alcohol the value of the spreading
constant was 1.5 x 10'0 molecules dyne-' sec.-l at 250 C.
A t the United States Bureau of Reclamation Laboratory the spreading rates of
monolayers of s o m e twenty commercial alcohols were evaluated (Timblin and
Florey, 1961; Timblin, Florey and Garstka, 1962). The method used was to measure
the increase in film pressure with time for a monolayer produced by a disk of alcohol
4 cm. in diameter placed on a crystallization dish. T h e initial slope of the film
pressure time curve was determined. The spreading rates thus obtained varied from
0.95 to 10.00 dyne cm.-l sec.-1, the highest figure corresponding to an 89 per cent
pure cetyl alcohol with 2 per cent of the oxide polyoxypropylene surfactant added.
Similar measurements on potential evaporation control materials were carried
out at the Institute of Physical Chemistry of the A c a d e m y of Sciences of the U.S.S.R.
(Trapeznikov and Ogarev, 1961; Petrov, Trapeznikov, Nikishin and Ogarev, 1961)
and at the W e i z m a n n Institute of Science, Rehovoth, Israel (Miller and Bavly-Luz, ,
1962). T h e latter measurements have shown that addition of smali quantities of
liquid paraffin oil improves the spreading properties. T h e kinetics of spreading from
mixtures of a normal and a secondary hexadecanol were discussed by Trapeznikov
and Ogarev (1963). Comparative results on the rate of spreading of long-chainalcohols
and alkoxy ethanols (glycolmonoalkyl ethers) were reported from India by Deo,
Kulkarni, Gharpurey and Biswas (1962a, 19626). T h e samples used in the measurements were prepared by dipping and then withdrawing glass rods from the melt of
a substance, and ageing them for at least two weeks. T h e reported data showed that:
1. T h e rate of spreading of a homologous series decreases with an increase in the
chain length.
2. T h e spreading rate for the alkoxy ethanols in thé range (C16-C2J
is greater than
that of the corresponding alcohols by at least one order of magnitude.
3. T h e spreading rate for the Cn + alkoxy ethanols is larger than that of the C,
alcohol. This makes possible the use of longer chain alkoxy ethanols for evaporation control.
In particular, the very high rate of spreading of ~,OC,H,OH (1.6
1014 molecules
em.-' sec.-l at 250 C.-as against 2.2 x 1013 molecules cm;l sec.-' for hexadecanol)
mixtures in
explains the excellent results obtained with Cls-and C2,-OC2H4OH
Japan (Mihara and Nakamura, 1961).
4
x
-
E F F E C T OF P O L Y M O R P H I S M
I
'The normal long-chain alcohols with an even number of carbon atoms can exist
in three solid phases (Kolp and Lutton, 1951): a, sub-a, ß. T h e a phase appears
from the melt at freezing point, and at a specific temperature transforms abruptly
to the sub-a phase. Both the a and sub-a phase gradually transform to the stable
ß phase. T h e ß phase is obtained directly on crystallization from a solvent. Recently,
a n additional phase (a') was found to exist in the mixtures of pure and commercial
hexadecanol and octadecanol, as intermediary between the a and sub-u phase on
cooling (Benton, 1962, 1963). This phase would preponderate in most commercial
cetyl-stearyl alcohols when water-surface temperatures are above 200 C. (Benton,
1963).
24
Physical and chemical principles
i
Vines and Meakins (1959) in their study of two commercial beaded forms of cetyl
alcohol containing 81 and 88 per cent of hexadecanol respectively, showed that
the materials underwent a-suba phase transformation at transition points which
differed appreciably for the t w o products, and that, furthermore, the transition
temperatures were lowered on wetting. They found some evidence that the material
with the higher transition point spread faster than the sample with the lower
transition point. On the basis of entropy considerations they suggested that the
alcohol in the sub-a phase would tend to spread more readily than in the u phase.
Stewart (1960) amplified these results in his studies of phase transformation
temperatures and spreading rates of a series of commercial products and cetyl
alcohol mixtures. H e has found that the phase in cetyl alcohol is apparently suppressed in favour of the sub-u phase, (a) by wetting and (b) by the addition of other
long-chain compounds. His spreading rate measurements indicated that alcohol in
the a phase spread more slowly than in the sub-u phase, as suggested by Vines and
Meakins (1959).
Brooks and Alexander (1962) in their measurements of spreading rates of
fatty alcohols as a function of temperature have observed discontinuities in the
usual linear dependency of the logarithm of spreading rate on the temperature for
Cl, and Cl, alcohols. They attribute these discontinuities to the u-sub-a phase
change in the crystal. In particular in the case of hexadecanol, abnormally high
spreading rates were observed after cooling through the temperature of this
phase change.
No data have been published yet concerning the spreading properties of the
a‘ phase relative to those of the sub-a and u phase.
E F F E C T O F PHYSICAL CHARACTERISTICS
McArthur and D u r h a m (1957)studied the effect of bead size and surface structure
on the spreading of commercial fatty alcohol mixtures. They found that while the
effect of size could be explained by the Variation of the perimeter of the solid-water-air
interface, the nature of the solid surface also had a marked influence on the initial
spreading velocities. Their conclusion was that the best results would be obtained
with fine crystalline particles.
In a later paper McArthur (1960) m a d e a distinction between ‘open crystals’
and ‘closed crystals’. Open crystals were those obtained by slow cooling from a
temperature of 800 C. They are large transparent rhombic crystals which break
readily along the crystal planes. Closed crystals are obtained by quick cooling from
the melt which has been stirred at crystallizing temperature. They form a dense
crystalline mass which is more difficult to break up. McArthur showed that open
crystals had a consistently higher initial spreading velocity. By spraying molten
alcohol into air (see page 44), spherical particles are obtained which have a smooth,
and in the main, non-crystalline surface. These have been shown to possess markedly
lower initial spreading velocities than the crystalline particles of corresponding
size.
McArthur also showed that pre-wetted particles had lower initial spreading
velocities as a result of the reduced air-water-solid interface from which spreading
takes place. This effect increased with the time of pre-wetting. Again, dispersions
of fatty alcohols in water had even lower, though still measurable, initial spreading
velocities. Similar results were found by Timblin, Florey and Garstka (1962).
Mansfield (1963) pointed out that spreading from floating single crystals (thin
plates) developed from crystal edges, and proceeded m u c h more slowly than from
the crystal face or from the surface of casting. (See figuresfor spreading rates given
on pages 22-23.)
i
,
’
25
Evaporation reduction
EFFECT OF . W I N D
The effect of wind on spreading was studied in the laboratory by McArthur and
D u r h a m (1957) as well as Cruse and Harbeck (1960)and Vines (1959~).The latter
has found that under the influence of wind a linear variation in film pressure is set
u p in the wind direction u p to the film front. The damping of water waves by a
monomolecular surface film was studied by Vines (1960~)and Goodrich (1962).
Both found that monomolecular films próduce effective damping of surface waves
although their results differed quantitatively.
The surface drift caused by wind was studied by Keulegan (1951) using a 60-ft.
wind tunnel, and V a n Dorn (1953) on an 800-ft. pond. They concluded that for
turbulent conditions the surface moves with a velocity about one-thirtieth that of
the wind velocity. They found this ratio to be independent of wind speed and the
presence or absence of waves on the water surface. Recently Fitzgerald (1964)
re-examined the question in a wind tunnel study. He measured the ratio of the
surface velocity to wind velocity at wind speeds between 3.50 to 7.50 m. sec.-l
He has found this ratio to be markedly affected by the damping-out of surface
waves. For a w a v y surface, as obtained with clean water, this ratio has a constant
value of about 0.03 in agreement with Keulegan’s and V a n Dorn’s results. By
adding detergent solution to the water the damping of surface waves yas achieved.
The value of the ratio was then found to increase, beginning with a particular
concentration of the detergent and a corresponding surface pressure, u p to about
0.045. For a fully damped surface the ratio was found to rise linearly with wind
speed for low wind speeds and tend to a constant value of 0.045 for wind speeds
greater than 5.50 m. sec.-l
EQUILIBRIUM - S P R E A D I N G P R E S S U R E
A n important property of a successful evaporation retardant is the ability to maintain a monomolecular film under compression or expansion. This property is usually
linked with a high equilibrium spreading pressure. The equilibrium spreadingpressure
is the surface pressure of a film in equilibrium with a surplus of the solid or liquid
film-forming material. By compressing the film further above the equilibrium
spreading pressure the film collapses and its structure is disrupted. The point of
collapse is k n o w n as the collapse pressure.
The equilibrium spreading pressures of long-chain alcohols were measured by
Brooks and Alexander (1962) as a function of temperature. This dependence was
examined both on cooling and on heating the alcohols. After cooling through the
temperature of the cc-sub-cc phase transition (see page 24) pressures considerably
above those corresponding to the values obtained on a rising temperature curve
were observed. Small additions of octadecanol were found to change markedly
the equilibrium spreading pressures of hexadecanol.
Mansfield (1963) has shown that the equilibrium spreading pressure of a crystal
edge is significantly less than that of a powdered hexadecanol.
At the United States Bureau of Reclamation laboratories equilibrium spreading
pressures of some twenty commercial alcohols were measured (Timblin and Florey,
1961;Timblin, Florey and Garstka, 1962). The values for all the alcohols were found
to be appreciably affected by the water temperature. In each case the film pressure
versus temperature curve had a m a x i m u m film pressure in the temperature range
of 22-330 C. The m a x i m u m film pressure varied from 30 to 43 dynes cm;l
Equilibrium spreading pressures (..s.p.)
of mixtures of a normal hexadecanol
and a secondary hexadecanol (see page 22) in different proportions were measured
by Trapeznikov and Ogarev (1963). They notcd that the curve of e.s.p. and that of
26
-
I
Physical and chemical principles
evaporation resistance as a function of the composition of the mixture are of the
same shape.
A comparative study of the equilibrium spreading pressures of saturated longchain alcohols and alkoxy ethanols has been m a d e by Deo, Kulkarni, Gharpurey
and Biswas (1962a, 19626). T h e y found that at 250 C. while the e.s.p. of alcohol
decreases from a value of 40 dynes crn.-l for hexadecanol to 25 dynes cm;'
for docosanol, the decrease for the alkoxy ethanols in the same range of alkyl chain length
(16-22)is only from about 50 dynes em.-' to 47 dynes cm;'
Ogarev and Trapeznikov (1963) measured the e.s.p. and evaporation resistance
of technical and pure alkoxy ethanols of alkyl chain length of 16 and 18 carbons.
They found that both the e.s.p. and evaporation resistance are m u c h higher for
pure alkoxy ethanols. Study of mixtures of a n alkoxy ethanol (CIS)with secondary
alcohols of a chain length C,&,,
or higher (as usually found in technical alcohols
prepared by the Bashkirov method) showed again that the curve of e.s.p. and
that of evaporation resistance as a function of the composition of the mixture are
essentially of the same shape.
COLLAPSE P R E S S U R E
1
McArthur and D u r h a m (1957)determined the collapse pressures of different commercial fatty alcohols which had been spread from solvent; they also measured the
equilibrium spreading pressures for films of these same alcohols when in equilibrium
with solid material. Recovery power was assessed by collapsing and expanding films
and observing the rate of build-up of the surface pressure.
It,was found that solvent spread films only recovered very slowly after collapse.
For films in equilibrium with crystals, recovery was rapid and was probably due
to spreading from crystals.
Brooks and Alexander (1962) studied the collapse of monolayers of pure longchain alcohols (C,,, C,,, C,,), which had been spread either from solution in petroleum
ether or from a solidified rod of the alcohol which was afterwards withdrawn from
the surface.
They found that in the absence of stable crystals the pressure could be maintained
at several dynes above the equilibrium spreading pressure for some hours without
film collapse becoming evident. If the film area was reduced fairly rapidly, very
high pressures could be obtained. Once collapse started, however, it proceeded at
constant pressure, at an increasing rate. T h e collapsed material always spread
m u c h faster than the stable crystal, and aged only slowly to the stable crystal.
Spreading pressure of the collapsed material depended o n the rates at which it had
been formed: film collapsed at a slower rate gave a higher spreading pressure.
With both collapsed and stable crystal present together, the collapsed crystal
spread first. If the collapse occurred in the presence of stable crystals, and the area
was then increased, it was found that the return to equilibrium conditions took
some time, the length of this time depending on the chain length of the alcohol and
the temperature.
I N F L U E N C E OF LOSSES B Y E V A P O R A T I O N A N D SOLUTION
Roylance and Jones (1959) drew attention to the fact that the losses of film b y
evaporation and solution would tend to prevent the 'true' equilibrium pressure
being exactly obtained. These losses were first evaluated by Mansfield (19596).
H e calculated that the rate of loss caused by solution would be small compared
with that by evaporation. This was on the assumption that the rate of loss of film
from both causes is proportional to the rate of diffusion of escaping molecules. Mans-
27
Evaporation reduction
field’s theoretically predicted rate of loss of cetyl alcohol at 250 C. was in agreement
with the experimentally observed value.
Brooks and Alexander (1960)measured the total losses from monolayers of long-
’
chain pure alcohols and also from some mixtures as a function of temperature.
T h e y experimentally confirmed Mansfield’s conclusions that the losses from saturated
alcohols were primarily due to evaporation. T h e rate of loss was found to increase
steeply with the temperature. For instance, the data o n fractional loss from hexadecano1 monolayer show a 24-hour loss of 35 per cent of one monolayer at 200 C.
and of 1,300 per cent at 400 C.
Roylance and Jones (1961)examined the rate of film loss as a function of surface
pressure. T h e results were in agreement with those of Brooks and Alexander. T h e
nature of loss w a s examined by a radio-tracer method using hexadecanol labelled
with carbon-14, and the results again confirmed that the loss is primarily due to
evaporation of the monolayer, the loss by solution into the substrate being negligible.
T h e effect of film loss w a s shown to reduce the steady-state spreading pressure
of a hexadecanol crystal to a value generally below that of the thermodynamic
equilibrium spreading pressure. T h e steady-state spreading pressure is attained w h e n
the net rate of spreading from the crystal is equal to the rate of loss from the film.
Since the rate of spreading is proportional to the crystal perimeter and the rate of
loss to the film area, the steady-statespreading pressure is a function of the perimeter/area ratio. Roylance and Jones have s h o w n that at 250 C. (rate of loss, 140
per cent of the monolayer in 24 hours) steady-state pressures exceeding 30 dynes
c m r 1 were attained only with a perimeterlarea ratio greater than about 2 x
cm.-l,
and only for a ratio m u c h larger than 2 x 10-1 cm.-l could the equilibrium spreading
pressure (of about 40 dynes cm.-l) be reached.
28
,
’
‘
f-
CHA-PTER II
Evaporimeter experiments and field trials
.
1. Evaporimeter experiments
Extensive evaporimeter experiments have been carried out in the United States
and on a more reduced scale in a number of countries, a m o n g t h e m Australia,
India and the U.S.S.R. These experiments will be reviewed below, but before doing
so it will perhaps be pertinent to quote t w o contrasting opinions of the value of
such tests, both taken from the book Retardation of Evaporation by Monolayers
(La Mer, 1962).
Professor L a M e r writes in the preface (p. XI):‘So-calledscreening tests performed
in the open, even on closely adjusted pairs of evaporator pans, frequently give
inconclusive and misleading results because what those pans measure are often
fluctuations in micrometeorology. It is the function of the chemist to k n o w the
composition and the effectiveness for evaporation of a given sample of retardant
offered for sale before large-scale field tests are undertaken. Evaporator pans are
not the best means of accomplishing this objective.’
O n the other hand, Timblin,Florey and Garstka (in L a Mer, 1962, p. 177-8)write:
‘ T h e evaluation of the evaporation reducing ability of monolayers and duplex films
of oil and surfactants has been studied with Class A pans. This method provides a
straight-forward,simple, reliable, and reproducible means of determining the ability
of a layer to reduce evaporation under limited field conditions.’
i
A USTR ALIA N E X P E R I M E N T S
Mansfield (1955, 1956) and Sutherland (1957) briefly mentioned results of tests
with evaporimeter pans 3 ft. in diameter using fairly pure hexadecanol. These
results showed good correlation between the (percentage reduction and the perimeterlarea (1/A)ratio: the evaporation reduction rising from 10 to 70 per cent with
cm.-l
the l/A ratio from 0.8 x 10-3 cm;l to 2 x
7
U N I T E D STATES E X P E R I M E N T S
Experiments at the Southwest Research Institute. Evaporimeter tests at the Southwest Research Institute of S a n Antonio, Texas, have been preceded by laboratory
screening of 152 compounds deemed to have some value as evaporation retardant
29
’
Evaporaiion reduciion
materials (Beadle and Cruse, 1957; Cruse and Harbeck, 1960). The components
to be tested were applied in solid or liquid form at a dosage equivalent to 1 pound
per acre of water surface. Water was held in jars, 9 inches in diameter, at a temperature of 300 C. with a constant slow-moving stream of dried air blowing over a test
period of about 110 hours. Such a test period gave results reproducible to within
5 per cent. Evaporation reduction of up to 68 per cent was recorded with homologous
straight-chain fatty alcohols.
T h e evaporimeter experiments consisted of further evaluation, in stock tanks
10 ft. in diameter, of those compounds or mixtures which offered promise in the
laboratory screening. Both hexadecanol and octadecanol in doses equivalent to
1 pound per acre broadcast, and 1.2 pounds per acre in reserve supply, gave evaporation reduction of some 25 per cent, but the effectiveness of the latter persisted
longer. In general, the life of the film was very short. It could be prolonged only
by use of some bacteriostatic or bactericidal material either in the water or in the
film itself. However, no fully satisfactory additive was evolved.
Experiments by Bureau of Reclamation. Extensive experiments using Class A pans
were initiated by the Bureau of Reclamation in 1955 to study the evaporation
reduction of hexadecanol and other monolayer-forming material in various dosages
(Timblin and Florey, 1957; Florey, 1957). In these tests, a reduction in evaporation
of up to 64 per cent was achieved over a four-week period after a single treatment
with hexadecanol flakes sprinkled on the pan at the rate of 60 pounds per acre
(Timblin, Moran and Garstka, 1957).
T o determine the effect of temperature on the performance of the film, indoor ’
tests with Class A evaporation pans were carried out. These were not successful,
probably because of dust which interfered with the film.During the s u m m e r of 1958,
tests were conducted with twenty-two selected fatty alcohol mixtures out-of-doors,
as it was decided to depend upon the natural, ambient temperature changes to
evaluate the temperature effect (Florey and Timblin, 1959). These screening tests
were conducted over two-week intervals, but only the results for the first 24 hours
after treatment were used for correlation with average temperature, the latter being
a time-weighted .average of water temperature at 8 a.m., 3.30 p.m. and the following
8 a.m. T h e analysis of the results (Timblin,Florey and Garstka, 1962 ;Timblin and
Florey, 1961) indicates that:
1. The performance of all monolayers of commercial C,,-C,, fatty alcohol is influenced
by temperature. An increase of water surface temperature is accompanied by a
decrease in evaporation reduction. T h e decrease in effectiveness with increasing
water temperature does not follow the s a m e relation for each material.
2. T h e presence of octadeeanol tends to deetease the influence of temperature on
evaporation reduction.
3. There is no clear relation between composition of long-chainfatty alcohol constituents and evaporation reduction.
4. T h e addition of a n ethylene oxide as a spreading agent does not appear to impair
significantly the evaporation-reducing ability of the monolayer.
by the Illinois Water Survey. These tests, which preceded experiments
on small lakes (see Section 2 below) were conducted on pans and on a 30-ft.diameter
tank at the Urbana, Illinois, evaporation station (Roberts, 1957).
Experiments
Experiments at Oklahoma Agricultural Experiment Station. In these tests (Crow
and Daniel, 1958) different methods of hexadecanol application were studied.
Alcohol in solution was found to be more effective than in the flake form.
30
Evaporimeter experiments and field trials
Experiments at Stanford University, Stanford, California. In these tests-which
were conducted in Class A pans-the effect of dosage and the m o d e of application ,
of powdered hexadecanol and the effect of chlorine residual was studied (Franzini,
1961).
Experiments at the Agricultural and Mechanical College of Texas. Evaporimeter
experiments were carried out at the Agricultural and Mechanical College of Texas
on the evaporation reduction effect of Aquasave (a mixture of hexadecanol a n d ’
octadecanol) both in solution and as an emulsion (Meinke, Waldrip, Stiles and
Harris, 1962) using small pans. Additional tests on the same material were conducted
in shallow pans and deep cans placed in a controlled-environmentchamber (Meinke
and Waldrip, 1964).
Experiments at Arizona Agricultural Experiment Station. In these tests, which were
carried out in Class A pans, different modes of application (as powder, solutions and
emulsions) of long-chain alcohols were studied (Resnick and Cluff, 1963).
INDIAN E X P E R I M E N T S
Extensive evaporimeter experiments using Class A pans have been carried out by
the Indian Meteorological Department since 1957 (Bose, 1962). In the first instance
these tests were concerned with cetyl alcohol only. Various methods for dispensing
the alcohol were tested. Solid cetyl alcohol in globules, flakes or in very fine powder
form were used in different quantities; different methods of dispensing the solid
mechanically were tried, as weil as solution in kerosene and turpentine oil. Generally
the evaporation reduction was measured for a period of 24 hours. It was found
that for longer periods the effectiveness of the film decreased rapidly. T h e results
obtained have shown very large variations despite close agreement between control
pans. It appears, therefore, that the effectiveness of cetyl alcohol depends on several
factors which have not been yet clearly resolved. Later tests using stearyl alcohol and
glycol mono-octadecyl ether have shown that the ether, either alone or in mixture
with some of the long-chain alcohols, effectively suppresses evaporation for periods
distinctly longer than those obtained with alcohol alone.
Similar results but of a more general nature have been obtained in screening tests
carried out at the National Chemical Laboratory, Poona (Katti, George, Deo,
Sanjana and Gharpurey, 1962; Katti, Kulkarni, Gharpurey and Biswas, 1962;
Deo, George, Sanjana, Kulkarni and Gharpurey, 1963). In these tests, using Class A
pans, monolayers of alkoxy ethanols (mono-glycolethers) were found to be superior
to those of corresponding alcohols as regards their resistance to evaporation as well
as durability. T h e alcohol film deteriorated rapidly within t w o to three days, whereas
the alkoxy ethanol films generally retained their efficiency consistently for about
a week or more. Alcohol-alkoxyethanol mixtures were also found to be more durable
than pure alcohols. During these tests it was observed that the films caused a relatively greater reduction in evaporation during the daytime than during the nighttime (Katti, Kulkarni, Gharpurey and Biswas, 1964). This effect has been noted
’ before (Bavly, Leitner and Miller, 1959).
Evaporimeter experiments were also carried out at the Defence Laboratory,
Jodhpur (Choudhury and Bhati, 1962).
E X P E R I M E N T S IN T H E U.S.S.R.
Following laboratory tests with twelve kinds of fatty alcohols at the Institute of
Physical Chemistry of the A c a d e m y of Sciences of the U.C.S.R. at Moscow, six
31
Evaporation reduction
alcohols were selected for experiments in the field (Petrov, Trapeznikov, Nikishin
and Ogarev, 1961;Trapeznikov and Ogarev, 1961). These were: 1-3,technical alcohols
produced by hydrogenation of cotton seed oil, whale oil and stearin, respectively;
4-5,secondary unsaponified with hydroxyl number 290 and 234 respectively, and
6, cetyl alcohol.
The tests were carried out first at the Valdai Hydrological Station of the State
Hydrological Institute during July and August 1960 (Makarova, 1960 ; Makarova
and Kuznetsov, 1961). Evaporimeters with 0.3 sq. m. water surface were used, as
well as concrete evaporation pans of 20 sq. m. and 100 sq. m. surface. During the
tests the m e a n daily air temperature varied from 110 C. to 240 C., and the m e a n
daily wind speed from 0.5 to 6.5 m./sec. (the m a x i m u m wind speed never exceeded
8 m. per second).
Whale oil alcohol proved to be the most effective evaporation retardant (30-40
per cent reduction), while cetyl alcohol was second best (24 per cent). In some
24-hourperiods the evaporation from evaporimeters covered with these films was
reduced by as m u c h as 50 per cent.
Additional evaporimeter tests were carried out that year in Armenia in co-operation
with the Institute of Energetics, Academy of Science of the Armenian S.S.R.,
with similar results (Makarova and Mkhitaryan, 1961 ; Makarova, Mkhitaryan,
Trapeznikov and Fedorova, 1962).
’
E X P E R I M E N T S IN ISRAEL
.r
Evaporimeter experiments in Israel were conducted at the Weizmann Institute
of Science, Rehovoth, using Class A pans, during 1956 and 1957 (Bavly, Leitner and
Miller, 1959). Cetyl and stearyl alcohols were tested in different proportions; they
were spread from crystals or from solid paraffin-alcohol mixtures, as well as from
various solvents. Best results were observed with mixtures containing four parts
of stearyl alcohol to one part of cetyl alcohol, with 20 per cent paraffin added;
the evaporation reduction of this mixture was 65 to 75 per cent with the water
temperature (of the control pan) of 150 C. whereas it was about 50 per cent only,
when the water temperature was 240 C.
E X P E R I M E N T S IN J A P A N
Extensive evaporimeter experiments with the recently synthesized OED compounds
have been carried out at the National Institute of Agricultural Sciences, Tokyo
(Mihara, 1962).
E X P E R I M E N T S IN O T H E R COUNTRIES
,
Tests in evaporation pans using Class A pans and larger pans were carried out in
South Africa by the Hydrological Research Division of the Department of Water
Affairs beginning in 1957 (Roberts, 1961). These tests have shown that:
1. The fatty alcohols should be applied as a very finely divided powder or in solution.
2. The mixture of hexadecanol and octadecanol in equal parts is as good as if not
superior to pure cetyl alcohol.
3. Daily or continuous dosing is necessary.
4. The m a x i m u m reduction of evaporation that was found under the best conditions
was 50 per cent.
In Canada evaporation control tests, using various physical forms of cetyl alcohol,
have been conducted by the Research Station, Canada Department of Agriculture,
32
Evaporimeter experiments and field trials
<
Lethbridge, Alberta. T h e tests which began in the s u m m e r of 1958 used 4 and 10 ft.
diameter evaporation pans (Hobbs, 1961).
In Algeria evaporimeter experiments were carried out with cetyl alcohol and a
synthetic material, pearled polystyrene ‘Styropor’ (Genet and Rohmer, 1961).
While cetyl alcohol showed very little evaporation reduction, ‘Styropor’ promised
to be a very effective retardant.
2..Field trials : general review
.’
Scarcely more than ten years have elapsed since the first field experiments were
carried out on the use of monomolecular layers for evaporation reduction. In that
comparatively short time m u c h work has been done in m a n y different countries.
This work will be described below under five headings. T h e general review of field’
experiments includes appropriate references, which, as a rule,.will not be repeated
in the following sections dealing with different aspects of field trials: Section 3,
materials; Section 4, methods of application; Section 5, detection and evaluation
of film coverage; and Section 6, effect of wind.
E A R L Y AUSTRALIAN EXPERIMENTS
T h e first report of ex/penments on the use of hexadecanol for control of reservoir
evaporation appeared toward the end of 1953 (Mansfield, 1953). It referred to field
tests o n small water bodies in Woomelang, Australia. T h e tests indicated the need
for further investigations of the technique of film application. During the s u m m e r
of 1954-55in southern Australia and during the dry season in northern Australia,
a number of investigations were conducted o n open reservoirs and large tanks
(Mansfield; 19556, 1956). Unfortunately, during the periods chosen, Australia
experienced heavy and unseasonal rainfall and this complicated the evaluation of
the results of these tests.
In the first set of experiments solid hexadecanol in the form of flakes was applied
to the water surface by means of small, gauze-coveredrafts anchored in the reservoir;
later, however, beaded material was used. T h e treated reservoirs varied in size from
a fraction of an acre to 22 acres and the periods of treatment from 3 to 11 weeks.
T h e results ranged from -15 per cent to +97 per cent reduction in evaporation,
the most probable values being in the range of 15 to 40 per cent’ (Mansfield,
1955b).
Experimentation on a large scale started with the field trial at Stephen’s Creek
reservoir, Broken Hill, New South Wales, in December 1955 (Mansfield, 1957).
This storage is a large shallow basin of m a x i m u m depth 16 ft. and m a x i m u m area
2,100 acres. T h e material applied was commercial cetyl alcohol in ,solution. Initial
results were disappointing, with virtually no effect on the rate of evaporation.
These were attributed to the insufficient dosage. After increasing the flow rate,
measurable results were obtained, and after further increase in application rate
during the s u m m e r of 1956-57 (when the average area of the storage was 930 acres)
an over-all reduction in evaporation of 37 per cent was observed over a period of
14 weeks (Sutherland, 1957).
1. A U figurei of evaporation reduction are quoted from the original papera: see Section 7.
33
,
Evaporaiion reduction
EAST AFRICAN E X P E R I M E N T S
In East Africa, the Australian method of applying solid alcohol in beads from rafts
was tried in several reservoirs during 1955 (Grundy, 1957a, 19576). A number of
practical difficulties were encountered; in particular the beads of cetyl alcohol were
either reduced in size by abrasion against the gauze and escaped from the rafts,
or they became covered with algae and silt.
Consequently it was decided to use cetyl alcohol in solution. First field trials were
carried out on two small reservoirs near Nairobi during the period August to October
1956. The first reservoir was 1 acre in area; an average reduction in evaporation
of about 25 per cent was reported over a period of 4 weeks. In the second reservoir,
6.5 acres in area, a reduction of evaporation of 30 per cent over a period of 4 days
was reported.
In August 1957 a fairly large-scale experiment was carried out on Malya reservoir
in Tanganyika (Grundy, 1958). At the time of experiment the surface area was
about 130 acres, with a m e a n depth of water of 7 ft. A n evaporation reduction of
11.5 per cent over 10 days was reported.
r1
U N I T E D STATES S M A L L - S C A L E E X P E R I M E N T S
In the United States the first evaporation reduction field test was conducted by
the Southwest Research Institute in co-operation with the United States Geological
Survey at the Essar Ranch Lake, Texas, in the s u m m e r of 1956 (Cruse and Harbeck,
1960). During the experiment the surface area of the lake was 4 acres. Film-forming
material (hexadecanol or octadeeanol) was stored on rafts. Evaporation savings
of from 4 to 18 per cent were achieved. Further tests in 1957 and 1958 on other
small lakes in Texas were inconclusive, due to various causes such as heavy rains,
and the difficulty of constructing a water budget. Additional tests were undertaken
in 1959 and 1960 on four 1-acre stock tanks situated near Laredo, Texas, and on
Essar Ranch Lake (Koberg, Cruce and Shrewsbury, 1963). Different materials and
dispensing methods were employed. T h e m a x i m u m reduction in evaporation of
27 per cent for a 2-week period was reported. Other small-seale field experiments
included:
1. Experiments on two small (about 2.5 acres each) adjacent lakes in Illinois (Roberts,
1959, 1962). T h e tests extended over the s u m m e r of 1957 and 1958, and showed
evaporation savings of 43 and 22 per cent respectively.
2. Tests on t w o adjacent reservoirs of 0.28 acre area each at Oklahoma Agricultural
Experiment Station, Stillwell, Oklahoma (Crow and Daniel, 1958; Crow, 1961,
1963). These tests have been conducted since 1956, first to develop and'test
equipment and techniques for applying the film, and later to investigate the effect
of wind on the application and maintenance of monolayers. An evaporation
reduction of 25 per cent was reported for a 66-day test in 1959 using continuous
application technique. W h e n closed barriers were used in later tests to reduce
air and film movement (see Section 6), the m a x i m u m evaporation reduction
rose to 31.3 per cent for an 80-day test in 1962, with only intermittent
film application.
3. Tests on two adjacent ponds, each 75 ft. b y 100 ft. (0.17 acre) at Texas Experimental Ranch near Seymour, Texas (Meinke, Waldrip, Stills and Harris, 1962;
Meinke and Waldrip, 1964). Fourteen tests were carried out on these ponds over
a period of four years from 1959 to 1963 with evaporation savings ranging from
O to 23 per cent.
4. A n experiment on the 40-acre Felt Lake, Stanford, California in s u m m e r 1960
(Franzini, 1961). A n evaporation reduetion of 19 per cent was reported.
34
Evaporimeter experiments and jield trials
5. Tests on duplicate ponds, each 53 ft. by 78 ft. (0.09 aire) at Arizona Agricultural
Experiment Station (Resnick and Cluff, 1963). These tests have shown that it
is almost impossible to maintain a film cover on small reservoirs when the wind
is above 8 m.p.h.
U N I T E D STATES L A R G E - S C A L E E X P E R I M E N T S
By far the largest share of the work on evaporation control in the United States
has been done by the Bureau of Reclamation (Anon., 1960) and all the important
field trials in America were carried out by that organization either alone, or in collaboration with other agencies. Summaries of these experiments have been given by
Garstka (1962a, 19626), and Timblin, Florey and Garstka (1962).
T h e first field test by the bureau was conducted at Kids Lake in preparation for
the large-scale tests later carried out at Lake Hefner, Oklahoma (Committee of
Collaborators, 1957). The City of Oklahoma City, the United States Public Health
Department, the United States Weather Bureau and the United States Geological
Survey co-operated in this test. Kids Lake is a small 6-acre lake adjacent to Lake
. Hefner. The primary object of the test was to determine the effects on water quality
of a treatment with commercial grades of hexadecanol. It was concluded that ‘insofar
as criteria of water quality including taste, odour, colour, toxicity and other chemical
qualities are concerned nothing has been determined from this study to preclude
further consideration of Lake Hefner for large-scale evaporation reduction investigations’.
Field studies of monolayer application and performance began in April 1957
with investigations of various techniques of application at Rattlesnake Reservoir,
a 100-acrelake in Colorado, and continued in August and September 1957 at Ralston
Creek Reservoir, a 150-acrereservoir near Golden, Colorado. In the latter experiment
Denver Board of Water Commissioners and the United States Public Health Service
co-operated. T h e results of these investigations led to the development of a method
of applying a water slurry of powdered hexadecanol from a boat. T h e technique
was tested in June 1958 at Carter Lake, a 1,000-acre lake s o m e fifty miles north
of Denver, Colorado (Timblin, Florey and Garstka, 1959).
T h e Lake Hefner investigations lasted from 7 July to 2 October 1958 (Committee
of Collaborators, 1959). This has been perhaps the most comprehensive and carefully
documented study on evaporation suppression that has been conducted so far. A
co-operative effort, it included as its participants in addition to the Bureau of
Reclamation, the City of Oklahoma City, the United States Public Health Service,
the Oklahoma State Health Department, the United States Weather Bureau and
the United States Geological Survey. Lake Hefner is a 2,500-acre lake in Oklahoma
City and forms a part of the city’s domestic water supply system. The major conclusion in this study was that a n evaporation-reducingmonolayer could be applied
and maintained on a large body of water for an extended period of time. The over-all
saving of evaporation for the 86-day period was 9 per cent.
Further monolayer behaviour studies were performed in the a u t u m n of 1959
at Boulder Basin of Lake Mead, Arizona-Nevada, and Sahuaro Lake, Arizona, to
test the persistence of the film (Florey, Backstrom and Ensign, 1961). T h e Boulder
Basin of.Lake M e a d has a surface area of.40,OOO acres. It was chosen for the study
because of its k n o w n offshore-onshorewind patterns. T h e Lake Sahuaro, a 1,000-acre
lake near Phoenix, Arizona, was chosen in preparation for the evaporation reduction
tests to be performed the following year. T h e persistence of film on both lakes w a s
good, reflecting favourable wind conditions.
The evaporation reduction test at Sahuaro Lake was conducted from 1 October
to 17 November 1960, with the collaboration of Salt River Valley Water Users
35
,
Evaporation reduction
i
Association, the United States Geological Survey, the United States Public Health
Service, the United States Air Force, the Arizona State Health Department and the
Arizona State Fish and G a m e Department (Bureau of Reclamation, 1961). The
evaporation savings reported were 14 per cent.
Further evaporation reduction studies were carried out at Lake Cachuma, a reservoir of 2,400 acres at the time of the experiment, near Santa Barbara, California
(Bureau of Reclamation, 1962). In this study, which extended from 31 July to
24 September 1961, the following agencies co-operated: Cachuma Operation and
Maintenance Board, the United States Public Health Service, State of California
Department of Public Health, Santa Barbara County Health Department, and the
Santa Barbara County Department of Parks. T h e evaporation savings reported
were 8 per cent.
In 1962, a monolayer behaviour and aerial application study was conducted
on the 8,000-acre Elephant Butte Reservoir, New Mexico (Bureau of Reclamation,
1963~).Evaporation savings were not determined.
A n evaporation reduction study at Pactola Reservoir which was also scheduled
for s u m m e r 1962 w a s only partially c a m e d out that year; it was resumed and
completed in s u m m e r 1963. N o report on this study was available at the time of
writing (July 1964).
R E C E N T 4A U S T R A LIA N E X P E RI M E NTS
I
T h e Australian field trials were resumed in 1959 by using a n e w technique of applying
a fine, dry powder of cetyl alcohol to the water surface (Vines, 1960a, 19606). T w o
major experiments were carried out: (a) at the 200-250acre Umberumberka Reservoir,
Broken Hill,New South Wales, during the period from March 1959 to February 1960;
and (b) at the 500-600acre Corella Lake, Mary Kathleen, Queensland, over the period
from June 1959 to April 1960 (Vines, 1962). Savings of some 40 per cent were attained
in calm weather, but m u c h smaller savings with persistent winds in excess of
5 m.p.h.
Further tests were conducted at Stephen’s Creek Reservoir in April-May 1962
and again in November-December 1962. In the first period application of cetyl
alcohol was m a d e for 6 weeks, and the estimated evaporation savings were approximately 20 million gallons, that is about 15-20 per cent. In the second period they
were less, probably o n account of higher wind velocities. -Treatment at U m b e r u m berka Reservoir was continued for another year until M a y 1961. During that period,
however, the reservoir level was low and the area of water small (below 100 acres),
and the effectiveness of the dusting process (see page 41) was thereby reduced.
Treatment was resumed in February and continued until M a y 1962 (Fitzgerald
and Vines, 1963).
INDIAN E X P E R I M E N T S
In India the preliminary field trials began with three relatively small reservoirs
(Ramdas, 1962). These are: (a) the 170-acre Badkhal T a n k at Faridabab, Punjab,
near Delhi; (b) the 28-acre Buderi T a n k at Poondi near Madras; and (c) the 165-acre
Kukrahally T a n k near Mysore City. The Badkhal experiment was carried out by
the Central Water and Power Commission, whilst the Buderi test was conducted
by Poondi Irrigation Research Station, P.W.D., and the Kukrahally test by the
Mysore Engineering Research Station. These experiments are described by Hoon,
Issar and Sachar (1962); Ganapathy (1962), Walter (1963); and Doddiali and
Shivanna (1962)respectively. Other field evaporation reduction studies were carried
out by the Soil Survey Division, Poona at a 40-acre Bhangoon T a n k and a 60-acre
.
Evaporimeter experiments and field trials
Kasurdi Tank, both near Poona (Kulkarni and Parashave, 1962). T h e first Iargescale field trial has been in progress since 1961 at the 1,500-acre W a l w h a n Lake
near Poona originally by the Tata Hydro-Electric Power Supply Co. Ltd., and later
by the Central Public Health Engineering Research Institute, Nagpur (Bapat
and Chakravarthy, 1962; Anon., 1962). As it has proved impossible to evaluate
the water budget of the lake, this experiment is aimed at perfecting the technique
of application and maintenance of the monolayer on large surfaces. For s u m m e r
1963 evaporation reduction tests were scheduled at the 470-acre Gorwala T a n k
near Nagpur. N o report on these tests was available at the time of writing.
I
{
O T H E R FIELD E X P E R I M E N T S
In Burma, a small-scale field test was carried out during the dry season (from
December 1958 to M a y 1959) on a 4.4-acre lake near Rangoon (Dr.P o E and U B a
Kyi,n.d.; U Thaung, 1962). Evaporation reduction from 8 per cent to 20 per cent on
a monthly basis was observed.
In Spain, t w o field trials were carried out in 1957 and 1958 on a 30-acre reservoir
in the south of the country. T h e tests were conducted by Price’s (Bromborough) Ltd.
in conjunction with the Rio Tinto Mines, and the evaporation savings were 35 per
cent and 31 per cent over periods of 7 weeks and log weeks respectively (McArthur,
1960).
In Britain, where climatic conditions are not suitable for field evaporation reduction tests, a field study was m a d e of different methods of application of the m o n o ’
layer and of the movement of film under the wind. T h e study was carried out in
1960 b y Price’s (Bromborough) Ltd. on the 900-acreeastern section of Loch Laggan
in Scotland (McArthur, 1962a, 19626).
In Japan, extensive field tests have been carried out o n paddy fields of 0.12-0.24
acre area with OED compounds aimed at increasing the temperature of the water
in the paddies. O n cloudless days increases of 70 to 80 C. at midday have been attained
(Mihara, 1958, 1962). Other tests have been concerned with the reduction of evaporation from wet soil and plants (Mihara, 1962; Mihara and Hagivara, 1960).
3. Field trials: materials
F A T T Y ALCOIIOLS
In the early Australian experiments by Mansfield, cetyl alcohol of high hexadecanol
(80-90 per cent) was employed. This material was considered necessary.
for successful application of the ‘beads in raft’ method (see Section 2) which was
recommended by the Commonwealth Scientific and Industrial Research Organization for use in small reservoirs of up to 2-acre surface (CSIRO,1956). T h e detailed
specification n a m e d ‘Si-ro-seal’was as follows:
’ content
SpeiJícalion
Cornpodion
%
Hexadecanol
Octadecanol
Cl*and Cl, alcohols
Lower alcohols
, Unsaturated alcohols
min. 80
.
max.
max.
max.
max.
10
5
0.5
4
Iodine value
Acid number
Saponification value
Hydroxyl value
Melting point
max. 3
max. 0.3
max.,0.5
225-230
45-500 C.
,
37
Evaporation reduction
For the Stephen’s Creek Reservoir a 10 per cent solution of flake commercial cetyl
alcohol was used, the solvent being a petroleum fraction of b.p. 95-1600C. containing
7 per cent of ethyl alcohol.
In the later Australian tests employing cetyl alcohol powder (see S2ction 4) use
was m a d e of a commercial product with the following properties:
Specijicafion
Composition
%,
I
Hexadecanol
Octadecanol
- Tetradecanol
Unsaturated alcohols
45
40
i0
5
Iodine value
Acid number
Saponification value
Hydroxyl value
Melting point
max. 4
max. 0.1
max. 0.5
22O 225
48-500 C.
-
In recent Stephen’s Creek Reservoir tests another product containing 35 per cent
cetyl and 65 per cent stearyl alcohol was also used. Altogether 1,400 pounds of cetyl
alcohol were added during the first 6-week test period.
In the experiments conducted in East Africa, after the initial failure with pelleted
cetyl alcohol, a commercial product similar to that used in later Australian experiments was used. T h e alcohol was applied in solution in kerosene, 30 grammes of
alcohol to 1 litre of kerosene, with 0.9 g r a m m e of spreading agent added. In the
Malya (Tanganyika) experiment 182 pounds of cetyl alcohol were used during the
ten days of the dosing period.
In the small-scale American experiments, various materials were used. For the ’
first Essar Ranch Lake experiment commercial hexadecanol (Adel 54) and octadecanol (Adel 62) were melted, cast into block, ground and screened to pass a
quarter-inch mesh and be retained on a No. 10 mesh. T h e evaporation reduction
with hexadecanol was found to be twice that obtained with octadecanol at the
s a m e dosage (18 per cent to 9 per cent at 20 pounds per acre), but the difference was
probably due to the addition of a bacteriostatic agent to the hexadecanol. In the
1959-60 experiments on the stock tanks and on Essar Ranch Lake the retardant
materials used were dodecanol, hexadecanol, and octadecanol. The best reduction
in evaporation was obtained with octadecanol. In the twin pond tests near Seymour,
Texas, a commercial mixture of hexa- and octadecanol in roughly equal proportions
(‘Aquasave’) was applied in the form of solution or emulsion.
For the Lake Hefner tests commercial high quality cetyl alcohol was chosen.
In the three months of film applications a total of 40,040 pounds of the material
was applied to Lake Hefner.
In Lake Sahuaro test two materials were used: (a) an octadecanol-hexadecanol
mixture in a proportion of approximately 2 :1, and (b) the same materials as
that used in Lake Hefner.
.
In the C a c h u m a test the fatty alcohol used was a tallow-based hexadecanol and
octadecanol material specifically manufactured for evaporation control. T h e chemical
analysis of the material was as follows (in percentages): octadecanol, 66.2; hexadecanol, 30.4; tetradecanol, 3.3; dodecanol, 0.1.
T h e chemical arrived at the reservoir in flake and powder form. It was then melted
by electrical heaters, p u m p e d into a boat storage tank and transported to the
automatic dispensers located at strategic points on the lake. T h e total amount of
the material applied to during the period between 20 July and 25 September 1961
w a s 59,650 pounds. In the Elephant Butte and Pectola Reservoir tests commercial
alcohol of the following approximate composition was used (percentages): tetradecanol, 2; hexadecanol, 30; octadecanol, 60; eicosanol, 8 (Michel, personal communication).
38
Evaporimeíer experiments and field trials
In Indian tests commercial cetyl alcohol was used either alone or in mixture with
stearyl alcohol.
In tests in Spain the fatty alcohol used had the following composition (percentages):
tetradecanol, 3; hexadecanol, 81; octadecanol, 16. In the 1957 test the alcohol was
dissolved in white spirit (b.p. 76-2120C.) to form a 4 per cent solution, while in the
1958 test a 3 per cent solution in kerosene was used.
M O N O - O X Y E T H Y L E N E ETHERS
J
i
In their experiments on evaporation suppression which were aimed at increasing
the water temperature in flooded rice fields, the Japanese investigators first tried
hexadecanol and octadecanol (Mihara, 1962). However, the results obtained with these
compounds were not considered satisfactory. Subsequently docosanol w a s tried.
Despite its excellent evaporation suppression potential, it too had to be abandoned
because of its poor spreading characteristics: nor could application in a solvent
to facilitate spreading be considered because no solvent was found which would
not be harmful to the rice plant. Finally, a n attempt was m a d e to combine docosanol
with some functional group which has a strong affinity for water, as it was assumed
that spreading characteristics of a polar compound depended on the balance of the
hydrophilic and hydrophobic group. This aim was achieved in 1956 by combining
the docosanol with ethylene oxide (CH,),O."he product, mono-oxyethylene docosyl
ether C,,H,,OC,H,OH, which w a s named OED-13, was found to be a highly efficient
evaporation retardant for temperatures above 200 C. (see Fig. 4). In colder water,
however, this compound did not spread well. T o remedy this disadvantage it was
mixed with its lower homologue C,,H,,OC,H,OH which had been prepared by
combining octadecanol with ethylene oxide. T h e mixture of 55 per cent of the C,,
product with 45 per cent of the Cl, product, which was n a m e d OED-70,exhibits
excellent evaporation reduction and spreading properties over a wide range
of temperature. For the past few years this material has been used in numerous
field experiments in Japan. Since 1960 it has been applied extensively by farmers
to w a r m the water in rice nurseries. Its application on large-scale rice fields is
hampered b y strong winds prevailing in Japan in late spring and early s u m m e r
(see Section 6).
'
1
~
4. Field trials: methods of application
B E A D S IN R A F T M E T H O D
The original m o d e of monolayer application to water storages, devised by Mansfield (1956)was to store a supply of solid cetyl in a n anchored raft fitted with wire
gauze sides and floating on the water surface. As the film was generated at the airsolid-waterinterface it moved out through the gauze. At first small flakes of cetyl
alcohol were used but these were found to be too fragile, resultini in loss of the
fragments of the material from the rafts. In later experiments flakes were replaced
with beaded cetyl alcohol, which had a stronger structure.
The method was at first considered in Australia as suitable for reservoirs of small
size, up to 2 acres in area; for larger reservoirs its use was precluded by the strong
abrasive action of waves on the beads through the gauze. However, even on small
areas the method proved to have two disadvantages: (a) the surface of the beads
was non-crystalline and the evolution of monolayer was slow, and (b) the beads
39
r
Evaporation reducíion
themselves tended to become coated with solids, both organic and inorganic, and this
coating prevented any further spread of the monolayer.
T h e beads in raft method was used in the Essar Ranch Lake test; but it was
found unsuitable in East African experiments by Grundy (1957a, 1957b), w h o first
pointed to its disadvantages; likewise, it was unsuccessfully tried in Illinois lakes
and in the first monolayer application studies of the Bureau of Reclamation at
Rattlesnake Reservoir. Today the method is of historical interest only.
i
S O L V E N T APPLICATION
This
,
._
method-the standard method of spreading a monolayer in the laboratorywas first applied in the field at Stephen’s Creek Reservoir in Australia. For this
purpose a commercial cetyl alcohol was dissolved in a volatile petroleum fraction
and ethyl alcohol (see Section 3). The solution was applied by means of a gravity
feed through fine-gauge piping from calibrated 40-gallon drums, which acted as the
dispensers. The drums were either placed on supports on the shore or were floated
on rafts anchored at suitable points on the reservoir.
The flow rate, controlled automatically according to wind direction from the
twelve land-based dispensers, was at first 0.007 gallon per acre per day. W h e n n o
effect on evaporation was observed,the rate was increased to 0.012 gallon per day and
subsequently to 0.017 gallon (equivalent to 0.8 monolayer) per acre per day. At this
rate of application evaporation savings of 37 per cent were observed (see Section 2).
In the Malya (Tanganyika) experiment, solution was applied from fifteen airgallon dispensers set u p on trestles erected in the water. This arrangement was
necessary because of the belt of vegetation growing in the water round the upwind
perimeter of the reservoir. A boat had to be used for filling the dispensers. Dosing
was commenced at the rate of 1.5 litres per acre per day. However, after three days
it became apparent that this dose was insu5cient. Thereafter the dose was increased
to 3 litres per acre per day, equivalent to 9 monolayers per day.
This figure m a y be compared with the dose of 2 monolayers per day used in the
small-scale tests in East Africa referred to in Section 2 (page 34) from which evapo-,
ration reduction of 25-30 per cent was reported.
In the tests in Spain two floating dispensers were used for spreading solution.
T h e dose was 2.8 monolayers per day in 1957 and 3 monolayers in 1958.
Solvent application was considered by the United States Bureau of Reclamation
but was rejected in favour of other methods on account of its health and fire hazard
and interference with any possible or potential recreational use of the reservoir.
However, solvent application using s o m e non-deleterious (but rather expensive)
solvents was tried in small-scale experiments by the Southwest Research Institute
(Cruse and Harbeck, 1960) and in the twin ponds near Seymour, Texas experiments
(see Section 2) with evaporation savings reaching 22.5 per cent.
In the preliminary Indian tests cetyl alcohol or its mixture with stearyl alcohol
was at first applied in solution either in mineral turpentine (at Badkhal, Buderi
and Kukarahalli) or in white petroleum (at Bhandgoon and Kasurdi). Later, the
solution in turpentine was abandoned in favour of the emulsion method. The latter
method has been used throughout at Walwhan Lake. Solvent application was used
in the Burmese experiment.
In the study of film application at Loch Laggan, spreading from a volatile petroleum fraction (S.B.P.3)was compared with spreading from kerosene and industrial
methylated spirits. Under the conditions at Laggan (water temperature about 100 C.)
none of the solutions gave evidence of marked superiority in performance. However,
kerosene solution had the advantage of a lower density than the other solutions,
and in rough water gave better coverage.
40
Evaporimeter experimenis and field trials
Solvent application possesses a number of limitations: the monolayer invariably
. suffersfrom contamination by retained solvent; not only does this reduce its efficiency
but the tendency of the film to collapse on compression is increased when it contains
impurities. T h e spreading pressure of a concentrated solution is only half the
spreading pressure of solid cetyl alcohol. And, not least, the cost of the solvent forms
a considerable addition to the cost of the material used. For these and other reasons,
mentioned above, the solvent method has been largely dropped in favour of the
methods to be described below.
/
P O W D E R APPLICATION
’
Powder application was first tried in the United States after it had been found that
a floating supply of solid hexadecanol was not practicable. First tests were m a d e
at Rattlesnake Reservoir by dusting the surface of the lake with powdered hexadecano1 from a boat in motion. The success of this method was immediately evident.
Tests were then transferred to Ralston Creek Reservoir. A series of tests was m a d e
to determine the amount of material required for film maintenance. At wind speeds
prevailing during the tests (5-15 m.p.h.) it was found that 0.2-0.4 lb./acre/day
(equivalent to 11 to 22 monolayers) was required to maintain significant coverage
of the lake. During these field studies it became increasingly evident that the use
of powdered material, although very effective in producing the film,presented s o m e
definite problems of handling and dispensing. T h e powder had a great tendency
to l u m p and cake. As a possible solution to this problem a separation with s o m e
inert-material was considered. No dry suitable material was found at the time,
and the dusting method was abandoned in favour of the use of water as separating
material.
In Australia also difficulties were experienced with storing and transporting
the cetyl alcohol powder (Vines, 1960~).However, addition of inert materials such
as talc or chalk was not considered advisable, for although these help prevent balling
of the alcohol they affect its spreading properties. As a remedy a fine ‘spray dried’
powder was produced in which the particles were globular, rather than flaky. This
material could be stored for long periods and transported over considerable distances
without any deleterious effects. Before applying it, the powder was freshly sieved
through wire screens passing particles of size 0.1 to 0.01 mm., cooled to 150 C. to
maintain a high proportion of sub-a phase in the individual particles and promote
rapid spreading (see Chapter I, Section 3) and then loaded into an agricultural duster
on a boat, from which it was blown out over the water.
This method was used at Umberumberka Reservoir from M a y to September 1959
and at Lake Corella from June to September 1959 (Vines, 1962).
The results of Umberumberka experiment were presented o n the basis of 28-day
periods. For the first few days, 20-30 pounds of powder were applied daily over
a n area of about 250 acres, and the whole reservoir could be completely covered
with a high-pressure film within an hour. Subsequently the feed rate was reduced
to 10 pounds per day (equivalent to about 2 monolayers). Under the prevailing
conditions of light wind (up to 5 m.p.h.), the evaporation savings exceeded 50 per
cent over a period of 28 days. In the second 28-day period, under similar conditions,
small quantities of the material were occasionally applied averaging 25-30pounds
per two weeks (equivalent to 0.4 monolayer/day). Evaporation savings of 40 per
cent were observed. In the third 28-day period, with calm persisting, the quantities
of alcohol applied were further reduced to 5 pounds per 2 weeks (equivalent to less
than 0.1 monolayer/day) and the evaporation savings computed were 35 per cent.
In the Lake Corella experiment even better results were achieved. During the
first 2-week observational period, in which 100 pounds of powder (equivalent to
41
-
.
Evaporation reduction
0.5 monolayerlday) were applied, the evaporation was reduced by more than
50 per cent. In the following two weeks, the results were even better. The film .
persisted thanks to the specially calm weather; however, after a month the effect
of the treatment began to wear off-though application of another 100 pounds
of powder quickly restored the film and brought about a further reduction in evaporation. Another 100 pounds of material was added in mid-August but by that
time the winds had sprung up, and less satisfactory evaporation savings of the order
of 30 per cent were obtained.
In September 1959 a n e w method of application was introduced at the two reservoirs. Its essential part is’the Robertson grinder-duster, named after its inventor.
It consists of a wire brush rotating at high speed to shred solid blocks of cast cetyl
alcohol (Vines, 1959, 1960b). Very fine powder with excellent spreading properties
is obtained. The powder is blown through a delivery tube by a fan. As the equipment
is mounted in the boat, the powder is not produced until actually requped: the
problem of sintering of cetyl alcohol powder in transport or storage is thus
obviated. The blocks are cast to the right size at the lake site and cooled to 150 C.
before use.
The introduction of a Robertson grinder-duster at the two reservoirs coincided
with the seasonal strengthening of winds and this was reflected in the figures for
evaporation savings; even so at Lake Corella savings of 20-90 per cent were
attained.
Tests with the Robertson grinder-duster were continued in recent experiments
at Stephen’s Creek Reservoir and the Umberumberka Reservoir.
At Stephen’s Creek, by using a fast speed-boat capable of 20-25 m.p.h., it was
possible to cover the entire area of more than 1,000 acres within about one hour:
under conditions of light wind 75 pounds of powder (equivalent to less than 4 monolayers) was sufficient to produce a uniform monolayer over the untreated reservoir.
During the first six weeks of monolayer application (9 April to 20 M a y 1962)
1,400 pounds of cetyl alcohol were added, equivalent to 1.7 monolayer/day on the
average.
In the United States, the dusting technique was again applied in 1959 film
behaviour tests at Boulder Basin of Lake Mead and at Sahuaro Lake. It was also
one of the two application methods used in the 1960 evaporation reduction investigations at Sahuaro Lake. T h e powder was dispensed through smali petrol-engined
blowers mounted on boats. It was found that in light winds of 7-8 m.p.h. or less
the entire lake could be covered with a monolayer in 1-2 hours. The usual procedure
was to take the boats to the windward side of the lake and traverse the lake in a
serpentine pattern perpendicular to the wind direction. Powder application was
also used in the Felt Lake test. The method was to dispense the material by hand
from a boat. It was distributed about once an hour in those areas of the lake where
it was needed. The dosage was from 0.15 to 0.75 lb./acre/day (equivalent to 40 monolayers), the average coverage was 50 per cent during daylight hours and 95 per cent
at night,
Loch Laggan tests (see page 40) have also included some experiments with
powder-generated monolayers using a modification of the Robertson device. It
was found that slicks (Section 5) obtained by powdering were similar to those from
application of Shell petroleum spirit (b.p. 102-1200 C.) or industrial methylated
spirit, carrying with them an excess of solid which after grounding could, with a
change in wind direction, be driven out from the shore giving rise to a fresh slick.
No difference was seen in the rate of movement by wind of the powder slicks as
compared with solvent slicks. The conclusion was that kerosene solution and powder
application were both well suited for large-scalework.
/
42
Evaporirneïer experiments and field trials
APPLICATION AS, A N E M U L S I O N
T h e possibility of using water as an inert material to separate powder particles was
studied by the Bureau of Reclamation both in the laboratory and in the field prior
to Lake Hefner tests (lïmblin, Florey and Garstka, 1962). Tests of separation and
stability of Èlurries were performed with from 10 to 50 per cent hexadecanol. For
every concentration of the alcohol it was found that the water and alcohol phases
began to separate, or a moisture gradient was set up in the slurry providing a fluid
of variable viscosity. Moreover, wetted particles were found to have a spreading
ability inferior to that of the dry particles.
In view of these results the technique finally developed for Lake Hefner investigations consisted of mixing the powder and water in a tank mounted in the boat.
T h e mixture was continuously agitated with mechanical stirrers to maintain uniform
consistency. This slurry was then sprayed on to the water surface as the boat moved
across the lake.
This technique was first tested at Carter Lake. Applications of 0.2-0.45 lb./acre/day
were made, and coverages of 50-75 per cent of the surface were achieved. At Lake
Hefner a monolayer of satisfactory coverage was established for 55 days out of the
86 days of the test. High winds prevented adequate coverage of the lake o n the
remaining days. Originally the applications were m a d e during the daylight hours, ’
but later in the tests the application schedule was modified to achieve the best
coverage during the period of greatest probable evaporation. T h e applications
were m a d e from both a boat and a barge. Because of its greater speed, the boat
was more effective. The daily application varied but the over-all average was about
0.3 Ib./acre/day (equivalent to 18 monolayers per day). The m a x i m u m coverage at
any time was 89 per cent, and the average coverage for the entire 86 days was
10 per cent. For the 55 days w h e n the film was applied, the average coverage was
16 per cent.
A device for the continuous application of powdered hexadecanol as a slurry o n
a small scale has been developed by Crow (1961)for his experiments at Oklahoma
Agricultural Experiment Station. The slurry is contained in a steel d r u m and agitated
continuously by a paddle-type agitator unit. Water from the pond, circulated
through a one-inch distribution line, serves as a diluting and transporting m e d i u m
for the slurry. The slurry is applied to the pond through application hoses perforated
at 10 ft. intervals. Automatic controls regulate the rate and the point of application
of the film in response to wind speed and direction. Metering of the slurry concentrate
is controlled by cup-contact anemometer.
T h e slurry technique was also compared.with other methods in the Illinois
experiment, and there it was found that it provides ‘the most efficient and effective
w a y to maintain a monomolecular film’ (Roberts, 1962). In the 1957 experiment
the slurry was applied for 35 days at the average rate of 0.3 Ib./acre/day (equivalent
to 16 monolayers) with evaporation savings of the order of 45 per cent. It should
be noted, however, that some copper sulphate was added to the slurries to counteract
biological attrition of the hexadecanol.
The application of the monolayer as emulsion was strongly advocated by Dressler
(1959, 1962; Dressler and Johanson, 1958), w h o carried out some evaporation
reduction tests in several reservoirs in Texas.
In some early Seymour, Texas, tests cast ‘Aquasave’ emulsion rods were tried
with results which wcre essentially negative. N o water savings were obtained,
and the rods became coated with an algae growth. Later tests on ‘Aquasave’emulsions
involved: (a) solid emulsion dispensed in floating nylon mesh bags; (b)liquid emulsion
metered to the water; and (c) emulsion powder placed in soluble bags in copper
43
Evaporation reduction
i
screen wire baskets. All of these applications gave at the m a x i m u m about 20 per
cent evaporation savings.
In the course of the Indian preliminary field trials at Badkhal, Buderi and Kukrahally, the emulsion technique superseded solvent application.
At Badkhal an emulsion prepared with the help of a high-speed.emulsifying
unit has been dispensed from three floating rafts and some shore units. The end
product of the emulsifying process has been usually thinned prior to use by diluting
with water to a consistency allowing it to flow freely from the dispensers. To supplement fixed dispensers a hand-operated spray p u m p has been employed.
At Buderi the emulsion was prepared by first melting the fatty alcohol with a
little water and then churning to make a paste after adding a little soap solution.
Water heated to 600 C. was then added to this paste and the whole mixture was
then agitated for about two hours. The dispensing was done by a number of shore'
line dispensers around the periphery of the lake; in addition, a boat was constantly
in operation for treating the uncovered regions of the lake surface.
At Kukrahally, a similar technique was employed.
At W a l w h a n Lake, after initial difficulties with the preparation of adequate
emulsion had been resolved, the process adopted was to mix in a high-speed emulsifier
4 pounds of powdered alcohol in 5 gallons of water with a little soap added to it.
After 80 minutes, a white emulsion was obtained which was then further diluted
with water to m a k e 80 gallons. The process was then repeated a number of times
daily in view of the large area to be covered. However, the consumption was limited
by the capacity of equipment to 40 pounds a day, this being equivalent to a rate
of 0.03-0.05 lb./acre/day, depending on the fluctuations in the surface area of the
lake. This rate (amounting to 2-3 monolayers/day) was- insufficient to keep
the lake covered with monolayer. Even in calm weather the coverage was only 50
per cent with the part covered exhibiting a surface pressure of 20-30 dynes. With
the installation of a n e w emulsifier it was proposed to step up the rate of
application to obtain complete coverage.
In Japan the emulsion application has been the standard method of dispensing
OED for warming water in the paddy field. T o facilitate spreading, carboxmethyl
cellulose is added to the 30 per cent emulsion of OED in water. Floating plastic
vessels are used for dispensing the material. The paste moves onto the water surface
through slots in the vessels (Mihara, 1962). Thus, although emulsion application
m a y be wasteful of material because of the reduced spreading power of pre-wetted
alcohol powder it seems to have s o m e advantages for small-sized reservoirs where
powder application is evidently not suitable.
H O T S P R A Y APPLICATION
j
Field studies on the behaviour of monolayer (Section 6)have indicated that the most
efficient method for treating a reservoir under windy conditions would be by the
J
use of automatic dispensers strategically placed around the banks and possibly
within the reservoir which would dispense the retardant at rates proportional
to the wind speeds.
A prototype dispenser of'this kind was m a d e in Australia (Vines, 1960~).It
sprayed molten cetyl alcohol from a pressurized-container heated by a small kerosene
burner, The spray solidified in the air into a fine powder,which fell on the water
surface to form a monolayer. Intermittent operation was achieved with a timing
device. T h e inferior spreading characteristics of the hot sprayed powder (see page 25)
in comparison with those of a cooled dusted powder, and the comparative complexity
of the equipment led the Australians to abandon this method in favour of nonautomatic application.
44
*
U
c
-
'
FIG.6. Block diagram: equipment for molten spray technique of monolayer application.
From Bureau of Reclamation (1962).
45
Evaporation reduction
In the United States the hot spray method was first tried in the 1960 tests at
Sahuaro Lake. The method of application was to first establish a layer on the lake
by spraying from the boat. Eight strategically located wind-operated automatic
dispensers sprayed additional material to maintain the film coverage. The automatic
unit consisted of a hot-water tank to hold the melted alcohol and battery-operated
equipment, which controlled the rate of spray from the tank. The melted alcohol
was forced through the spray nozzle by gas pressure applied b y bottled gas, initially
butane, and later compressed air. The controlling equipment regulated the rate of
application between the low and high wind velocity cut-Offs which were set for
4 m.p.h. and 17 m.p.h. respectively.
Between these two velocities, the rate was proportional to the wind velocity.
No applications were m a d e with onshore winds.
The performance of automatic dispensers at Sahuaro Lake was not satisfactory;
malfunction of electrical and mechanical parts occurred repeatedly; in particular
freezing of the spray nozzles caused considerable difficulty. These nozzles were
heated by a small gas burner mounted in a heater box under the nozzle. The burners
were'very difficult to protect from strong wind gusts and almost never burned
through an entire day,
These difficulties were corrected and other improvements were m a d e in the
dispensers for the 1961 Lake Cachuma tests. Four additional units were constructed.
The twelve dispensers were installed at Lake Cachuma in July 1961, and served as the
only source of film-generating materials for the lake during the 9-weektest period.
A block diagram for the dispenser and auxiliary equipment is shown in Fig. 6. The
improvements over the original design included an electric nozzle heater to prevent
freezing of the nozzle between spray bursts, and a transistorized controller unit.
1 The twelve dispensers were each placed on raftsasupported by six 55-gallon oil
drums. In the final disposition they formed t w o lines across the breadth of the lake
.with eight units in one line and four units in the other. These lines were normal
to the prevailing wind direction thus taking full advantage of the wind to spread
the monolayer the length of the lake. The dispensers discharged from 10 to 15 gallons
of alcohol per day, the rate of discharge being proportional to the wind speed and
also influenced by the dispenser tank pressure, the electric control setting, and the
nozzle orifice size.
At the beginning of the tests four m e n were required to service the dispensers
and associated equipment. Later, after the personnel became more familiar with
the equipment and procedures, two m e n could carry on this work in an eight-hour
shift. The operation of the dispensers proved satisfactory and only minor malfunctions were experienced, such as dirt plugging the nozzles and dust and grease coating
the triggering contacts of the anemometers and wind vanes. The former was eliminated by double filtering of the melted alcohol, while the latter was taken care of
by a routine cleaning schedule.
The performance of an automatic dispenser in what amounts to almost ideal
wind conditions is illustrated in Fig. 7.
A prototype of another molten alcohol dispenser was built in 1959 in the Stanford
University Engineering Laboratories (Franzini, 1961). In the summer of 1960 this
dispenser was mounted on a raft and tested 'at Felt Lake. Its performance was
fairly satisfactory but some problems were encountered necessitating improvements
in design.
AERIAL APPLICATION
The methods of application of the evaporation retardants described so far such as
the use of floating rafts, boats and automatic dispensers have been found suitable
46
Evaporimeter experiments and field trials
for smaller reservoirs. Their use on large bodies of water, however, could prove
difficult and costly.
In 1961, under the sponsorship of the United States Bureau of Reclamation,
Utah State University undertook the task of determining the feasibility of applying
evaporation retardants from the air (Israelsen and Hansen, 1963; Newkirk, 1963a,
19636). T h e first year was spent primarily in developing a dispenser that would
handle the retardants in a liquid state. Subsequently, an application capable of
applying the retardants in the powder form was also developed. The two dispensers
were tested in a series of monolayer applications on Utah Lake, and H y r u m Reservoir,
Utah and in Elephant Butte Reservoir, New Mexico. It was found that films formed '
from powder spread more rapidly than films formed from liquid, but usually both
films ultimately spread to approximately equal width and had the same degree
of compression as determined with indicator oils (see Section 5). Both powders and
50
?
40
-
30
20
Average wind speed
4.5 m.p.h.
10
Dispensing rate
0.6 Ib./min.
c
YI
!
O
u
m
e
>
$ 0
O
20
40
60
80
Time (minutes)
FIG. 7. Coverage versus
time. From Florey, Hansen and Cleaver (1961).
47
Evaporation reduction
sprays most suited for aerial application appear to be in the 75 to 200 micron m e a n
particle-diameter size. Retardants with a large percentage of particles having
diameters smaller than 75 microns are often carried by the wind away from the
surface of the reservoir while powders and sprays with a preponderance of particles
having diameters larger than 200 microns are less effective in film formation. The
comparison of powder dispensers with liquid dispensers for aerial application shows
a number of advantages of the powder dispenser, such as easier handling, no safety
hazards inherent in handling hot liquids, lower capital investment and less labour
required. Its chief disadvantage is the tendency of powder to lump and to bridge
across the outlet from the hopper. The use of the Robertson grinder-duster for aerial
application was rejected because of the considerable extra weight involved and the
difficulty of maintaining a constant rate of feed.
T h e conclusion was that the use of aircraft to apply evaporation retardants
appeared to be a n effective method for large reservoirs; however no definite preference for this method could be claimed until further tests were m a d e involving
both computations of evaporation savings and the refinement of the techniques
of aerial application.
’
5. Field trials: detection and-evaluation of film coverage
T o pvaluate the effectiveness of monolayer application it is necessary to have a
method for detecting a monolayer and determining the degree of its compression.
For this purpose a set of calibrated indicator oils has been devised by the Bureau
of Reclamation, making it possible to determine the film pressure of the surface to
within 5 dyneslcm. (Timblin,1959).
Field tests were conducted at Ralston Creek, Rattlesnake, and Carter Lake
reservoirs with the indicator oils. These tests demonstrated that the presence of
even a slightly compressed monolayer was visible as a ‘slick‘ (a smooth patch on the
water surface). Conversely, apparent slicks were often observed on these mountain
lakes which were not produced by the film but simply because the wind was not
blowing on that particular part of the lake surface. Likewise, the photographic
technique of film detection, developed during these studies, could not always be
relied upon since m a n y factors such as thé angle at which the picture was taken,
the position of the sun, and cloud and haze condition significantly influenced the
results obtained. However, it was also found that when any wind at all was blowing
the monolayer m o v e d about the lake and remained fully compressed except for
some feathering-out and decomposition in a narrow transition zone along the
windward edge of the film (Florey, Foster and Townsend, 1959). Consequently use
of the indicator oils was not generally required. E v e n under calm conditions, through
careful examination of the surface by an experienced observer, apparent or no-film
ahks and fully compressed film slicks could usually be differentiated.
The extent of the coverage of the water surface is one of the principal factors
that enter into the method of evaluation of evaporation savings which has been
used by the Bureau of Reclamation (see Section 7). Consequently, m u c h care has
been taken in the Bureau’s field tests to assess this factor with m a x i m u m
accuracy.
At Lake Hefner the extent of the coverage was determined by several methods:
(a) observations and photographs made from a vantage point above the water
surface; (b) observing the location of the film w h e n driving periodically around the
lake; (c) aerial photographs coupled with shore observations.
O n the basis of all this information the film coverage of the water was mapped
48
~
’
Evaporimeter experiments and Jield trials
as frequently as the time and weather conditions permitted. Usually from t w o
to five mappings per day were made.
At Sahuaro Lake, in addition to routine visual observations of the lake, several
aerial photographic techniques of measurement were investigated. Photographs
from different angles and altitudes, colour transparencies, oblique and vertical
black and white photographs were taken. It was noted in some of the sequence
photographs, where one was taken from a slightly different angle than the other,
that the photographs gave quite different indications of film coverage and that
therefore not too m u c h reliance could be put on aerial photography alone.
Since the presence of the monolayer by reducing evaporation causes a rise in water
temperature, the possibility of using infra-red scanning and photographic technique
to determine film location was also investigated. However, no conclusive evidence
of superiority of infra-red over ordinary photography was obtained.
Still another photographic technique has been tried. This was based on the property of thin films or monolayers on a water surface to affect the rotary polarization
of the reflected light. A polarizing filter was used, and it was found that in m a n y
instances this technique was helpful both for locating the monolayer and for determining its relative degree of compression. It was observed that the areas on the
water surface that were covered by only a partially compressed monolayer quite
often had the same appearance, both visually and in unfiltered pictures, as the
areas covered by a fully compressed monolayer. In the pictures taken through a
polarizing filter the areas covered by a partially compressed film appeared as uncovered areas, and the boundary between the covered and uncovered portions of
the water surface was m u c h more sharply defined.
At Lake Cachuma the visual method of estimating lake coverage was systematized
by using a plane table and alidade to plot the film boundaries on a prepared base
map. T h e film coverage of the lake could thus be obtained at hourly intervals
throughout the daylight hours.
At Elephant Butte Reservoir the methods of detecting and evaluating dm coverage
were again: (a) observational m a p drawing from ground vantage point; (b) observations from boat and pressure measurements; (c) photographs taken from the ground
vantage point and from the air (Kuehn, 1963). T h e use of a plane table and alidade
was satisfactory on calm days but proved very difficult w h e n stronger winds were
blowing as the film pattern changed too rapidly for film boundaries to be determined
with any degree of accuracy. Aerial photographs were m a d e using K-20 aerial
cameras for the first time in this type of study. These photographs did show a definite
contrast between the areas covered and areas non-covered whenever lighting conditions were favourable.
6. Field trials: effect of wind
Of all the factors adversely influencing filmcoverage, the wind is probably the most
important. This seems to be the inescapable conclusion of the analysis of all recent
field investigations.
T h e problem was not fully realized at first, probably because the early field trials
took place in Australia under conditions of very little wind.
The 1957 tests at Rattlesnake and Ralston Creek reservoirs showed that full
coverage could be obtained with mild winds, but the coverage would not persist
because winds gradually decreased the area covered by the layer. With winds of
5-10m.p.h. the coverage decreased appreciably during the day, and for any adequate
coverage almost continuous application of hexadecanol was required. Winds of
49
Evaporation reduction
15 m.p.h. quickly swept the monolayer from the water, and all evidences
of the
film disappeared.
At Lake Hefner conditions were rarely calm, and winds of 5-17 m.p.h. were the
normal condition. The influence of the wind on the film behaviour confirmed previous
observations. It was frequently observed that when winds reached 20 m.p.h. the
film would s e e m to disappear suddenly. This was believed to be due to decompression
of the film by wave formation and mixing of the hexadecanol in the water by wave
action and white-capping.A study of the daily average percentage coverage achieved
in relation to daily average wind velocity showed a definite trend, although the
correlation coefficient was only 0.56 and the standard deviation was 12 per cent
(n = 57). This wide spread of points was probably due to different operational
procedures used during different days of the experiments, some of which were more
effective than others.
At Lake Cachuma where a single operation procedure (automatic dispensing)
was employed, a very high correlation between average coverage and average wind
speed was obtained (see Fig. 8). T h e wind speed followed a well developed diurnal
pattern increasing in the morning from 4 to 6 m.p.h. during the night to about
14 m.p.h. in the afternoon; the percentage coverage showed a similar trend, decreasing
from 60 per cent in the early morning to about 20 per cent in the late afternoon.
During the night, w h e n there was little wind, the dispensers were able to replenish
the film blown off the previous day (see Fig. 9).
T h e rate of movement of fatty alcohol films under the action of wind were investigated by Vines (1962) and McArthur (1962a, 19623).
Vines’ measurements, which were made at Umberumberka Reservoir, were
concerned with the rate of retraction of the monolayer. Retraction takes place when
a monolayer bounded downwind by a shore line is compressed by the wind and
collapses. Vines’ technique was to measure the time taken by the trailing edge of
an extended film to traverse the distance between two buoys separated by a thin
rod pointing in the wind direction. Although in this way drift rates were determined,
retraction rates might equally be derived from the results as the retraction rate
of an extended film is very nearly equal to its rate of drift on an open surface
80 -
~~74.3-3.64~
Coefficient of correlation
Standard deviation
Fz0.95
3=3.90
Average wind speed (rn.p.h.)
FIG.8. Average monolayer coverage versus average wind speed (Lake Cachuma investigations,
25 July to 24 September 1961). From Newkirk and Hansen (1962).
50
,
Evaporimeter experiments and field irids
2
4
6
8
IO
12
14
16
18
20
22
24‘
Time of day
FIG.9. Average monolayer coverage versus time of day (Lake Cachuma investigations).
From Newkirk and Hansen (1962).
O
.
(Mansfield, 1959b). The results of measurement in different wind speeds are given
in Fig. 10. Extrapolation to zero wind speed leads to the result that the film spreads
on a still surface at a rate of 1-2in./sec.; this is in fact the spreading velocity of the
monolayer, when applied b y the dusting technique in calm conditions. T h e slope
of the line is 1/30;hence the rate of retraction of a film is about one-thirtieth of the
amount by which the wind velocity exceeds 2 m.p.h. (=180 ft./min.).The retraction
rate at low wind speed is therefore less than one-thirtieth of the wind velocity
though it approaches this ratio at high wind speeds. This ratio of 1 :30 was explained
by Fitzgerald (1964)as due to the fact that the measurement at the trailing edge
of the slick was unaffected by the damping of the surface (see page 26).
McArthur’s investigations, conducted at Loch Laggan, Scotland, dealt with the
lateral and forward movement of the film slicks (see Section 5) under the influence
of yind of measured velocity. The length of the slick was found to depend mainly
on the quantity of spreading source present in relation to the condition of the wind.
With insufficient cetyl alcohol present a condensed film and a defined slick could
not be formed. Steady winds of up to 15 m.p.h. were found to assist the spread of
the film when sufficient spreading source was present, but very gusty winds of higher
wind velocity tended to disperse the slick. The width of the slick depended both
on the wind speed and the initial spreading rate of the source which had to overcome
the lateral stress of the wind. High wind velocities, other factors remaining constant,
gave narrower slicks. For winds of 5-9 m.p.h. and surface temperature of about
100 C. the width of slicks grew with time from about 30 yards at 10 minutes, from
the start of application to about 50 yards after 80 minutes. It was expected that
at higher water temperature higher spreading rates would result in wider slicks.
The influence of wind was separately examined for solvent application and powder
application methods. It was found that w h e n solids form in the slicks by evaporation
or by solution of the solvent the wind moved them more slowly than the film itself.
As the solid particles became wetted they spread film more slowly and thus formed
a narrow tail to the slick. In rough water under wind of about 12 m.p.h. the solids
tended to be submerged. Except for the tendency for powder particles to be sub-
51
Evaporation reduction
-
1400
1200
1 O00
..-.
,
800
600
400
C
.-
E
\
-
O
10
~
20
30
40
Drift (ft./min.,
FIG.10. Drift rate of the monolayer veraua wind speed. From Vines (1962).
-
merged by eddy currents in rough water, the behaviour of the powder-generated
slicks was similar to those formed from solution.
I
T h e rate of movement of the slicks was measured at intervals throughout runs
and compared with the corresponding average wind velocities measured in the path
of the slick. T h e ratio of film velocity to wind velocity was found to vary from
0.04 to 0.07. Over distances varying from 200 yards to 900 yards in runs of up
to 3,300 yards, only in the initial sections of the runs was this ratio approximately
constant with a value of 0.045. During consecutive sections of any run the ratio
was found to increase progressively; this was demonstrated for full pressure slicks
spread from both solution and powder. The rate of acceleration was shown to be
associated with water temperature. A possible explanation of these results was
given by Davies (1962). H e suggested that w h e n the waves were damped by the
surface-active agent, conditions of laminar flow existed at the surface and in the
underlying water layers, and that the flow of water under the surface led to an
increase in m o m e n t u m and so to an acceleration of the surface.
This explanation was questioned by Fitzgerald (1963, 1964) who, while agreeing
that conditions near the surface were considerably modified by the presence of
surface-active material, nevertheless asserted (on the grounds of experimental
evidence to be published shortly) that the laminar flow conditions near the surface
did not extend right down into the body of water. As McArthur's observations of
52
Evaporimeter experiments and field trials
the drift velocity were made on the leading edge of the slicks, they were affected
by errors due to the spreading rate of the alcohol film itself. Fitzgerald (1964)
suggested that the increase of the ratio of film velocity to wind velocity was due to
differing conditions at the points of measurement.
;
With a knowledge of the average width of slick formed and its approximate rate
of movement under wind, it was possible to estimate, in terms of wind velocity, the
rate at which cetyl alcohol should be added continuously to the water surface to
maintain a fully condensed monolayer. This rate-under the conditions of the experiment, and assuming a ratio of film speed to wind speed of 0.045, and-a m e a n slick
width of 120 ft. was found to be u1840 grammes per minute, where v is wind speed
in feet per minute. This is equivalent to 1.16 x 10-4 V pounds per hour. per foot
of upwind shore line normal to the wind, where Vis the wind speed in miles per hour.
The figure given was an estimate for the m i n i m u m dosage rate required for continuous
application assuming complete conversion of alcohol to monolayer for the conditions
of the experiment.
This semi-empirical formula m a y be compared with the results of actual experiments to determine the effect of wind on the application rate required for a complete
film cover. Such experiments were carried out by Crow (1961)on his experimental
i
.O025
1
.O015
b
R = .O000093 U2*02
.o010
’
.O006
t
i
i
.o004
A
h
2 .O0025
t
C
.c
O
.Q
f
.O0015
2.5
Wind speed (U),m.p.h.
1.5
4
6
10
15
25
FIG.11. Application rate (inpounds per hour per foot of upwind shore line normal to the wind
direction) required versus wind speed. From Crow (1961).
53
.
Evaporation reduction
*
ponds at Stillwell, Oklahoma. The results are given in Fig. 11 from which it is seen
that the application required is proportional to the square of wind velocity. Each
point on the curve represents a test of several hours’ duration during which the wind
speed and direction remained constant. Throughout each test the application rate
of slurry of k n o w n concentration was controlled in such a manner that the rate of
replacement of the film equalled but did not exceed the rate of removal by the wind.
Crow’s and McArthur’s figures are about equal for V
10 m.p.h. Crow’s quadratic
law m a y be due to the short length of the fetch in experimental ponds (Mansfield,
19596). This seems to be confirmed by the results of Crow’s recent experiments
(Crow, 1963) in which he placed a network of wind baffles on the ponds to reduce
the removal of the monolayer by the wind. For a ratio of baffle spacing to baffle
height equal to 12 :1, the application required was found to depend on an even
higher power of the wind speed (about 2.5) than with an open pond.
It is also possible that the quadratic law might approximatelyhold for both large
and small reservoirs. Recently experiments were undertaken at Stephen’s Creek
(Fitzgerald and Vines, 1963), when wind conditions were constant during periods
of an hour or more. Extensive cover was established over the reservoir, and the
boat was then driven continuously up and down, close to the windward shore,
distributing powder at a rate just sufficient to maintain full coverage. Thus estimates
were obtained of the amount of the alcohol required to replace the monolayer
as it was being retracted in winds of different velocities. These are given as: 6 pounds
per mile of shoreline per hour in a 7-8m.p.h. wind (i.e. 150 Ib./day); 25-30 pounds
per mile of shoreline per hour in a 12.5 m.p.h. wind (i.e. 650 Ib./day). With a previous
estimate (Vines, 1962) of the amount of retardant needed per square mile of reservoir
in the absence of wind and bacterial attack (see Section 8), Fig. 12 was obtained.
Influence of the wind on the maintenance of a monolayer was considered by
Grundy (1961, 1962) in the light of his experiments in East Africa. He found that
under conditions locally prevailing (wind speeds of 4 to 20 knots): (a) the f
ilm was
moved over the water surface at a speed of about 400 yd./hr.; (b)the width of the
film (perpendicular to wind direction) varied with the rate of application and the
wind speed; (c) the film was damaged by waves, the effect varying with wind speed
and wind fetch.
=
Cetyl alcohol per day (Ib.)
FIG.12. Amount of cetyl alcohol powder required per day
area 1 mile square. From Fitzgerald and Vines (1963).
54
to maintain full cover over an
,
Evaporimeter experimenís and field trials
Wind is the principal reason why the method of water-warming b y evaporation
reduction, which has been widely used for rice seed beds in Japan since 1960, has
not been adopted o n a large scale for transplanted rice fields (Mihara, 1962). E v e n
moderate wind was found to sweep off easily the film on water more than 5 cm.
deep. Only films on water layers of 3 cm. and less were resistant to the action of
the wind (Mihara, 1961).
Perhaps the best illustration of the influence of wind on the efficiency of m o n o layers as evaporation retardants is given by Fitzgerald and Vines (1963). S u m m i n g
up the Australian work in evaporation control over the previous three years they
come to the following conclusions:
1. For winds up to 5 m.p.h., evaporation savings of 40 per cent or more.
2. For winds u p to 10 m.p.h., evaporation savings of 10-20 per cent though occasionally the savings m a y be somewhat less, depending on prevailing conditions.
3. For winds in excess of 15 m.p.h. the savings approach zero.
These figures illustrate very clearly the limitations of the evaporation reduction
method by use of monolayers.
7. Evaluation of evaporation savings
T h e use of open expanses of water to test the effectiveness of monomolecular layers
in reducing evaporation immediately poses the problems of the reliability of the
methods which are available for evaluating the results, that is the accuracy of
methods of estimating any reduction in evaporation.
Fully controlled conditions d o not exist in the open even in tests on adjacent
evaporation pans in which the evaporation from treated pans is compared to that
from untreated control pans (see Section 1); but there, at least, the observed variability is the result of fluctuations in microclimate which can be largely accounted
for or compensated by suitable design. N o such possibilities exist for large expanses
of water where the seepage is an important and u n k n o w n item.
Up to a certain size it is feasible to construct two identical ponds, or tanks, one
of which would serve as a control. Such experimental twin facilities have in fact
been built at the Oklahoma Agricultural Experiment Station, Stillwell, Oklahoma
(Crow and Daniel, 1958; Crow, 1961), at Texas Experimental Ranch, Seymour,
Texas (Meinke, Waldrip, Stiles and Harris, 1962; Meinke and Waldrip, 1964) and
at Arizona Agricultural Experimental Station (Resnick and Cluff, 1963). At Stillwell
the t w o 100 by 120 b y 7 ft. ponds were lined with a buried vinyl plastic membrane,
and numerous calibration tests showed seepage to be negligible. At Seymour the
t w o 75 by 100 by 6 ft. tanks were lined with 5 mil polyethylene sheeting to prevent
seepage. During both tests consistent results were obtained. At Tucson the two
53 by 78 ft. ponds were lined with vinyl plastic. Such tests, however, cannot be a
substitute for large-scale field trials because of the influence of the factors which
cannot be scaled in size.
With larger ponds and lakes no duplication of the important characteristics
is practicable. O n e w a y to take account of the inherent differences between t w o
natural lakes is to use first one of the lakes and then the other as the control. This
method was used by Roberts (1959,1962) in his experiments on t w o small adjacent
lakes in Illinois. Apart from natural limitations of this scheme, additional disadvantages are that the length of time required for the experiment is doubled and the
uncertainty that all the film-forming material has been removed from the treated
reservoir at the conclusion of the first part of the experiment (Cruse and Harbeck,
1960).
55
Evaporation reduction
In large-scale field trials conducted so far three methods of evaluation of evaporation savings have been used: (a) the pan coefficient method; (b) the combined
energy budget and mass-transfer method; (c) the ‘simplified’ method.
i
,
P A N COEFFICIENT M E T H O D
T h e pan coefficient method which was recommended originally by Mansfield (1955a,
1956), consists of plotting pan evaporation against change in reservoir level during
a pretreatment calibration period. T h e slope of the line is the pan coefficient, and the
intercept is the seepage:
Ea = S + kE,
where
E, = the
E,
S
k
J
apparent evaporation from the reservoir (change of level corrected
for inflow and withdrawal);
= the evaporation recorded by the evaporimeter;
= the average seepage loss;
= the ‘pan coefficient’.
I
1
T h e seepage loss and pan coefficient thus determined during the pre-treatment
calibration are assumed to remain valid during the period of treatment with the
monolayer. T h e apparent evaporation during that time, corrected for seepage, is
compared with the evaporation that would have occurred had the film not been
applied by using the evaporimeter data multiplied b y the pan coefficient.
T h e assumption of the constancy of pan coefficient over periods as short as a
m o n t h or less, which is the basis of Mansfield’s method, w a s at variance with the
evidence obtained during the Lake Hefner studies (Kohler, 1954). Lack of consideration of seepage losses or differences in types of evaporimeter pans was advanced
by Mansfield (1956) at first as a possible explanation of the discrepancy. In a later
conceded that constancy of pan coefficient was
paper, however, Mansfield (1959~)
not the general rule. Although the coefficient m a y be constant with annual values
of evaporation, for monthly values this is only the case for reservoirs not more than
4 m. deep. T o obtain the values of the seepage and the pan coefficient for reservoirs
4-8m. deep s o m e corrections to values obtained by the linear correlation method
had to be applied, the correction depending on the depth of reservoir and the climate
characteristics. For storages deeper than 8 m. the linear correlation method was not
valid.
Mansfield’s original method of evaluation of evaporation reduction was presumably
ill,
applied in the early field trials and at the Stephen’s Creek Reservoir at Broken H
but no details of the method of evaluation of evaporation savings used were published.
A similar method of evaluation was used by Grundy (1958), in his experiment
on Malya Reservoir in Tanganyika. H e assumed seepage losses to be constant, in
view of the small change in water level and in the area covered by water during the
experiment. T h e pan coefficient was also assumed to be constant, and moreover,
as no pan and reservoir records of evaporation existed at the site, a value of pan
coefficienthad to be assumed. Thus, if E, and E,, refer to change in water level
as measured and E and E, to the actual evaporation from the reservoir, during
the treated and untreated periods respectively, and S the seepage losses
resulting in
56
-
Evaporimeter experiments and field trials
If now E, is the evaporation that would have occurred during the treated period
had no monolayer been present, then the reduction in evaporation is given b y
E, -E,:
where E, and E,, represent the pan evaporation during the treated and’untreated
period, respectively, and k the pan coefficient. To the value of evaporation reduction
so obtained Grundy added the evaporation equivalent of the increased heat storage
‘inthe water, obtaining total equivalent reduction of evaporation of 11.5 per cent
of which 4.5 per cent was due to the added heat storage. However, there is little
justification for considering the warming effect of the monolayer as additional
evaporation reduction. Indeed, the opposite is true: increased heat storage represents
potential additional evaporation once the monolayer is removed (see below and
page 58). O n this basis Grundy’s experiment would show almost no evaporation
reductio? O n the other hand, there m a y have been some rise in the water tcmperature even if the film were not present, as the average air temperature rose during the
period under treatment. T h e above considerations, added to the uncertainty of
the value of the pan coefficient, clearly show that the Malya experiment-the first
in which details of the method used in computing results were [published-was
not conclusive.
1
The pan coefficient method was employed in Indian, Burmese and Spanish field
trials as well as in the Felt Lake, California, experiment. It was also used in recent
Australian experiments (Vines, 1962;Fitzgerald and Vines, 1963). At Umberumberka
Reservoir, pan and reservoir evaporation records were available for a long period,
and by grouping the records according to different levels of water in the reservoir,’
it was possible to determine seepage at these levels, assuming that the seepage
depended on the level only. A statistical analysis of the records showed the pan
coefficient to be 0.80 0.05, with uncertainty limits at 5 per cent level. At Lake
(Corella no long-term pan and reservoir data existed, and the pan coefficient w a s
assumed to be approximately 0.8, corresponding to that found for Umberumberka.
A critical evaluation of Vines’ data showing the uncertainties of the method even
when the pan coefficient remains sensibly stable was given by Frenkiel (1962).
There seems little doubt that the difficulty of constructing a water balance and
the uncertainty introduced into the calculations by the use of the pan coefficient,
restrict the validity of the pan coefficientmethod to some exceptional experimental
sites.
&
C O M B I N E D E N E R G Y B U D G E T A N D MASS T R A N S F E R M E T H O D
In a n attempt to overcome these limitations Harbeck and Koberg (1959)developed
an indirect method that is based upon both the energy-budget and mass-transfer
evaporation measurement techniques. T h e energy equation is:
or inflow minus outflow equals change in storage, where:
Qb = short-wave solar radiation incident to the water
Q, = reflected solar radiation;
Qa = incoming long-wave atmospheric radiation;
Qu,= reflected atmospheric radiation;
surface;
57
Evaporaiion reduction
Qv = net
energy advected into the body of water by inflow and withdrawal;
Qbs =’
long-wave radiation emitted by the body of water;
Qe = energy utilized by evaporation;
Q h = energy conducted from the body of water as sensible heat;
Qw = energy advected in the evaporated water;
Qg = heat conducted through the lake bottom;
Qe = increase in energy stored in the body of water.
/
’
They assumedithat the application of a monomolecular film would not affect the
items grouped together as inflow. This is obvious in respect to QS,Qa and Qv,and
they have shown that any effect of a film on Q, would be largely counterbalanced
by a compensating change in Obs.They assumed further that over a long period
of time, the film would have no appreciable effect on Q0.
T h e last assumption is open to some doubt. It appears that the rise of temperature
caused by the film is not confined to the water surface only, where the energy is
immediately dissipated, but that it extends further d o w n as was noted by Grundy
(L958),Bavly, Leitner and Miller (1959) and Crow (1961). This would indicate that
there is some change in the amount of energy stored below the surface, and’a further
complication could arise if water were withdrawn from the reservoir as in this case Qv
would also be affected. T h e possible magnitude of this effect was discussed by
Koberg (1962) in his evaluation of evaporation savings in the field test at Lake
Cachuma:
Koberg assumed that a period of one month without the treatment was sufficient
to dissipate the energy stored in the lake as the result of the film,and evaluated the
evaporation savings for the whole period of treatment plus that month; in this w a y
he found that the savings as computed, without taking into account the energy
stored in the lake as the result of the film, had to be reduced by as m u c h as
a third.
In this connexion Crow’s (1961)results should be cited. H e performed an experiment to determine the effect of intermittent application of the film. When the
film was applied only 12 hours per day, from 9 a.m. to 9 p.m., evaporation was
reduced by 6.5 per cent, compared with 25 per cent reduction when the film was
applied continuously. T h e apparent cause for this difference is the higher energy
content of the reservoir on which evaporation has been suppressed resulting in
higher evaporation rates when the film is removed.
This effect of change in heat storage on evaporation was discussed in a recent
paper by Wolbeer (1963).
Admittedly, there is still m u c h uncertainty concerning the influence of additional
energy storage on evaporation reduction, Very precise thermal surveys before,
during and after the treatment could possibly help in elucidating the matter. Alternatively, such surveys could be m a d e concurrently on treated and untreated portions
of a reservoir.
Assuming that the application of a monomolecular film will not affect QI, Qa,Qv
and Q0, and assuming the same about Qw and Qg which are, anyhow, minor items
in the energy budget, leads to the ‘change in outflow budget’ equation:
,,
,
-
(Q’bs
-
Qbs)
(Q’e
-
Qc)
+ (Q’r -Qd = 0
/
in which the symbols with primes refer to the reservoir with film and the symbols
without primes to the same reservoir without film.
This equation can also be given in the ‘expanded’ form, thus:
0.9700 [(Po
+ 273)’
58
/
-(To+ 273)‘]
+ [QIo-Nu(eo-e,)] + Ku(TIo-To)= O
~
- Evaporimeìer experiments and field trials
where
a = Stefan-Boltzman constant for black-body radiation;
T’,= water surface temperature observed (in OC.);
To = water surface temperature that would have been observed if film had not
been applied;
QrC= observed energy utilized for evaporation from the energy budget (Qrc+Q’h)
and heat transfer formula (Q’h)
established in the pretreatment calibration i
period;
N = empirical constant obtained during the pretreatment calibration period
from the mass transfer formula: Qc= Nu(eo e,)
Qc being obtained from energy budget (which gives Qc+ Qh) and the B o w e n
ratio: Qh/Qc= 0.61 x 10-3P(T0 Tu)/(e, eu);
P = atmospheric pressure;
Ta = air temperature;
, u
wind speed;
e, = saturation vapour pressure at To;
,e = water vapour pressure in the air;
K
empirical constant obtained during the pretreatment calibration period
from the heat transfer formula: Qh= Ku(To Ta);
Qh being obtained, like Qc from the energy budget and the B o w e n ratio.
-
-
-
=
=
-
T h e ‘change in outflow budget’ equation in its expanded form serves to determine
To,and eo which is a one-valued function of To.As the relationship between the t w o
is not simple, different values of Toand the corresponding tabular values of eo are
assumed until the equation is satisfied. T h e mass transfer formula then gives Qc,
which can be compared with QrC
to determine the reduction in evaporation.
No indication was given originally by Harbeck and Koberg of the estimated
accuracy of their method. T h e results of the first field tests evaluated by this method
bore accordingly n o confidence Limits to the percentage reduction claimed. These
field tests included the test at Essar Ranch Lake, in Texas (Cruse and Harbeck,
1960) and the large-scale test at Lake Hefner (Harbeck, 1959).
Later, however, Koberg (1961)raised the question in connexion with the evaluation
of the evaporation reduction in field tests at Sahuaro Lake. H e stated that there
is a f 5 per cent uncertainty o n the percentage of evaporation reduction, the main
reason being that the actual evaporation could only be determined with a n accuracy
of about 10 per cent. This uncertainty was also assumed in the case of the Lake
C a c h u m a results (Koberg, 1962).
Frenkiel (1962, 1963) showed that the f 5 per cent uncertainty limits on the
percentage of evaporation reduction was valid only on the assumption that the
coefficient of mass transfer N, and the coefficient of heat transfer K were constant
and k n o w n exactly. This assumption was probably approximately correct on a
yearly basis. On a monthly basis the standard error of N in Lake Hefner w a s found
to be about 5 per cent (Marciano and Harbeck, 1954) corresponding to about
f 10 per cent confidence limits at the 95 per cent level; while the variability of N
at other lakes examined was at least twice that figure. T h e variability of K w a s
of the same order but its influence on the evaporation reduction was, in general,
considerably less o n account of the usually low values of Bowen’s ratio @,IQc.
T h e conclusion from Frenkiel’s study was that in general there was no firm basis
for regarding any evaporation reduction of less than 20 per cent as falling outside
the range of random experimental errors if calculated by Harbeck and Koberg’s
method. T h e possible exception was Lake Hefner, where, in view of highly accurate
measurements, a f 10 per cent range of random errors would be assured. T h e
measure of uncertainty in the results of evaluation is illustrated in Table 1 below,
59
Evaporation reduczion
comparing evaporation reductions computed by the combined energy budget and
mass tranfer method and the 'simplified method'.
TABLE1. Comparison of evaporation
reductions (from Florey, 1962)
Computed evaporation iavingi
Combined method
Teit and period
Lake Hefner (1958)
8 July to 1 October
Sahuaro Lake (1960)
1 October to 17 November
19 October to 17 November
9&5
3.4
L
22
O
10
11
8
'
-
SIMPLIFÌED
%
O
'
~
%
14 &5
22 f 5.
Lake Cachuma (1961)
25 July to 31 July
31 July-to14 August
14 August to 28 August
28 August to 5 September
5 September to 11 September
11 September to 18 September
25 July to 24 September
Simplified method
,
19
23
9
16
19
23
24
18
19
.
)
METHOD
A new simplified method for computing the percentage reduction in evaporation
was formulated by Florey, Garstka and Timblin (1959), at first primarily to interpret
evaporation saving results as obtained by Harbeck and Koberg's method in terms
of the effectiveness of the film cover,*and since used to give independent estimates
of evaporation savings. T h e method, in its latest form (Florey, Teter and Hansen,
1961; Florey, 1962) consists in measuring: (a) the coverage factor c (i.e., the fraction
of the lake covered with a fully compressed monolayer); (b) the evaporation reduction
factor f (i.e., the film effectiveness at the water surface temperature), found empirically using evaporation pans at the test site; (c) wind speed u; (d) water vapour
pressure gradient e, -eu. All these quantities are measured or estimated at threehour intervals.
Percentage evaluation savings are then given by:
-_
The summation
can be extended over any period for which the data exist.
Strictly speaking, the values of u and of e, appearing in the nominator and the
denominator of this expression are not identical. As surface films hinder the development of waves (Vines, 1960a), there is bound to be a considerable difference in the
.
roughness parameter between a treated and untreated water surface. T h e untreated
water surface will usually be aerodynamically rough (Marciano and Harbeck, 1954).
T h e treated surface, on the other hand, if effectively covered by the film, w
ill
probably be aerodynamically smooth. As a result, wind velocity profiles near the
water surface in the two cases will differ, but this difference will gradually diminish
with the height over the water surface and is likely to be quite small at the standard
60
Evaporimeter experiments and field irials
height of 2 metres, the lowest usual height of measurement. More important, the
simplified formula implies that the mass transfer coefficient is the s a m e in both
the treated and the untreated case. This is, of course, not strictly true w h e n the
nature of air flow over water surface differs as explained above, and could cause
s o m e error as w a s first pointed out by Mansfield (1962). T h e difference in e, could
also, if not taken into account, cause appreciable errors. Notwithstanding these
remarks, the success of the method depends essentially on the accuracy with which
e andfcan be determined. For estimating film coverages different visual and photographic (including aerial infra-red and polarized) methods were tried. These have been
described previously (Section 5). Wolbeer (1963) pointed out that the reduction
factor will be overestimated in the method because of the heat flow through the pan
walls and because the turbulent mixing factor is larger for a pan than for a lake.
T h e simplified method does not suffer from the inherent limitations of the previously described methods. If the corrections in N, u and e, could be estimated,
and the techniques of estimating c and f were further improved, there seems to be
no reason why this method should not be more widely adopted. True enough there
is the disadvantage of having the result given in percentage only; for the estimate
of the volume of water saved w e need again the mass transfer formula. B u t n o w the
percentage uncertainty of the mass transfer coe5cient will be related to the percentage of savings, and no longer to the full unhindered evaporation.
8. Biological aspects
T h e possibility of adverse biological effects following the application of m o n o molecular films w a s recognized at an early stage. Thus Mansfield (1955) postulated
that, a m o n g other qualifications, materials used for evaporation reduction should
not markedly restrict the access of air and sunlight to the water, and must not be
toxic. Considerable work has since been carried out, especially in the United States,
on these biological aspects of evaporation control. These studies have been concerned
with: (a) effects on physical and chemical factors influencing life in reservoirs;
(b) toxicity to h u m a n and animal life; (c) influence on organisms dependent upon
the natural surface film of water at some stage in their life cycle; (d) changes in the
plant and animal communities as a result of continuous film application.
PHYSICAL A N D C H E M I C A L FACTORS
Physical factors influencing plant and animal life in water include temperature,
natural surface tensions and water transparency. Chemical factors include dissolved
mineral constituents and dissolved gases. Of these, the last factor is perhaps the
most important.
Timblin (1957) studied the rate of diffusion of oxygen and carbon dioxide from
supersaturated gas solutions treated with hexadecanol. H e observed no difference
in rates between the treated and the control solutions.
’
Linton and Sutherland (1958)examined the influence of a hexadecanol monolayer
on the rate of transfer of oxygen from the air into the water, under conditions simulating those obtaining in water reservoirs. T h e y found that in the absence of wind
the monolayer, both on stirred and unstirred surface, caused little or no reduction
in the oxygen transfer coefficientin the range of values of importance in water
reservoirs. If a jet of air was blown on the centre of the water, the monolayer w a s
found to reduce the oxygen transfer coefficient by s o m e 40 per cent. T h e y concluded
that it appeared safe to spread hexadecanol on any water surface which w a s initially
61
,
Evaporation reduction
90 per cent saturated with oxygen, as a 50 per cent reduction in oxygen transfer
coefficient would only lower the oxygen content to the satisfactory limit of 80 per
cent saturation.
These conclusions are in agreement with the result of Kids Lake studies (Committee
of Collaborators, 1957) and those of Hayes (1959)w h o found that hexadecanol caused
a small diminution in the rate of diffusion of oxygen across the air-water interface
in both field and laboratory experiments. However, it should be noted that at
higher gas transfer rates, monolayers considerably reduced the passage of oxygen
and other gases (Water Pollution Board, 1957; Downing and Melbourne, 1957;
Blank, 1962; H a w k e and Alexander, 1960).
.
I
TOXICITY
Statements asserting that there is no toxicological effect of hexadecanol applied
in the quantities used for evaporation reduction have been made by a number of
American public health authorities (Committeeof Collaborators, 1957; Eaton, 1958;
r
Cruse and Harbeck, 1960).
During the large-scale field experiments at Lake Hefner in 1958, a comprehensive
study of the influence of monolayer application on water quality was carried out.
T h e results of three months’ tests indicated (Matthews, 1959) that there were no
deleterious effects from the application of monolayer.
Attempts to recover hexadecanol from the water outlets, first in 1957 during the
preliminary tests at 150-acre Ralston Creek Reservoir and again during the Lake
Hefner tests in 1958 (Florey, Timblin and Garstka, 1960) were unsuccessful despite
the use of very sensitive methods. The conclusion was that, if hexadecanol was
present, the concentration was less than 5 parts per billion. A similar conclusion
(less than 10 parts per billion) was reached in respect to the octadecanol in Lake
Sahuaro in 1960 (Middleton, 1961).
Preliminary toxicity investigations with respect to fish, ducks, aquatic and
emerging insects and aquatic plants were reported by Timblin (1957). No toxic
effect of hexadecanol was noted. Similar results as regards fish were reported by
Berger (1958). Later an extensive field and laboratory study by Hayes (1959) on
the effect of hexadecanol monolayers on certain g a m e fish and the insect population
- upon which the fish feed also failed to reveal any toxic effects.
S U R F A C E TENSION R E D U C T I O N
This is the only large effect of biological significance. Consequently organisms
dependent upon support by the surface film at s o m e stage in their life history m a y
be adversely affected. This was shown to be the case by Timblin (1957) and Hayes
(1959)in respect to the emergence of immature aquatic insects.
C H A N G E S IN T H E P L A N T A N D ANIMAL C O M M U N I T I E S
-
Grundy (1957a, 19576) in his experiments in East Africa observed that beads of
cetyl alcohol became coated with algae and silt which prevented the normal spread
of monolayer. On examination in the laboratory (Price’s, 1959) these beads yielded
a large number of spore-bearing organisms. It was further observed that bacteria
Pseudomonas sp. growing on demineralized water on which floated fresh cetyl alcohol
beads, coated the beads within 24 hours and continued to multiply with cetyl
alcohol as the only nutrient source.
Indication of biochemical oxidation of cetyl alcohol was found by Boon and
Downing (1957). Ludzack and Ettinger (1957) found definite evidence that micro-
62
Evaporimeter experiments and field trials
organisms (primarily bacteria, protozoa and fungi) could metabolize hexadecanol
in significant amount (0.25 to 3.3 lb./acre/week). T h e y also observed the encrustation
of the hexadecanol pellets with a shell of organic matter.
T h e effect of the presence of bacteria and proteins on the evaporation-reduction
ability of fatty alcohol monolayers has been studied by Jones and Stephens (1960).
Both proteins and bacteria were shown to have a very deleterious effect on the films;
in tests with natural waters bacteria seemed to be the main cause of the drastic
reduction in efficiency. No suitable bactericide has been discovered.
Preliminary water quality studies with hexadecanol at Kids Lake, Oklahoma,
in the s u m m e r of 1956 (Committee of Collaborators, 1957) indicated that the growth
of certain bacteria-Pseudomonas
sp. and Alcaligenes sp.-was
promoted by the
presence of hexadecanol.
This observation was confirmed in the large-scale field study on water quality,
-lake biota and bacterial population in Lake Hefner, Oklahoma, in the s u m m e r of
1958 (Cunningham et al, 1959; Silvey, 1960) where a n even greater increase in the
Aero bacter sp. population was also shown.
Laboratory investigations (Chang, Walton, W o o d w a r d and Berger, 1959a, 1959b)
further showed that: (a) hexadecanol supported the growth of certain bacteria,
especially the Pseudomonas and/or Flavo bacterium sp. which are capable of utilizing
hexadecanol as a food source; (b) growth of these bacteria was accompanied by
destruction of the hexadecanol film and interference with its repair; (c) incorporation
of certain additives in the hexadecanol prolonged the life of a hexadecanol film by
preventing the rapid growth of the micro-organisms;however, the efficiency of such
a monolayer in evaporation suppression w a s generally lower in comparison with
untreated hexadecanol during the first t w o days of application; it was higher over
longer periods. Although considerations of the potential toxicity to h u m a n s resulting
from the additives suggested were not evaluated in these experiments (Florey,
TimbIin and Garstka, 1960), they are an essential preliminary to acceptance of any
proposed additives to fatty acids.
Later experiments (Chang, McClanahan and Kabler, 1962) were aimed at studying
the growth patterns of both Pseudomonas and Flavobacterium sp. in distilled water
covered with monolayer films of hexadecanol and octadecanol and their effect on
the evaporation-suppression efficiency of these compounds. It w a s found that:
(a) hexadecanol and octadecanol on distilled water supported a limited growth of
both Pseudomonas and Flavo bacterium; (b) the impairment of the evaporation
suppression efficiency of these films appeared to b e more closely related to the
isolation of the alcohol solids preventing the spreading needed for film repair than
to the damage done to the film. The authors’ conclusion is that while an effective
means for controlling the growth of the bacteria is being developed, the best method
of obtaining high efficiency in the field is to apply the evaporation retardant continuously.
J
9. Economic evaluations
T h e large measure of uncertainty as regards the results of field trials (Section 7)
is of necessity reflected in any realistic evaluation of the cost of saving water by
evaporation reduction.
This cost is computed by dividing the sum total of the operational expenses
incurred during the test period by the amount of water saved. It would not be
justified, at this early stage of the art, to include overhead costs or development
costs of any kind.
63
.
Evaporation reduction
/
In ali the evaporation reduction experiments conducted by the United States
Bureau of Reclamation very careful accounts were kept of the expenses involved.
At Lake Hefner records were maintained on costs of: (a) hexadecanol applied;
(b) petrol, oil and repairs for operation of boats; (c) salaries and wages of operators
and labourers; (d) motor vehicle operation; (e) rental of barge; (f) equipment depreciation; (g) miscellaneous expenses. ,
Of the total cost of $27,500, the cost of hexadecanol amounted to 73.5 per cent,
while the cost of labour w a s 15 per cent. T h e cost per acre-foot saved for the entire
period of the test was computed at $61 (equivalent to $0.05 per CU. m.).
At Sahuaro Lake, where n e w techniques were tried, similar cost accounting was
kept (Teter and Florey, 1961). Of the total cost of the treatments of $7,400 about
52 per cent w a s cost of materials and 32 per cent labour. T h e cost per acre-foot
saved w a s $120 but it w a s estimated that on an operational basis this would be
reduced to $69 per acre-foot.
A t Lake C a c h u m a the s a m e procedure w a s employed (Hamburg; 1962). Of the
total cost of $20,800 about 62 per cent was the cost of material, and 16 per cent
labour. T h e average material used per acre-day amounted to 0.37 pound, compared
to 0.29 pound used at the Sahuaro Lake test. This difference is ascribed to different
method of treatment and to the stronger winds prevailing during the Lake C a c h u m a
tests.
T h e cost per acre-day w a s $0.13 at Lake Cachuma, compared to $0.17 at Lake
Sahuaro and $0.14 at Lake Hefner. The cost of water saved for the entire test
period at Lake C a c h u m a w a s about $68 per acre-foot (equivalent to $0.055 per
CU. m.).
,
It should be borne in mind that the above figures for the cost of water saved were
derived on the basis of evaporation savings computed by combined energy budget
and mass transfer method. If these evaporation savings were too low, as indeed
they very well might have been in the case of the two mountain lakes, in view of
the high coverage rate achieved, the corresponding cost of water saved could go
d o w n considerably.
T h e cost of saving water is governed largely by the cost of materials. This is bound
to go d o w n with increasing use. A t Sahuaro Lake the fatty alcohol used was
purchased at $0.25 per pound. T h e cost of the material used at Lake C a c h u m a was
less than $0.22 per pound.
It seems reasonable to assume on the basis of the foregoing American experiments
(Franzini, 1961) that it is entirely possible to save water by monolayer process at
a cost ranging from $20 to $35 per acre-foot (equivalent to $0.016-0.028 per CU. m.)
at selected reservoirs in areas of moderate wind and high evaporation. These
conclusions seem to have been confirmed by the Spanish tests in 1957 and 1958
where the cost of material only was equivalent to $0.33 and $0.018 per cu.m
respectively.
However, other estimates of the cost of water saved have been obtained from the
Australian experiments and these give m u c h reduced costs. A t Stephen’s Creek
Reservoir (Sutherland, 1957) the estimated cost was equivalent to $0.002 per cu.m.
Again, the s a m e estimate was given in connexion with Umberumberka and Corella
Lake studies (Vines, 1962). More recent estimates (Fitzgerald and Vines, 1963),
however, put the cost of material only at about $0.007 per cu.m. of saved water.
It is not easy to reconcile the American and the Australian figures. Obviously,
they are based on entirely different dosages deemed necessary for maintaining the
monolayer on the water surface. These dosages depend largely on wind conditions;
but wind conditions alone, as they are known, cannot explain the difference in dosage
ratio of 1 0 to 1, or more. Other factors such as bacterial content m a y contribute,
but the full explanation is still lacking.
64
i-
Evaporimeter experiments and jîeld irials
It should be stressed that the figures quoted above relate to large reservoirs only.
For small reservoirs such as stock tanks of 1 acre or less in surface area, the amount
of retardant material needed per unit area to obtain a reduction in evaporation is
m u c h greater than that needed for a larger reservoir because of the shorter travel
distance of the film over the small area. Consequently, the cost of water saved might
well be higher by one or even two orders of magnitude. Meinke and Waldrip (1964)
give costs of $1.02 to $2.45 per 1,000 gallons (equivalent to $0.29 to $0.70 per CU. m.)
of water saved from a one-sixth acre pond with a 100 feet of shoreline normal to the
prevailing wind. Koberg, Cruse and Shrewsbury (1963) simply state that the feasibility of applying monolayers to small stock tanks to reduce evaporation is very
questionable at this time.
.
10. Reduction of evaporation from soil and transpiration
from plants by means of fatty alcohols
T h e possibility of applying long-chain alcohols like hexadecanol and octadecanol to
reduce water losses from soil and plants received little attention until very recently.
Roberts (1961)suggested that hexadecanol might form a film on the evaporating
water surfaces within a plant or soil similar to the monomolecular film on the open
water surface, and thus reduce evapotranspiration. H e reported on preliminary
experiments in which hybrid corn grown in soil enriched by fatty alcohols had
required up to 40 per cent less water during its growth than control plants. However,
subsequent experiments by Olsen, Watanabe, Kemper, and Clark (1962) and Woolley
(1962) failed to show any effect of fatty alcohols on transpiration.
. The effect of hexadecanol on the evaporation of water from soil appears to depend
on the nature of the soil. Woolley (1962)has found that water evaporation from sand
was reduced by 33 per cent for 2 mm. sand, and by 18 per cent for 0.2 mm. sand,
while there was no reduction at all for clay or loam soil. Similar results (26.5 per cent
for sand, 4.1 per cent for loam) were obtained by Atsatt (1963)and Mistry and Bloodworth (1963). This is in contrast with results reported by Mallik (1962) and more
recently by Olsen, Watanabe, Clark, and K e m p e r (1964). Mallik found hexadecanol
to be effective in reducing evaporation from Poona black soil to the extent of 30
per cent, and Olsen et al. reported a 43 per cent decrease of water loss in a ten-day
period from a Weld loam. According to Olsen et al., the mechanism of evaporation
suppression from soil differs from that observed on a free-water surface. In soil,
hexadecanol allows the surface layer to dry and creates a diffusion barrier to water
loss by vapour transfer.
Another mechanism by which hexadecanol m a y reduce evaporation of water
from soil is that of changing the soil properties that influence the capillary rise of
water. This effect was noted by Lemon (1956).
A method of reducing transpiration without any ill effects on the growth of a
wide variety of plants using alkoxy ethanols was reported from Japan (Mihara,
1962; Mihara and Hagivara, 1960). In these experiments plants were sprayed with
a dilute emulsion of OED-70 (see page 39) which, on evaporation, formed a multimolecular layer of the alkoxy ethanol on the plants.
Obviously the available results are insufficient to draw definite conclusions on
the feasibility of reducing evapotranspiration by means of fatty alcohols or their
derivatives. Further experiments are needed. If successful, this field could offer
more promise than the reduction of evaporation from open water surfaces, chiefly
because of the absence of the wind effect.
65
,
.
Evaporation reduction
11. Evaporation reduction by means other
>
than monolayers
,
For the last ten years the use of monomolecular filmsto reduce evaporation from
reservoirs has focused attention to the detriment of other methods. This is due
to the fact that the monolayer technique is the only one which does not call for
' extensive installations or heavy investment in structures. Moreover, it does not
interfere with recreational uses of water bodies or with fish and wildlife.
However, other approaches to reduce reservoir evaporation losses have also
been studied (e.g., Garstka, 1962b; Bureau of Reclamation, 1963ó):
1. Locating reservoir at highest elevation possible. There is increasing evidence that
higher altitude reservoirs lose less water per unit surface than do reservoirs in
lower altitude.
2. Shaping the reservoirfor the lowest arealuolume ratio. This includes both the choosing
of a site with steep banks and diking-off shallow areas.
3. Reservoir regulation. In a reservoir and river system consisting of both high
altitude and low altitude reservoirs, it might be possible to operate the system
in a w a y to present the least exposed surface, for the system as a whole, during
the seasons of high evaporation loss.
4. Covers. These include continuous floating covers such as polyethylene films and
floating covers of microscopic beads. The use of polyethylene films on stock pond
tests has shown that the material becomes inundated by the weight of dirt and
dust deposited on it. T h e apriori disadvantages of using beads are: (a) the evaporation m a y be increased b y the spinning of the beads; (b) they will be subject
to drift by wind.
5. Windbreaks. Windbreaks, to be effective, must present a dense barrier from the
ground level. Vegetable windbreaks will involve evapotranspirational losses
which will have to be subtracted from whatever evaporation saving3 m a y result
by reduction of evaporation loss from the reservoir. T h e effectiveness of nonvegetable windbreaks on small reservoirs w a s studied by Crow (1963). H e found
that with the barrier spacing-to-height ratio of 16 :1, the evaporation was
reduced by 9 per cent when the average wind speed was 10 m.p.h.
6. Air-bubbling. For deep reservoirs where a considerable proportion of the water
in storage remains cold throughout the year the air-bubbling technique might
be practical. T h e technique consists essentially in bubbling air from the bottom
of the reservoir to artificially mix the water and break u p the stratification. In
this w a y colder water rises to the surface and the evaporation thereby is reduced.1
Recently a more detailed study of the technique was carried out in 1961 and 1962
on Lake Wohlford, California (Koberg, in press). During the test period of 1962
the contents of the reservoir averaged approximately 2,500 acre-feet with a
surface area of 130 acres. The elimination of thermal stratification during May,
June and July reduced the evaporation by 15 per cent. Although the evaporation
was increased by 9 per cent in September, October, and November, the net reduction was about 6 per cent. This reduction, however, might well be non-significant
(see Section 7).
1. The bubbling technique
66
wni
the subject of several preliminary itudiei in the field.
i
C O N C L U S 1-0N S
I
In the light of the results of the experiments reviewed in the present survey, there is
no longer any doubt that it is possible to cover the surface of a reservoir with a
fully compressed monomolecular layer of fatty alcohol series of compounds which
act as an effective evaporation retardant. Such a retardant will have no adverse
effecton the quality of water in the reservoir and will not disturb its utilization for
recreational and other purposes. Even more powerful retardants of the family of
!
alkoxy ethanols have more recently been synthesized and used successfully for agricultural purposes; they have yet to be tried on large reservoirs.
T o maintain an effective layer of the retardant on the water surface for extended
periods of time is still a major problem. W i n d is deleterious and is chiefly responsible
for the difficulties experienced. New substances, with higher capability of re-forming
a monolayer from crumpled state than the compounds used at present, would be
very helpful. But other factors than wind m a y also be involved.
Different techniques for applying monolayers to the water surface have been
developed. The suitability of these techniques for spreading and maintaining a
monolayer under different conditions has yet to be firmly established.
'
Several methods of the evaluation of evaporation savings have been developed
but no fully satisfactory technique has as yet been evolved. Both improved techniques and new ideas in this field are needed.
T h e analysis of the cost of water saved has shown that in some areas of the world
there is already a firm economic basis for water saving by the monolayer method; '
elsewhere some improvement in technique is still needed to m a k e the method
commercially worth while.
67
B I B L I O G R A P H Y
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