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ARID ZONE RESEARCH - XXVII EVAPORATION REDUCTION

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Titles in this series:

I.

II.

III.

IV. V.

VI. VII.

VIII.

IX.

x. XI.

XII.

XIII.

XIV.

xv. -.' XVI.

XVII. XVIII. XIX. xx.

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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 Directory of institutions engaged in arid zone research Utilization of saline water. Reviews of research Plant ecology. Proceedings of the Montpellier Symposium colloque de Montpellier Plant ecology. Reviews of research 1 8cologie végétale. Compte rendu de recherches Wind and solar energy. Proceedings of the New Delhi Symposium I Energie solaire et éolienne. Actes du colloque de New Delhi I Energía solar y eólica. Actas del coloquio celebrado en Nueva Delhi Human and animal ecology. Reviews of research 1 Ecologie humaine et animale. Compte rendu de recherches Guide book to research data on arid zone development Climatology. Reviews of research Climatology and microclimatology. Proceedings of the Canberra Symposium et microclimatologie. Actes du colloque de Canberra Arid zone hydrology. Recent developments Medicinal plants of the arid zones 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 Plant-water relationships in arid and semi-arid conditions. Reviews of research ' 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 The problems of the arid zone. Proceedings of the Paris symposium Nomades et nomadisme au Sahara (in French only) Changes of climate. Proceedings of the Rome symposium organized by Unesco and WMO 1 Les changements de climat. Actes du colloque de Rome organisé par 1'Llnesco et l'OMM Bioclimatic map of the Mediterranean zone. Explanatory notes Environmental physiology and psychology in arid conditions. Reviews of research Agricultural planning and village community in Israel Environmental physiology and psychology in arid conditions. Proceedings of the Lucknow symposium 1 Physiologie et psychologie en milieu aride. Actes du colloque de Lucknow Methodology of plant eco-physiology. Proceedings of the Montpellier symposium 1 Métho- dologie de I'éco physiologie végétale. Actes du colloque de Montpellier Land use in semi-arid Mediterranean climates Evaporation reduction

Ecologie végétale. Actes du

Climatologie

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XXVIII. A geography of coastal deserts (in preparation) J

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. FRENKIEL 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

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0 Unescn 1965 Printed in France NS. 6.l/III.32/A

F O R E W O R D

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 U support for certain institutions in the arid regions. Though 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 was 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 . . . . . . . . . . . . . . .

Organizational framework of evaporation control research . . . .

CHAPTER I . Physical and chemical principles of evaporation control : laboratory experiments . . . . . . . . . . . . . . . .

2 Evaporation resistance of monolayers

1 Historical review . . . . . . . . . . . . .

. . . . . . . . 1 Film pressure . . . . . . . . . . . . .

Temperature . . . . . . . . . . . . . Wind velocity . . . . . . . . . . . . . . Purity of the. material . . . . . . . . . . .

3 Spreading properties . . . . Spreading rates . . . . Spreading velocities . . . Effect of chemical composition . Effect of polymorphism . . Effect of physical characteristics Effect of wind . . . . Equilibrium spreading pressure . Collapse pressure . . . . Influence of losses by evaporation

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CHAPTER II . Evaporimeter experiments and field trials . . . . .

1 Evaporimeter experiments . . . . . . . . . . . Australian experiments . . . . . . . . . . . United States experiments . . . . . . . . . . Indian experiments . . . . . . . . . . . . Experiments in the U.S.S.R. . . . . . . . . . Experiments in Israel . . . . . . . . . . . Experiments in Japan . . . . . . . . . . . Experiments in other countries . . . . . . . . . .

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11

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17 18 18 21 21

22 22 23 24 24 25 26 26 27 27

29

29 29 29 31 31 32 32 32

2 Field trials: general review . . . Early Australian experiments . . East African experiments . . . . United States small-scale experimenta United States large-scale experiments Recent Australian experiments . . Indian experiments . . . . . Other field experiments . . .

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. . . . . . . 37 3 Field trials: materials . . . . . . . . . . . . . 37

Fatty alcohols . . . . . . . . . . . . . 37 Mono-oxyethylene ethers . . . . . . . . . . 39

4 Field trials: methods of application . . . . . . . . 39 Beads in raft method . . . . . . . . . . . 39 Solvent application . . . . . . . . . . . 40 Powder application . . . . . . . . . . . 41

43 Hot spray application . . . . . . . . . . . 44 Aerial application . . . . . . . . . . . . 46

48 6 Field trials: effect of wind . . . . . . . . . . 49 7 Evaluation of evaporation savings . . . . . . . . . 55

Pan coefficient method . . . . . . . . . . . 56 Combined energy budget and mass transfer method . . . . 57 Simplified method . . . . . . . . . . . . 60

8 Biological aspects . . . . . . . . . . . . 61 Physical and chemical factors . . . . . . . . . 61 Toxicity . . . . . . . . . . . . . . . 62 Surface tension reduction . . . . . . . . . . . 62 Changes in the plant and animal communities . . . . . 62

9 Economic evaluations . . . . . . . . . . . 63

Application as an emulsion . . . . . . . . . .

5 Field trials: detection and evaluation of film coverage . . . .

10 Reduction of evaporation from soil and transpiration from plants by means of fatty alcohols . . . . . . . . . . . 65

11 Evaporation reduction by means other than monolayers . . . .I 66

Conclusions . . . . . . . . . . . . . . . 67

Bibliography . . . . . . . . . . . . . . 69 I

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I N T - H O D U C T I O N .

The 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 demand for water all over the world. To 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 new methods to conserve existing water supplies. In any region where water supply from natural resources is not uniformly abundant,

its maximum 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 new is the realization that evaporation losses need not necessarily be 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 pheno-

menon, but it was not until 1952, in Australia, that the first attempts were made to use this principle to reduce evaporation of water under natural conditions (Mans- field, 1953). In the wake of the initial Australian experiments, which aroused world- wide attention, other investigations have followed in many 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 (South- west 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 Dominy (1962), Michel (1963a, 19636) and Bunker (1963). All these manifestations of research activity point to the very considerable interest

that has been focused lately on the problems of evaporation control both from the 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 many 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

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Organizational framework of evaporation control research

The 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 some properties of monolayers bearing

on their water conservation ability has been carried out at the Department of Physical Chemistry of the University of Sydney.

In 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, an agency of the Department of the Interior, began

its extensive laboratory and field investigations in 1955. Much 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.

The 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. The 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.

In Great Britain, Price’s (Bromborough) Ltd., in co-operation with other firms, has been active in laboratory and field studies in an attempt to learn more about the potentiality of the filming technique. Some biological aspects of evaporation control has been studied by the Water Pollution Research Laboratory of the Department of Scientific and Industrial Research.

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In East Africa, field experiments in evaporation control were conducted in the years 1954-59 by the East African Meteorological Department in co-operation with 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 on 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 Academy 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 on natural bodies of water were made by the Hydrological Research Division of the Depart- ment of Water Affairs. In Canada, evaporation pan and field tests have been carried out by the Leth-

bridge (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

laboratory experiments ~ evaporation control :

1. Historical review

It has long been known 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, Abbe (1914) reports on the work of Onofrio, who 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 carboxyl- COOH) 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). The 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

the first time to observe the reduction of evaporation caused by monolayers. His , apparatus consisted of an inverted U-tube, one arm of which contained water at

13

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room temperature upon which the monolayer was formed, the other arm being cooled in an ice bath. The system was evacuated. By comparing the rates of conden- sation in the cold arm 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 much 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. The 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 monolayer- forming substances were made by Baranaev (1937), Sklyarenko and Baranaev (1938), Glazov (1938), Kheinman (1940), Docking, Heyman, 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. They 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 by 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 by 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 by 60 per cent. Thesameauthors contrasted the mechanism of evaporation reduction by an 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 was given to the use of multimolecular films of oil mixtures for evaporation control.

Heymann 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, new instrument which measure# evaporation wntinuouuly hai been deicribed by Walker (1963).

14

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 minimum value. Gilby and Heymann (1948) made 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 more 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; Heymann 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 new 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 La 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 more effectively. This conclusion has been verified recently in respect to other compounds by means of another technique of measuring evaporation reduction, that of measuring the reduction of surface temperatures (Jarvis, Timmons, and Zisman, 1962). Further laboratory

film pressure, on the evaporation resistance of monolayers has been reported by La Mer and his co-workers (La Mer and Robbins, 1958; Robbins and La Mer, 1959; La Mer and Barnes, 1959; Barnes and La 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 most 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

work on the influence of the spreading technique, the purity of the material, and the -

15


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 x 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-sealing limited the types of compounds likely to form useful monolayers to those forming liquid films on natural water at normal temperature. The only such compounds known 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. The 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 maximum of 35 per cent, under hot, arid conditions (water temperatures 260 C. and more).

The 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 Durham (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 (Durham 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 maximum value after which a constant efficiency was reached. The 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. The influence of dosage on 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, on 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 by their progressively higher melting points which reduce their ability to spread on 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.

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7

1. Or dtoxy-cthanols.

16

Physical and chemicai principles

2. Evaporation resistance of monolayers

The evaporation resistance of monolayers on a water surface has been found to depend on many factors, among 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 thefie different factors will be reviewed.

FILM PRESSURE

.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. The 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; La Mer and Barnes, 1959, Barnes and L a Mer, 1962~; La Mer, Aylmore and Healy, 1963). The 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). The 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

U u

.- L .-

2 0 . o 10 20 30 40

' Surface pressure (dynes/crn.) '

FIG. 1. Evaporation resistance as a function of surface pressure of octadecanol and hexa- decano1 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 by succensive purifications.

17

Evaporaiion reduction

4

3

.- E , 'i

9:l

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 La Mer (1962a).

The 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 Mer (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 same 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). The evaporation resistances of long-chain alcohols containing up 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 same pressure. At high film pressures, however, the position is reversed and the glycol ethers exhibit higher evaporation resistances.

TEMPERATURE ,

From their studies on monolayers of fatty acids, Archer and La Mer (1955) concluded that the logarithm of the specific resistance of a given monolayer could be repre- sented 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


20

15

10

r: 5 - > SI - 5 U C O

m m

U

U u

e .- e .- - .- 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 hexa- decano1 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 ,500 C. 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.

Ramdas 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 made with two glass petri dishes exposed to a steady horizontal wind of 2 m.p.h., one of which served as a control. The 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

'


and Shukla, Deo, Sanjana and Kulkarni (1962) as well as Shukla, Kulkarni, Ghar- purey 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. Some 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, The efficiency of the monolayer increased with decreasing temperature for all the alkoxy-ethanol films except for C,,-OC2H4OH in which case the reduction 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). The anomaly is probably due to spreading difficulties at lower temperatures. The 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 -greater relative ease of spreading. This result, too, is confirmed by Mihara and Naka- mura'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). The flattening of the curve

1 O0

80

,

60

- 6p

z 40

Y

ó g n O o s c .- .- + U a -0

Bi 20 15 20 Temperature (OC.)

30 40 50

FIG. 4. Evaporation reduction 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 com- 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 was 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 by the wave rather than being collapsed into islands of ‘duplex’, film.

2

I W I N D VELOCITY

The influence of wind velocity on the evaporation reduction by monolayers was considered by Mansfield (19586). His 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. The 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 La Mer (19625) and La Mer and Aylmore (1962). Their conclusion is that these

90

- 80

70 s C .- c

60 g >

+ C,,OCH,CH,OH

e C,,OCH,CH,OH

.-E- 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 .


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 some extent secondary alcohols, Trapeznikov and Ogarev (1963) investigated the ability to reduce evaporation of mixtures of normal hexa- decano1 and secondary hexadecanol CH,-(CH,),-CHOH-(CH,),-CH,. They found that the evaporation resistance of mixtures of up to 1 : 1 proportions of the two 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 an alkoxy ethanol (CI8H,,OC,H,OH) and secondary alcohols of C,,-C,, chain length.

-3. Spreading properties

The ability to suppress evaporation, essential though it is, is not, by 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, new 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 on chemical composition but also on polymorphic structure and physical characteristics.

In this section laboratory work on spreading properties such as spreading rates, spreading velocities, equilibrium spreading pressure and collapse pressure, and factors influencing them will be reviewed.

SPREADING RATES

Several spreading rate measurements have been made 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 of 2.4 x lo1, molecules dyne-l sec.-l. Roylance and Jones (1959), on the other hand, have found the rate of spreading from the periphery of a single floating crystal to be about 0.2 x 10la molecules cm;l sec.-l, equivalent to a spreading coefficient

22


of 5.1 x 10'0 molecules dyne-1 sec;' or some 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 made 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.). Inci- dentally, 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) who 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, up to the equilibrium spreading pressure. This result applied for a spreading both from the edge of a crystal of hexadecanol and from powdered hexadecanol.

'

I

SPREADING 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. up to 12 cm./sec. at a temperature of 24 & lo C.

McArthur and Durham (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

Evaporation reduction. ./ Ø

Ì EFFECT OF CHEMICAL COMPOSITION

The 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. At the United States Bureau of Reclamation Laboratory the spreading rates of

monolayers of some 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. The 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 Academy of Sciences of the U.S.S.R. (Trapeznikov and Ogarev, 1961; Petrov, Trapeznikov, Nikishin and Ogarev, 1961) and at the Weizmann Institute of Science, Rehovoth, Israel (Miller and Bavly-Luz, , 1962). The latter measurements have shown that addition of smali quantities of liquid paraffin oil improves the spreading properties. The 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-chain alcohols and alkoxy ethanols (glycolmonoalkyl ethers) were reported from India by Deo, Kulkarni, Gharpurey and Biswas (1962a, 19626). The 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. The reported data showed that: 1. The 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. The 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 x 1014 molecules em.-' sec.-l at 250 C.- as against 2.2 x 1013 molecules cm;l sec.-' for hexadecanol) explains the excellent results obtained with Cls- and C2,-OC2H4OH mixtures in Japan (Mihara and Nakamura, 1961).

-

4 -

EFFECT OF POLYMORPHISM

'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. The 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. The ß phase is obtained directly on crystallization from a solvent. Recently, an 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).

I

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 two 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.

EFFECT OF PHYSICAL CHARACTERISTICS

McArthur and Durham (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) made 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. i,

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 much more slowly than from the crystal face or from the surface of casting. (See figures for spreading rates given on pages 22-23.)

’ 25


EFFECT OF .WIND

The effect of wind on spreading was studied in the laboratory by McArthur and Durham (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 up in the wind direction up 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 Van 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 wavy surface, as obtained with clean water, this ratio has a constant value of about 0.03 in agreement with Keulegan’s and Van 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, up 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 -SPREADING PRESSURE

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 known 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 maximum film pressure in the temperature range of 22-330 C. The maximum 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


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 long-

chain alcohols and alkoxy ethanols has been made by Deo, Kulkarni, Gharpurey and Biswas (1962a, 19626). They 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 much higher for pure alkoxy ethanols. Study of mixtures of an 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 PRESSURE

McArthur and Durham (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 long-

chain 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. The collapsed material always spread much faster than the stable crystal, and aged only slowly to the stable crystal. Spreading pressure of the collapsed material depended on the rates at which it had

1 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.

INFLUENCE OF LOSSES BY EVAPORATION A N D SOLUTION

Roylance and Jones (1959) drew attention to the fact that the losses of film by 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


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. They experimentally confirmed Mansfield’s conclusions that the losses from saturated alcohols were primarily due to evaporation. The rate of loss was found to increase steeply with the temperature. For instance, the data on fractional loss from hexa-

’ decano1 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. The results were in agreement with those of Brooks and Alexander. The nature of loss was 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. The effect of film loss was shown to reduce the steady-state spreading pressure

of a hexadecanol crystal to a value generally below that of the thermodynamic equilibrium spreading pressure. The steady-state spreading pressure is attained when 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-state spreading pressure is a function of the perimeter/area ratio. Roylance and Jones have shown that at 250 C. (rate of loss, 140 per cent of the monolayer in 24 hours) steady-state pressures exceeding 30 dynes cmr1 were attained only with a perimeterlarea ratio greater than about 2 x cm.-l, and only for a ratio much larger than 2 x 10-1 cm.-l could the equilibrium spreading pressure (of about 40 dynes cm.-l) be reached.

’

‘

28

,

f-

C H A - P T E R I I

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, among them 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 two contrasting opinions of the value of such tests, both taken from the book Retardation of Evaporation by Monolayers (La Mer, 1962). i

Professor L a Mer writes in the preface (p. XI): ‘So-called screening 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 know 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:

‘The 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.’

A USTR ALIA N EXPERIMENTS

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 the l/A ratio from 0.8 x 10-3 cm;l to 2 x

’

cm.-l 7

UNITED STATES EXPERIMENTS

Experiments at the Southwest Research Institute. Evaporimeter tests at the South- west Research Institute of San 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.

The 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).

tests with Class A evaporation pans were carried out. These were not successful, probably because of dust which interfered with the film. During the summer 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. The 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. The decrease in effectiveness with increasing water temperature does not follow the same relation for each material.

2. The presence of octadeeanol tends to deetease the influence of temperature on evaporation reduction.

3. There is no clear relation between composition of long-chain fatty alcohol constituents and evaporation reduction.

4. The addition of an ethylene oxide as a spreading agent does not appear to impair significantly the evaporation-reducing ability of the monolayer.

T o determine the effect of temperature on the performance of the film, indoor ’

Experiments 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 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


Experiments at Stanford University, Stanford, California. In these tests-which were conducted in Class A pans-the effect of dosage and the mode 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 and’ 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-environment chamber (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 EXPERIMENTS

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. The 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-glycol ethers) were found to be superior to those of corresponding alcohols as regards their resistance to evaporation as well as durability. The alcohol film deteriorated rapidly within two to three days, whereas the alkoxy ethanol films generally retained their efficiency consistently for about a week or more. Alcohol-alkoxy ethanol 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 night- time (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).

’

EXPERIMENTS IN THE U.S.S.R.

Following laboratory tests with twelve kinds of fatty alcohols at the Institute of Physical Chemistry of the Academy of Sciences of the U.C.S.R. at Moscow, six

31


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 mean daily air temperature varied from 110 C. to 240 C., and the mean daily wind speed from 0.5 to 6.5 m./sec. (the maximum 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-hour periods the evaporation from evaporimeters covered with these films was reduced by as much 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).

’ EXPERIMENTS 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.

- EXPERIMENTS IN JAPAN

Extensive evaporimeter experiments with the recently synthesized OED compounds have been carried out at the National Institute of Agricultural Sciences, Tokyo (Mihara, 1962).

EXPERIMENTS IN OTHER 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:

2. The mixture of hexadecanol and octadecanol in equal parts is as good as if not

3. Daily or continuous dosing is necessary. 4. The maximum reduction of evaporation that was found under the best conditions

In Canada evaporation control tests, using various physical forms of cetyl alcohol, have been conducted by the Research Station, Canada Department of Agriculture,

, 1. The fatty alcohols should be applied as a very finely divided powder or in solution.

superior to pure cetyl alcohol.

was 50 per cent.

32

Evaporimeter experiments and field trials <

Lethbridge, Alberta. The tests which began in the summer 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 much work has been done in many different countries. This work will be described below under five headings. The 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.

EARLY AUSTRALIAN EXPERIMENTS

The 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 on small water bodies in Woomelang, Australia. The tests indicated the need for further investigations of the technique of film application. During the summer of 1954-55 in southern Australia and during the dry season in northern Australia, a number of investigations were conducted on 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-covered rafts anchored in the reservoir; later, however, beaded material was used. The treated reservoirs varied in size from a fraction of an acre to 22 acres and the periods of treatment from 3 to 11 weeks. The 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, 1955 b). 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 maximum depth 16 ft. and m a x i m um area 2,100 acres. The 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 summer 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. AU figurei of evaporation reduction are quoted from the original papera: see Section 7.

33


EAST AFRICAN EXPERIMENTS

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 mean depth of water of 7 ft. A n evaporation reduction of 11.5 per cent over 10 days was reported. r- 1

UNITED STATES SMALL-SCALE EXPERIMENTS

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 summer 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. The maximum 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). The tests extended over the summer of 1957 and 1958, and showed evaporation savings of 43 and 22 per cent respectively.

2. Tests on two 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 maximum 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. by 100 ft. (0.17 acre) at Texas Experi- mental 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 summer 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.

UNITED STATES LARGE-SCALE EXPERIMENTS

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). The 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 inves- tiga tions’.

Field studies of monolayer application and performance began in April 1957 with investigations of various techniques of application at Rattlesnake Reservoir, a 100-acre lake in Colorado, and continued in August and September 1957 at Ralston Creek Reservoir, a 150-acre reservoir 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. The technique was tested in June 1958 at Carter Lake, a 1,000-acre lake some fifty miles north of Denver, Colorado (Timblin, Florey and Garstka, 1959). The 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 an evaporation-reducing monolayer 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 autumn 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). The Boulder Basin of.Lake Mead has a surface area of.40,OOO acres. It was chosen for the study because of its known offshore-onshore wind patterns. The 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 was 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


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. An evaporation reduction study at Pactola Reservoir which was also scheduled

for summer 1962 was only partially camed out that year; it was resumed and completed in summer 1963. N o report on this study was available at the time of writing (July 1964).

i

RECE NT 4A US TRA LIA N E XP E RI M E NTS

The Australian field trials were resumed in 1959 by using a new 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 200-250 acre Umberumberka Reservoir, Broken Hill, New South Wales, during the period from March 1959 to February 1960; and (b) at the 500-600 acre 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 much 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 made 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 on account of higher wind velocities. -Treatment at Umberum- 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).

I

INDIAN EXPERIMENTS

In India the preliminary field trials began with three relatively small reservoirs (Ramdas, 1962). These are: (a) the 170-acre Badkhal Tank at Faridabab, Punjab, near Delhi; (b) the 28-acre Buderi Tank at Poondi near Madras; and (c) the 165-acre Kukrahally Tank 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 Tank and a 60-acre


Kasurdi Tank, both near Poona (Kulkarni and Parashave, 1962). The first Iarge- scale field trial has been in progress since 1961 at the 1,500-acre Walwhan 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 summer 1963 evaporation reduction tests were scheduled at the 470-acre Gorwala Tank near Nagpur. N o report on these tests was available at the time of writing.

I

{ OTHER FIELD EXPERIMENTS

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, two field trials were carried out in 1957 and 1958 on a 30-acre reservoir

in the south of the country. The 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 reduc-

tion tests, a field study was made of different methods of application of the monolayer and of the movement of film under the wind. The study was carried out in 1960 by Price’s (Bromborough) Ltd. on the 900-acre eastern section of Loch Laggan in Scotland (McArthur, 1962a, 19626). In Japan, extensive field tests have been carried out on 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

FATTY ALCOIIOLS

In the early Australian experiments by Mansfield, cetyl alcohol of high hexadecanol ’ content (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 Organi- zation for use in small reservoirs of up to 2-acre surface (CSIRO, 1956). The detailed specification named ‘ Si-ro-seal’ was as follows:

Cornpodion SpeiJícalion

% Hexadecanol min. 80 Iodine value Octadecanol max. 10 Acid number Cl* and Cl, alcohols max. 5 Saponification value Lower alcohols max. 0.5 Hydroxyl value

, Unsaturated alcohols . max. 4 Melting point

max. 3 max. 0.3 max., 0.5 225-230

45-500 C. , 37


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. 95-1600 C. containing 7 per cent of ethyl alcohol. In the later Australian tests employing cetyl alcohol powder (see S2ction 4) use

was made of a commercial product with the following properties:

Composition Specijicafion

% - ,-I Hexadecanol 45 Iodine value max. 4

Octadecanol 40 Acid number max. 0.1 - Tetradecanol i0 Saponification value max. 0.5 Unsaturated alcohols 5 Hydroxyl value 2 2 O - 2 2 5

Melting point 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. The alcohol was applied in solution in kerosene, 30 grammes of alcohol to 1 litre of kerosene, with 0.9 gramme 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. The evaporation reduction with hexadecanol was found to be twice that obtained with octadecanol at the same 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 Cachuma test the fatty alcohol used was a tallow-based hexadecanol and

octadecanol material specifically manufactured for evaporation control. The chemical analysis of the material was as follows (in percentages): octadecanol, 66.2; hexadecanol, 30.4; tetradecanol, 3.3; dodecanol, 0.1. The chemical arrived at the reservoir in flake and powder form. It was then melted

by electrical heaters, pumped into a boat storage tank and transported to the automatic dispensers located at strategic points on the lake. The total amount of the material applied to during the period between 20 July and 25 September 1961 was 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 com- munication).

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. 76-2120 C.) to form a 4 per cent solution, while in the 1958 test a 3 per cent solution in kerosene was used.

J

MONO-OXYETHYLENE ETHERS 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 was 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, an attempt was made 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 was 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. To 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. The mixture of 55 per cent of the C,, product with 45 per cent of the Cl, product, which was named 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 warm the water in rice nurseries. Its application on large-scale rice fields is hampered by strong winds prevailing in Japan in late spring and early summer (see Section 6). '

1

~

4. Field trials: methods of application

BEADS IN RAFT METHOD

The original mode of monolayer application to water storages, devised by Mans- field (1956) was to store a supply of solid cetyl in an anchored raft fitted with wire gauze sides and floating on the water surface. As the film was generated at the air- solid-water interface 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

r

39

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), who 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

._ SOLVENT APPLICATION

This method-the standard method of spreading a monolayer in the laboratory- was 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 air-

gallon dispensers set up 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.

The 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 some 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 petro-

leum 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 . suffers from 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. The 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 made at Rattlesnake Reservoir by dusting the surface of the lake with powdered hexa- decano1 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 made 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 some definite problems of handling and dispensing. The powder had a great tendency to lump and cake. As a possible solution to this problem a separation with some 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 on the basis of 28-day

periods. For the first few days, 20-30 pounds of powder were applied daily over an 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 25-30 pounds 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


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 new 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. The 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-scale work.

42

Evaporirneïer experiments and field trials

APPLICATION AS, A N EMULSION

The 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 investi-

gations consisted of mixing the powder and water in a tank mounted in the boat. The 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 on the remaining days. Originally the applications were made 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. The applications were made 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 maximum 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 when the film was applied, the average coverage was 16 per cent. A device for the continuous application of powdered hexadecanol as a slurry on

a small scale has been developed by Crow (1961) for his experiments at Oklahoma Agricultural Experiment Station. The slurry is contained in a steel drum 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 medium 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. The 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 way 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), who 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


screen wire baskets. All of these applications gave at the maximum about 20 per cent evaporation savings. In the course of the Indian preliminary field trials at Badkhal, Buderi and Kukra-

hally, 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 supple- ment fixed dispensers a hand-operated spray pump 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 Walwhan 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 make 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 new 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 some advantages for small-sized reservoirs where powder application is evidently not suitable.

i

HOT SPRAY 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 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 made in Australia (Vines, 1960~). It

J 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. The 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 non- automatic application.

44

*

U

c -

FIG. 6. Block diagram: equipment for molten spray technique of monolayer

application. From Bureau of Reclamation (1962).

'

45


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 by 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 made 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 made 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-week test 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 two 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 men were required to service the dispensers

and associated equipment. Later, after the personnel became more familiar with the equipment and procedures, two men could carry on this work in an eight-hour shift. The operation of the dispensers proved satisfactory and only minor malfunc- tions 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 elimi- nated 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


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

10

c YI !! O

u - m e >

$ 0 O 20 Time (minutes)

Average wind speed 4.5 m.p.h.

Dispensing rate 0.6 Ib./min.

40 60 80

FIG. 7. Coverage versus time. From Florey, Hansen and Cleaver (1961).

47


sprays most suited for aerial application appear to be in the 75 to 200 micron mean 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.

The conclusion was that the use of aircraft to apply evaporation retardants appeared to be an effective method for large reservoirs; however no definite prefe- rence for this method could be claimed until further tests were made 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 many 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 moved 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. Even 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, much care has been taken in the Bureau’s field tests to assess this factor with maximum 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 when 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 two 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 much 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 pro-

perty 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 many 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 much 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. The 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 when 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 made 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 film coverage, 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 5-10 m.p.h. the coverage decreased appreciably during the day, and for any adequate coverage almost continuous application of hexadecanol was required. Winds of

49


80 -

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 seem 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, when there was little wind, the dispensers were able to replenish the film blown off the previous day (see Fig. 9).

The 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

~~74.3-3.64~ Coefficient of correlation Fz0.95 Standard deviation 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

O 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).

(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 1-2 in./sec.; this is in fact the spreading velocity of the monolayer, when applied by 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 when 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


- 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.

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. The 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 when 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 momentum 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

I

-


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 mean 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 minimum dosage rate required for continuous application assuming complete conversion of alcohol to monolayer for the conditions of the experiment.

ments 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

;

This semi-empirical formula m a y be compared with the results of actual experi- i

.O025

1 .O015

.o010

’ .O006

t

i .o004 i

A - h

2 .O0025 t C .- c O .- - Q

f .O0015

b R = .O000093 U2*02

1.5 2.5 4 6 10 15 25 Wind speed (U), m.p.h.

FIG. 11. Application rate (in pounds per hour per foot of upwind shore line normal to the wind direction) required versus wind speed. From Crow (1961).

.

53


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 known 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 approximately hold 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 7-8 m.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 film 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 to maintain full cover over an area 1 mile square. From Fitzgerald and Vines (1963).

54

Evaporimeter experimenís and field trials

Wind is the principal reason why the method of water-warming by evaporation reduction, which has been widely used for rice seed beds in Japan since 1960, has not been adopted on a large scale for transplanted rice fields (Mihara, 1962). Even 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 monolayers as evaporation retardants is given by Fitzgerald and Vines (1963). Summing 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 up to 10 m.p.h., evaporation savings of 10-20 per cent though occa-

sionally 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

The 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 do 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. No such possibilities exist for large expanses of water where the seepage is an important and unknown 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 two 100 by 120 by 7 ft. ponds were lined with a buried vinyl plastic membrane, and numerous calibration tests showed seepage to be negligible. At Seymour the two 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. One way to take account of the inherent differences between two 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 two 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


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

,

PAN COEFFICIENT METHOD

The 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. The slope of the line is the pan coefficient, and the intercept is the seepage:

Ea = S + kE, where E, = the apparent evaporation from the reservoir (change of level corrected E, = the evaporation recorded by the evaporimeter;

k = the ‘pan coefficient’.

for inflow and withdrawal);

S = the average seepage loss; I 1

J The 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. The 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 by the pan coefficient.

The assumption of the constancy of pan coefficient over periods as short as a month or less, which is the basis of Mansfield’s method, was 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 paper, however, Mansfield (1959~) conceded that constancy of pan coefficient was 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. To obtain the values of the seepage and the pan coefficient for reservoirs 4-8 m. deep some 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 applied in the early field trials and at the Stephen’s Creek Reservoir at Broken Hill, 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. The 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 coefficient had 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


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 by 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 ‘in the 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). On 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 tcmpera- ture even if the film were not present, as the average air temperature rose during the period under treatment. The 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 was 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 coefficient method to some exceptional experimental sites.

COMBINED ENERGY BUDGET A N D MASS TRANSFER METHOD

In an 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. The energy equation is:

or inflow minus outflow equals change in storage, where:

Qb = short-wave solar radiation incident to the water surface; Q, = reflected solar radiation; Qa = incoming long-wave atmospheric radiation; Qu, = reflected atmospheric radiation;

57


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 down 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. The 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 way 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 much as a third. In this connexion Crow’s (1961) results should be cited. H e performed an expe-

riment 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

This effect of change in heat storage on evaporation was discussed in a recent paper by Wolbeer (1963).

Admittedly, there is still much 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. Alter- natively, such surveys could be made 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:

,,

,

higher evaporation rates when the film is removed. -

(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)’ - (To + 273)‘] + [QIo - Nu(eo - e,)] + Ku(TIo - To) = O 58

/

~ - Evaporimeìer experiments and field trials

where a 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 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 Bowen ratio: Qh/Qc = 0.61 x 10-3P(T0 - Tu)/(e, - eu);

P = atmospheric pressure; Ta = air temperature;

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 Bowen ratio.

= Stefan-Boltzman constant for black-body radiation;

i

, u = wind speed;

The ‘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 two is not simple, different values of To and the corresponding tabular values of eo are assumed until the equation is satisfied. The 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. The results of the first field tests evaluated by this method bore accordingly no 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 on the percentage of evaporation reduction, the main reason being that the actual evaporation could only be determined with an accuracy of about 10 per cent. This uncertainty was also assumed in the case of the Lake Cachuma 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 known exactly. This assumption was probably approximately correct on a yearly basis. On a monthly basis the standard error of N in Lake Hefner was 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. The variability of K w a s of the same order but its influence on the evaporation reduction was, in general, considerably less on account of the usually low values of Bowen’s ratio @,IQc. The 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. The possible exception was Lake Hefner, where, in view of highly accurate measurements, a f 10 per cent range of random errors would be assured. The 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'.

TABLE 1. Comparison of evaporation reductions (from Florey, 1962)

Teit and period

Computed evaporation iavingi

Combined method Simplified method

Lake Hefner (1958) 8 July to 1 October

Sahuaro Lake (1960) 1 October to 17 November 19 October to 17 November

Lake Cachuma (1961) 25 July to 31 July

' 31 July-to 14 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 -

% %

9 & 5 3.4

14 & 5 19 22 f 5. 23 .

9 O 16 22 19 O 23 10 24 11 18 ) 8 19

L ,

~ SIMPLIFÌED 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 empiri- cally 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 three- hour 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. The untreated water surface will usually be aerodynamically rough (Marciano and Harbeck, 1954). The treated surface, on the other hand, if effectively covered by the film, will 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 same in both the treated and the untreated case. This is, of course, not strictly true when the nature of air flow over water surface differs as explained above, and could cause some error as was first pointed out by Mansfield (1962). The 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.

The 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. But 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

The possibility of adverse biological effects following the application of monomolecular films was recognized at an early stage. Thus Mansfield (1955) postulated that, among 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 human 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 CHEMICAL 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 simu- lating those obtaining in water reservoirs. They found that in the absence of wind the monolayer, both on stirred and unstirred surface, caused little or no reduction in the oxygen transfer coefficient in the range of values of importance in water reservoirs. If a jet of air was blown on the centre of the water, the monolayer was found to reduce the oxygen transfer coefficient by some 40 per cent. They concluded that it appeared safe to spread hexadecanol on any water surface which was initially

61

’


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) who 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; Hawke 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 (Committee of Collaborators, 1957; Eaton, 1958; Cruse and Harbeck, 1960). r

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 game fish and the insect population upon which the fish feed also failed to reveal any toxic effects. -

SURFACE TENSION REDUCTION

This is the only large effect of biological significance. Consequently organisms dependent upon support by the surface film at some stage in their life history m ay 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.

CHANGES IN THE PLANT A N D ANIMAL COMMUNITIES - 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


organisms (primarily bacteria, protozoa and fungi) could metabolize hexadecanol in significant amount (0.25 to 3.3 lb./acre/week). They also observed the encrustation of the hexadecanol pellets with a shell of organic matter. The 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 summer 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 summer of 1958 (Cunningham et al, 1959; Silvey, 1960) where an even greater increase in the Aero bacter sp. population was also shown.

Laboratory investigations (Chang, Walton, Woodward and Berger, 1959a, 1959 b) 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 was generally lower in comparison with untreated hexadecanol during the first two days of application; it was higher over longer periods. Although considerations of the potential toxicity to humans 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 was 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 be 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 The 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


/

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 depre- ciation; (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 was 15 per cent. The 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 new 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 was cost of materials and 32 per cent labour. The cost per acre-foot saved was $120 but it was estimated that on an operational basis this would be reduced to $69 per acre-foot.

At Lake Cachuma the same procedure was employed (Hamburg; 1962). Of the total cost of $20,800 about 62 per cent was the cost of material, and 16 per cent labour. The 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 Cachuma tests.

The cost per acre-day was $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 Cachuma was 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 down considerably.

The cost of saving water is governed largely by the cost of materials. This is bound to go down with increasing use. At Sahuaro Lake the fatty alcohol used was purchased at $0.25 per pound. The cost of the material used at Lake Cachuma 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 much reduced costs. At Stephen’s Creek Reservoir (Sutherland, 1957) the estimated cost was equivalent to $0.002 per cu.m. Again, the same 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 10 to 1, or more. Other factors such as bacterial content m a y contribute, but the full explanation is still lacking.

i- 64

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 much 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 from plants by means of fatty

and transpiration alcohols

The 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 Blood- worth (1963). This is in contrast with results reported by Mallik (1962) and more . recently by Olsen, Watanabe, Clark, and Kemper (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


11. Evaporation reduction by means other > than monolayers ,

For the last ten years the use of monomolecular films to 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 reservoir for 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 way 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. The apriori disadvantages of using beads are: (a) the evaporation m a y be increased by 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. The effectiveness of non- vegetable windbreaks on small reservoirs was 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. The technique consists essentially in bubbling air from the bottom of the reservoir to artificially mix the water and break up 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 wni the subject of several preliminary itudiei in the field.

66

i C O N C L U S 1-0 N 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 effect on 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. Wind 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 make the method commercially worth while.

'

'

67

B I B L I O G R A P H Y

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ANDERSON, H. W.; WEST, A. J.; ZIEMER, R. R.; ADAMS, F. R. 1963. Evaporative loss from soil, native vegetation, and snow as affected by hexadecanol. In: General Assembly of Berkeley, p. 7-12. Gentbrugge, Belgium, International Association of Scientific Hydrology. (Publ. no. 62.)

ARCHER, R. J.; LA MER, V. K. 1954. The effect of monolayers on the rate of evaporation of water. Ani. New York Acad. Sci., vol. 58, p. 807-29. -- .- . 1955. The rate of evaporation of water through fatty acid monolayers. J. phys. Chem., vol. 59, no. 3, p. 200-8.

ATSATT, P. R. 1963. Some effects of emulsified hexa-octadecanol on gemination establishment and growth of Kentucky bluegrass. In: General Assembly of Berkeley, p. 49-58. Gentbrugge, Belgium, International Association of Scientific Hydrology. (Publ. no. 62.)

BAPAT, M. N.; CHAKRAVARTEY, K. s. 1962. Evaporation control experiment at Walwhan. Paper presented at the Unesco/CSIR Symposium on Water Evaporation Control, Poona, India.

BARANAEV, M.K. 1937.The effect of surface layers of insoluble substances on the rate of evaporation of water. [In Russian.] Zhurnal Firicheskoi Khimii (Moskva), vol. 9, p. 69-76.

BARNES, G. T.; LA MER, V. K. 1960. Evaporation resistance measurements for investigating the molecular architecture of monomolecular films. 3rd Int. Congr. of Surface Activity,, Cologne, vol. 2, p. 192-5. Mainz. Verlag der Universitätsdmckerei.

and their mixtures. In: La Mer, V. K. (ed.). Retardation of evaporation by monolayers: transport processes, p. 9-33. N e w York and London, Academic Press.

the evaporation of water, In: La Mer, V. K. (ed.). Retardation ofevaporation by monolayers: transport processes, p. 35-9. New York and London, Academic Press.

BAVLY, A.; LEITNER, M.; MILLER, I. R. 1959. Monolayers for reduction of water evaporation. Progress report April 1956-February 1958. Tel Aviv. Weizmann Institute of Science and Tahal. (Tahal P. No. 78.)

BEADLE, B. W. and CRUSE, R. R. 1957. Water conservation through control of evaporation. J. Amer. Water Works Assoc., vol. 49, no. 4, p. 397-403.

BENTON, E. J. 1962. X-ray diffraction studies of mixtures of n-hexadecanol and n-octadecanol. In: La Mer, V. K, (ed.). Retardation of evaporation by monolayers: transport processes, p. 235-44. New York and London, Academic Press. - . 1963. Investigation ofphase transformation of mixtures ofn-hezadecanoi with n-octadecanol and of seven Commercial n-alcohols. Denver, Col., U.S. Bureau of Reclamation, 12 p. (Chem. Eng. Branch. rept. no. Pet. 130.)

-. , -. 1962a. The evaporation resistances of monolayers of long-chain acids and alcohols

-* , -. 1962b. The laboratory investigation and evaluation of monolayers for retarding

69


BERGER B. B. 1958. Use of hexadecanol in reservoir evaporation reduction. J. Amer. Water Works Assoc., vol. 50, no. 7. p. 855-8.

BLANK, M. 1962. The permeability of monolayers to several gases. In: La Mer, V. K. (ed.). Retardation of evaporation by monolayers: transport processes., p. 75-95. New York and London, Academic Press.

BOON, A. G.; DOWNING, A. L. 1957. Observations on the use of cetyl alcohol for conservation of water. J. Inst. Water Eng., vol. 11, no. 5. p. 443-8.

BOSE, E. J. 1962. O n certain experiments pertaining to suppression of evaporation from free- water surfaces by means of monomolecular layers of long-chain compounds under semi-jield conditions. Paper presented at the Unesco/CSIR Symposium on Water Evaporation Control, Poona, India.

BROOKS, J. H.; ALEXANDER, A. E. 1960. Losses by evaporation and solution from monolayers of long-chain aliphatic alcohols. In: 3rd ínt. Congr. of Surface Activity, Cologne, vol. 2. sect. B, p. 196-201. Mainz, Verlag der Universitätsdruckerei.

-; -. 1962. The spreading behaviour and crystalline phases of fatty alcohols. In: La Mer, V.K. (ed.). Retardation of evaporation by monolayers: transport processes. New York and London, Academic Press. Pt. I (Brooks and Alexander): Collapse of the monolayer, p. 245-51, Pt. II (Brooks): Spreading rates, p. 251-4. Pt. III (Brooks): Thermal curves, p. 255-8. Pt. IV (Brooks): Equilibrium spreading pressure, p. 259-69.

BUNKER, R. J. 1963. Selection of material for use in water evaporation reduction by monolayers. Paper presented at the Water Resources Engineering Conference of ASCE, Milwaukee, Wise., 13-17 May, 1963.

BUREAU OF RECLAMATION. 1961. 1960 evaporation reduction studies at Sahuaro Lake, Arizona, and 1959 monolayer behavior studies at Lake Mead, Arizona-Nevada and Sahuaro Lake Arizona. Denver, Col., U.S. Bureau of Reclamation. (Chem. Eng. Lab. rept. no. SI-32.) - . 1962. Water-loss investigations: Lake Cachuma-I961 evaporation reduction investigations. Denver, Col., U.S. Bureau of Reclamation. (Chem. Eng. Lab. rept. no. SI-33.)

-. 1963a. Aerial application technique development and monolayer behaviour study. Elephant Butte reservoir-1962. Denver, Col., U.S. Bureau of Reclamation. (Water Cons. Lab. rep. no. WC-i.)

-. 1963b. Proceedings, Conference of Collaborators, Evaporation Reduction Program, January 30-February I, 1963. Denver, Col., U.S. Bureau of Reclamation.

CARY, A.; RIDEAL, E. K. 1925. The behaviour of crystals and lenses of fats on the surface of water. Proc. Roy. Soc. (London), ser. A, vol. 109. Pt. I: The mechanism and rate of spreading, p. 301-17. Pt. II: The effect of temperature on the equilibrium pressure, p, 318-30. Pt. III: The effect of the polar group on the equilibrium pressure, p. 331-8.

CEANG, S. L.; MCCLANAHAN, M. A.; KABLER, P. W. 1962. Effect of bacterial decomposition of hexadecanol and octadecanol in monolayer films on the suppression of evaporation loss of water. In: La Mer, V. K. (ed.). Retardation ofevaporation ofmonolayers: transport processes, p. 119-31. N e w York and London, Academic Press.

-; WALTON G.; WOODWARD, R. L.; BERGER, B. B. 1959a. Evaluation of hexadecanol for- mulations, containing various disinfecting agents in evaporation reduction. In: Water-loss investigations: Lake Hefner 1958 evaporation reduction investigations, p. 71-83. Denver, Col., U.S. Bureau of Reclamation. -- , -- -- - . 1959b. Effect of disinfecting agents on evaporation reduction witb hexadecanol. J. Amer. Water Works Assoc., vol. 51, no. 11, p. 1421-32.

CHOUDHURY, J. C.; BEATI, O. P. 1962. Control of water evaporation in Rajasthan desert. Paper presented at the Unesco/CSIR Symposium on Water Evaporation Control, Poona, India.

COMMITTEE OF COLUBORATORS. 1957. Water quality study with hexadecanol, Kids Lake, Oklahoma City, Oklahoma. Denver, Col., U.S. Bureau of Reclamation, 18 p. Also published in: Water-loss investigations: Lake Hefner 1958 evaporation reduction investigation. p. 93-106. -

- \

/

Denver, Col., U.S. Bureau of Reclamation.

Denver, Col., U.S. Bureau of Reclamation.

1 - . 1959. Water-loss investigations: Lake Hefner 1958 evaporation reduction investigations. CROW, F. R. 1961. Reducing reservoir evaporation. Agriculiural Eng'g. vol. 42, no. 5, p. 240-3.

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