THE EFFECT OF WIND ON EVAPOIATION SUPPRESSING FILMS

AND METHODS OF MODIFICATION (*)

Franklin R. C R O W (**)

RÉSUMÉ

Quand Oklahoma State University entreprit un programme de recherche pour augmenter l'approvisionnement en eau des fermes par contrôle de l'évaporation nous avons concentré nos efforts sur les méthodes d'application et l'évaluation de produits chimiques capables de former une pellicule monomoléculaire. Il devint bientôt évident que le vent était un sérieux obstacle à tout progrès dans le domaine du contrôle de l'évaporation par pellicule chimique. Ceci aboutit a un projet de recher­che, en collaboration avec le U.S. Bureau of Reclamation, sur les effets du vent dans l'application et le maintien de pellicules monomolécuïaires. Nous avons surtout donné notre attention à de petits réservoirs, semblables à des étangs de ferme. Nous présentons dans ce compte-rendu, le résultat de nos recherches.

Les expériences ont été conduites sur un jeu de deux réservoirs construits dans ce but. Nous avons menénotre étude du problème du ventde trois manières différentes, comprenant l'installation et les essais de : 1) un système d'application de la pellicule, installé sur la rive sous le vent. 2) un réseau de barrières à interstices pour réduire la vitesse du vent près de la surface de l'eau. 3) un réseau d'écrans étanches pour réduire la vitesse du vent et maintenir la pellicule à l'intérieur de petits compartiments.

Nous avons établi des courbes illustrant le ratio de la vitesse du vent par rapport à l'application des pellicules. Nous avons trouvé que le taux d'application de la pelli­cule nécessaire est fonction de la vitesse du vent suivant l'équation : R = 93U2-02

X 10-6 , où R représente le taux d'application du produit chimique en livres par heure et par pied perpendiculaire au vent, et U représente la vitesse du vent en miles par heure.

Dans notre étude de l'efficacité de la barrière à interstices pour réduire la vitesse du vent et les mouvements de la pellicule, nous avons ancré sur la surface de l'eau des barrières de 1,2 pieds de hauteur avec 57 pourcent de vide, disposées en carrés de 14,5 pieds de long. Celles-ci ont réduit le déplacement de la pellicule de 20% par vent de 8 miles par heures, mais était sans effet par vent de 12 mph. L'équation de cette courbe était : R = 2.8 f/2.51 X 10-°. Les résultats de ce test indiquent que la barrière à interstices a peu de valeur pratique, puisque l'application continue de la pellicule est indipensable.

Un réseau d'écrans étanches d'est prouvé beaucoup plus efficace contre le vent. Avec ces écrans arrangés d'une manière semblable aux barrières à interstices décrites plus haut, nous avons trouvé que l'évaporation était réduite de 9%, sans l'aide de pellicule. La vitesse moyenne du vent pendant ces tests était de 10 miles par heure. Les relations linéaire entre le retard d'évaporation et la vitesse du vent montrent une augmentation du retard d'évaporation quand la vitesse du vent augmentait.

Les écrans étanches empêchent efficacement la pellicule de s'échapper, et par conséquent au lieu de la renouveler continuellement nous avons pu reculer le renou­vellement jusqu'à 3 jours. Nous avons trouvé que la surface de chaque compartiment occupé par la pellicule est fonction de la vitesse du vent et du rapport de l'espacement des écrans à leur hauteur (LjH). Les relations entre ces variables ont été développées et illustrées graphiquement. Le rapport de l'espacement des écrans à leur hauteur que nous avons trouvé le plus efficace est, LjH= 16, qui maintenait une couverture de pellicule sur toute la surface par tous vents au-dessous de 10 miles par heure.

Les tests de suppression de l'évaporation ont été conduits avec des pellicules formées par un mélange de Cie Hexadecanol, et de Cis Octodecanol. Le rapport de l'espacement des écrans à leur hauteur a été un facteur important de ces tests. Quand les écrans étaient posés à L/H = 58, l'évaporation était réduite de 11%. Les meilleurs résultats de suppression de l'évaporation ont été obtenus quand la pellicule chimique

(*) Paper prepared for presentation at the International Union of Geodesy and Geophysics Assembly, Berkeley, California, August 19-31, 1963. Paper approved as Oklahoma Agricultural Experiment Station Manuscript No. 887.

(**) The author : Associate Professor, Agricultural Engineering Department, Oklahoma State University, Stillwater, Oklahoma.

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était employée en combinaison avec les écrans étanches, placés à LjH~ 16. La réduc­tion de l'évaporation atteignait 31%. Cette très satisfaisante réduction a été obtenue par vents normaux pour FOklahoma, et avec seulement des remplissages périodiques de la pellicule. C'est pourquoi il est apparent qu'un système d'écrans à vent et de pellicule de ce genre permet beaucoup d'espoir pour le contrôle de l'évaporation dans les petits réservoirs.

In the past decade water conservation research has been intensified. One aspect of this research has been the search for ways to increase water supplies by reducing losses through evaporation from impoundments. Laboratory and field experiments by numerous research organizations in the USA and abroad have demonstrated that evaporation losses can be reduced through the use of long chain fatty alcohols that have the ability to form monomolecular films on the water surface.

In 1956 Oklahoma State University began a long range research program in an attempt to increase farm water supplies by evaporation control. The principal results of the first four years covering the development of application systems and conduct of long duration tests of the effectiveness of chemical films for evaporation suppression on farm reservoirs have been reported previously C1-2»3). In that inves­tigation wind was found to be an important factor, and some preliminary relationships were developed.

In 1961 a further investigation of the effect of wind on monolayers was initiated, aided by a research contract with the Bureau of Reclamation (***) which supplemented the regular support of the Oklahoma Agricultural Experiment Station. The objectives of this research project were to : (1) Determine the effect of wind on the application and maintenance of monolayers on small impoundments and (2) Test the effectiveness of floating wind and film barrier systems for reducing the attrition of monolayers by wind. It is the purpose of this paper to report results of this research.

Previous Investigations

Published literature on evaporation control research contains many references to the adverse effects of wind on monolayers. In field studies at Lake Hefner (4), U. S. Bureau of Reclamation researchers found wind to be the most important single factor in the application and maintenance of the film. Applying hexadecanol as a slurry from a boat they concluded that it appeared to be impractical to attempt to maintain any appreciable coverage when winds exceeded 15 mph. Similar difficulties were experienced in later studies at Lake Cachuma (5) where the monolayer forming material was applied as a fine powder by automatic dispensers. Adverse effects attributable to wind are reported in many other studies.

CROW and SATTLER (2) found an inverse relationship between the portion of reservoir covered by film and wind velocity. Using a special system for applying hexadecanol in water slurry, CROW (S) presented data showing the relationship between wind velocity and film application rate required to maintain a monolayer on an experimental reservoir. Field studies of rate of film movement in the presence of wind have been reported by VINES (6) and MCARTHUR (7). Measuring film movement over a very short distance (20 ft), VINES found the ratio of film velocity to wind velocity to be 0.036. McArthur made trials over much longer distances (2500-6700 ft). He found that the ratio of film velocity to wind velocity was not a constant, but showed a progressive increase from 0.045 to 0.07.

(***) The assistance of the Office of Engineering Research, Oklahoma State University, in the administration of the Bureau of Reclamation Contract No. 14-06-D-4275 is gratefully acknowledged.

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Possible Approaches to the Wind Problem

Since monolayers are so readily transported by wind the reduction or elimination of this adverse effect should be a logical goal of research. Possible approaches to the problem are : (1) Continuously replenish the monolayer at the upwind shore, (2) Reduce the wind speed near the water surface by wind-breaks along the shore or floating on the surface, and (3) Restrict the movement of the monolayer by confinement within a network of floating compartments.

It is generally agreed that some method of continuous application is the only feasible approach to the wind problem on large reservoirs. Regardless of application method, whether by plane, boat, or shorebased dispensers, economics would favor large area reservoirs if the monolayer were applied continuously.

The small farm reservoir covering one to five acres constitutes an important livestock and domestic water supply in the southwestern United States. Evaporation is a serious problem, but due to their small size continuous application of monolayer forming chemicals would be an expensive process. Therefore for impoundments of small area and shallow depth approaches No. 2 and No. 3, namely, reducing the wind speed or restricting film movement may offer an economical means of reducing evaporation losses.

Experimental Facilities

The experimental facilities for this research were designed as a compromise between a purely laboratory approach on one hand and field trials on existing lakes on the other. Two identical and adjacent ponds, 100 x 120 x 7 feet, were constructed on a broad crested ridge subject to wide variation of wind speeds. While there may be some danger in extrapolating the results to much larger lakes especially with regard to length of fetch, the data obtained from these small experimental ponds will apply to most farm reservoirs, and may point to significant trends in larger reser­voirs, where the variables cannot be controlled as well.

An accurate water budget is essential for evaporation control research. This was made possible by the design of the ponds in which seepage was eliminated as variable through the use of buried plastic membranes. There was no surface runoff into the ponds, water being supplied by the University water system. Change in water level during a test was recorded by sensitive stage recorders and laboratory point gages located at nearby still wells.

Wind speed was measured by a standard cup type anemometer located on the center dike between the two ponds at a height 6.5 ft above the mean water surface. Each mile of wind passage was recorded by an event marker on a strip chart recorder, which was also used to continuously record water surface temperature, and wet and dry bulb air temperatures.

Slurry Application System

An apparatus was developed for continuous application of powdered fatty alcohols to the pond surface as a slurry. Automatic controls regulated the rate and point of application of the monolayer forming material in response to wind speed and direction. The principal components of the apparatus were the rate control system (fig. 1) and the distribution system (fig. 2). The slurry was agitated continuously. Discharge from the mixer occurred when the solenoid valve opened in response to a signal from a cup-type anemometer after each 1/10 mile of wind passage. The time interval during which the valve remained open was controlled by an automatic reset time-delay process timer, variable from one to 15 seconds.

The slurry v/as distributed to the pond as shown in Figure 2. Water from the

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RATE CONTROL SYSTEM

C>-<jH?

Anemometer S-'Variable Timer

To distribution pipe

From Pond

•Û Solenoid Valve

• Pump Fig. 1 — Rate control system of experimental apparatus for continuous application

of monolayer forming chemicals. Response to wind is provided by an anemometer which actuates solenoid valve for each 1/10 mile of wind passage.

DISTRIBUTION SYSTEM

T ; X

•Application Pipe (orifices 10 ft. o.c]

"Solenoid Valver

Jy

3 Direction Z) Control / Switch

Distribution Line'

Recirculation Pump

Fig. 2 — Distribution system for delivery of monolayer forming chemical to experi­mental ponds. Up-wind application is obtained by solenoid valves in the distri­bution line which are actuated by wind operated direction control switch.

pond, circulated by an electric pump through a one-inch distribution line, served as a diluting and transporting medium for the slurry. The concentrate was introduced into the suction side of the pump and applied to the pond through application hoses perforated at 10 ft intervals. The hoses were located for maximum coverage when any three adjacent solenoid valves between the distribution and application lines were opened by the wind-direction sensing switch.

Wind Baffles and Film Barriers

The second phase of the study dealt with systems to reduce or prevent the removal of the monolayer by wind. Two types of wind and film barrier systems were tested. Type A was an open type wind baffle constructed from a picket type "snow fence" with two inches of open space between the 1-% inch pickets. A grid network was

29

formed by erecting the baffles on 14.5 ft centers with one axis parallel to the prevailing wind. The baffle was supported by floats and cables with provision for adjusting the height. This arrangement permitted testing the effect of the barrier spacing to height ratio (L/H).

tC .

Fig. 3 — Closed wind/film barriers (Type B) for reducing wind speed and confining monolayer within each bay of the experimental pond. Barriers were formed by fastening sheet plastic to "snow fence". Spacing, 14.5 ft. Height 0.9 ft. L/H ratio = 16. Wind speed at time of photo, 12.4 mph. Film indicator oil check showed 33% monolayer cover. Note rough surface of check pond at upper left.

Type B barrier, Figure 3, was a true wind and film barrier. It was formed by securing vinyl chloride plastic to the pickets of the Type A barrier. This had the effect of sub-dividing the surface of the pond into 48 small compartments, 14.5 ft square. These closed barriers proved to be effective for confining the monolayer within each bay.

Test Procedure

Several barrier type and height combinations were tested. For each arrangement there were two tests : These were :

1. Evaporation tests without monolayer forming chemicals. 2. Evaporation tests with monolayer forming chemicals. The first test served to determine the effect of the barrier system alone on the

rate of evaporation. This type of test was made when the ponds were known to be free from residual monolayers from previous tests. The second test measured the combined effect of the chemicals and of the barriers on the rate of evaporation.

Experience with previous tests showed that thermal equilibrium did not exist in the treated pond until several days after the beginning of the test. Therefore all tests reported in this paper were of fairly long duration, ranging from 19 to 50 days. Periods when rainfall occurred were eliminated from consideration due to the uncer­tainty of the amount of direct rainfall into the ponds and of runoff from the dikes.

JO

Except as specifically noted the chemical used for the second test series was a blend of straight-chain saturated higher alcohols with 5% Cj.4, 44% Ci6, 46% Cis , aàd 5% C20- The chemical was applied either as a powder in water slurry or directly as a powder according to the nature of the test.

RESULTS

The Effect of Wind Speed on Film Movement

Films created by fatty alcohols reduce the surface tension of water, resulting in the smoothing of small waves when film is applied. The smoothing action also serves the practical purpose of providing an easy and reliable method of determining whether a film is present. This characteristic was used in tests to determine the mini­mum application rate required to maintain a film on the experimental pond for various wind speeds.

Figure 4 shows the minimum application rate plotted as a function of wind speed for the conditions both with and without the open wind baffles. These data were obtained using the slurry application apparatus during periods of constant wind speed and direction. Throughout each test the application rate of slurry of known concentration was manually controlled in such a manner that the rate of film repla­cement equalled but did not exceed the rate of removal by wind. Each point on the curve represents a test duration of 30 to 60 minutes. The equation for the curve obtained during the tests without wind baffles is 7? = 9.3 £/2-02 x 10-6. The appli­cation rate i? is expressed in pounds of chemical per hour per foot normal to the wind. Wind speed U is expressed in miles per hour. The point of wind measurement was 6.5 ft above the water surface.

.00025

•00015 _ 4 6

WIND SPEED

Fig. 4 — The effect of wind speed on the minimum application rate of hexadecanol slurry required to maintain a complete monolayer cover on the experimental pond surface.

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Also shown in Figure 4 is the curve obtained when film application rate tests were made with the Type A (open) wind baffles in place. The equation for this curve was J? = 2.8 t/2-51 x 10-6 . It will be seen that at 12 mph the chemical requirement was the same, whether or not baffles were present. At 8 mph, with the baffles in place, the film requirement was 20 percent less than without baffles. This is an appreciable decrease. Yet when viewed from practical considerations there appears to be little advantage in using this type of open baffle because the film requirement remains large at the higher wind speeds and equipment for continuous application would still be required.

The form of the equations is of special interest. The required application rate is shown to be a power function of the wind speed. Therefore at high wind speeds an excessively large application rate is required. The highest wind speed for which it was possible to obtain data for any test was 19 mph, which required 0.006 lbs of chemical per hour per foot normal to the wind. This is consistent with field obser­vations made at Lake Hefner and Lake Cachuma where it was found impractical to attempt to maintain a film when wind speeds increased beyond 15 mph.

Effect of closed Barriers (without monolayers) on Evaporation

Evaporation reduction tests were made for two different heights (0.25 ft and 0.90 ft) of the Type B (closed) barrier. Monolayer forming chemicals were not used. The results and conditions of the tests are shown in Figure 5. A significant decrease in evaporation (9.1%) was obtained when the 0.9 ft high closed barriers were tested during the relatively high winds of May compared with no reduction for the 0.25 ft barriers during June. Unfortunately the limitations of the test facilities prevented making simultaneous tests on the two barriers heights and therefore direct comparison could not be made.

1 2 3 4 5 EVAPORATION FROM UNTREATED POND (INCHES)

Fig. 5 — The effectiveness of closed wind barriers of different spacing/height ratios for reducing evaporation. These tests were made without monolayers. Double mass plotting.

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The wide range of daily wind speeds during the May tests provided an opportunity to further examine the evaporation retardation potential of the wind barriers. These data are plotted in Figure 6, where the percent daily evaporation reduction is plotted as a function of the average wind speed for that day. The correlation coefficient r = 0.64, shows that the fit of the trend line is far from ideal, but since wind speed is only one factor in the evaporation process the fit must be considered fair. The impor­tant aspect here is that at high wind speeds evaporation was reduced significantly. By contrast at the higher wind speeds evaporation retardation by chemical monolayers was difficult to achieve because of blowoff (2). Thus under certain circumstances for small ponds, the use of this type wind barriers may be a better evaporation control practice than application of chemical monolayers.

20

o i— o =3

o •=1

12

1 1 TREATMENT:

j CLOSED WIND BARRIERS WITHOUT MONOLAYER BARRIER L /H = 16

o

o ER= I . 5 5 U - 6 . 5

r = 0 . 6 4 o

o

i

3 o

o o

o

>o y O jr

o

o i 0 2 4 6 8 10 12 14

AVERAGE DAILY WIND SPEED (U) MILES/HR.

Fig. 6 — Evaporation retardation by closed wind barriers as a function of wind speed. Barrier height, 0.9 ft. Test was made without a monolayer.

Effect of closed Barriers on monolayer Coverage

One of the expected benefits of the barriers was preventing film losses by wind action. Therefore, tests were made to determine this characteristic for two L/H values. The procedure was to apply 10 grains of the fatty alcohol powder in each bay. Then during periods of steady wind speed and direction the percent of bay covered by the fully compressed monolayer was determined by the indicator oil technique (8). Results of a series of tests at various wind speeds are shown in Figure 7.

Also shown in the figure is the relation between film area and wind speed for an L/H value of 150. This curve was obtained from earlier tests by CROW and SATTLER (2), in which the entire pond was considered as one bay. This curve is parti­cularly helpful since it provides confirmation of the nature of the relationship, espe­cially at the higher wind speeds. The same type of relationship is in evidence for L/H values of 58 and 16 although one can be less certain about the points for the higher wind speeds.

The importance of the L/H ratio in restraining film movement is readily apparent. When L/H — 150 a complete film cover would exist only when the wind speeds are less than 2.0 mph, a rare condition for Oklahoma. Daytime wind speeds of 8 to 10 mph are common. Therefore, even with barriers set at L/H =58 only 20 to 40 percent

33

film coverage would be obtained. However, for L/H=16 a complete cover would exist for all wind speeds less than 10 mph.

1.5 2.5 5 10 15 WIND SPEED (U) MILES/HR.

Fig. 7 — The effect of wind speed and barrier spacing/height ratio on the area of the pond covered by the monolayer.

3 4 5 6 7

AVERAGE DAILY WIND SPEED (U) MILES/HR.

Fig. 8 — The relation between evaporation retardation and wind speed during a test when the monolayer was confined within closed wind/film barriers 0.25 ft high. Barrier spacing 14.5 ft.

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The poor evaporation suppression results obtained with the lower barrier height is undoubtedly caused by incomplete film cover. There was a good correlation between evaporation suppression and average daily wind speed as shown in Figure 8. When this figure is examined in connection with the area coverage versus wind speed relation of Figure 7 it will be seen that a large portion of the pond surface was exposed much of the time.

Evaporation Suppression Tests

The ultimate purpose of barriers and monolayers is to suppress evaporation. Therefore, their combined effect was determined for the closed barrier arrangement for L/H values of 16 and 58. The same chemical was used throughout but the mode of application was different. For the LjH.= 58 tests the chemical was applied as a slurry at two to three day intervals. Powder was used for the L/H= 16 tests, with applications at 3 to 6 day intervals as the season progressed toward cooler tempera­tures. In each case a surplus quantity of chemical was applied to assure that a film would be present if wind conditions would permit.

The results of the evaporation suppression tests and the test conditions are shown in Figure 9. Here it is seen that although the average wind speeds for the two tests were identical the monolayer in combination with the L/H= 16 barriers yielded three times greater evaporation reduction than the L/H = 58 setting. Evaporation was reduced 31.3 percent when the chemical was confined within the higher barriers. This is probably as much reduction as can be expected, since previous tests (3) with continuous chemical application resulted in reductions of this magnitude. However, with the barrier system, smaller amounts of chemicals were required, and there was no need for expensive application and control apparatus.

_ 7 IS)

o 5 6

TEST DATE BARRIER (1962) L/H RATIO

_ Aug. 3-Sept. 1 58 Sept. II-No». 1 16

|

~ . i

/ / i T

/ / £•

- /rif'rff^— /dÇcr

dp

AVG. WIND PERCENT j SPEEO MPH REDUCTION . i

5.84 11.3 / A

5.82 31.3 / y

j ^-sf-' [ / < V L / H = 5 8

! / ' ,/f REDUCTION'11.3%

1 / X i / f If i

••/ J* j F

/ j/T <T ^ L / H = I6 '• / sP REDUCTION =31.3%

rfl \

H [ :

J j _...! TREATMENT: |

CLOSED WIND BARRIERS WITH MONOLAYER

i t i ( i

2 3 4 5 6 7 8 9 EVAPORATION FROM UNTREATED POND (INCHES)

Fig. 9 — Relative evaporation reduction resulting from confinement of CIG and Cis fatty alcohol monolayer within closed wind/film barriers of different L/H ratios. Double mass plotting.

Summary

When Oklahoma State University began a research program to increase farm water supplies by evaporation suppression the emphasis was on methods of appli-

35

cation and the evaluation of monolayer forming chemicals. It soon became apparent that wind was a major obstacle to progress in evaporation control by chemical mono­layers. This led to a research project, cooperative with the U.S. Bureau of Reclamation, to investigate the effect of wind on the application and maintenance of monolayers. Particular emphasis was placed on small impoundments similar to the farm pond. The results of this investigation are reported in this paper.

The experimental investigation was carried out on paired ponds designed for this purpose. Three different approaches to the wind problem were undertaken. These included the development and testing of : (1) Apparatus to continuously replenish the monolayer at the upwind shore; (2) Open picket type baffles to reduce the wind speed near the water surface, and (3) Completely closed barriers to reduce the wind speed and confine the monolayer within a small bay or compartment.

Wind speed versus film application rate curves were determined through the use of the experimental monolayer application apparatus. It was found that the required film application rate is a power function of the wind speed having the equation R = 9.3 t/2-02 x 10~6, where R is the rate of chemical application in pounds per hour per foot normal to the wind, and U is the wind speed in miles per hour.

The effectiveness of open picket type baffles to reduce the wind speed and film movement was studied. Baffles with 57 percent open spaces and 1.2 ft height were moored on the water surface on a 14.5 ft grid. They reduced film removal about 20 percent during 8 mph winds but had no effect on the film requirement at 12 mph winds. The equation for this curve was R=2.S C2-5 1 x 10~6. The results of these tests indicate that this type of open baffle has little practical value, since continuous application of the monolayer is a necessity.

A much more effective anti-wind measure was a network of completely closed barriers. With these barriers, arranged similarly to the open baffles described above, it was found that evaporation was reduced nine percent, without the aid of mono­layers. Average wind speed during this test was 10 mph. Linear relations between the evaporation retardation and the wind speed were developed which showed an increase in evaporation retardation with increasing wind speed.

The closed barriers were effective in preventing the film from escaping and also had the advantage of requiring replenishment of the film at intervals of 3 to 6 days. The area of each compartment occupied by the monolayer was found to be a function of the wind speed and of the barrier spacing to height ratio, (LjH). Relations between these variables were developed and shown graphically. The most effective barrier spacing/height ratio tested was L/H= 16, which provided a complete film cover during all wind speeds less than 10 mph.

Evaporation suppression tests were made with monolayers formed by a mixture of Cie and Cis fatty alcohols. The barrier spacing to height ratio was an important factor in these tests. When the barriers were set at L/H= 58 evaporation was reduced 11 percent. The best evaporation suppression results were obtained when the mono­layer was used in combination with the closed barriers set at L/H— 16. Evaporation reduction then amounted to 31 percent. This quite satisfactory reduction was achieved during normal Oklahoma winds and with only periodic replenishment of the film. Therefore it is apparent that a system of wind/film barriers of this type holds much promise for evaporation control on small impoundments.

36

REFERENCES

C1) CROW, F. R. and E. R. DANIEL. Chemicals for controlling evaporation from open water surfaces. Transactions of The American Society of Agricultural Engineers, Vol. 1, No. 1, pp. 72-75, 1958.

(2) CROW, F. R. and Harold SATTLER. The influence of wind on chemical films for reservoir evaporation retardation. Paper presented at the meeting of the South­west-Southeast Sections of the American Society of Agricultural Engineers at Little Rock, Arkansas, 1958.

(3) CROW, F.R. Reducing reservoir evaporation; application of surface films cuts losses. Agricultural Engineering, Vol. 42, No. 5, pp. 240-243, 1961.

(4) Water-loss investigations : Lake Hefner-1958. Evaporation reduction investi­gations, Report of the collaborators. Bureau of Reclamation, U. S. Department of the Interior, 1959.

(5) Water-loss investigations, Lake Cachuma-1961. Evaporation reduction investi­gations. Chemical Engineering Laboratory Report No. SI-33, Bureau of Recla­mation, U.S. Department of the Interior, 1962.

(6) VINES, R. G. Evaporation control : A method of treating large water storages. Retardation of evaporation by monolayers : Transport Processes, Academic Press, New York, pp. 137-160, 1962.

(7) MCARTHUR, I.K. H., Cetyl alcohol monolayers for water conservation : Methods of application and the influence of wind. Research, Vol. 15, pp. 230-238. June 1962.

(8) Determination of the presence and degree of compression of a monomolecular layer using indicator oils. Division of Engineering Laboratory Report No. SI-13. Bureau of Reclamation, U.S. Department of the Interior, 1957.

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