I.
EVAPORATION REDUCTION BY MONOMOLECULAR FILMS *THE INFLUENCE OF WATER TEMPERATURE AND APPLICATION
RATE ON THE EFFECTIVENESS OF CETYL ALCOHOLby
E. H. Hobbs
Member C.S.A.E.
Canada Agriculture Research Station, Lethbridge, Aiberfa
INTRODUCTION
If evaporation from free-water surfaces could be reduced, more waterwould be available for useful purposes. Scientists in several countriesHave been investigating the use ofmonomolecular films as evaporationretarding agents. Hexadecanol, alsoknown as cetyl alcohol, appears to bethe most promising material for thispurpose.
Evaporation is an endothermic process. Thus, when evaporation is reduced the water temperature rises.Several authors, when reporting experiments concerned with evaporationcontrol, have commented on this fact.Hayes (6) found that the surfacewaters of hexadecanol-treated pondswere from 2.6° to 3.8° F. warmer than
those of untreated ponds. Roberts (8),using a high-accuracy thermistor thermometer, recorded a temperature difference of 4° to 7° F. between treatedand untreated lake surfaces. Computations by Harbeck and Koberg(5) indicated that the 18 per centreduction in evaporation achieved ina field test was necessarily accompanied by a temperature rise of 3.4°F.
Others have noted that the successot evaporation retardant processes isdependent in part on the water temperature. Mansfield (7) reported thatthe rise in water temperature whichfollows application of a surface filmreduces the degree to which that filmmay retard evaporation. Vines (9)observed that, as a water surface became excessively warm in hot weather,the efficiency of the evaporation control process dropped. Florey et al. (4)were unsucessful in attempts to establish a temperature-evaporation ratiousing 4-foot-diameter pans indoorswhere air temperature was controlled.They suggested that dust contamination was the chief factor that inter
fered with film continuity. Floreyand Timblin (3) conducted similartests out-of-doors and depended uponseasonal changes in ambient temperature to provide temperature differences. From these tests they determined evaporation reduction factors
based on a correlation ot evaporationand water temperature.
Preliminary studies, conducted bythe author, using 4-toot-diameter pansexposed to tielu conditions producedresults similar to those reported in theliterature. Methods that appearedquite promising early in the seasonoecame mucn less eltective during theIieat ot tlie summer, therefore, laboratory tests were conducted with filmsot cetyt aicoliol to determine, morespeciiicaliy, the effects of water temperature and application rates on thereduction of evaporation from a free-water surface. The results of these
laboratory tests are reported in thispaper.
MATERIALS AND METHODS
One-gallon plastic pots equippedwith point gauges were placed in eachol tour constant-temperature waterbaths housed in an enclosed greenhouse bay. The water baths weresimilar to those described by Cooperet al. (1) and operated at temperatures ol 41.5°, 54.5°, 67.0° and 79.0°,±1°, F. Air temperature within thegreenhouse was tnermostaticatly controlled at 69.5° F. No attempt wasmade to modify the existing relativehumidity, which was comparativelyconstant and averaged 40 per cent.
The four refrigeration units whichprovide cooling for the water bathswere installed in such a manner thattheir cooling fans help to produceuniform air circulation over the waterbaths. This circulation of air appeared to prevent air stagnation above theevaporating surfaces. The water levelin the plastic pots was approximatelythe same as that in the surroundingbath, and neither was allowed to fluctuate more than one-quarter of aninch. This required daily addition ofwater to the pots held at 79.0° F.and less frequent additions to the remaining pots. The volumes of wateradded were converted to inches of
evaporation. Five treatments, randomized in six replications, were carriedout in each of the four water baths.
Cetyl alcohol, mixed in a 1:5 solution with methyl alcohol, was appliedto the test pots at the following rates:
17
Treatment No. Cetyl alcohollb./ac.
0 (check) nil
1 0.026
2 0.052
3 0.104
4 0.260
the No. 1 treatment rate was
sliglitly in excess of that required toform a compressed monolayer on thewater surlace. Treatments were re
newed at the initial rate of application whenever a comparison ot theevaporation loss from treated andcheck pots indicated that a treatmenthad lost, or was losing, its effectiveness. Under this definition it wasnecessary, during the 30-day test period, to renew treatment ten times onthe pots held at 79.0°, five times onthose held at 54.5° F. No additionaltreatment was applied to the pots heldat 41.5° F.
A thermistor thermometer was usedto determine whether a temperaturegradient existed within the pots oramong treatments. The failure to findeither was assumed to be the resultof rapid conduction of heat throughthe walls of the pots to the surrounding water bath.
RESULTS AND DISCUSSION
The effects of the different treat
ments on evaporation at each of thefour water temperatures are shown inFigure 1, as well as the significantdifference between treatments as de
termined by Duncan's (2) multiplerange test.
The differences in evaporationamong check treatments were a resultof the temperature maintained ineach water bath. The increased air
movement under field conditions
would cause more evaporation thanwas recorded in this experiment. Theadditional water lost, however, wouldprobably be of little practical concernuntil that part of the season whenwater temperatures approached 55° F.
No significant differences due totreatments were found for the potsheld at the 41.5° F. temperature.
X
At 54.5° and 79.0° F. the evaporation from treatment 1 did not differsignificantly from the correspondingcheck. The inability of this low rateof application to maintain an evapor-
I "1 *3
Figure I. Effect of a monomolecular film of cetylalcohol on evaporation, and significant differences between treatments, at four water temperatures. (Any two means underscored by thesame line are not significantly different at theb% level.)
ation retarding film was attributed toan evaporative loss of the cetyl alcohol and the fact that there was littleor no excess material available for
film replenishment. Treatments 2 and3, at 54.5°, 67.0°, and 79.0° F., weresignificantly different from their respective checks although not necessarily from one another. Treatment4, the maximum rate employed, provided a significantly greater reductionin evaporation than did all othertreatments at the above temperatures.
The evaporation reduction efficiency of each treatment at each watertemperature is shown in Figure 2.Although the difference in efficiencyof comparable treatments at watertemperatures of 54.5° and 67.0° F.was not great, there was a marked decrease in efficiency for all treatmentsat 79.0° F. This lower efficiency occurred despite the fact that almosttwice as many applications of cetylalcohol were made to the pots heldat 79.0° F. than to those held at 67.0°F.
D
RATE (ib/oei
0026
0 052
0(04
0 260
u dTREATMENT I 2 3 4 list
54 5 6T0 1WATER TEMPERATURE "F
ure 2. Percentage evaporation reduction bytreatments at three temperatures.
CONCLUSIONS
Fig
P
four
The amount of evaporation occurring at 41.5° F. was too slight to beof practical concern. Application ofcetyl alcohol had no significant effect
upon evaporation at this temperature.At all other water temperatures studied, high rates of treatment application were more effective than lowrates. As water temperature increasedbeyond 67.0° F., the efficiency of alltreatment rates declined. In order to
compensate for the higher water temperatures occurring during midsummer, heavier and more frequent treatment applications would be requiredto maintain evaporation control at apractical level.
REFERENCES
1. Cooper, D. J., K. F. Nielsen, J. W.White and W. Kalbfleisch. Note onan Apparatus for Controlling SoilTemperatures. Can. J. Soil Sci.40:105-107. 1960.
2. Duncan, D. B. Multiple Range andMultiple F Tests. Biometrics 11:1-42. 1955.
3. Florey, Q. L., and L. O. Timblin,Jr. Screening Tests, Water-Lossinvestigations: Lake Hefner 1958Evaporation Reduction Investigations. Oklahoma City, Okla. pp.125-131. 1959.
4. Florey, Q. L., W. U. Garstka andL. O. Timblin, Jr. Computation ofEvaporation Savings. Water-LossInvestigations: Lake Hefner 1958Evaporation Reduction Investigations. Oklahoma City, Okla. pp.37-42. 1959.
5. Harbeck, G. E., Jr., and G. E. Ko-burg. A Method of Evaluating theEffect of a Monomolecular Film inSuppressing Reservoir Evaporation. J. Geophys. Research 64:89-93. 1959.
6. Hayes, M. L. Biological Effects ofHexadecanol used to SuppressWater Evaporation from Reservoirs. Final report. Submitted tothe U.S. Bureau of Reclamation.98 pp. 1959.
7. Mansfield, W. W. The Influence ofMonolayers on Evaporation fromWater Storages. I. The PotentialPerformance of Monolayers ofCetyl Alcohol. Australian J. Applied Sci. 9:245-254. 1958.
8. Roberts, W. J. Reducing LakeEvaporation in the Midwest. J.Geophys. Research 64:1605-1610.1959.
9. Vines, R. G. Evaporation Control— The Mansfield Process. WoolTechnol. and Sheep Breeding 5:51-53. 1958.
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Multi-plane Draft hxture.
urements the drawbar of the tractorshould be pinned so that the normalsoil variations and resulting changesin the line of draft will not move theplane of the transducer from theplane of the direction of travel. Formost work, however, this is not necessary as the error introduced is smalland for small angles can be readilyignored.
An electronic integrator is an extremely valuable edition to any draftrecording system, especially in themeasurement of torque which mayhave one or more peak values perrevolution. Any time base may beselected for the limits of integrationup to the maximum capacity of theelectronic counter (upper rack Figure6). For draft measurements a ten-second time base has been selected asan estimate of the reaction time ofan operator to an overload conditionof his tractor.
The integrator operates on theprinciple of converting the voltage(lower rack Figure 6) applied to thepen of the ascillograph to an oscillating voltage which is then fed to theelectronic counter. As the frequencyof oscillations is directly proportionate to the pen voltage (pen deflection) the count is then the integrationor summation of the area under thecurve drawn by the pen.
Fig. 6. Integrator.
SUMMARY
The use of transducers provides anextremely accurate method for deter-
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