1. T h e figures in brackets refer to the bibliography at the end of this chapter.
Climatology and microclimatology / Climatologie et microclimatologie film must be self-sealing. Accordingly the monolayer
must be fluid.
A measure of fluidity is required also if the monolayer is to resist wave action. The development and relaxation of waves upon a water surface produces increases and decreases of surface area. A liquid film can accommodate expansions and contractions readily, whereas a rigid film is broken up b y such variations.
Finally, the film should exert a surface pressure across the surface greater than that exerted b y the wind, otherwise downwind compression will cause the film to collapse.
F r o m this preliminary survey, it is clear that a liquid monolayer exerting as high a surface pressure as possible and providing a resistance to vapour transfer of several hundred units is required.
It is obvious that any monolayer generated should be reasonably stable. Monolayers m a y be formed either by spontaneous generation from solids or Uquids floating on the water surface, or b y the spreading of solutions in volatile solvents. B y the former technique an equilibrium surface film is established, and m a y be maintained against losses from the monolayer b y appropriate further generation from excess solid or liquid. Although films more potent than equilibrium films m a y be generated initially b y the solvent tech-nique, reversion to equilibrium is rapid; with this technique also some form of replenishment is necessary.
SELECTION OF C O M P O U N D S
It has been noted that compounds forming monolayers contain both hydrophilic and hydrophobic groupings within the molecule. O f the c o m m o n hydrophilic groupings, only the hydroxyl allows formation of liquid films on natural water. Thus the compounds desirable are alcohols of high molecular weight.
The hydrophobic portion of the molecule of such an alcohol m a y be a straight paraffin chain, a branched chain, or there m a y be present fused ring groupings.
Experiment indicates clearly that straight-chain alcohols provide the greatest resistance to the transfer of water vapour because of the more compact structure of the film.
As the chain length of the molecule of such an alcohol is increased, several significant changes take place. The compound becomes more insoluble in water, and at a fixed surface pressure the resistance to evapo-ration increases. Beyond 16 carbon atoms in the chain, however, the equilibrium surface pressure generated at normal temperatures declines, and the film becomes less fluid. For equilibrium films, optimum resistance is obtained with cetyl (Clg) and stearyl (C18) alcohols.
ESTIMATES OF P E R F O R M A N C E
F r o m measurements in a suitable laboratory apparatus, it is found that the resistance of an equilibrium cetyl-alcohol monolayer at normal temperatures is about
1,800 units. Under natural conditions there is present an atmospheric resistance of about 600 units. Thus in the presence of such a film the resistance to evaporation is increased fourfold, and on first principles a fourfold reduction in evaporation should follow.
In practice automatic compensation decreases this expected reduction of 75 per cent. A reduction in evapo-ration means that the loss of latent heat is decreased, and surface temperatures increase to provide increased losses b y sensible and radiative transfer. This rise in temperature increases the vapour pressure difference between surface and atmosphere, and as a secondary effect reduces the intrinsic resistance of the film. T h e actual extent of compensation m a y be estimated for particular conditions b y the construction of heat balances.
Assuming that bulk and surface water temperatures are always equal, a condition approached with very shallow bodies of water, the effects of compensation are most marked. In the absence of compensation the expected reductions at a water surface for 30° C . and 22.8 per cent. R . H . are 77 per cent into a wind of 10 feet per second and 45 per cent into a wind of 2 feet per second. Reductions of this extent are approached w h e n there is no input of solar energy, but for an input obtainable at high noon on a summer's day reductions fall to 30 per cent and 3 per cent respectively.
With deep bodies of water, however, rises of surface temperature trail those required for complete compen-sation, so that the degree of reduction in evaporation lies between the m a x i m u m and m i n i m u m possible.
Approximate calculations indicate that the m e a n reduction in evaporation to be expected in Australia is about 50 per cent.
THE SIGNIFICANCE OF DOSAGE
These results indicate that the presence of a complete monolayer of cetyl-alcohol on a water surface should reduce the evaporation of water to an appreciable degree. It remains to establish the conditions necessary for the formation and maintenance of such a monolayer.
The spontaneous generation of a monolayer from solid cetyl-alcohol floating on the water surface has been mentioned. B y suitable experiment w e find that the rate at which molecules leave the solid is dependent only on the perimeter of solid in contact with water surface; in fact, the rate of generation of monolayers is:
0.196 ( T ) monolayers/sec. at 25° C.
where 1 c m . is the perimeter of solid in contact with the surface of water of area A sq.cm.
Although cetyl-alcohol is virtually insoluble in water and relatively non-volatile, losses from the monolayer b y dissolution and sublimation are signi-ficant simply because a monolayer contains very little mass in unit area. Introducing these losses, the rate of generation becomes modified to:
Reduction of evaporation of stored water
0.196 {j — 2.7 x 10-M monolayers/sec. at 25° C.
Thus w e would expect a complete monolayer to be formed and maintained on a water surface provided the dosage of cetyl-alcohol, measured as 1 / A c m . , exceeds about 3 X 1 0- 4 c m . O n a natural surface subjected to agitation, however, this dosage is insufficient.
W h e n waves form upon a previously still surface, the actual surface area is increased. Accordingly any monolayer present is diluted and becomes less compact, and both the surface pressure and resistivity of the film decline markedly. W h e n this surface relaxes, however, surface pressures and resistivities above the equilibrium values are obtained only temporarily if more material has been added to the monolayer during its expanded stage. Accordingly the m e a n resistivity of a monolayer present on a natural water surface is always less than that obtained on a still surface unless new material can be supplied to the monolayer at a sufficient rate whenever the water surface is increasing in area.
B y using the data of Sverdrup and M u n k [10] de-scribing the characteristics of wind-generated waves, it m a y be estimated that an additional dosage of roughly 2.2 X 10~3 c m . of solid cetyl-alcohol should provide a sufficient additional rate of supply to cover average rates of surface disturbance in Australia.
Adding the dosage of 2.7 X 1 0- 4 c m . required to meet losses b y solution and sublimation, dosages of 2.5 X 1 0- 3 c m . should engender reductions in evapo-ration approaching the expected value.
T E C H N I Q U E S O F APPLICATION
There are several considerations determining the method of application of solid cetyl-alcohol to a natural water surface.
Unless a storage has vertical weed-free banks (say, of concrete) broadcasting of material is not desirable unless regular replacement is permissible. Material is soon washed up on the banks, out of contact with the water surface, and is not returned w h e n the direction of the wind reverses.
If the material be not broadcast, it must be stored
Réduction de Vévaporation de Veau emmagasinée ( W . W . Mansfield).
L'auteur énumère les conditions auxquelles il faut nécessairement satisfaire pour établir et maintenir une
on the surface. Although at the present time alternative procedures are being investigated, the most obvious e technique, and the one used to date, is to store the e material in floating containers fitted with gauze s windows.
} The use of gauze windows necessitates compromising.
As the perimeter of material is the significant measure , of dosage, economically it is desirable to use very small j particles and hence very fine meshes. A mesh, however, , offers a resistance to the passage of surface film,
approxi-; mately proportional per unit length of gauze to 1/d1, , where d is the mesh opening. Thus fine particles of
; cetyl-alcohol require extremely long, cumbersome and f expensive containers. W e have compromised with I containers, approximately three feet square, containing r beads of cetyl-alcohol 2 to 4 m m . diameter.
5 With this technique applications are quite satis-i factory on areas of a few acres, provsatis-ided the float i remains quite buoyant. O n large areas, however, agi-' tation leads to abrasion sufficient to destroy a complete
dosage in a few months. Accordingly alternative methods of application are required.
, It is convenient to indicate here that it m a y be shown F readily that the rate at which molecules are liberated I to a water surface is influenced but slightly by the dispo-r sition of the dispo-requidispo-red dosage. Thus, fodispo-r a dose of thdispo-ree
pounds per acre, it matters little if w e design and locate t floats so that w e have 0.3 lb./O.l acre, 3 lb./l.O acre, F or 30 lb./10 acres.
R E S U L T S O P FIELD TRIALS
Trials conducted to date and providing reliably assess-able results indicate that the technique does reduce
> evaporation and to an extent approximating that [ predicted, at least up to an area of 20 acres. Experiments on m u c h larger areas have been conducted, but over-, flows following soon after treatment have destroyed
their significance.
[ Assuming the most gloomy picture possible from the present results, the cost of treatment is estimated at L 2.4 pence per thousand gallons of water saved. F r o m the two viewpoints of performance and economy, the i process accordingly m a y be described as promising.
pellicule monomoléculaire sur une surface aquatique naturelle et donne un aperçu de l'effet d'une telle pellicule sur le taux d'évaporation de l'eau.
Il indique comment on emploie des pellicules d'alcool cétylique et quels résultats on obtient.
R É S U M É
Climatology and microclimatology / Climatologie et microclimatologie