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Use of a monomolecular film to retard evaporation from water surfaces Pohjakas, Kaljo 1959

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THE USE OF A MONOMOLECULAR FILM TO RETARD EVAPORATION FROM WATER SURFACES by Kaljo Pohjakas B.S.A., University of B r i t i s h Columbia,  1951  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science i n Agriculture i n the Department of A g r i c u l t u r a l Mechanics  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA February, 1959  II  ABSTRACT  With the increasing demands on water f o r a g r i c u l t u r a l , domestic and i n d u s t r i a l purposes, water conservation has become an e s s e n t i a l feature of our economy i n Western Canada. Water conservation has been under study f o r many years, and research workers have been experimenting various conservation methods.  with  Lately, the use of monomol-  ecular f i l m s has shown great promise i n providing an economical means f o r retarding the rate of evaporation from water surfaces. This study was undertaken to test the e f f e c t i v e ness of c e t y l alcohol films i n retarding the rate of evaporation from free water surfaces. Four c i r c u l a r tanks were i n s t a l l e d at the Summerland Experimental Station and the rate of evaporation was recorded d a i l y f o r each one of these tanks.  The study was  carried out during the summer of 1958. Varying quantities of c e t y l alcohol were added to the water surface and t h e i r effect on the rate of evaporation was recorded. Besides recording the rate of evaporation, other meteorological information was also collected and used i n evaluating and i n interpreting the experimental  results.  iii  I t became evident from the experimental r e s u l t s that a twenty to t h i r t y per cent retardation i n the rate of evaporation can be achieved with the use of c e t y l alcohol f i l m s .  Department of A g r i c u l t u r a l Mechanics The University of B r i t i s h Columbia Vancouver 8, Canada  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  the requirements f o r an advanced degree at the  University  o f B r i t i s h Columbia, I agree that the  L i b r a r y s h a l l make  it  study.  f r e e l y a v a i l a b l e f o r reference  agree that  and  I further  permission f o r extensive copying of t h i s t h e s i s  f o r s c h o l a r l y purposes may  be granted by the  Department or by h i s r e p r e s e n t a t i v e s .  Head of  my  I t i s understood  that  copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l  gain  s h a l l not be allowed-without my  Department  of  The U n i v e r s i t y of B r i t i s h Vancouver 8 , Canada.  Columbia,  written  permission.  i  I  ACKNOWLEDGEMENTS  The writer wishes to take t h i s opportunity to thank Professor T. L. Coulthard, Chairman of the Department of A g r i c u l t u r a l Mechanics, f o r his permission to undertake t h i s project and f o r the use of the departmental f a c i l i t i e s . Sincere thanks are expressed to Dr. J . C. Wilcox, Head of the Department of I r r i g a t i o n and Plant N u t r i t i o n at the Summerland Experimental  Station, f o r his d i r e c t i o n ,  assistance and c r i t i c i s m during the course of t h i s  study.  The writer also would l i k e to thank Dr. J . D. Beaton, Instructor, Department of S o i l Science, and Professor E . S. Pretious of the Department of C i v i l Engineering f o r t h e i r valuable assistance and c r i t i c i s m during  the preparation of t h i s t h e s i s .  iv  III  TABLE OF CONTENTS Page  I  ACKNOWLEDGEMENTS  II  ABSTRACT  III  TABLE OF CONTENTS  IV  LIST OF NOMENCLATURE  1  V  TABLE OF ILLUSTRATIONS  3  VI  INTRODUCTION  4  VII  HISTORICAL  6  VIII  THEORY OF EVAPORATION  7  A  7  Evaporation from Free Water Surfaces  i i i iv  1. Dalton's Law 2. Meteorological d e f i n i t i o n of evaporation 3. Effect of d i f f u s i o n versus turbulent transfer 4. E f f e c t of s a l t s on the rate of evaporation 5. V a r i a t i o n of evaporation with altitude B IX  Monomolecular Theory  REVIEW OF LITERATURE A  Type of Evaporimeter versus Rate of Evaporation  13 15 15  1. "Gross" and "net" evaporation 2. Size of evaporimeter versus evaporation rate B  Evaporimeters 1. 2. 3. 4.  Class "A" Weather Bureau Land Pan Plant Industry Sunken Pan Colorado Sunken Pan The United States Geological Survey Floating Pan 5. Four-foot Ground Tank 6 . Black B e l l a n i Plate  19  V Page 7. White B e l l a n i P l a t e 8. P i c h e Evaporimeter  X  C  Comparison of Evaporimeters  22  D  E v a p o r a t i o n Formulae  23  E  I n s o l a t i o n Method  26  MATERIALS AND A  METHODS  27  Materials 1. 2. 3. 4. 5. 6.  27  F o u r - f o o t tanks Solar Radiation recordings Sunshine r e c o r d e r Wind v e l o c i t y r e c o r d e r R e l a t i v e humidity r e c o r d i n g s Temperature r e c o r d i n g s a) Ambient a i r temperature b) S o i l temperature c) Water temperature  7. Barometric B  XI  readings  Methods  34  1. Temperature r e c o r d i n g s 2. T e s t i n g o f s u r f a c e f i l m RESULTS AND DISCUSSIONS  37  A  The e f f e c t of i m p u r i t i e s on the monomolecular f i l m  37  B  Water temperature v a r i a t i o n i n ground tanks and i n a l a n d pan  40  The e f f e c t of monomolecular f i l m on t h e d i u r n a l v a r i a t i o n of the evaporat i o n rate  41  The minimum amount o f c e t y l a l c o h o l r e q u i r e d t o a f f e c t m a t e r i a l l y the rate of evaporation  44  C  D  XII  pressure  SUMMARY AND  CONCLUSIONS  47  vi  Page A  Summary  47  1 . Film pressure v a r i a t i o n 2. The e f f e c t of wind 3. The cumulative effect of c e t y l alcohol f i l m B  Suggestions f o r Further Studies  49  1 . Lack of experimental data with large bodies of water 2. The e f f e c t of wave action 3. Need f o r continuous temperature recording XIII  LIST OF REFERENCES  51  XIV  APPENDIX  54  IV  LIST OF NOMENCLATURE  Mean barometric reading i n inches of mercury at 3 2 ° Fahrenheit constant c o e f f i c i e n t evaporation i n inches per twenty-four hours evaporation i n centimeters per twenty-four hours evaporation i n inches per t h i r t y days saturation pressure of aqueous vapour at the temperature recorded by the wet bulb mean pressure of saturated vapour at the temperature of the dew point i n inches of mercury mean vapour pressure corresponding to temperature of water surface i n inches of mercury vapour pressure at the mean wet bulb i n inches of mercury  temperature  incoming radiation i n c a l o r i e s per square centimeter of horizontal surface latent heat of water i n c a l o r i e s per cubic centimeter correction f o r the interchange of heat through the walls surrounding water barometric pressure i n millimeters of mercury barometric pressure of a i r i n inches of mercury vapour pressure of saturated vapour at t-, i n millimeters of mercury p a r t i a l pressure of actual vapour i n a i r i n millimeters of mercury vapour pressure i n a i r i n inches of mercury maximum vapour pressure corresponding to temperature of water i n inches of mercury  r e l a t i v e humidity Bowens r a t i o , r a t i o of sensible heat to latent heat heat storage i n water i n calories per square centimeter, cross-section to f u l l depth of water temperature i n degrees Fahrenheit water temperature at the surface i n degrees centigrade ambient a i r temperature i n degrees centigrade mean temperature of a i r above water surface i n degrees Fahrenheit dry bulb temperature i n degrees Fahrenheit mean temperature of the water surface i n degrees Fahrenheit wet bulb temperature i n degrees Fahrenheit maximum vapour pressure corresponding to the monthly mean a i r temperature i n degrees fahrenheit monthly mean wind v e l o c i t y i n miles per hour t h i r t y feet above ground surface mean v e l o c i t y of wind at the surface of ground or water i n miles per hour actual vapour pressure i n the a i r i n inches of mercury i n s o l a t i o n i n gram c a l o r i e s per square centimeter per twenty-four hours  3 V  TABLE OF ILLUSTRATIONS Page  Figure No. 1  v  V a r i a t i o n of evaporation with altitude.  12  2  Size of pan versus evaporation rate.  16  3  Comparison of evaporation from tanks of d i f f e r e n t diameters.  18  4  Three widely used evaporimeters.  21  5  The r e l a t i o n s h i p between recorded wind v e l o c i t y and ground wind velocity.  25  6  Row of tanks at the Summerland Experimental Station.  27  7  MSC Type G Bimetal Actinograph at the Summerland Experimental Station.  30  8  The anemometer located within the Weather Station enclosure.  32  9  Floating organic matter on the water surface a f t e r twenty-four hours.  38  10  Floating organic matter on the water surface a f t e r seventy-two hours.  3$  11  The effect of impurities on monomolecular f i l m s .  39  4  VI  INTRODUCTION  Future increases i n human and livestock populations i n the world w i l l create increasing demands upon water supplies.  In many areas, including the semi-arid and a r i d  regions of Western Canada, the available water supply i s limited.  In order to u t i l i z e the water resources to t h e i r  maximum e f f i c i e n c y , water conservation becomes a necessity. Water i s l o s t through the process of evaporation from free water surfaces, evaporation from moist s o i l surfaces and i s u t i l i z e d by plants i n t h e i r process of transpiration. To conserve moisture we must concentrate on eliminating i t s unnecessary waste.  Evaporation from the s o i l sur-  face can usually be considered pure waste. through the process of t r a n s p i r a t i o n .  Water i s l o s t  For every pound of dry  matter produced by plants several hundred pounds of water are (27)  required.  Evaporation from plants occurs primarily be-  cause the stomata have to be open to allow adequate entry of CO2 into the leaves.  Moisture by moving up the plant and  evaporating c a r r i e s plant nutrients up from the roots.  Dur-  ing the t r a n s p i r a t i o n process large quantities of water are released as water vapour. The losses from free water surfaces perform  little  useful function except replenishing the a i r vapour storage with moisture necessary t o complete the hydrologic cycle. I n s emi-arid and i n a r i d regions the potential evaporation  5 losses exceed the t o t a l p r e c i p i t a t i o n . In order to produce crops water conservation  i n those areas becomes a necessity.  There are several ways to control the evaporation losses from natural and a r t i f i c i a l r e s e r v o i r s . 1.  The construction of reservoirs with maximum depth and minimum surface  2.  area.  The concentration of water into single reservoirs.  3.  The elimination of shallow areas i n the existing reservoirs.  4.  The storage of water i n the groundwater r e s e r v o i r s .  5.  The construction of reservoir roofs and floating  covers.  6.  The construction of windbreaks.  7.  The use of a monomolecular f i l m on the surface.  The l a s t item of the above seven methods has received p a r t i c u l a r attention i n t h i s study.  6 VII  HISTORICAL  The effect of o i l films to smooth the waves has been known to mariners for 2,000 years.  It is also a well  known fact that a film of o i l on the water surface reduces the rate of evaporation.  Unfortunately the amount of o i l  to prevent evaporation from a large water reservoir is very great and the cost becomes prohibitive in addition to the undesireable effect of o i l on the fauna and flora.  Obviously  water so treated would not be f i t for recreational nor domestic consumption. The f i r s t attempts to use the monomolecular film technique to reduce evaporation were made in the United States in 1924 which, however, were not  successful.  The f i r s t successful results were reported by Langmuir in 1927.  The use of a hexadecanol film gave a f i f t y  per cent reduction in the rate of evaporation.  '  Since that  time, many experiments have been conducted using various chemical substances in creation of monomolecular films.  The  most widely known work has been done by the East African Meteorological Department under W. W. Mansfield working with the Commonwealth Scientific and Industrial Research Organization in Australia.  The results have been encouraging and  the cost has been small per volume of water saved.  7  VIII A  THEORY OF EVAPORATION  Evaporation from Free Water Surfaces 1.  Dalton* s Law Evaporation i s a process by which a l i q u i d or  s o l i d i s changed into vapour.  Any body consists of a large  number of molecules, each one being i n constant motion at varying v e l o c i t i e s and i n d i f f e r e n t d i r e c t i o n s .  The average  v e l o c i t y of a l l the molecules determines the temperature of the body.  Usually at lower temperature the v e l o c i t y of  molecules decreases. In any given mass, molecules are attracted to other molecules by various forces.  Many of the forces are included  i n the term "van der Waal's forces".  In water the most  important van der Waal's force i s the orientation effect which results from dipole-dipole interactions.  The molecules  that are closest to the surface of a l i q u i d are acted upon by more of those forces from underneath than from above. Molecules that are near the surface must overcome t h i s additional force i n order to escape into the a i r as water vapour.  I t appears that only the rapidly moving molecules  escape.  As a result the average mass w i l l have a lower  temperature than before, t h i s explains why evaporation i s a cooling process. Although molecules are continuously leaving the water surface others are returning and the rate of evaporation i s determined by the excess rate of those leaving over  those returning. ing,  I f more molecules are returning than leav-  condensation i s taking place. Immediately adjacent to a water surface i s a t h i n  layer of a i r whose temperature i s the same as that of the water. vapour.  This layer quickly becomes saturated with water I f the vapour pressure of the a i r above the layer  i s equal to that of the layer, there can be no further evaporation.  However, i f the vapour pressure i n the a i r  above i s l e s s than that i n the layer, the vapour near the surface w i l l be dispersed by d i f f u s i o n , convection and wind action and consequently the evaporation process w i l l continue. The rate of evaporation depends upon the two pressure d i f f e r ences or the pressure gradient.  This p r i n c i p l e i s known as  Dalton's law and i s expressed by the formula: E = C (pw - Pa)  (17)  E  evaporation i n inches per 24 hours  Pw  maximum vapour pressure  corresponding  to the temperature of water i n inches of mercury Pa  vapour pressure i n a i r i n inches of mercury  C  c o e f f i c i e n t depending upon wind v e l o c i t y , barometric pressure, etc.  9  2.  Meteorological D e f i n i t i o n of Evaporation Evaporation can be defined i n terms of meteoro-  l o g i c a l f a c t o r s which control i t .  Such a d e f i n i t i o n involves  a wetted surface amply supplied with water and exposed to the elements i n such a manner so that a maximum response i n evaporation i s produced by changes i n the meteorological factors with a minimum of influence being produced by the evaporating surface i t s e l f .  Such a surface would of neces-  s i t y have to be f l a t , horizontal and constructed of t o t a l l i g h t absorbing material.  Surfaces of any other shape and  i n c l i n e d to the horizontal would give an a r t i f i c i a l v a r i a t i o n i n the rate of evaporation, depending upon the angle of i n c l i n a t i o n to the h o r i z o n t a l .  Surfaces other than black  would not absorb a l l incident energy and would, therefore, introduce arbitrary v a r i a t i o n i n the rate of evaporation. Latent or potential evaporation can be defined as "the maximum possible rate of change of water to water vapour from a black horizontal surface f u l l y exposed to a l l (72)  meteorological factors."* ' Such a d e f i n i t i o n would possibly be more c o r r e c t l y termed the "latent drying a b i l i t y of the a i r " , since the process so defined i s almost e n t i r e l y dependent upon the existing weather conditions.  Measurements of latent evapora-  t i o n would be r e l a t i v e and f o r s p e c i f i c problems the relationship of a c t u a l evaporation to latent evaporation (22) would have to be determined.  10  3.  E f f e c t of d i f f u s i o n versus turbulent transfer Molecules of water move away from the v i c i n i t y  of an evaporative surface by the phenomenon of d i f f u s i o n . D i f f u s i o n i s a very slow process and as a r e s u l t , there would be almost as many molecules returning as leaving the free water surface i f these molecules were not removed by the turbulent transfer and carried into higher layers of a i r . The e f f e c t of turbulent transfer i s by f a r the most effect i v e factor i n carrying water molecules away from an evaporating surface. Transfer by turbulence i s usually at least 25,000 times more e f f e c t i v e than molecular d i f f u s i o n and i t may be considered wholly responsible f o r any loss of moisture from an evaporating surface f r e e l y exposed to the a i r . 4.  Effect o f s a l t s on the rate of evaporation Various substances dissolved i n water w i l l  lower  the vapour pressure by d i f f e r e n t amounts. In a l l cases, as shown i n the following table, the equilibrium i s maintained at less than one hundred per cent r e l a t i v e humidity.  11 Relative Humidity maintained at s p e c i f i e d temperatures by various saturated saline solutions.  Temperature  KNO3  104  90%  92% 94% 95% 95%  95  86  77  68  59  95%  50 41 32  95% 96% 96%  Relative Humidity obtained by using a saturated solution o f : NaCl MgfNC^Jg 6H 0 MgCl 6H 0 L i C l 2  75% 75% 75% 75% 75%  76fo 76%  76% 76%  51% 51% 52%  52% 53% 53%  53% 54% 54%  2  2  32% 32%  16% 16$ 16% 16% 16%  31% 30% 30%  16% 16% 16%  33% 33% 33% 31%  16%  Generally, the evaporation from a free water surface w i l l be reduced i n proportion to the s a l t concentration.  This r e s u l t s  from the fact that the s a l t ions are at the water surface and i n t e r f e r e with a free movement of the water molecules.  The  suppression of evaporation may also be due to the a t t r a c t i o n that may exist between the solute and water molecules.  Also  there are less water molecules on the surface reducing the potential evaporating  area.  There i s always an exchange of water molecules between the l i q u i d water and the water vapour i n the a i r . The return of molecules from the a i r i s not influenced by d i s solved solute; however, the movement from the water surface i s suppressed which accounts f o r saturation at lower r e l a t i v e humidities. Sea water has a rate of evaporation two to three  (17) per cent less than s i m i l a r i l y exposed fresh water.  12  5.  V a r i a t i o n of evaporation with a l t i t u d e . At higher elevations evaporation should increase  as the result of the decreasing barometric pressures; however, due to the decrease i n temperature,  the rate of evapora-  t i o n frequently decreases with an increase i n elevation because the temperature  of the a i r and water are usually lower  at higher a l t i t u d e s .  VARIATION OF EVAPORATION WITH ALTITUDE. Evaporation i n per cent(5,ooo feet as 100%)  1  Q\  1  1  1  1  Diagram from O.E.Meinzer" Hydrology »,page 6 5  Figure No. 1  #  V a r i a t i o n of evaporation with a l t i t u d e .  13  B  Honomolecular Theory Certain types of organic compounds (fatty acids,  f a t t y amides, f a t t y alcohols)possess the property of forming a f i l m of one molecule thickness when applied to the water surfaces.  These molecules have i n t h e i r molecular structure  a hydrophilic portion which i s attracted by the water and a hydrophobic radicals  portion attached to one of the above hydrophilic  which i s repelled by the water. When packed together, the molecules stand on end  closely and form a f i l m which helps r e s i s t evaporation of the (2)  water thus covered.  An excess of the f i l m forming  material i s necessary at a l l times to keep the f i l m t i g h t l y packed and to repair i t i f i t should be broken. The rate of spreading of the monomolecular f i l m depends on the surface temperature of water and the characteri s t i c s of the chemical compound.  The average rate of spread-  ing f o r alcohols recommended f o r water conservation i s approximately twenty to t h i r t y centimeters per second at twenty degrees Centigrade. The f i l m of c e t y l alcohol i s about two m i l l i o n t h s of a millimeter thick, i s not v i s i b l e to the naked eye but i t reduces r i p p l e s  on the water surface so that the extent of  spreading can be observed.  As the spreading proceeds, the  surface pressure increases and the rate of spreading  decreases  u n t i l an equilibrium pressure of forty dynes per square centimeter i s reached.  14 To be p r a c t i c a l , a surface f i l m must meet the f o l lowing requirements: 1.  To form an unbroken surface f i l m over the entire water surface.  2.  To be impervious to water vapour.  3.  To be able to regenerate the f i l m when broken.  4.  To be i n v i s i b l e , odorless and non-toxic.  5.  To be e a s i l y generated by using chemicals of reasonable cost. In addition, i t must not seriously retard the d i f -  fusion of oxygen into and carbon dioxide out of water.  A  reduction i n the rate of movement of such gases would seriousl y upset the b i o l o g i c a l balance of bodies of water so treated. S c i e n t i s t s have reported c o n f l i c t i n g r e s u l t s regarding the e f f e c t of monomolecular f i l m s , some report even to the extent where f i s h and w i l d l i f e w i l l s u f f e r .  South  African reports indicate a reduction i n the oxygen supply i n water due to monomolecular f i l m s to the extent where the  (9) b i o l o g i c a l balance i s upset. Other research workers from A u s t r a l i a report no harmful e f f e c t s to l i v i n g organisms from (2) the monomolecular f i l m .  This difference i n r e s u l t s i s  doubtless due to the type of f i l m used.  15 IX A  REVIEW OF LITERATURE  Type of Evaporimeter versus Rate of Evaporation 1.  "Gross" and "net" evaporation. Evaporation i s usually measured from small metal  pans and translated to equivalent evaporation f o r large water areas through the use of reduction c o e f f i c i e n t s , which must f i r s t be determined  experimentally.  Evaporation  recorded from pans i s the true evaporation, with r a i n f a l l i n g on the pan accounted  f o r as water added.  This i s the  common method of c a l c u l a t i n g pan evaporation.  In some cases,  evaporation from reservoirs i s designated as "gross" or "net" evaporation.  Gross evaporation i s the actual depth of water  lost to the atmosphere.  Net evaporation i s the gross evapor-  ation minus the depth of water f a l l i n g on the water surface. Gross evaporation i s always p o s i t i v e but net evaporation i s negative i n months when true evaporation i s less than the rainfall.  The terms "gross" and "net" are used i n connection  with estimating reservoir losses but not i n connection with (17)  pan evaporation. 2.  Size of evaporimeter versus evaporation rate. Evaporation i s controlled by the nature of the  container through i t s response or reaction to the meteorologi c a l factors.  A deep body of water absorbs a large percentage  of the solar and sky radiation incident upon i t s surface. This energy i s distributed throughout  a large mass of water to  16  a depth of several feet.  The result i s that the diurnal  temperature v a r i a t i o n of the water surface i s small.  The  warming of such depths of water i n the early part of the summer u t i l i z e s some of the heat supply that vrould otherwise be available for evaporation.  Also the release of the  stored heat i n the f a l l and winter makes available a heat supply i n excess of that received at such times from the sun.  SIZE OF PAN VERSUS EVAPORATION RATE.  Diagram from C.W.Thornthwaite"The 'Water Balance" Drexel Inst.of Technology-T9^5.  Figure No. 2  Size of pan versus evaporation r a t e .  17 The preceding diagram shows that the size of the pan does not affect the evaporation to a great extent when the r e l a t i v e humidity i s high.  As the r e l a t i v e humidity  decreased, the size of the pan became more important since i t has been shown that higher losses occurred from smaller areas. The o v e r a l l evaporation rates from deep and shallow bodies of water of the same area are approximately  identical  but the deeper bodies of water have t h e i r rate of evaporation delayed i n the early part of the summer.  This same  phenomenon i s evident when the Glass A Land Pan i s compared with the Four Foot ground Tanks.  18  COMPARISON OF EVAPORATION FROM TANKS OF DIFFERENT DIAMETERS.  Diameter of tank i n f eet.  Mean C o e f f i c i e n t Deviation of c o e f f i c i e n t from annual mean(in %\ to be applied to Monthly Weekly each size to get evaporation from Mean Range Mean Range 12 foot tank.  3.8  -5.5 to +U.5  2.2  U.2  -10.7 to+U.6  3.8  to +17.0  U.7  -19.8 to+7.3  5.3  -15.9  to +9.6  h.I  -13.0 to+5.3  -19.5  to +23.6  3.6  -IlwO to+I3.0  to +15  9  99.0  -8  6  91.3  -10.5  3.39  52.9  -15.6  2  77.9  I  6U.5  to +13.0  5.6  Data by R.B.Sleight at Denver f o r 12 months 1915/1916 using round tanks three feet deep.  Figure No. 3  Comparison of evaporation from tanks of d i f f e r e n t diameters.  Generally, the smaller pans tend to show a higher rate of evaporation than the larger ones.  This i s due to  the higher r e l a t i v e humidity which exists above a larger body of water.  The wind more e f f e c t i v e l y removes the satur-  ated surface l a y e r of water vapour from smaller pans.  Where  the r e l a t i v e humidity i s high the difference i n rate of  19  evaporation between smaller and larger pans i s reduced. B  Evaporimeters 1.  Class A U.S. Weather Bureau Land Pan. Different types and sizes of evaporation pans are  i n common use.  The most popular one i n North America, and  with the most extensive l i s t of records, i s the Class A Weather Bureau Land Pan.  U.S.  It i s c i r c u l a r , four feet i n  diameter with a depth of ten inches.  The bottom of the pan (31)  i s raised s i x inches above the ground surface.  The  Water surface i s maintained within two or three inches of the top of the pan.  Evaporation i s measured with a hook  gauge i n a s t i l l i n g well. 2. . Bureau of Plant Industry Sunken Pan. Second i n popularity i n the United States i s the Bureau of Plant Industry Sunken Pan which i s six feet i n diameter and two feet deep.  The upper edge protrudes four  inches above the ground surface and the water l e v e l i s maintained within one h a l f of an inch of the ground l e v e l . Evaporation i s measured with a point gauge i n a s t i l l i n g well. 3.  Colorado Sunken Pan. The Colorado Sunken Pan i s three feet square  and  one and one-half to three feet deep, i t s upper edge i s four inches above the ground and the water l e v e l i s maintained within one inch ;J of the ground surface.  20  4.  The United States Geological Survey Floating Pan. The United States Geological Survey Floating Pan  i s three feet square and one and one-half feet deep.  It i s  i n s t a l l e d on a rectangular r a f t sixteen feet by fourteen feet dimensions, i n the lake or i n the reservoir and i t i s f i t t e d with necessary b a f f l e s to damp out surges or splashes. The upper edge of the pan Is three inches above the water surface.  The l e v e l of water i n the pan i s maintained at the  same l e v e l as the l e v e l of the surrounding water. 5.  Four-foot Ground Tank. The Experimental Farms Service i n Canada use as  t h e i r standard a tank four feet i n diameter and two feet deep.  I t i s sunk i n the ground with the rim extending above  the ground l e v e l by two inches.  The water l e v e l i s maintained (22)  at or close to the ground l e v e l . 6.  '  Black B e l l a n i Plate. It consists of a black porous c i r c u l a r disc of  seven and one-half centimeters i n diameter f i t t e d over a glazed porcelain funnel.  Water i s conducted to the funnel by  glass and rubber tubing from a reservoir below the l e v e l of the plate.  Because the whole system i s a i r t i g h t , the water  i s held i n contact with the plate by c a p i l l a r y tension. Black B e l l a n i Plate Atomometer was  The  suggested f o r t h i s purpose  by the F i e l d Husbandry D i v i s i o n of the Experimental Farm Service at Ottawa.  7///^/'/  s Roy N O  ^UJ  H_I_.^J  -  -GROUND  U.S. CLASS A  cm  4-FOOT  GROUND  M-J H_J \  ^JJ-TJ^CJ  U  S U R F A C E  V/EAJHSR BUREAU  - v - -  05 S3 O  DISC  A  THREE tfATE* Id  SUPPLY  GRADUATED  CONTAINER  B E L L A N I  P L A T L  WIDELY  USED  EV/lPORJMETERS.  /"  LAND PAN  T A N K  POROUS  v  22  7.  White B e l l a n i Plate. The White B e l l a n i Plate Atmometer i s i d e n t i c a l with  the Black B e l l a n i Plate i n i t s construction with the exception of a white evaporating surface replacing the black as described i n the former type. 8.  Piche Evaporimeter. This instrument consists of a graduated glass tube  open at one end and closed at the other.  The tube i s f i l l e d  with water and a porous paper disc i s placed over the open end.  The tube i s then inverted.  The popularity of t h i s  instrument has declined i n recent years. C  Comparison of Evaporimeters As would be expected the Black B e l l a n i Plate gave  the highest actual evaporation rate. Surface Area i n Square Inches  Evaporimeter Black B e l l a n i Plate White B e l l a n i Plate Piche Atmometer Summerland Tank Four-Foot Ground Tank  Amount Evaporated i n inches/month  6.85 6.85 1.60 113.10 1809.56  S.18 5.21 4.#1 4.67 3.44  Results f o r September 1953^" Black surfaces are believed to absorb solar energy more comp l e t e l y and thus should provide more energy f o r the conversion of water to water vapour.  1  G. W. Robertson, Latent Evaporation: I t s Concept, Measurement and Application, pp. 4-5.  23 It i s obvious that the measurement of evaporation i s only r e l a t i v e regardless of the instrument type.  In  order t o determine actual evaporation from measured latent evaporation i t i s f i r s t necessary to determine the r e l a t i o n ship between the latent evaporation and the actual evaporation. D  Evaporation Formulae There are several formulae f o r the c a l c u l a t i o n of  evaporation from water surfaces.  Some of these  formulae  include the barometric pressure but disregard the effect of wind, others include the wind e f f e c t s but ignore the barometric pressure.  The common part of each of the formulae  i s the pressure gradient derived from the o r i g i n a l Dalton principle.  As each of the following formulae was derived  under different climatic conditions, i t i s rather d i f f i c u l t to f i n d one equation that would meet universal requirements. (31)  The most commonly used formulae a r e :  w  '  Fitzgerald s T  E  =  (0.40 / 0 . 1 9 9 w) (pw-pa)  E  =  1.96 W  E  =  0.4 (X pw-pa) Where X = 2 - e '  Russel*s 43.88  ( p w  _  p a )  Horton's - 0  2 w  24 Rohwer* s E =  0.771  (I.465-O.0186B)(0.44 / 0.11&V) (pw-pa)  Meyer s 1  E =  15  (V-v)(1 / W ) TO"  See Nomenclature as l i s t e d on page 1-2. The formula proposed by Meyer i s recommended to be used f o r calculating evaporation rates from small bodies of water including evaporation pans. In order to compare the above formulae,  the  meteorological information from the Vancouver Airport Meteorological Station f o r the month of August 1956  was used  and the evaporation rates -were calculated by using the above mentioned formulae.  The calculated values d i f f e r e d as much  as one inch to f i f t e e n inches per t h i r t y days. Where the wind v e l o c i t y at the ground surface was required, i t was necessary to calculate such information that was not normally recorded at the Vancouver Meteorological Station.  The standard height of the wind measuring i n s t r u -  ments was t h i r t y to seventy feet above the ground surface. By using a graph prepared by C a r l Rohwer ( 3 ) , ^he recorded 2  wind v e l o c i t y was converted to the wind v e l o c i t y at the ground l e v e l .  25  Ratio- of Observed Wind to around Wind  Height  versus  wind  speed  Figure No. 5 The relationship between recorded wind v e l o c i t y and ground wind v e l o c i t y .  26  E  Insolation Method Another approach i n c a l c u l a t i n g evaporation i s the  Insolation Method.  This method i s based on the p r i n c i p l e  of conservation of energy i n a body of water.  A balance  must exist between (a) i n s o l a t i o n , (b) heat transferred by the water surface by r a d i a t i o n , conduction or convection, (c) the heat required to raise or lower the water temperature and (d) the heat dissipated by evaporation or acquired by condensation.  Factors a, b and c can be measured and d  can be calculated. For example, Cummings and Richardson^ 3) have 2  proposed a formula f o r c a l c u l a t i n g evaporation: E  r C  =  I - S - 0 L (1 / R)  R - Bowen's Ratio i s a r a t i o of sensible heat to latent heat computed by using a formula: R  -  0.46  (P  x  (t]_ - t ) P 2  - P ) 2  760  This formula i s independent of the wind but contains terms that are affected by the a l t i t u d e .  27  X A  MATERIALS AND METHODS  Materials 1.  Four-foot tanks. For the f i e l d experiment, four separated four-foot  diameter galvanized tanks as shown i n Figure No. 6 , were i n s t a l l e d i n the ground within the enclosure of the Summerland Experimental Station Weather Station. Two of the tanks were i n use previously and the other two were i n s t a l l e d f o r use i n t h i s  Figure No. 6  experiment.  Row of tanks at the Summerland Experimental Station.  28  The tanks were of c y l i n d r i c a l design - twenty-four inches deep with an inside diameter of 47-1/2 inches.  The  upper edge was two inches above the ground surface when the tanks were i n position.  For each tank a s t i l l i n g well was  made and the f i x e d point was i n s t a l l e d exactly two inches below the upper edge of the tank, which corresponded to the ground l e v e l around the tank.  The amount of water evaporated  every day was determined by adding water from a p l a s t i c gauge.  One d i v i s i o n of the p l a s t i c gauge was equivalent to  one hundredth of an inch of evaporation.  The readings were  taken to the nearest one hundredth of an inch. The time of f i l l i n g the tanks, which was between seven A.M.  and seven t h i r t y A.M.  P a c i f i c Standard Time, co-  incided with the time the other meteorological equipment was read.  The experimental tanks were f i l l e d every morning and  any f l o a t i n g foreign matter was removed from the surface except during the periods when the effect of the impurities upon the c e t y l alcohol f i l m was being studied. Water f o r f i l l i n g the tanks was obtained from the domestic supply, which was pumped from the Okanagan Lake. There was a water hydrant located within the Weather Station enclosure. Tanks were i n s t a l l e d i n a row approximately i n an East-West  d i r e c t i o n and numbering started from the East side.  28  The tanks were of c y l i n d r i c a l design - twenty-four inches deep with an inside diameter of 47-1/2 inches.  The  upper edge was two inches above the ground surface when the tanks were i n p o s i t i o n .  For each tank a s t i l l i n g  well was  made and the fixed point was i n s t a l l e d exactly two inches below the upper edge of the tank, which corresponded to the ground l e v e l around the tank.  The amount of water evaporated  every day was determined by adding water from a p l a s t i c gauge.  One d i v i s i o n of the p l a s t i c gauge was equivalent to  one hundredth of an inch of evaporation. The readings were taken to the nearest one hundredth of an inch. Tlje time of f i l l i n g the tanks, which was between seven A . M . and seven t h i r t y A . M . P a c i f i c Standard Time, coincided with the time the other meteorological equipment was read.  The experimental tanks were f i l l e d every morn-  ing and any f l o a t i n g foreign matter was removed from the surface except during the periods when the effect of the impurities to the c e t y l alcohol f i l m was being studied. Water f o r f i l l i n g the tanks was obtained from the domestic supply, which was pumped from theOkanagan Lake. There was a water hydrant located within the Weather Station enclosure. Tanks were i n s t a l l e d i n a row approximately i n an East-West d i r e c t i o n and numbering started from the East side.  29  2.  Solar r a d i a t i o n recordings Radiant solar energy i s the ultimate source of  p r a c t i c a l l y a l l energy on the earth.  The study of r a d i a -  t i o n from the sun i s of great value i n the f i e l d of meteorology. Solar energy i s received at the outer edge of the atmosphere at an average rate of 1.94 square centimeter per minute. "solar constant".  gram c a l o r i e s per  This rate i s known as the  Only a portion of the t o t a l energy  reaches the earth's surface.  Insolation i s the term used  f o r the amount of energy received on the horizontal surface.  I t depends upon the s o l a r constant,  a l t i t u d e of the  station, i n c l i n a t i o n of the incident rays to the h o r i z o n t a l (influenced by l a t i t u d e , time of the day and time of the year), moisture i n the a i r , dust and cloud effect and depletion while passing through the atmosphere. factors influence the rate of i n s o l a t i o n .  A l l these  Figure No. 7  MSC Type G Bimetal Actinograph at the Summerland Experimental Station.  For recording the solar radiation the MSG Type G Bimetal Actinograph was used at the Summerland Experimental Station Weather Station.  I t was a continuous s e l f record-  ing instrument i n d i c a t i n g the rate of r a d i a t i o n i n gram c a l o r i e s per square centimeter per minute.  From the graph  the rate of solar r a d i a t i o n can be determined f o r each hour.  The column of hourly values t o t a l l e d to give the  t o t a l radiation per day.  31  3.  Sunshine Recorder. The chief purpose of sunshine recorders i s to  enable the hourly t o t a l s of the duration of bright sunshine to be measured accurately to the nearest tenth of an hour. At the Summerland Experimental Station Weather Station a Campbell-Stokes Recorder was used.  The recorder  consisted e s s e n t i a l l y of a glass sphere four inches i n diameter, mounted concentrically i n a section of a spherical bowl, the diameter of which i s such that the sun's rays were focused sharply on a card held i n grooves i n the bowl.  The  focused rays from the sun burned a trace i n the chart.  The  movement of the trace i s opposite to that of the sun.  Three  overlapping grooves were provided i n the bowl to take cards suitable f o r d i f f e r e n t seasons of the year.  Cards i n the  recorder were changed d a i l y between seven and seven t h i r t y A.M. P a c i f i c Standard Time. 4.  Wind v e l o c i t y recordings. Wind mileage passing the anemometer was recorded  daily.  A three-cup anemometer recorded continuously the  wind mileage within one one-hundredth of a mile.  By d i v i d i n g  the d a i l y t o t a l wind mileage by twenty-four the average wind v e l o c i t y was calculated f o r each twenty-four hour period.  Figure No, 8 The anemometer located within the Weather Station enclosure. 5.  Relative humidity recordings The r e l a t i v e humidity was determined by means of  a s l i n g psychrometer.  The wet and dry bulb temperatures  were recorded every morning and the r e l a t i v e humidity was found by the use of United States Weather Bureau tables.  33  The r e l a t i v e humidity values i n the United States Weather Bureau t a b l e s have been calculated by the formula: e - el= 0.000367 P (t -t )(l / t 0  d  w  w  - 32)  1571  See L i s t of Nomenclature on page 1 - 2 6.  Temperature Recordings. a)  Ambient a i r temperature  The ambient maximum and minimum temperature was recorded as a part of the routine meteorological observations made at the Summerland Weather Station. The thermometers were housed i n a thermometer screen which keeps out the direct rays of the sun but enables a i r to c i r c u l a t e f r e e l y .  The height of the thermometers  above ground was f i v e feet. b)  S o i l Temperature  S o i l temperature readings were continuously recorded by automatic recorders at s i x inches, twenty-four inches and forty-eight inches of depth.  For the purposes of  t h i s paper, only readings of s i x inches and twenty-four of depths were considered necessary.  inches  Mercury thermometers  have been i n s t a l l e d within the Weather Station enclosure and the wires were connected to the automatic recorders i n the Weather House. c)  Water temperature  Minimum and maximum water temperatures  i n the four-  34  foot diameter tanks were measured by a minimum and maximum thermometer supported on a stand holding the thermometers three inches below the water surface. 7.  Barometric Pressure  Readings.  Due to the absence of a barometer at the Summerland Experimental Station Weather Station, barometric readings i n values of m i l l i b a r s were obtained from the Penticton Airport Weather Station. B  Methods 1.  Temperature recordings. During the early part of the recording period,  neither minimum nor maximum temperature i n the tanks.  Within the enclosure of the Weather Station  recording of water temperature r i e d out regularly. temperatures  readings were taken  i n a Class A Land Pan was car-  I t was believed i n i t i a l l y that the water  i n the Class A Land Pan and i n the four-foot  tanks were i d e n t i c a l .  This assumption was found to be wrong  and minimum and maximum thermometers were i n s t a l l e d i n tank # 1 .  The thermometers were supported by a stand holding  them approximately three inches below the water surface.  Fol-  lowing the i n s t a l l a t i o n of thermometers, water temperature was recorded every morning. During the f i r s t two weeks, readings were taken without the addition of c e t y l alcohol to the tanks.  By com-  35  paring the evaporation figures the tanks were calibrated i n relationship to each other.  (See appendix I) Following the  correlation between tanks, varying quantities of c e t y l alcohol were added. 2.  Testing of surface f i l m . When the surface pressure of the monomolecular  f i l m i s low the molecules are dispersed and do not form a tight surface cover.  At pressures below f i v e dynes per square  centimeter there i s l i t t l e retardation of evaporation but i t increases u n t i l the effect reaches i t s maximum at approximately f o r t y dynes per square centimeter.  The surface pres-  sures can be tested by the use of indicator o i l s .  Many o i l s  or o i l y substances when placed on the water surface spread and t h e i r spreading pressures can be evaluated.  I f a drop of  such indicator o i l i s placed on the water surface covered with c e t y l alcohol f i l m i t w i l l spread i f i t s pressure i s greater than that of the f i l m on which i t i s placed.  The spreading  pressures of various o i l y substances are known and by t e s t ing the treated surface with various o i l s the f i l m pressure can be  determined.^ ^ 2  36  Name of Substance  Spreading pressure i n dynes per square centimeter  Hexadecyl Acetate Oleic Acid Tri-Olein Laurie Acid Castor O i l M y r i s t i c Acid Carbon Disulphite Stearic Acid  34.4 30.0 22.0 21.0 17.0 11.0 2.3 1.5  During t h i s investigation Oleic Acid and Castor O i l were used as the t e s t i n g substances.  These two l i q u i d s were  selected because i t was believed that seventeen to t h i r t y dynes per square centimeter represented a c r i t i c a l pressure range.  Cetyl alcohol was  dissolved i n a gasoline which did  not contain any lead compounds.  The concentration  of the  c e t y l alcohol varied between f i v e per cent and t h i r t y per cent by weight.  The method of a p p l i c a t i o n of c e t y l alcohol  to water surface i n gasoline solution has been referred to as the "Mansfield Process" which has been successfully used i n Australia.^ ) 2  37  XI A  RESULTS AND  DISCUSSIONS  The effect of impurities on the monomolecular f i l m As shown i n Figure No.  11 the effectiveness of the  monomolecular f i l m on evaporation retardation i s greatlyreduced by the accumulation of floating organic material the surface.  I t was  found by t r i a l s that i f the  on  surfaces  of the tanks were not cleaned d a i l y , the differences i n the evaporation rate from free water surfaces and from surfaces covered with a monomolecular f i l m decreased r a p i d l y and a f t e r a lapse of three days no difference could be observed i n the evaporation rate.  The observations  were made during  the  month of July when during dry weather the amount of pollen, dust and airborne material was  believed to be unusually high.  It seems that the impurities have a rapidly deteri o r a t i n g effect upon the a b i l i t y of c e t y l alcohol to retard the rate of evaporation.  I t was  also observed that the  f l o a t i n g material accumulated more rapidly on the tanks covered with the c e t y l alcohol f i l m .  Clusters of f i l m  and  vegetable matter were formed which interrupted the continuity of the f i l m upon being moved about the surface under the influence of the wind.  FEB  •  59  Figure No. 9 Floating organic matter on the water surface a f t e r twenty-four hours.  Figure No. 10 Floating organic matter on the water surface a f t e r seventy-two hours.  ;  Cthjl akohol odctzd, surfacecleaned daily.  i  Coiij/ alcohol added so rface cleaned ctoily,  alcohol addedj &/y7 alcohol addad \ sorfa.ee oof' cleaned ^Surface not cleaned  1  t  0.4-  OA  0.3  03  T  H-  (ft 5: CD  0.1  —t  0.2  o o o  f ree  hnafar  Surface.-  water covered hiitHi hionomoltcolar film  0.  f  4o.l  V  O.Q\-  4  6  7  8  9 / 0  . DAYS : 3  rd  TM  EFFECT  OF  to 20*  /;  July  IMPURITIES  12,  A3  /4  /5  • '6  -4 (7  , /&T£ ON.  MONOllQiECUL&R  FILM,  J8  '9  20  40  The impurities became heavily coated with the c e t y l alcohol molecules, which may have resulted i n the formation of an incomplete monomolecular  film.  Although when excess  amounts of c e t y l alcohol were added to the tanks, while covered with impurities, the reduction of evaporation was not B  increased to any appreciable extent. Water temperature v a r i a t i o n i n ground tanks and i n a land pan The high s o i l temperatures that prevailed during  the  summer kept the water temperature high i n the ground  tanks.  There was a marked difference i n the temperature  range i n the ground tanks and i n the Class A Land Pan.  The  minimum water temperature i n the ground tanks was found to be ten to f i f t e e n degrees Fahrenheit higher than i n the Class A Land Pan.  The spread i n the minimum temperatures  was most noticeable when the a i r temperature dropped low at night and the Class A Land Pan without direct contact with warm s o i l did not receive any heat from i t as compared with the  ground tanks which were supplied with heat from a source  of stored solar energy.  (Refer to Appendix II)  Differences i n the maximum temperatures between the two classes of evaporimeters varied a few degrees only, the temperature of water i n the Class A Land Pan being s l i g h t l y higher.  This could be due to a smaller volume of water that  would warm up more rapidly when receiving the same amount of  41 solar energy per unit area.  Also the Class A Land Pan i s  insulated by an a i r layer and the ground tanks are i n t o t a l contact with s o i l having a lower temperature than the maximum  temperature of the water.  The ambient a i r temperature  and the temperature of the water i n the Class A Land Pan were quite similar to each other, varying only a few degrees. Diurnal variations i n the water temperature i n the Class A LandPan a r e more pronounced than i n the deeper Four-foot Tank, and consequently a r e l a t i v e l y high percentage of evaporation occurs from the Four-foot Tank a f t e r the sunset. C  The effect of a monomolecular  f i l m on the diurnal  v a r i a t i o n of the evaporation rate The information obtained from the experiments revealed that a greater diurnal difference occurred i n the rates of evaporation from the surfaces covered with a monomolecular f i l m than i n the evaporation rates from a free water surface (Refer to Appendix I I I ) . There was a markedly lower evaporation rate during the day through a monomolecular from an untreated surface.  f i l m of c e t y l alcohol than  At night the rate of evaporation  from a free water surface and from a surface covered with a monomolecular  f i l m were approximately the same, with the loss  from free water surfaces being s l i g h t l y higher. This observat i o n applies to the four-foot ground tanks as these were used  42  as evaporimeters i n the monomolecular  f i l m experiments.  Read-  ings were taken at 7:00 A.M. and 7:00 P.M. P.S.T., dividing a twenty-four hour period into day and night. The duration of bright sunshine after 7:00 P.M. was n e g l i g i b l e , mostly due to the shading effect of the mountains that surrounded the Weather Station i n a southerly and westerly direction. Since the rate of evaporation depends upon the vapour pressure gradient and the water temperature i n the ground tanks the evaporation rate was at the maximum around 7:00 P.M. P.S.T.  The higher evaporation rate during the  early part of the night can be e a s i l y explained.  At that  time the water temperature was at i t s maximum and the greatest number of molecules escaped to the a i r due to t h e i r higher velocity. Temperature of the Black B e l l a n i Plate has been found t o be usually between the ambient a i r temperature and the wet bulb temperature.^ ^ 22  The temperature of the four-  foot ground tank appeared to remain higher than a i r temperature when the l a t t e r was low.  Thus at 8:30 A.M. the water tempera-  ture i n the tank averaged one to four degrees above the ambient a i r temperature. On the other hand the effect of retardation of evaporation depends upon the spreading pressures of the monomolecular f i l m .  With an increase i n temperature, the spread-  ing pressure increased.  During the hours of sunlight the d i r -  43  ect  rays of the sun penetrated the f i l m and the water s u r -  f a c e was warming up the t o t a l volume o f water. passed through  As the energy  the s u r f a c e , the h i g h e s t temperature p r e v a i l e d  at and near the s u r f a c e making the monomolecular f i l m more effective i n controlling  evaporation.  During the n i g h t the c o o l i n g process s t a r t e d at the s u r f a c e and f i r s t lower temperature.  the f i l m was Secondly,  w i t h i n the water i t s e l f was  rendered  l e s s e f f e c t i v e due  to  a continuous molecular movement  t a k i n g p l a c e as the  molecules  c l o s e r to the s u r f a c e cooled, more r a p i d l y and moved down due to the i n c r e a s e i n t h e i r s p e c i f i c weight.  As long as such  process continued, d i s t u r b a n c e i n the s u r f a c e f i l m can expected,  a  be  a f a c t t h a t could have c o n t r i b u t e d c o n s i d e r a b l y t o  the decrease  of e f f e c t i v e n e s s o f the s u r f a c e f i l m .  During the n i g h t , the e f f e c t of c e t y l a l c o h o l as a monomolecular f i l m was ing evaporation.  comparatively  i n e f f e c t i v e i n prevent-  I t can be concluded  from the  experimental  r e s u l t s that the main e f f e c t of r e t a r d a t i o n of the r a t e of evaporation by c e t y l a l c o h o l o c c u r r e d d u r i n g the hours of sunl i g h t when high a i r temperatures U n f o r t u n a t e l y i t was  prevailed.  not p o s s i b l e to f i n d e x p e r i -  m e n t a l l y the e f f e c t of a monomolecular f i l m on the r a t e from a C l a s s A Land Pan.  evaporation  That i n f o r m a t i o n c o u l d be  v a l u e from the r e s e a r c h p o i n t o f view.  The p r a c t i c a l  ance of i t would be l i m i t e d t o areas where the r a t e exceeded the annual p r e c i p i t a t i o n  of  import-  evaporation  as i n shallow  bodies  44  of water resembling the Class A Land Pan. D  The minimum amount of c e t y l alcohol required to affect materially the rate of evaporation During t h i s experiment various amounts of c e t y l  alcohol were added to the water surface and the r e s u l t i n g effects were observed.  See table IV i n Appendix.  Five drops of a twenty per cent solution by weight of c e t y l alcohol were added to 1 2 . 3 0 square feet of water surface which i s equivalent to 0 . 1 1 pounds of chemical per acre.  This provided a f i l m on tanks with a f i l m pressure i n  excess of t h i r t y dynes per square centimeter.  Duration of the  f i l m formed with such an addition was short and the surface pressure dropped to less than t h i r t y dynes per square meter pressure i n a lapse of one day.  centi-  I t was therefore  necessary to replenish the f i l m d a i l y and before the end of a twenty-four hour period i t was s u f f i c i e n t l y weakened to be only p a r t i a l l y e f f e c t i v e .  I t was also observed that the f i l m  became progressively less e f f e c t i v e with the passing of time from the time of application even before the spreading pressure dropped below t h i r t y dynes per square  centimeter.  Part of the explanation of t h i s phenomenon can be traced back to the v a r i a t i o n i n s o l u b i l i t y of the c e t y l alcohol.  At a higher temperature the amount of c e t y l alcohol  dissolved i n a gasoline solution was higher than at a lower temperature.  Applications of c e t y l alcohol and gasoline  solutions to the water surface permitted the gasoline to  45  evaporate r e a d i l y leaving the molecules of c e t y l alcohol f l o a t i n g on the surface. Any excess of c e t y l alcohol seemed, to form s o l i d c l o t s that floated on the surfaces of the water. temperature of the water decreased  As the  at night, more c e t y l  alcohol collected around the e x i s t i n g lumps formed on the inside surface of walls of tanks or formed new lumps.  Dur-  ing the next day the water temperature increased, but since there was no solvent present, the process could not be reversed and only the amount of c e t y l alcohol that remained on the surface as a monomolecular f i l m during the lowest d i urnal temperature was e f f e c t i v e i n retarding the rate of evaporation from a water surface during the following day. There was a d e f i n i t e c o r r e l a t i o n , within l i m i t s , between the amount of cetyl alcohol added and r e s u l t i n g reduction i n evaporation.  A f t e r a complete monofilm was  formed a d d i t i o n a l amounts of c e t y l alcohol caused l i t t l e , i f any change i n the rate of evaporation. During the test period, an approximately  26 per  cent reduction i n evaporation rate was achieved, with an application of two and. one t h i r d pounds per acre.  When  smaller amounts than t h i s were used, the surface f i l m often dissipated i n less than a 24 hour period and the rate of evaporation increased before a d d i t i o n a l s o l u t i o n was added. When the rate of two and one t h i r d pounds per acre was doubled the increase i n the reduction of the  evaporation  46  rate was q u i t e small and. the saving i n water would hardly warrant the increased expense.  Refer to Appendix I V .  47  XII A  SUMMARY AND CONCLUSIONS  Summary 1.  Film pressure v a r i a t i o n . During the experimental period the r e s u l t s varied  within a wide range from day to day. Some of the factors causing the v a r i a t i o n were determined but s t i l l some questions remained unanswered. As observed during t h i s experiment i t was quite d i f f i c u l t to correlate the effectiveness of the monomolecular f i l m to the surface pressure of the f i l m .  The surface  pres-  sure was maintained i n excess of t h i r t y dynes per square centimeter but the evaporation rate varied greatly depending upon the t o t a l amount of c e t y l alcohol added.  When the f i l m  pressure dropped below t h i r t y dynes per square centimeter there was l i t t l e e f f e c t on the rate of evaporation and under these conditions the surface appeared to behave s i m i l a r l y to a free water surface. f i l m pressure,  Instead of being able to measure the  some other way of assessing the effectiveness  of the f i l m i s necessary.  The use of indicator o i l s w i l l i n -  dicate the presence or absence o f the monomolecular f i l m and w i l l also make i t possible to f i n d the pressure range, but i t does not help to evaluate the effectiveness of the f i l m . 2.  The e f f e c t of wind. The. action of the wind had a d e f i n i t e effect on the  monomolecular f i l m as observed by v i s u a l inspection.  There  48  were no means at the disposal of the author by which an accurate measurement could be made regarding the effect of the wind.  The t o t a l wind mileage was recorded f o r each twenty-  four hour period but the e f f e c t of sudden gusts of wind on the  monomolecular f i l m cannot be evaluated with the available  information.  I t was observed that the monomolecular f i l m was  disturbed even more by these gusts of wind when f l o a t i n g organic matter was present, but there was no means available to correlate the duration and v e l o c i t y of the wind and i t s effect on the monomolecular layer. 3.  The cumulative effect of a c e t y l alcohol f i l m . Also the cumulative e f f e c t of c e t y l alcohol was  d i f f i c u l t to determine. By the nature of the experiment i t was not possible to remove the o l d monomolecular f i l m and apply  a new one d a i l y .  As a rule the additional amount of c e t y l  alcohol was added to the existing f i l m .  There were no means  for determining the amount of c e t y l alcohol i n the active monomolecular f i l m l e f t from the previous application, at the time a new a p p l i c a t i o n was made.  As previously discussed a  certain amount of c e t y l alcohol was rendered i n e f f e c t i v e due to  i t s formation into s o l i d c l o t s , but the exact amount was  impossible to determine. So i t was therefore possible that a d a i l y application, f o r example, of 1.09 pounds per acre, a f t e r a lapse of a few days, could have had the effect of a single application of 2.34 pounds per acre, due to the cumulative effect.  49  B  Suggestions f o r Further Studies 1.  Lack of experimental data with large bodies of  water.  In order to obtain r e s u l t s of a p r a c t i c a l economic value, t e s t s need to be conducted with larger bodies of water.  A small lake or an a r t i f i c i a l reservoir would be  s a t i s f a c t o r y f o r that purpose. 2  •  The e f f e c t of wave action. The e f f e c t of wave action on the s t a b i l i t y of the  monomolecular f i l m warrants f u r t h e r studies.  It i s antici-  pated that the waves w i l l destroy the f i l m and a more frequent replenishment of the f i l m would be necessary. The effect of waves i n r e l a t i o n s h i p to monomolecular f i l m s i n larger bodies of water i n Western Canada has not as yet been determined experimentally. 3.  Need f o r continuous temperature recordings. Only the minimum and maximum a i r and water  temperatures were recorded f o r each twenty-four hour period. Due to sudden overcast periods or rainshowers the temperature dropped occasionally f o r a short period and then i n creased again soon after the previous reading.  Such v a r i a -  tions were l e f t e n t i r e l y unrecorded, regardless of the fact that these had a definite a f f e c t on the rate of evaporation.  50  In future studies of evaporation an automatic  continuous  temperature recording device would enable one to evaluate the effect of temperature on the rate of evaporation more accurately.  51  XIII  LIST OF REFERENCES  1 Baier, W., Neue Ergebnisse von vergleichenden Verdustungsmessungen i n F r e i l a n d , International Associat i o n of Hydrology, Rome, Agrometeorologische Forchungss t e l l e , Stuttgart, Hohheim. 2 Beale, B. W. and Cruise, R. R. , Water Conservat i o n through Control of Evaporation, 1947, Journal of American Water Works Association, v o l . 49, No. 4. 3 Birge, Edward A., Solar Radiation and Inland Lakes, 1933, Wisconsin Academy of Sciences, Arts and L e t t e r s (Madison) Transactions, v o l . 31. 1  4 Birge, Edward A., Transmission of Solar Radiation by the Waters of Inland Lakes, 1929 Wisconsin Academy of Sciences, Arts and Letters (Madison) Transaction, v o l . 24,  pp. 510-530.  5 Bowen, I . S., The Ratio of Heat Losses by Conduct i o n and by Evaporation from any Water Surface, 1926^ Phys. Review, v o l . 27, pp. 779-7871 6 Cummins, N. W. and Richardson,B., Evaporation from Lakes, 1927, Phys. Review (2) 30, pp. 527-3JC 7 Cummins, N. W,, Evaporation from Water Surfaces, 1936, Trans. Am. Geophysical Union, v o l . 17, pp. 507-509. 3 Dressier, R. G. and Johanson, A. G., Water Reservoir Evaporation Control, 1953, Chemical Engineering Progress, v o l . 57, No. 1 January. 9 Department of Water A f f a i r s , Chemicals Combat Evaporation, Farming i n South A f r i c a , March, 1958. 10 Follansbee, R., Evaporation from Reservoir Surfaces, 1934, Trans. A. S . C. E., p. 99.  52 11 Hederstrand, G., The Influence of Thin Surface Film on the Rate of Evaporation of Water Below the B o i l ing Point, Journal of Physical Chemistry 28:1945 (Copyr i g h t 1924). 12 Hickcox, G. H., Evaporation from a Free Water Surface, 1946, Trans. A. S. C. E., v o l . 3, pp. 1-33. 13 Langmuir, I. andLangmuir D. B., The E f f e c t of Monomolecular Films on the Evaporation of Ether Solutions, 1927, Journal of Physical Chemistry, p. 1719. 14 Loveland, C. A. and Perrin, S. W., Evaporation of Water from Free Surface at Lincoln, Nebraska, 25th Annual Report Nebraska A g r i c u l t u r a l Experimental Station, 1957, p. 193. 15 Mansfield, W. W., Influence of Monolayers on the Natural Rate of Evaporation of Water. 1955, Nature, pp. 175-245. l6_Mansfield, W. W., Influence of Monolayers on the Natural Rate of Evaporation of Water, 1955, Nature, pp. 180-250. 17 Meinzer, 0. E., Hydrology, New York, McGraw-Hill, 1942 p. 65. 18 McEwen, G. F., Results of Evaporation Studies, 1931, Scripps I n s t i t u t e of Oceanology Technical Service, v o l . 2, pp. 401-415. 19 Penman, H. L., Natural Evaporation from Open Water. Bare S o i l and Grass, 1948, Proc. Royal Society, An. 193, p. 120. 20 Prescott, J . A., A Relationship between Evaporation and Temperature, 1943, Trans. Royal Society, A u s t r a l i a , p. 67.  53 21 Price (Bromborough) Ltd. Technical B u l l e t i n No. Fatty Alcohols f o r Water Conservation.  1,  22 Robertson, G. W., Latent Evaporation: I t s Concept, Measurement and Application, pp. 4-5, Unpublished. 23 Rohwer, C., Evaporation from Free Water Surfaces, 1931, United States, Department of Agriculture, Technical B u l l e t i n No. 271. 24 Rosano, H. L. and LaMer, V. K . , The Rate of Evaporat i o n of Water through Monolayers of Esters, Acids and Alcohols, 1955 > Columbia University, August, p. 23. 25 Rosano, H. L. andLaMer, V. K., The Rate of Evaporat i o n through Monolayers of Esters, Acids and Alcohols, The Journal of Physical Chemistry, v o l . 59 and 60. 26 Sleight, R. B., Evaporation from the Surfaces of Water and Riverbed Materials, Journal, A g r i c u l t u r a l Research, 1917, v o l . 10, No. 5, op. 209-262.  1949,  27 Thorne, D. W. and Peterson, H . B„, Irrigated S o i l s , New York, Blakiston Company.  28 Thornthwaite, C. W., The Water Balance, Drexel Institute of Technology, Centerton, N.J.  1955,  29 Thornthwaite, C. W., Evaporation from Land, D. A. Technical B u l l e t i n 817.  U.S.  30 Whitlow, E. P. and. Cruise, R. R., Reduction of Water Losses by Retardation of Evaporation, Paper presented at the Eighteenth Annual Water Conference, Pittsburg, Pennslyvannia, October 1957. 31 Wisler, C. 0., Wiley Publishing Co.  and Brater, E. F., Hydrology,  1949,  32 Young, A. A., Evaporation from Water Surfaces i n C a l i f o r n i a , 1947, Department of Public Works, B u l l e t i n No. 54, p. 57.  54 Appendix I S t a t i s t i c a l Analyses to Compare Individual Evaporating Tanks with Each Other. C a l i b r a t i o n period from June 6 to 15, 1958. Date  Tank #1  Tank #2  Tank #3  Tank #4  0.23  0.23 0.23 0.19 0.16 0.07 0.11 0.07 0.17 0.15 0.24  0.25 0.27 0.19 0.16 0.07 0.16 0.07 0.19 0.15 0.25  0.21 0.30 0.18 0.16 0.07 0.16 0.07 0.18" 0.14 0.24  June 6 7 8 9 10 11 12 13 14 15  0.29 0.18 0.16 0,07 0.11 0.07 0.18 0.15 0.24  92 114 74 64  28  54 28 72 59 97  ANALYSIS OF VARIANCE V a r i a t i o n Due to:  Sum of Squares:  Degrees of Freedom  Mean Squares  5  3  1.666  1769  9  Error:  38  27  Total:  1812  39  Tanks: Days:  196.555 1.407  F  Not significant Highly significant  -  The analysis was performed by Dr. T. H. Anstey at the Summerland Experimental Station, Surnmerland, B.C. As a result of t h i s analysis i t was found to be unnecessary to apply any corrections to the measured evaporation values.  55 Appendix I I Table V. Temperature V a r i a t i o n i n Ground Tanks As Compared with the Class A Land Pan. Minimum A i r Ground Land Tank Pan Water Water Op Op o  Date  F  June 24 72  July  25 26 27 28 29 30 1 2 3 4 5 6 7 8  64 60 60 50 54 52  53 59 58 59 64 58 62 62 9 56 10 61 11 68 12 64 13 53 14 55 15 57 16 61 17 65 18 65 19 64 20 63 21 68 22 73 23 61 24 58  73  58 59  70 70  53 51 54 54 54 56 58 59 61 60 62 61 56 59 63 61 52 58  65 67 66 66 71 73 74 73 68 69 70 69 69 70 74 68 67 69 69 67 72  55 57  72 72 71  *  79 80 85 69 68 68 71 76 81 89  87 83  81  81  80 85 90 91 84 77 84 86 95 98  61 64 61 58  97 97  64 54 55 61  87 85 89  61  73 68 68 68  Maximum A i r Ground Land Tank Pan Water Wat er Op Op °F  94 98  85  77 78 73 72 75 77 82 81 83 83 82 82  80 79 80 84 83  80  80 80 82 85 84 82 85 84 82 82 79 82  79 76 80 67 66 71 71  80 79 85 86 83 84 77 79 84 89 86 78 75 83  86 89 90 86  89 88 85  84 82 87  Solar Radiation in cal/cm^ per day*  Hours 0 : Bright Sunliglr  640 636 650 693 497 167  11.2 13.5 11.3 13.1 8.8 9.8  414 661  13.2  5.0 9.7  417  697 565 628 590 737 723 712 697 562 700 685 614 717 736 717 520 688 720 436  13.2 11.6 9.0 13.5  _  13.5 13.3  664  14.4 14.2 14.3  12.9 7.7  13.8 13.5 13.0  11.3 14.1  11.7 11.1  13.1 13.2  Approximate values before correction.  6.6 7.4  56 Appendix I I I Diurnal V a r i a t i o n i n the Evaporation Rates Ambient A i r Ternperature  4 FOOT GROUND TANK Water Evaporation Temp. Loss MonomoMin. Max. Min. Max. Free l e c u l a r Op Op Op Op Water Film  Date  Time 58 July 24 Day Night  89  68  82  44.8% 35.0% 55.2% 65.0% 50.0% 30.0% 50.0% 70.0%  25  Day Night 68  89  71  80  26  Day Night 70  88  72  82  66  94  73  84  28 Night 67  89  72  80  42.5% 41.1% 57.5% 58.9% 46.8% 30.0% 53.2% 70.0%  Day Night 63  78  70  77  40.0% 60.0%  30 Night 59  90  80  81  56.5% 43.5%  31 Day Night 57  90  69  81  46.0^  54.0%  39.4% 60.6%  Day Night 63  87  69  81  66.7% 66.7%  33.3%  33.3%  93  67  81  45.0%  47.6%  78  65  75  67.0% 33.0%  64.0% 36.0%  Day  27 Night Day  29  Day  Aug. 1  Day  2 Night 59 Day  3 Night  60  Day Night  Evaporation Losses From Black Class A B e l l a n i Pan Plate  51.5% 37.4% 62.6%  48.5%  41.6% 58.4% 46.0% 54.0%  55.0% 52.4%  67.5% 32.5% 58.8% 63.0% 41.2% 37.0% 52.0% 75.0% 48.0% 25.0% 31.0% 71.0% 69.0% 29.0% 58.4% 41.6%  75.0% 25.0%  48.6%  70.0%  56.0% 71.0% 29.0%  40.0% 86.0% 14.0%  51.4% 30.0% 38.0% 66.0% 62.0% 34.0% 44.0% 60.0%  From 7:00 A.M. t o 7:00 P.M. P.S.T. From 7:00 P.M. to 7:00 A.M. P.S.T.  57 Appendix IT The E f f e c t of the Amount of Cetyl Alcohol on the Rate of Evaporation.  Date June 20 21 22 23 24 25 26 27 28 29 July 3 4 5 6 7 8 9 10 11 23 24 25  Evap. from Free Water (in.) 0.26 0.30 0.295 0.31 0.28  0.26 0.26  0.18 0.14 0.10 0.21 0.22 0.21 0.21 0.19 0.16 0.21 0.23 0.28 0.36 0.29 0.30 26 0.33 27 0.25 28 0.25 Aug. 2 0.365 3 0.295 4 0.265 5 6 0.26 7 0.29 8 0.325 9 0.31 10 0.35 11 0.27 12 0.26 13 0.28 14 0.31 15 0.285 0.295  A i r Temp. Min. Max.  Evaporation rate i n inches per day through a Monomolecu l a r f i l m and application i n lbs./acre  Reduction i n Evaporation  °F  °F 0.11 0.263 0.655 1.09 2.34 4.68 %  65 62 67 72 64 60 60 50 54  90 96 97  52  58 59  64 58 62 62 56 61 68 61 58 68 70 66 67 59 60 50 56 66 64 57 62 60 63 59 59 60 61  3.8  ,25*  ,28* ,275*  T 85  ,30*  84 79 72 73 71  6.7 6.7 .23'  89 87 83 81 81 80 85 90  91  85 89 89 88  .25  .215 .205 .245 .255 .205  94  89 93 78 78 88  92 86 86 90  89 87 88 87 86 90  .245* .215* .12* .11* .08* .14* .20* .13* .16* .16* .065* .14* .155 .185  .345 .295 .270  .220* .230*  .30*  17.3 33.3 21.4 20.0 33.3 9.1 38.0 23.8  15.8 58.3  33.3 32.4 . 34.0 30.5 25.9 31.6 25.8 22.7 18.0 5.5 0.0 17.0 11.5 6.9 7.7  6.4  .29 .30* .25 .245*  .26 .28  17.8 5.8  .26* .275  14.3 7.4 5.8 7.2 9.7 8.8 6.8  58 Appendix IV (cont'd.) The E f f e c t of the Amount of Cetyl Alcohol on the Rate of Evaporation.  Date  Evaporation rate i n inches Eva p. from per day through a Monomolecu l a r f i l m and application i n Free A i r Temp. Water Min. Max. lbs./acre (in.) 0.11 0.263 0.655 1.09 2.34 4.63  Aug. 16 17 18 19 20 21 22 23 24 2  0.25 0.24 0.30 0.32 0.315 0.285 0.305 0.295 0.255 5 0.28  26 0.275  27 28 29 30 31  0.145 0.205 0.185 0.185 0.20  63 63 63  65 61 62 67 62 65 65  67 59 53. 53  52  61  85 39 39 91 93 92 93 93 95 97 34 72 77 74 77 73  .22 .22  %  Reduction i : Evapor ation  12.0 3.4 .29 3.3 .29* 9.4 .29 7.9 3.5 .275 .220* 27.3 .200 32.2 .210 17.6 19.2 .225 .21* 23.3 .12* 17.2 .17 17.1 .10* 46.0 .10* 46.0 .12 40.0  A new application of Cetyl Alcohol Film.  LEATHER ! i  j  J AT E  £. H.  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