L

 

EVAPORATION STUDY IN

GRAND RIVER BASIN :‘ i .L
.. ThesisLfof.’tl1.e.lD.egfeer-§f€ ' '

i,’ o

I) . H K Hamilton; :_ a __

.
v .
' ' ,. ' n '
: - I ' , I I . . _
n . . O . ‘
. _ - . ‘ V ‘
. . .5 ‘ > _ ‘ z - - - :
" ‘ U ‘ O . I . .
.' ‘ . n ‘ I . . I . ‘
. . . ' . _ ' . . ‘
I J ‘ . 2 ‘ _ .
. ,
;. , ‘ ‘ ' .
. . _ ‘
' - r ,, . , g
l . . V
. . ‘ -
. - ~._ 4 _ _ 1 ‘ . . .
. . . v
‘ . . 1 fl ‘ _
I l v - . ' I _
‘ u
5 . . . . _ 0 ‘ A ‘ . .
‘ .- i . _ . ‘ . - ‘ I ‘ ’
l ‘ ‘ . . ‘ ' 1 4 .
. I ‘ l ‘ ‘ ‘ a ‘ ‘ ‘ ‘
I 1 . . . . . ‘ z _
. n . . . o a , , _ . ‘ . ~
. . 9 . , v A — . ' .- , ‘
I I I ' ' ‘ x ' . a .
n : . c . 3 ’ . ' A .
- g V l A _ n I . ' J ‘ ' , _ -
l ‘ I I. ' l O l
I . A " ' . ' : A .' I - I
- ‘ x ' ‘ ’ ‘ . ' z ‘ l _ u
‘ ‘ ' ' ’ ~ ' I _ . I . 1
. . . ' ‘ l ‘ .
x 4 . . ' I . ‘ . _ ‘ ‘
. - ' f '4 . a . ' ‘ . l
- _ . _ . 3 _ , l ‘ . ‘ ‘ . i -. ‘
> - . . ' _ . . I
X 4 ' v _ _ 0 ¢ . Q r a ‘
' - ' s ‘ . a . , . | .
‘ I I I a ‘ ‘ ‘ . .
‘ I ‘ ‘ . ‘ ' - I _ 7 . .0 ‘ ~
_ ‘ _ c u ‘ — ’ ‘1 ’ o .
. , . A ~ I. - - '
. ' ' . I ~ ' .
. _ . _ A ' - . .
. ' . _ y . u » - -
. . . .
n . .4 - ¢ _ . ' ‘ I - - . -
I ‘ ' ' ‘ I . .
. ‘ ¢ . . I ‘ .. I . — . a
. ' - - A ‘ . ‘ ' . l
. - ‘ e D ‘ o ‘ - a ' ~ g 2
- _ ' ‘ _ I ' ' .
x ‘ . . - -
. n . V _ . ' . . I .
.
- ‘ '- n - J
. _ ‘ a o . . , . . ‘ , '
' - ‘ - _ - _ .
' - ‘ I ' . —
‘ I
A I . . ~ ~ I ..
. I . fl . -
1 I I _ ‘ ' l - .
u ‘ . _ . I . . ‘ ‘
. - ‘ . n _ O _ . E ‘
. ' . . -
. , . - D .
. 3 . ‘ ‘ . '
. . . . _ z . _ . ‘ ,-
u n . d . . _ ' ’ . _ . , _
. - _ — ‘ I .
’ . 3 , — 0 .
. ' - ‘ s . < . . ‘ _ , -
t v ‘ o ‘ u . . .
. . . . , . . — '
| ' .' - . - o ‘
‘ “ 1 ‘ O .
' . . ‘ , 4 :
. . . ,
. . ‘ , - ' ‘ .
_ - _ ' . . v ‘ ' _ .
. . . I , .
. . ‘ ’ ‘ ‘ .' A _
' . - . ‘ ‘ a g .
I .
l ‘ . , _ ‘ ‘ ' ‘ . ‘ ‘ '
C '2 ' A
.
. I . . . - A -. ‘
. , ‘
’ ‘ . r _ . _ . ‘
. . ‘ J V ‘ ‘ I
. l .
' . - - I
n , . . _ . ‘ A
‘ I ' . . I
O ' . ‘ .
. . 5
- ' g _ . . ~ ‘
l .
' . ‘ a ' . ' ‘ .
‘ O
I . ! ' ‘ l ' -
. ‘ I ‘ ‘ ‘ _ -
' ‘ ‘ o - - ‘ , ‘ a ‘ ‘
' D ' - . - _ ‘ ' '
. - ' -
- l 1 o ' ‘ ' ‘
. = . v ‘
A ' . C l
I

 

 

4. 4-

1"”
“5:!

fl’

      

SF

. '. o -
_ o
_ - y A‘ ‘ ' '. -2
‘A 0
P ‘ '7 '_ f
b... -. ‘ l
l _ ’ ,< t ‘- _T
“. an}... '. ‘.. ' no; ‘
'0 a. A n. -
4.-‘.' ‘ ‘
. _ . v. ' ‘-..‘ *
_ . .. .' -
.‘o' -_-A "'.-V .
... , _ _ _ _ '
' V 'A ' . S
' ,, . .
b o

)
,Q t

| ,b' ' -
. .v .m.‘ , a: VI
’3‘ " . - ‘0‘; .R ' ‘| V ‘

I
$ _ .L, 9'4
'h-«Ixa .
., “'4!

‘ ~
i? .

u . .3 \
. U "1 "(as 23143)?
2 ' .3 . D.
U . . .
‘ Iv." ' o

"

".." o
'I, :4
r
I‘D I _
.44
a
a

25:.
’ ‘3‘ 31¢
:24 ~ 4%.:

. " ' )‘ ' -
filo. _' { 2', .t - ‘ ' , . ‘..,‘4{ -
. _, ‘7 . ‘ . . ‘

O z 1'; in 3. " -"' w." " “ . C "I‘
,3": .v ,2"? .. ‘\ ‘ .. “’1-1-227 :2
'3'"; U} ‘ I. ‘20“ ’ ":~ 2""
. .& .‘J."""'I ’I_ : r \ bI’ 'Iflx' ' '
31' '. ‘ v7" '_L-A I ‘ ‘x; t 1.40 5

J”.‘P' - ’_-: . 3.. .' ‘ 4
3" r '4"); n .Y 4
02"
I V‘

1' " 1'3" ' ' " ’ ‘ .2
- . .1. . cw J .
.J\ . 3' __;\c' f‘ 4 I 35:: \’$ ‘ ‘

fl- .- ‘:'.I, ' 7‘ 'u.‘ $’
. ' .'_ ' 35.") {4133‘} 7 " . ‘
U 2 J a .. Wu 5*. 2%”Lé ILA . l
. .1; x 3 .I. . W .-.
v r' " V 2 ‘9’!" ”. ‘J. d J"
B. . .
4 A

r L ." 91”; ‘l
. (I?

   
    
 

      

    

L
i

1

'V

'3 2 9:. . ' I
& (3’2, '_'. J; ,_.
247‘

2‘ .A

._ L In . . _- . '
"H "‘2. -- '2, 3;? 935:2?“ x“; '

. 9:7. ?§(}%fi 3;?) SEQUHfigtl .

.'.:.-2 22 4'2 .. .
222224 2 222222222 '22. 24:2. .. .
.. $fi,~(«‘.a _ 273‘”; Egg? .125: . .
. gym" " "
'& W .r‘ .

e '5“: .. v

.' fl '3'". f‘.“ ' ' k-I'“ 43'.
.h’ \43'3 I) 'o-‘v t‘r‘hv" “k-i

.2: é ~22.- - M -
" TE".
4‘ 2’31"!"-

,\
3
”v

23 3
Jr

I
. v; 2
Q on
'V'.
.
.3." ~

.,.I
‘u‘ .

   
  
   
   

 
    
    
   
   
  
  

 

- I A
~4. - ‘
. .<
. ~.

 

 

Evaporation Study
in

Grand River Basin

A Thesis Submitted to

The Faculty of
MICHIGAN STATE COLEEGE
of
AGRICULTURE AND APPLIED SCIENCE

BY

H. R. Hamilton
Candidate for the Degree of

Bachelor of Science

June 1935

 

TH E315

CbQK/ql

 

TABLE OF
Preface
Introduction
General Discussion
Theoretical Considerations
Experimental Considerations
Formulas, General Form
Methods Of Observation

Procedure Of Gathering Data

Variables And Their Effects On Evaporation Rate

Daily, Monthly And Annual Variation In Evaporation Rate
Variation Due To Geographical Location

Evaporation From Ice And Snow

Variation Of Evaporation Due To Specific Gravity
Discussion Of Grand River Experiment

Bibliography

CONTENTS

4-5
6-8:
99-10
11-14
15-17
18-21
22-25

'24

25
26
27-37
38

 

...I.I lit-IIIILIDIEIIFJjII!2 2 I
I . i 1 L. A
x —

PREFACE

Purpose and Scope:

This thesis problem was begun in the Spring of 1935
by H. R. Hamilton. It was the desire to gain some knowledge
of the evaporation rate in the Grand River Basin from water
surfaces. Before this time such an experiment had not been
attempted in this area, so the purpose was to obtain results
that would give those interested in this field a practical
application to base future experiments on.

Comparable observations were taken over a period of
time. The results of this experiment are considered of
great value by the author and the hope exists that the work

will be carried further on.

Acknowledgement:

The author wishes to acknowledge the help and very
helpful suggestions of Mr. C. M. Cade of the Civil Engineer-
ing staff and his readiness at any time to give advice. The
author is also indebted to the Board of Water and Electric
Light Commissioners for the use of their equipment and their
source of data prior to the beginning of this experiment.
Also many helpful suggestions were given by the Engineering

staff of the Board of Water and Electric Light Commissioners.

INTRODUCTION

Since there had not been any experiments made on this
problem before in the Grand River Basin, all results had to
be compared with areas outside of the Grand River Basin. A
systematic study was begun of methods as to how to determine
the various effects of the different variables. There is a
great amount of literature on various phases of evaporation
but not a great deal has been compiled on evaporation rate

from water surfaces.

Considerable difficulty was experienced in obtaining
the observations by the floating pan method. This study
should really be made in an isolated area where the observ-
er does not have the problem of watching his experiment all
of the timme to keep the various sources of error from dis-

turbing the recordings of data.

GENERAL DISCUSSION

A great deal of work has been done in connection with
evaporation from water surfaces, but the literature is quite
limited on this phase of evaporation. In late years however,
more attention has been given to the study of rates of evap-
oration and an effort has been made to evaluate these factors
so they could be used in practical manner by those interested

in this particular field.

In the different regions evaporation varies because of
precipitation rates are greater or lower, that is in the East
where it is humid, precipitation depth equals or exceeds the
evaporation, where in the arid West evaporation may exceed
precipitation greatly. It has been shown from a number of ex-
periments that the evaporation in the West may exceed that in
the East from two to four times in the months of May to Sep-
tember. Therefore, it can be said that climatic conditions

must be considered in the study of evaporation.

THEORETICAL CONSIDERATIONS

In the theoretical considerations evaporation from a
water surface consists of two distinct processes; the trans-
ferring of the molecules of water from a liquid to a vapor-
ous state and the removal of the water vapor from proximity
to the water surface. The molecules of liquid water are in
a state of constant movement which is very rapid, the rate
of a movement depending upon the temperature of the water
which in turn depends upon the amount of heat energy receiv—
ed from the sun and atmosphere. The rapid rate of vibration
of the molecules located near the water surface causes them
to break through the surface film of the water. They do not
return by continue to remain in the air as water vapor. The
molecules continue to vibrate at a rapid rate, pounding on
the walls of any enclosing medium which results in a pres-
sure upon the walls of the medium, this pressure is known as
vapor pressure. Increase in the temperature of the air con-
taining water vapor, or increase in the amount of water vap-

or will increase the vapor pressure.

At any given temperature of the air there exists a
maximum amount of water vapor which the air can hold, when
this amount is reached the air becomes saturated. If temp—
erature is increased further the vapor holding capacity is
increased also; whereas a decrease will reduce the vapor

holding capacity and, if the air has been saturated, will

cause condensation. As the water temperature increases to-

ward the surface the rapidity of movement of the water mole-
cules increases and more of them leave the water surface.

The fewer the number of water molecules in the air near the
water surface the less the resistance will be to those leav—
ing the surface, so more molecules will leave the airface un-
der this condition. It then becomes evident that any factor
which reduces the number of water molecules near the water
surface, such as wind movement or rise of temperature of wet.
er at the surface, will naturally increase the rate of evap-

oration.

EXPERIMENTAL CONSIDERATIONS

Experimental evidence as well as theory indicate that,
everything else being equal, the rate of evaporation from a
water surface varies directly with the difference in vapor
pressure of saturated air corresponding to the temperature
of the water at the surface of the body, and the actual va-
por pressure of the air in contact with such a surface, and
also with the rate of removal of the moist air from the wet-
er surface. Vapor pressure is a function of temperature and,
temperature being constant, of the amount of vapor contained
in the air; and temperature is likewise a function of the
amount of radiant energy received by the water surface from
the sun and atmosphere less the amount lost through various
agencies. Barometric pressure also affects evaporation, as
the latter is a measure of the combined pressure exerted by
all of the gases included in the atmosphere, and a decrease
in this pressure results in fewer molecules of air to impede
the departure of the molecules from the water surface. An
increase in density of the water, due to various types of
matter in the water, will slow down the movement of the mole-
cules in the water and cause greater difficulty in leaving
the water. Basically speaking, it may be said that four
factors govern the rate of evaporation from a water surface.
These four factors are:

1. Difference in temperature of the water and of
the surrounding air, which influences the dif-

ference in vapor pressure.

2. Amount of water vapor in the air in prox-
imity to the water surface, which also in-
fluences the difference in vapor pressure.

3. Barometric pressure of the air.

4. Density of the water.

The pressure of saturated vapor at ordinary tempera—
tures which occur, doubles for every increase of approxi-
mately 20 degrees fahrenheit. Due to this fact a tempera-
ture differential of a given amount between water and air,
when a constant vapor content exists in the air, will cause
much greater evaporation at high than at low temperatures.
Little evaporation will occur with cool air in proximity to
cool water, but with air of same vapor content, a consider-
able amount will occur with both the water and air warmed a
few degrees. In regions of fairly high temperatures the vap-
or pressure differences are considerable and evaporation is
high. The effect of wind in increasing evaporation rate is
primarily to remove the overlying moist air from the water
surface from which it is constantly receiving water vapor,
and replace it with air of smaller moisture content. Were
the overlying air to remain still it would soon become sat-
urated and evaporation would cease. The greater the velo-
city of the wind the greater will be the rate at which the
moisture laden air is removed from the water surface and dry
air introduced. Therefore, differences in vapor pressures

are maintained, and evaporation is a function of wind velocity.

Variations in barometric pressure at a given elevation
and their effect upon evaporation are relatively small. With
increase in altitude, while the decrease in barometric pres—
sure is marked, the decrease in temperature has so much great-
er effect that for all practical purposes barometric pressure

is a negligible factor.

FORMULAS - GENERAL FORM

Many formulas have been developed for computing the
rate of evaporation from other known factors through experi-
mental research and other theoretical considerations. Those
most frequently used are based on Daltons' law and follow
the general form of

E'= (ew - ed) (1 . CK)

the rate of evaporation

where E

ew2= vapor pressure of saturated vapor correspond,
ing to temperature of water surface.

ed,= vapor pressure of atmosphere at water surface.
K = wind velocity.

C = Constant.

The values of the constant depend upon the units used

in expressing the other factors in the formula.

Meyer modified this formula into the following result:
Em = 15(3w ' ed) (1 e Ig)

this giving evaporation in inches per month.

Bigelow's formula which is expressed in English units:

Em - 75.8 (31_ 23!) (1 2 £2
(ed dad)

where dew is the rate of charge with temperature of maximum
de
d
vapor pressure at the temperature of water surface. The con-
stant 75.8 being a factor introduced to take care of small

water surface areas. When getting the data together to in-

troduce into these formulas,the following things must be gath—
ered and considered: air temperature, water temperature, 1"
above water, 1' above pan water temperature, pan water temp-

erature, air velocity and relative humidity.

For expressing the rate of evaporation over a relative-
ly short period of time, such as a few hours, these formulas
are quite satisfactory, but to the research man who is pri-
marily interested in monthly or annual rate of evaporation,
they are of little practical use. This is due to the varia—
tions of the factors in the equations when used over longer
periods of time and the difficulty in evaluating these, for
if such evaluation is attempted it would take considerable
time, during which actual evaporation experiments could be

run which would yield more complete and satisfactory results.

METHODS OF OBSERVATION

Much work has been done on evaporation experiments to
determine the rates of evaporation through field experiments
carried on by the United States Weather Bureau, Bureau of
Reclamation and many other private agencies during recent
years and a great deal of discussion has followed these exper-
iments and their results in the technical press. Most of the
work has been carried on by use of evaporation pans, either
situated on land or floating in water such as a pont, lake,
or reservoir. The results of these experiments show that
there exists a substantial difference in the evaporation rate
so determined, depending on the way the pan is exposed, its
size, shape and setting. The problem then involves the find-
ing of a correlation between the surface area of pan and some

large surface area from this experimental work.

The location of the evaporation pan should be given
careful consideration; otherwise comparable and reliable data
can not be obtained. The emplacement of land pans should be

on a level area on which there are no trees or obstructing

buildings, and if the obBeIVaticns are for the purpose of e-
termining the equivalent reservoir evaporation the location
for the pan should be representative of conditions at the
reservoir. Isolated places where there is no satisfactory
water supply available or where a competent observer ccnnot

be had, should be avoided. The standard pan used at Class A

12

stations of the United States Weather Bureau is made of gal—
vanized iron and is circular in shape, a feet in diameter and
10 inches in depth, set somewhat above the surface of the
ground upon a wooden support so that the sides and bottom are
exposed to the air. The other pans used for land observations
range from 3 to h feet in diameter for circular pans to the
same side dimensions for square pans, and in depths from 2 to

3 feet. They are set in the ground the entire depth.

Floating pans are not recommended;if it is necessary
to install one the location should be carefully chosen. The
location should be in an area protected from wave action.
The exposure to the wind should be representative of the av-
erage conditions of the reservoir or body of water that is un-
der observation. Floating pans also vary in shape, size and
depth and are supported by metal floats attached to the pans
so as to keep the lip of the pan a few inches above the water
surface. Protection from splashing by wave action in the case
of floating pans can be obtained by placing a wooden float a-
rcund.the outside of the pan. A suitable float may be made
from.4“x 4" timbers or by setting on edge planking l"x 12",
16" long, forming a square with cross members so arranged as
to form an opening invvhich the pan may be located and cross

braced to make float rigid.

A vertical rod with a sharp pointed top is attached
to the bottom of the pan, which must be braced so as to make

it rigid, and the water surface is maintained at the top of

13

the rod which must be near the top of the pan. Every few
days or once a week,water is added to the pan to bring the
water level to the top of the rod and make up for the water
evaporated. The added water may be measured by using a
anall cup of such a size that each cupful of water is equiva—
lent to 0.01 in depth on the surface of the pan. The use of
the calibrated cup simplifies the work of keeping notes and

making computations.

The rain gauge for evaporation stations should be of
the Weather Bureau type and should be located adjacent to the
pan in order to determine the amount of precipitation caught
be it. This amount should be added to the observed evapora-
tion to give the actual quantity. The evaporation gauges used
in connection with the different pans are usually of special
forms, but any gauge giving readings within 0.001 feet should

be satisfactory.

The evaporation from different types of pane is depend-
ent to some extent on the color of the pan. With this in view
it is recommended that no paint, tar or other coatings be used.
To reduce the growth of Algae and other plants in the water,
the pans should be cleaned once a month and oftener if neces—
sary. The cleaning of the tanks also removes the salts,thus
lowering the concentration of the water which would otherwise

occur due to the evaporation.

In order to obtain comparable evaporation data the

floating pan record should be checked by installing a land
pan as near to it as possible, because even when protected
by a float, waves may sometimes splash into the pan and cause
erroneous results. Where pan cannot be kept under constant
observation it should be covered by a covering of checken
mesh and surrounded by a fence. Instances have been known
to occur where unprotected pans have been used as bathing
pools by birds, resulting in a loss of water by splashing.
A complete meteorological record should be kept and a stand-
ardized procedure followed in taking observations. The me—
teondogical record should include the air and water tempera—
tures, the humidity, the wind movement, the precipitation and
the evaporation loss. Other added items might be the tempera—
ture 1.0 inch above water, pan water temperature and 1.0 inch
above pan water surface. The following instruments are re-
quired:
2 - Sets of "maximum and minimum" thermometers
for determining the mean air and water
temperature, or '
2 - Ordinary thermometers where observations
can be taken at 12 hour intervals or often-

er,

1 - Pychrometer for determining the humidity of
the air,

- Anemometer for recording the wind movement,
Rain gauge for measuring the precipitation,

- Gauge for measuring the evaporation, and

HHHH
I

- Instrument shelter for the air thermometers
and psychrometer.

?ROCEDURE OF GATHERING DATA

The standard.method used by the Weather Bureau should
be used in taking observations on the pan. This is, the read-
ings on the instruments are taken twice daily at approximate-
ly 7:00 A.M. and 7:00 P.M. The mean temperature of the water
may be determined either by readings of "maximum-andpminimum"
thermometers floating in the water with their bulbs immersed
1/4 inch below the surface of the water, or by readings on an
ordinary thermometer similarly exposed. The ”maximumsand-
minimum" thermometers require only one reading per day to ob-
tain a fair average of the temperature while the ordinary
thermometer requires two readings a day at 12 hour intervals
to get similar accuracy. uaximum thermometers are easily
broken and so for this reason it should not be used by an ob-
server not accustomed to using this type of thermometer. If
possible, the sling or rotating psychrometer should be used
for determining humidity, but where readings cannot be taken
at regular intervals a hair hygrometer may be used to obtain

approximate results.

The velocity of the wind varies with the elevation a-
bove the ground surface. With this face in consideration, a
standard anemometer setting should be adopted for each type
of evaporation pan in order to obtain comparable results.
The standard practice at Class A Weather Bureau stations is

to mount the anemometer on the grillage of timbers which sup-

port the evaporation pan so that the cups of the anemometer
are 6.0 inches above the top of the pan. At some places in
the United States the practice has been to mount the anemom—
eter so that the cups are 24.0 inches above the top of the
pan. When in doubt as to what distance to set anemometer a-
bove pans that are not of a standard type, it is suggested
for uniformity that the cups of the anemometer be set 13.0
inches above the top of the surface of the ground or water.
Either the 3—cup or the u-cup anemometers, of the Weather
Bureau pattern, may be used. The 3-cup type are more accur-
ate at high velocities, but near the ground the velocities
are never so high that the u-cup anemometer is not suffici-
ently accurate. When considering this variable, that is wind
velocity and its effect on evaporation rate, most evaporation
formulas contain an expression relative to some of the gener-
al form

(1 2 CK)
where C is a coefficient depending in magnitude upon the ele—
vation above the ground and decreasing with increase in eleva-
tion above the surface at which.the wind velocity was record-
ed, and K is the wind velocity expressed in suitable units.
Very often velocities of wind given by Weather Bureau or oth-
er agencies are observed at elevations ranging from 50 to 150
feet above the ground surface where the ground records were
taken from 2 to 3 feet above the surface. A value of C of
0.10 may be taken as a fair average provided the velocity of

the wind is expressed in miles per hour as observed at regu-

1?:

lar Weather Bureau Stations. If wind velocity is taken near
the ground this value should be increased from two to four
times. It was found from experiment that in the Grand River

Basin that the best value for C was 0.20.

The wind factor should be used only as a guide to judg-
ment, in comparing evaporation at a location under considera-
tion. In such cases the ratio

1+CK
'I‘I“EI‘k1

1 K1 being

should be applied to the observed records; CK and C
quantities at the location under consideration and’at the lo-

cation where the records were obtained, respectively.

VARIABLES AND THEIR EFFECTS ON EVAPORATION RATE

Wind effect is less on water areas of large extent
than on smaller bodies of water. Wind serves merely to re-
move water vapor from the water surface and replace if with
dry air. It may be said then that when the wind sweep over
a water surface is great, the vapor content becomes large
before the leeward side is reached and the drying effect of

the wind is lessened.

The effect of size and shape of the pan upon evapora—
tion rate may be shown through experiments carried on in con-
nection with this subject. The results show that with an in-
crease in size of the pan used the recorded evaporation is
reduced and from this it may be assumed that this condition
more closely approaches that of a large open water surface.
This is probably due to the fact that the larger and deeper
pans hold the heat better and maintain a more constant temp-
erature than the smaller pans, which causes a higher rate of
evaporation. The thin capillary film creeping up the sides
of the pan has a higher temperature than the water in the
pan and evaporation occurs from it also at a much higher
rate than from the cool water surface. With larger pans,
the ratio of this area around the water's edge to the total
exposed area of water surface becomes smaller and the effect

is reduced.

- Experiments having to do with the effect of size and
snaps of pan upon evaporation rate indicated quite clearly
the effect of these factors upon the result. Data gathered
showed a decreasing rate of evaporation in relationship to
an increasing size of pan until pan surface area reaches 100
square feet, then the evaporation rate does not further in-
crease. Although these experiments are not conclusive of
the fact that the rate from pans of 100 square feet in.sur-
face area is equal to that from a large open water surface
such as a reservoir or lake, and confirmatory experiments
are necessary before this fact can be generally accepted
and a relationship established, they do quite clearly bring
out the fact that such a distinct relationship exists and
it is essential in using results of evaporation experiments
that careful consideration be made of SlCh factors as size

and shape of pan, exposure, location and. similar effects.

The relationship between rate of evaporation from
land and floating pans shown by both experimental results
and theoretical considerations indicate that the rate of
evaporation from pans floating in a body of water is less
than from same pans set upon the airface of the ground or
in the ground. A floating pan is covered with a blanket of
water vapor which reduces the evaporation rate, and the
temperature of the surrounding water serves to maintain a
lower temperature in the water in the pan. Land pans are

not so likely to he covered by a vapor blanket and the wat—

'

20

er will attain a higher temperature than that in a floating

pan. Pans set upon the surface of the ground or upon boards
attain a higher temperature than those set in the ground and
therefore show a higher evaporation rate than those insulat—
ed by surrounding soil.

The conclusions that are drawn as to the results from
the different types of pans, as far as the gathering of
comparable data is concerned, are, that the records from
floating are not as consistent or reliable as land~pan
records, nor is the evaporation from a floating pan any
nearer the evaporation from a large water airface than that
from a sunken pan of the same size and shape. Comparison of
the evaporation from different types of pans with that from
large water surfaces of different sizes, shows that the size
of the pan has a prOportionately smaller effect on
evaporation as the size of the surface increases, and that
when the diameter is greater than 12 feet, the size of the
pan has practically no effect on the evaporation. In order
to obtain comparable evaporation results, standard equipment
installed under representative conditions should be used, and
a standard procedure should be followed in making the
observations.

The relationship between mean temperature and the
rate of evaporation may be developed. When an attempt is
made to determine the rate of evaporation at a given locality
for the use in water supply studies, the only physical data

usually available are the mean daily or monthly temperature

-21-

records as given by the U. 8. Weather Bureau, and it is
often necessary to combine these with experimental data of
nearby stations to attain the result desired. A relation-
ship between evaporation and mean temperature may be
developed which will be sufficiently accurate for use as a
guide, particularly when other factors such as wind velocity,
relative humidity, and others are known. These figures,
though, should be used only as a guide in aiding judgment,

in cases where local data are not available.

DAILY MONTHLY AND ANNUAL VARIATION IN EVAPORATION RATE

The daily variation in rate of evaporation, as a
general prOposition, includes all factors tending to cause
an increase in the evaporation rate are subject to increase
during the daylight hours and decrease during the night.
Temperature, wind, and relative humidity are usually greater
during the daylight hours. The great decrease in the rate
of evaporation at high altitudes, even though the daytime
temperatures may reach fairly high figures, is due to the
fact that the low night temperatures reduce the average
temperature to such a degree that the mean evaporation is
small.

When making a study of the monthly variation in rate
of evaporation, it will be found that those stations which
have a mean monthly temperature around the freezing point,
indicate a monthly rate of evaporation during the winter
periods to be from & to % of an inch per month for land pans,
and from 1 to 2 inches per month from floating pans. The
latter pans give higher figures due to the insulating effect
of the surrounding water upon the contents of the pan.
monthly variation of evaporation should be studied in areas
such as the Arid West Where evaporation rate is greater than
the precipitation rate. In northern areas, the freezing
weather prevents securing of records during the winter months,
but because of the fact that the total rate of evaporation

is so small during this period, the evaporation during these

months may be disregarded.

Annual variation of evaporation rate, like most
meteorological and hydrological data, little thought usually
is given to obtaining experimental data relative to evaporation
until the time it is needed, and then usually it is possible

to obtain records of only one or two seasons. It, therefore,
becomes of great importance to determine whether the annual
variation in evaporation rate is great enough to render a
short record of doubtful value as an indication of mean rate,
and also whether the use of mean values when determined will
greatly affect the results desired. In regions where large
annual variations are indicated, it is advisable to develop
a relationship between mean annual temperature and rate of
evaporation for the location, and use the resulting figures
for each year, rather than to use the mean value of evapora-

tion throughout any computations of water supply.

VARIATION DUE TO GEOGRAPHICAL LOCATION

Due to theeffect upon evaporation rate of the size
of the pan, exposure temperature and wind velocity, it is
impossible to obtain an idea of the variation of evaporation
in different localities by comparing results of observations
at different stations. A factor has to be applied to reduce
to a common denominator the observations from other stations.
This factor is due to the locality of the station. In general
it may be said that in the United States the rate of.

evaporation decreases with increase of latitude.

EVAPORATION FROM SNOW AND ICE

It is very difficult to make year—round observations
of evaporation at stations where sub freezing weather occurs
during several months of the year. For that reason, records
lat such stations are very often incomplete. When an attempt
is made to keep a record of evaporation during this freezing
period, it will be found that the average monthly rate of
evaporation is so small that it cannot be considered a factor
in the losses from a large body of water.

Evaporation from snow surfaces is also difficult to
derive, and because of the greater area eXposed to the air,
the losses are greater than those from an ice surface. It
very often happens that a warm, dry wind blowing over snow
fields for several days will result in their complete dis-
appearance. This is often credited to the evaporation of the
snow, but this theory is generally in error and the fact is
the warm air causes the snow to melt and be absorbed by the

ground.

f

20
VARIATION OF EVAPORATION DUE TO SPECIFIC GRAVITY

Specific gravity of water and its effect upon the rate
of evaporation must also be considered as a factor. When
water is heavily charged with mineral substances, the internal
motion of the molecules is decreased and their rate of
escape from the solution is retarded. When the mineral content
reaches such a point that crystallization or the formation of
a film of crystals at the water surface occurs, the rate is
reduced to practically zero. The specific gravity at which
crystallization commences will vary with the chemical composi-
tion of the mineral constituents.

The importance of this phenomenon from a water supply
standpoint develops from the use of evaporation rate to
determine the water supply of areas tributary to lake or
moist areas.

many lakes in the Arid West are saline in character,
with no outlets, and, based upon their fluctuations, area
and an assumed or observed rate of evaporation, the annual
inflow is sought. Also in areas where a stream may be highly
polluted and a great portion of this water is held in a
reservoir, it can readily be seen that an allowance should
be made for the specific gravity of the water content in
determining the evaporation rate from the surfaces of water

of this nature.

27

DISCUSSION OF GRAND RIVER EXPERIHENT

With the theoretical and practical application
mentioned before, this problem of the evaporation rate in
the Grand River Basin was studied and results noted. The
equipment that was used was a 4 foot diameter floating pan
and the other standard equipment already mentioned. A
floating pan was used so as to get the most nearly ideal
conditions necessary to make a study of this kind. Observa-
tions were taken over a period of three years and were found
to be comparable to other observations taken at various
points in other areas.

Variation of evaporation rate with temperature was
noted and results may be noted on Chart I. All the variables
such as wind movement, temperature, and relative humidity
were included in Chart II. These results may be considered
as the most complete because of the fact that all variables
were included. A curve cannot be made of one of these effects
and have the curve be of any practical value, because the
actual evaporation rate recorded includes all of the
variables. Recordings were not taken in the freezing months
because these values may be neglected for the reason that
evaporation is so low during this period. Mean daily
evaporation during the three years was A inch per day. This
value fluctuated very little and the reason for this was the
fact that during the seasonal changes, both temperature and
wind movement varied so as to cause little change in the

daily evaporation rate.

28

Using this 4 foot pan, a coefficient of reduction

to apply to the open water surface was used.

the coefficient was 9.

evaporation recorded daily.

COEFFICIENTS OF REDUCTION; PAN EVAPORATION
RECORDS TO OPEN WATER SURFACES

Table I

Depth of
Tank or Pan
in Feet

1.R

J

0.83

1.83

Depth of Dimensions
Water in of Pan or
Tank or Pan Bank in Feet
in Feet
1.25 3x3, sg.
circular
.58 4.0

3.5

This was applied to the total

Coefficient
of Reduction
to Open Water
Surfaces

0.91

0.92

0.91

The value of

Remarks

Pan set in
water pro-
tected by
raft to re-
duce wave
action.

Pan set in
water sub—

merged to
a depth of
7".

-29-

The general equation derived by Dalton may be used

in the Grand River Basin

E=(cu—ed) (1,1cx)
This formula having been discussed before, there is no need
of further explanation of its use. The reason that this
formula serves the purpose of getting the evaporation rate
is that it entails all of the variables and it also deals
with the vapor pressures of the air and water vapor. Vapor
pressure has a marked effect on evaporation rate in this
area due to the slight mists that hang over water surfaces
during certain months of the year. These mists cause a
-vaporous state of the air conditions close to the water
surface. The molecular action of the water particles is
reduced by the steaming surface to such an extent that there
is a marked reduction in the evaporation rate.

One thing in particular was noticed as to water
temperatures, that is that evaporation rate has a cooling
effect on the body of water. This was noted when making a
study of River temperature below Moores Park Power Plant.
As the discharge of cooling water cane into the river, the
water in the stream increased in temperature, and as the
flow went downstream, the water gradually cooled.

On days when evaporation was the greatest, the river
cooled more rapidly, therefore making evaporation a major

factor in the location of future power stations downstream.

30

Comparison of observations was made with the U. S.
Weather Bureau at East Lansing, andin this way a check was
kept on recordings at experimental station on the river.
Difficulties constantly come up with an eXperiment of this
kind such as, the station can't be constantly watched so
interference comes from intruders who some of the time disturb
the setup.

Loss of equipment is another item that must be
watched closely.

When considering the specific gravity of the Grand
River, it may be considered high. The reason for this is
that the stream is highly polluted causing a density some-
what higher than that of ordinary water. Although in this
particular case this higher specific gravity doesn't affect
evaporation greatly because of the stream flow. A crust
cannot form on this moving surface and inpede evaporation.
The water in the pan, though, has to be changed every three
days so as to have a solution under comparable conditions,
under observation.

The accompanying charts will show the results of the
observations gathered and give reason for the foregoing
conclusions. Chart I was compiled from observations taken
over a period of three years, 1931, 1932, 1933, and the
spring of 1935. A direct variation in evaporation may be
noted with the temperature. Chart II was compiled from the

same observations and that in itself gives a more complete

description of what taken place under conditions set up by

the variable effects on evaporation rate. The purpose
of this chart is to give a basis for judgment and computa—
tions on evaporation rate from a water surface. Chart III
will show a factor that exists between the rate of
evaporation and the area of the water surface. This curve
can be used to apply to any size of pan that is used for
the eXperiment, when doing this the results from the pan
may be changed into terms of a large water surface. Chart IV
shows the effect of humidity on evaporation rate from a
water surface. The one difficulty that arises in compiling
a humidity effect curve lies in the fact that the effect
of wind movement causes the humidity to give a widely differ-
ent effect at the same conditions of humidity. Chart V
shows the effect of wind movement on the evaporation rate.
This curve very nearly gives a direct result of evaporation
because of the direct effect that the wind movement has on
the evaporation rate. All of these results are to be
considered as mean values, however, because observations
extended over a period of 24 hours each.

This fact may be noted from data sheet. Again too
much stress cannot be laid on the fact that extreme careful—
ness must be followed in taking observations by the floating

pan method.

 

v .

 

0'-

0
o
o
..
5.
,.
o
l

a
A

 

.,....DO

sA~

‘0'.

‘Quo

_-..-—.-

 

 

'OQ
0".

 

 

 

. i, _

..--F1L- Fl .11 A.

Ly In. .L.

a 1.5.

lLJL- L

milk I k. -

.l

k

L

OBSERVER

EMPERATURE
WER PAN

H
b-

C
H

 

rans o

5. Am.

. Ame

dons,

 

 

 

GAUGI:

 

 

 

 

't Gradient

her

Stations

News,

0P3:

. D.

'now Sur-
11917, p. 565.
ghen Used

eview,

. Nichigan.

: chOo

 

58

BIBLIOGRAPHY
1. Monthly'Weather Review, Vol. 55, p. 55, 1907
2. Evaporation On Reclamation Projects, Sleight, Trans.
Am. Soc. c. E., Vol. 90, p. 305 (1927).
3. Mead, Elements Of Hydrology, p. 196
4. Houk, Evaporation On Reclamation Projects, Trans. Am.
Soc. c. 3., Vol. 90, p. 336 (1927).
5. Lee, Evaporation 0n Reclamation Projects, Trans. Am.
Soc. C. E., Vol. 90, p. 556 (1927)
6. Evaporation As A Simple Index To Weather Conditions,
Monthly Weather Review, 1925, p. 570.
7. Method Of Computing Evaporation From TEmperature Gradient
In Lakes And Reservoirs, G. E. McEwen, Monthly Weather
Review, 1924, p. 108.
8. Records Of Evaporation Obtained At 25 Different Stations
In Various Parts Of The United States, Engineering News,
J une 16, 1910.
9. Evaporation Losses And Water REquirements 0f Crops,
Samuel Fortier, Bull. 177, Off. Of Exp. Stat.,U. S. D.
A.. p. 64 (1907).
10. Some Field Experiments Upon Evaporation From Snow Sur-
faces, F. S. Baker, Monthly Weather Review, July, 1917, p. 563.
11. Evaporation From Snow And Errors Of Raingage When Used
To Catch Snowfall, R. E. Horton, Monthly Weather Review,
feb., 1914, p. 99.
12. Board Of Water And Light Experiment, Lansing, Michigan.
13. Kootania Valley Experrment, Kootania Valley, Idaho.

311:: ROOM USE ONLY

WE,

W”

 

1

CH
16
A
N
3
T
A
T
E
U
N
IVER
5
IT
Y
L
IS
R
A
R
IC
Q

MINI ‘ IIIII
I3I III lzIa

III 0IIISIOI 8I4IIII8I I