Evaporation losses in sprinkler irrigation

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Title:
Evaporation losses in sprinkler irrigation
Series Title:
Florida Water Resources Research Center Publication Number 12
Physical Description:
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Myers, J. M.
Baird, C. D.
Choate, R. E.
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University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Notes

Abstract:
Water conservation, distribution of chemicals through irrigation water and the increasing popularity of low application rate irrigation systems are all important factors pointing up the need for more precision in irrigation management which in turn is dependent upon accurate estimates of expected evaporation losses. Data has been obtained to predict the independent effect of water application rate, air (wind) velocity, water temperature and dry bulb and dew point temperature of the ambient air on evaporation losses by water droplets and water droplets in combination with plant intercepted water. By far the most influential factor on evaporation losses is the rate of application. Results indicate evaporation losses are about 60% for low application rates (0.15 iph) with climatic conditions typical of Florida and when·plant foliage is present to intercept most of the applied water. Evaporation losses by water droplets in motion is relatively insignificant in comparison to losses from extensive wetted surfaces afforded by dense vegetation. It is unlikely that evaporation by water droplets in transit could amount to more than 5% of a water application. The independent effect of water temperature and several important climatic factors on evaporation losses were determined and are presented in graphic form in this report.

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EVAPORATION LOSSES IN SPRINKLER IRRIGATION


by


J.M. Myers
Professor, (Agricultural Engineer)

C.D. Baird
Research Instructor, (Agricultural Engineer)

R.E. Choate
Professor, (Agricultural Engineer)


PUBLICATION NO.


/2


of the

Florida Water Resources Research Center


RESEARCH PROJECT TECHNICAL COMPLETION REPORT

OWRR Project Number A-003-FLA

Annual Allotment Agreement Numbers

14-01-0001-580 (1965)
14-01-0001-779 (1966)
14-01-0001-903 (1967)
14-01-0001-1077(1968)

Report Submitted: December 31, 1970

The work. upon which this report is based was supported in part
by funds provided by the United States Department of the
Interior, Office of Water Resources Research as
Authorized under the Water Resources
Research Act of 1964.
















TABLE OF CONTENTS


Page
. 1


ABSTRACT . . .


SUMMARY . . . .

INTRODUCTION . . .

REVIEW OF LITERATURE . .

METHODS AND EXPERIMENTAL FACILITIES

Climatic Control Chamber .

Air Straighteners . .

Test Section of Chamber .

Water Droplet Generator .

Air Conditioning and Heating .

Dew Point Temperature Control

Dry Bulb Temperature . .

Air Flow Rate Control . .

Instrumentation . .

PROCEDURE . . .

DISCUSSION AND RESULTS .. .

Evaporation by Water Droplets

Evaporation of Plant Intercepted

Interpretation of Results .

CONCLUSIONS . . .

ACKNOWLEDGMENTS . .. .

LITERATURE CITED . . .


-4
.. . 2







. . 10

. . 11

.,14
S . 14

S . 14



. . 19

S . 21

S . 21







. . 23

. . 24

. . 25




. . 32

S . 37

. . 38

S . 39


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. R












ABSTRACT


EVAPORATION LOSSES IN SPRINKLER IRRIGATION


Water conservation, distribution of chemicals through
irrigation water and the increasing popularity of low appli-
cation rate irrigation systems are all important factors
pointing up the need for more precision in irrigation manage-
ment which in turn is dependent upon accurate estimates of
expected evaporation losses. Data has been obtained to pre-
dict the independent effect of water application rate, air
(wind) velocity, water temperature and dry bulb and dew point
temperature of the ambient air on evaporation losses by water
droplets and water droplets in combination with plant inter-
cepted water. By far the most influential factor on evapora-
tion losses is the rate of application. Results indicate
evaporation losses are about 60% for low application rates
(0.15 iph) with climatic conditions typical of Florida and
when plant foliage is present to intercept most of the applied
water. Evaporation losses by water droplets in motion is
relatively insignificant in comparison to losses from exten-
sive wetted surfaces afforded by dense vegetation. It is
unlikely that evaporation by water droplets in transit could
amount to more than 5% of a water application. The independ- -
ent effect of water temperature and several important climatic
factors on evaporation losses were determined and are presented
in graphic form in this report.







Myers, J.M., C.D. Baird and R.E. Choate
EVAPORATION LOSSES IN SPRINKLER IRRIGATION
Completion Report of the Office of Water Resources Research,
Department of Interior, December, 1970, Washington, D.C. 20240
KEYWORDS: irrigation/ evaporation losses/ water droplets/
wind velocity/ water temperature/ dew point/ air temperature/
application rate/ intercepted water.









SUMMARY


The objectives of these studies were to evaluate
independently the climatic factors of air temperature,
dew point temperature and wind velocity, and the physical
factors of water temperature and application rate in
terms of evaporation losses for sprinkler irrigation.
Evaporation losses were separated into two sources, that
from the spray (water droplets) and that from the wetted
surface of plant material. Supporting tests were conducted
under controlled conditions in an environmental control
chamber built in the laboratory of the Department of Agri-
cultural Engineering at the University of Florida. The
results of the study are not directly applicable to field
conditions for sprinkler irrigation but the data in this
report are interpreted so that it supplies a basis for
making decisions concerning design and operational manage-
ment of sprinkler irrigation systems.

The results of tests on evaporation losses for spray are
consistent with those of several other investigators in
that the losses are very small when compared to the total
amount of water applied and also small when compared to.
evaporation losses from plant surfaces. Spray evaporation
losses, expressed as a percentage of the amount applied,
Ranged from 0.20% to 1.13% while that from the plant inter-
cepted water ranged from 3.5% to 60.3%. Based on laboratory
tests and in consideration of adjustments for droplet size,
time of exposure and relative velocity between droplets and
air, it is estimated that evaporation from this source should
not exceed 5% of the amount applied under typical field
conditions in Florida. In past studies of evaporation the
initial water temperature was not considered in most cases.
However, the laboratory tests on this factor indicate that it
is important even though the droplets approach the wet bulb
temperature very rapidly. For field conditions where the
droplet exposure time will be greater than that for the lab-
oratory tests, the effect of initial water temperature on
evaporation will be reduced. The influence of air (wind)
velocity on evaporation from droplets appears to be more
closely related to the movement of high-moisture-content-
air from the general vicinity of the droplets rather than
to the increase of the relative velocity between the air and
the droplet. The effect of air temperature on evaporation
losses is strictly related to the rate at which the droplet
temperature changes and the equilibrium temperature (wet
bulb) which is reached by the droplet. As the dew point of
the air increases, both vapor pressure and wet bulb tempera-
ture of the air increases at increasing rates. The two
occurrences have an opposing effect on the evaporation rate
and it appears that the relationship is almost linear.









The relationships of the effect of air velocity, dry
bulb temperature and dew point to evaporation losses from
plant surfaces is similar to those found for water droplets,
however, the order of magnitude of evaporation from plant
surfaces is many times greater. Important factors contri-
buting to these larger evaporation rates are the larger
wetted areas and longer exposure time for plant intercepted
water when compared to water droplets during transit.
Evaporation losses from plant surfaces are primarily a
function of the rate of application. Evaporation losses
in terms of percent of the total application can vary from
10% for application rates of 5 iph to more than 60%-for
application rates of 0.15 iph for typical Florida climatic
conditions.

Publications that have resulted from this project thus
far are:


Baird, C.D. Measurement of Water Evaporation Rates Utilizing
an Electrolytic Condensation Hygrometer. Unpublished.
M.S. Thesis, University of Florida, Gainesville,
Florida. 1967.

Baird, C.D., J.M. Myers and I.J. Ross. Precision Measure-
N ment of Dew Point Changes with Electrolytic Conden-
sation Hygrometer. Transactions of the ASAE. 12(6):
849-853. 1969.

INTRODUCTION

Water losses by evaporation from sprinkler irrigation
can vary from practically nothing to more than half the
volume of water delivered to the sprinkler nozzle. It is
believed that many users of irrigation have only a general
appreciation of the magnitude of evaporation losses. In the
management of irrigation, overall application efficiencies,
i.e. the relative proportion of the water that is removed
from the source and placed in the soil for crop use, of
70 to 80 percent are in standard usage in Florida.

Rule-of-thumb criteria for estimating irrigation
efficiencies may not have affected significantly the economics
of irrigation in the past, but, with anticipated new techno-
logical advancements and the increased awareness of the neces-
sity for water conservation, the demand for more precise
information on irrigation efficiency and evaporation losses
will be required for the years ahead.

The potential is promising for effective and low cost
application of chemicals through irrigation for plant growth
regulation and insect and disease control. Evaporation from









Sthe foliage is a needed value in calculating the amount
of chemical residues remaining on the plants after the
cessation of the irrigation application. The concentration
of chemical solutions at the moment of application is
dependent upon evaporation losses from the water droplets
as they move through the air. When plant foliage are
sensitive to salt residues resulting from evaporation of
saline irrigation water, the rate of evaporation with respect
to rate of application becomes one of the important factors
to consider in adjusting to this problem. At times, evapo-
ration from irrigation spray is desirable as a medium for
cooling plants and the surrounding air. Certainly there
is a trend toward greater control of the micro-environment
for growing agricultural crops. More knowledge about water
evaporation losses associated with sprinkler irrigation is
a significant entity towards coping with these situations.

...-. OQf course, any water that is removed from its source
for irrigation purposes and does not reach the zone of
intended application reflects unfavorably-upon irrigation
operating costs and is not in..concert with the philosophy
of water resource conservation.

Air temperature, dew point temperature and wind velocity
are-climatic factors that can greatly influence evaporation
Slosses-while irrigating. -Water temperature, water droplet.
size,evelocity of water droplet, time duration that the drop-
let is in transit between-the sprinkler nozzle and the inter-
ception point .and interception characteristics are other
factors playing a part in this phenomenon. It was the object
of the study reported herein to independently evaluate these
factors in terms of their influence on evaporation losses
from sprinkler irrigation. --

SREVIEW OF LITERATURE

_:. 'Irrigation has been practiced in the United States for
more -han 100 years. Eortier (10) stressed the importance
for increasing irrigation efficiency as-early as 1915 when
he-stated that "measurements and experiments show that for
every three gallons of water diverted from natural streams
one gallon serves a useful purpose in nourishing plant life."

Irrigation principles and practices have advanced to the
point that water application efficiency is primarily controlled
by:the amount.of evaporation losses. Water application effic-
iency-mmay be defined-as the.-ratio of the quantity of water
effectively put into the crop root zone and utilized by
growing crops to the quantity delivered to the field (34).
e s. ... n .
... Interest in evaporation and evapotranspiration is not of
recent origin. Dalton (8) in 1798 showed that the rate of
evaporation was proportional to the difference between the









water vapor pressure at the evaporating surface and in the
atmosphere. Essentially all vapor transport formulas since
that time take this principle into account.

According to Frost (11) the operational factors which
may influence the losses during sprinkling are droplet size,
application rate, crop, crop height, soil moisture retention
and water temperature. He also lists the following climato-
logical factors as influencing evaporation losses during
sprinkling: vapor pressure deficit, wind velocity.and cloud
cover. Evaporation spray losses by sprinklers have been
studied by Frost and Schwalen (13). A test plot was set
up for collecting the discharge from sprinklers by using
collecting containers from which the total volume of water
reaching the ground surface could be computed. The discharge
from the sprinkler nozzles was measured through a calibrated
meter and the spray loss was determined by the difference
between the metered discharge and the computed amount of
water reaching the ground surface. Using these tests as a
basis a nomograph was developed which showed the relationship
of relative humidity, air temperature, nozzle diameter,
nozzle pressure and wind velocity to evaporation losses.
The spray losses computed by using this nomograph include
wind drift losses for small droplets that were blown out of
the collecting area. Since this nomograph was computed for
a single sprinkler and because wind drift losses might not
be actual losses in a large area, Frost has suggested that
a value approximately 25% of those computed from the nomo-
graph could be used for a solid set system. Under extreme
conditions this nomograph shows evaporation losses as high -J
as 20%.

..Results of studies with a single lateral by Krause (2) -
and with two laterals by Sternberg (31) at Davis California
show that Frost and Schwalen's nomograph. may result in low
estimations of spray losses. Kraus' data show losses up to
20% higher than the nomograph and Sternberg's studies show
losses up to three times greater than the nomograph. It
should be noted that one would expect the greatest loss from
the single lateral. In order to be consistent with Frost
and Schwalen's interpretation of their nomograph, Kraus'
and Sternberg's results should have shown less evaporation
loss. On the other hand, Christiansen (5) in his work with
sprinkler irrigation in California studied direct evaporation
loss from the spray and concluded that this loss should be
less than 2%.

The Sprinkler Irrigation Association (SIA) Proceedings
(9) point out other apparent conflicts in the results of
evaporation studies which are perplexing to the engineer.
One of these is the amount of evapotranspiration during




5


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sprinkling. Sternberg (31) reported it was negligible for
grass. On the other hand, Frost and others have found that
evapotranspiration 'is approximately equal for sprinkling and
nonsprinkling periods. SIA has also pointed out that many
have concluded that the combination of evaporation, drift
losses and interception by vegetation do not significantly
reduce evapotranspiration from normal dry leaf values and
that losses from wet leaves are equal to losses from dry
leaves. Hence, interception of water by closely growing crops
is not a loss. Paul-and Burgy (27) on the other hand found
that interception evaporation losses might approach 60% of
the gross interception for widely spaced plants.

Wiser (33) tested the hypothesis that evaporation loss
during sprinkling is approximately the same as evaporation
from a free water surface under similar meteorological condi-
tions, and found it to be true. He attempted to do this in
field tests at Oxford, North Carolina, where he accounted for
all the water applied except that lost due to evaporation, and
took the difference to be equal to evaporation. Evidently
there was no crop, so this evaporation was for spray losses,
and soil surface evaporation but not evaporation and transpi-
ration from plants.

In order to test his hypothesis, he compared his results-
with that of several other investigators who had developed
equations for estimation of evaporation from free water surfaces.
An equation by Leeper (23) showed a good overall relation to
the test results.

E = 0.0207 (ea ed) X []

where E'= evaporation, inches per day

ea = saturation vapor pressure, millibars

ed = water vapor pressure of the air, millibars

X = relative duration of daylight, hours of actual
daylight/12

Weaver and Pearson (32) suggested a similar equation
including the wind velocity
0.76
E = 0.011 (e ed) X W76 [2]
a d
where W = wind velocity at a four foot level in miles per hour-

Wiser also compared these results with some equations by Penman
(28) which were more complicated and harder to use.

Most designers of sprinkler irrigation systems are using
a water application efficiency of approximately 70% which
includes all water losses (1,15,34). Cannell (3) gave a


Is -~-~.--;Y-~-YC~i~--surP~stl(~






summary of water application efficiencies obtained from
sprinkler systems as reported by several investigators.
These efficiencies range from 26 to 84% with a mean value
of 55%.

The lack of uniformity in irrigation efficiency is
partly due to disregard of rate of application. The use of
any uniform figure for efficiency as a design criteria
assumes that the total loss is proportional to the applica-
tion amount but depends on no other factor. Published data
do not bear this out. Several investigators, Christiansen
(4), Mather (24), Hammilton and Schrunk (18) and Somerhalder
(30) have pointed out that the efficiency is increased with
higher application rates as long as the ilfiltration rate of
the soil is not exceeded. For example, Mather got approximately
the same rate of evaporation loss from two systems although
one had a water application rate of 4 times the other.

Christiansen (5) developed an indirect method of esti-
mating evaporation loss from the spray through the use of
thermodynamic principals. Evaporation of water requires heat.
Three sources of heat are available for evaporating water
from a spray: (1) heat from the water, (2) heat absorbed from
the air, and (3) radiant heat [principally from the sun]. If
all the heat came from the water it would require a temperature
drop of about 10.5 F to evaporate 1% of the water.

When the water is cooler than the air, which is normally
the case in the daytime, the water will absorb heat from the
air and the temperature drop will be less than 10.5 F for a
loss of 1%. Absorption of radiant heat will increase the
evaporation for the same temperature change, however, this has
been shown to be negligible in most cases. When the initial
water temperature is the same as the wet bulb temperature of
the air an equilibrium temperature exists, in which case, all
the heat required for evaporation comes from the air and the
water remains at a constant temperature. When the initial
temperature of the water is lower than the wet bulb temperature,
the temperature of the water will increase even though some
evaporation still takes place. The evaporation would be zero,
however, if the water temperature was at the dew point; and if
it were lower, condensation would occur and there would be a
gain rather than a loss of water. Neglecting radiant heat,
Christiansen developed the following expression for the evapora-
tion loss from the spray.

= 100 C At P w -. .. [3]
r P Pa 0.00037B (t tw)


where E = the evaporation of water from the spray expressed
as a percentage of the amount discharged.

C = the specific heat of water, caloriea per gram per
degree F


-- ---------- ac~anrx-BVIPi~WLH~~r~a~:~l*F*li*f--i~







r = the heat of vaporization, calories per gram


SAt = drop in temperature of the water from the time
it leaves the nozzle until it reaches the ground

tw =the mean water temperature, degrees F

ta = the air temperature, degrees-F

P = the vapor pressure at temperature t in. Hg
w w
Pa = the pressure of water vapor in the air, .in. Hg

B = the barometric pressure, in. Hg

This equation fails to take into consideration the very
small droplets which are completely evaporated or blown away
by the wind and which do not contribute to the final tempera-
ture of the water as it reaches the ground. A study of the
distribution of droplet size indicates that only a very small
part of the water discharge is in the form of tiny droplets
that are lost in this manner. Thus, it is believed that this
loss would not cause an appreciable error in the determination
of evaporation loss from irrigation spray. Tests have been
conducted in which the change of water temperature as a result
of evaporative cooling has been measured. Tests on rotating
sprinklers with initial water temperature of 69.5 to 84 F and
1 with air temperature ranging from 75 to 101 F show decreases
of water temperature from 1 to 7 F corresponding to evaporation
losses of 0.23 to 0.81%. Another test with the initial water
of 98.7 F and an air temperature about 105 F resulted in a
temperature drop of 20.7 F, corresponding to a loss of about
2%. Christiansen concludes that evaporation loss from the
spray is negligible in comparison with subsequent losses from
the wet soil and vegetation.

Mather (24) made a field investigation of evaporation
from sprinklers by observing the actual increase in moisture
content of the air moving through an irrigated spray area.
His values were obtained from measurements of the dew point
upstream and downstream of the air entering the irrigated area.
He calculated the evaporation loss through the use of the
absolute moisture gain of the air as it passed through the
irrigated area. He either estimated the amount of moisture
movement upward or assumed that it was negligible. His results
show that as the distance downwind from the irrigated area
increased- the amount of evaporation into the air becomes less.
For example, his data indicate that only within the first
40 meters is there much gain in the moisture content of the
air. Thus, the percent of water lost would be minimized by
making the size of the field to be irrigated as large as
possible. The evaporation loss from the spray and moist soil




8


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ranged from 4 to 30% of the water applied. However, the
application rate in some cases was less than 0.1 of an inch
per hour. From actual observations and evaporation computa-
tions, Mather suggests that from a water conservation point
of view the application of water by irrigation should occur
as rapidly as is economically possible.

Ingebo (19), in his studies of vaporization rates of
iso-octane sprays, developed a semiempirical equation for
the prediction of spray losses during the initial period
after atomization at the nozzle.



de H U

where m =mass of water

8 = time

D = diameter of droplet

K = coefficient of thermal conductivity

At = difference of temperature between the surface of
the droplet and the surrounding gas atmosphere.

H = heat of vaporization of liquid

AV = the relative velocity between the droplet and the
surrounding gas

W = density of surrounding gas atmosphere

U = viscosity of the surrounding gas atmosphere

This equation was derived by converting and simplifying a
mass transfer equation.

Peters (29) studied the relative magnitude of evaporation
from soil surfaces and the transpiration by plants. He con-
cluded that in the midwest where frequent summer showers occur,
as much of 50% of the total water loss in a season can be
accounted for by evaporation from the soil surface. He used
plastic covered plots in his experiments and determined that
the amount of transpiration from a particular crop is within
rather narrow limits. He pointed out that photosynthesis has
been thought to be such a minor fraction of the total heat
budget that it could be neglected and that the net radiation
is used up principally in heating air and evaporating water.

Fortier and Beckett (10) conducted experiments to determine
evaporation losses after an irrigation from undisturbed and
cultivated soils at Davis, California. They found that one



9

^-I







to two inches of water evaporated from a soil within three
to four weeks after an irrigation, and that more than half
Sof it occurred during the first five days. According to
other experiments conducted by these men, the rate of evapo-
ration from saturated soils is about the same as that from
a free water surface or about 0.3 inches per day in the
Sacramento Valley during the summer.

When relatively small amounts of water are applied to
exposed soils at frequent intervals by sprinkling, much of
the water can be lost by evaporation. In some instances,
irrigation applications of about 1 inch are made at weekly
intervals to aid in the germination and starting of a crop
and to prevent the drying out and crusting of the surface
soil.. Most of this water may be lost directly from the
soil by evaporation. It is estimated that if application
rates of 0.25 to 0.5 inches per hour are used, more than
10% of the water may evaporate when it is applied during
daylight hours.

When crops are sprinkled part of the water is inter-
cepted by the foliage and later evaporated without reaching
the soil. Clark (6) determined the maximum interception
capacity of many plants and it appears, from his data, that
few crops can attain more than 0.1 inches of water.

Kraus (21) reported spray evaporation losses as a
function of vapor pressure deficit and separated the losses
into evaporation loss and drift loss. He measured the drift
losses by detecting and measuring impressions made by droplets
falling on a layer of magnesium oxide which was smoked on a
glass slide. This method is applicable for drops ranging
from 10 to 200 microns in diameter.

Total losses ranged from 3.4 to 17.0% for vapor pressure
deficits of 0.123 and 0.673 in. Hg, respectively. The
average drift loss was 36% of the total.. Through the use of
lysimeters he determined that evapotranspiration in the drift
zone, as compared to a dry control area, was increased under
high wind speed conditions, and was decreased under low wind
speed conditions.

George (14) studied spray evaporation losses by deter-
mining the salt content of the water in the lateral and-in
catchment bottles. Drift losses were not considered. The
author reported a correlation between relative humidity and
evaporation loss.

METHODS AND EXPERIMENTAL FACILITIES

Studies of irrigation evaporation have been conducted in
the field under natural conditions and in the laboratory. Each


PP~6BY







of these locations has advantages and disadvantages. In the
field, precision must be sacrificed in controlling and measuring
) the properties of the surrounding atmosphere, however,
adequate space is available to operate irrigation sprinklers
in the conventional manner. In the laboratory it is possible
to control the properties of the atmosphere during the
experiment and make precise measurements, however, it is
costly to provide adequate space within.an atmospheric control
facility in which to operate a standard agricultural irrigation
sprinkler. In either case, the experimental values that are
obtained must be projected and related in order for them to
provide practical information on irrigation evaporation. A
climatic control chamber was used in conducting all the tests
supporting this study.

Climatic Control Chamber

The climatic control chamber, shown in Figure 1 in
cutaway perspective view, was built specially for conducting
the experiments supporting this study. The chamber is equipped
for control of a range of dry bulb temperatures, dew point
temperatures, and air flow rates. Also, a water droplet
generator with the capacity for controlling precipitation
rates and water temperatures at different levels was constructed
as an integral part of the chamber.

The approximate outside dimensions of the chamber are
50 feet long, 16 feet wide and 10.feet high. It was designed
to minimize heat and vapor transfer between the surrounding
atmosphere and the air inside the chamber. Typically, as
shown in Figure 2, the exterior consisted of 4 inch thick
panels of paper honey-comb insulation with sheet aluminum
bonded to both sides and a 3 inch layer of polyurethane foam
poured in place on the inside to assure a good air and vapor
seal. Holes made through the exterior for water, electric
and refrigeration conduits were sealed by pouring polyurethane
foam around them. Nevertheless, there was some air leakage
through small openings around the blower shafts, door seals,
etc. However, the quantity of air transfer was determined
for various operating conditions and taken into consideration
for all data presented. The magnitude of the air transfer was
estimated by operating the chamber with an inside dew point
lower than outside and measuring the rate of moisture removal
required to maintain a constant dew point temperature within
the chamber. This value, along with the inside and outside
dew points, was used to compute the quantity of air transfer.
Air leakage under the most adverse conditions was approximately
125 cfm and this quantity was not considered significant except
for tests where evaporation rates were low.

As indicated in Figure 1, air moves in a closed circuit
from the blower discharges through the air conditioning section,
airflow measurement section, air straighteners, test section



11


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Air Flow Rote
Control Domper


0~o

4,~;


Primary Blower
(15 h.p.)


Figure 1. Climatic control chamber


Air Flow
'Measurement




















- OUTSIDE--


24 GA.'
ALUMINUM


-INSIDE-


22 GA. ALUMINUM


TYPICAL WALL SECTION

Figure 2. Typical wall section indicating construction materials and dimensions









and back to the blower intakes. The reason for using two
blowers was primarily for convenience in manipulating air
flow velocities through the heating and cooling coils with
minimal interference with air flow rates through the test
section.

Air Straighteners

Air straighteners and resistance layers were installed
at both ends of the test section as shown in Figure 3. They
consisted of stacks of 3 inch diameter by 12 inch long sheet
metal tubes, layers of aluminum honey-comb type material and
10-mesh screen wire. Trial and error techniques were used to
attain the desired degree of uniformity of air velocity through
the test section. The technique used required that air
velocity measurements, obtained with a hot wire type velocity
meter, be made at each intersection of an imaginary 12 inch
square grid across the test section in the vicinity of the
water droplet generator. On a basis of these values, unsatis-
factory air flow patterns were altered by adjusting the
location, size and number of layers of screen wire patches
that were-placed on the leading air side of the air straighten-
ers. The level of uniformity of air flow was considered
satisfactory when none of the individual velocity measurements
varied by more than 25% from the mean.

Test Section of Chamber

The test section is approximately 24 feet long, 8 feet
high and 5.5 feet wide. Figures 3, 4 and 5 are section drawings
of the climatic control chamber on which the location and
relative size of the test section is indicated.

Ten viburnumm" plants were placed in the test section
immediately beneath the water droplet generator to provide
vegetative material for tests involving evaporation losses
from intercepted water. Individual plants were approximately
3 feet high and 2 1/2 feet in diameter and conformed generally
to an ellipsoidal shape. In elevation, shown schematically
in Figure 5, they were located at three levels so as to fully
occupy the volumetric space beneath the generator. On each
plant there were approximately 500 leaves, each with a surface
area of about three square inches. Leaf density appeared to
be about the same-as that found in a mature citrus grove. During
test, it appeared that 80 -90% of the droplets were intercepted
by the plants.

Water Droplet Generator

A special apparatus was developed to generate water droplets
that could be used to simulate irrigation spray. Figure 6 shows


~-or--s~ul ------I-I^-Y--sC"~Y"ieannrr~ll
































SECTION THRU CLIMATIC
CONTROL CHAMBER


Figure 3.


Section through climatic control chamber indicating relative position of
air straighteners and test section











ELECTRICAL
PANEL BOXES

SECOND
B LLOWE


ENTRANCE


CLIMATIC CONTROL
CHAMBER LAYOUT


Figure 4.


Layout drawing through, the climatic control chamber indicating relative
position of the component parts of the chamber














WATER DROPLET GENERATOR
*': ="" ^ l ^ L- "_ Z- ^ I T J -- T -- ,, r i -


ACCESS
SPASSAGEW,


I- j .- r --, / -







.... .' OBSE
' "..T CT-
'ICO


S ilCA EC

---- ^----- ^2


OVATION


WINDOW


NTROL ROOM -


SECTION THRU
CLIMATIC CONTROL CHAMBER AT CONTROL ROOM


Figure -5.;


Section view through the climatic control chamber at the test section
indicating the positioning of plants


AIR RETURN
DUCT




/'r~-rr^-r~rrrr4


I- I-----7


~mun~--- LI---~~- -n~ ~*-r~un ~ 1.


Ir -


IccT(DE~J1II -.I



























^^^^*^.^^......

*', ,- .%-,-,i-
1 -- t ..




WATERDROPLET GENERATOR



WATER DROPLET GENERATOR'


Figure 6. .Schematic of water droplet generator








a view of this apparatus. The reservoir with disposable type
syringe needles projecting through the bottom, was recessed
into the ceiling of the test section of the climatic control
chamber so that the tips of the needles were flush with the
ceiling. It was constructed of plexiglass panels fastened
into a fabricated aluminum framework. Four hundred and three
needles spaced on two inch centers in a square pattern were
required to dispense droplets uniformly over an area 62 inches
wide and 26 inches long. When in operation the reservoir was
vented to the atmosphere through a stand pipe. The rate of flow
was controlled by adjusting the rpm of the paristaltic type
pump until the desired water pressure head was obtained over
the needles.

Number 20 gage X 1 inch syringe needles were used for all
tests. Preliminary tests had revealed that this size needle
produced droplets that were approximately 3 mm in diameter
which is also approximately the same average diameter as water
droplets produced by many irrigation sprinklers (13,17). Flow
rates equivalent to precipitation depths of 0.1 to 5.4 inches
per.hour could be obtained by adjusting the water pressure head
between 0.1 and 6.0 inches.

In order to have instant shut off of flow from the needles
it was necessary to install solenoid valves in the standpipe
and inlet water lines. The valves were electrically wired in
series-with the pump motor and thus were open or closed when
the pump was on or off. It was necessary for the entire water
droplet generating system to be purged of air in order to
obtain sudden shut off of flow from the needles.

Water that was not evaporated was collected in a pan
recessed-into the floor of the test section of the chamber.
A trap was installed in the pan drain pipe line so that the
depth of water in the pan was maintained at a constant level
of about three inches above the bottom. In order to minimize
evaporation from the water collected in the pan, a layer of
type I hydraulic fluid (oil), 3/8 inch in depth, was maintained
over the water surface during all tests.

A small electrical resistance type emersion water heater,
equipped with rheostat, was used to maintain the water tempera-
ture in the reservoir at the desired temperature.

Air Conditioning and Heating

Air leaving the test section of the climatic control
chamber is divided so that part of it goes through the air
conditioning-heating system and the remainder is recirculated.
The quality of air passing through the air conditioning-heating
system is dehumidified, cooled or heated to a level so that
when it is mixed with the recirculated air, the two will combine
to produce air with the desired physical properties for a
particular test. Figures 1 and 7 show the relative location of





































DEHUMIDIFYING COOLING AND HEATING. COMPONENTS
(Located in the return section of the CHAMBER)



Figure 7. Schematic indicating layout of mechanical components of the dehumidifying,
cooling and heating section of the climatic control .chamber







the components of the air heating, cooling and dehumidification
system as well as the distribution and direction of air flow.

If the dew point of the air coming in contact with the
water droplets is less than the temperature of the water
droplets, some of the water is evaporated from the droplets
and becomes water vapor in the air stream. In order to keep
the dew point of the air within the chamber from increasing
it becomes necessary to remove water from the chamber at the
same rate that it is evaporating. Water is removed from the
chamber as condensation on refrigeration coils. This conden-
sation is collected and weight measurements made with respect
to time to determine the rate. Since the only significant
source of moisture added to the system comes from the water
droplets, the rate of condensation is also the rate of
evaporation. This measurement is the primary criteria for
evaluating treatment responses presented in this report.
Two precautionary measures had to be taken to assure accuracy.
First, instrumentation had to be monitored to assure that the
.chamber had been operated for a sufficient length of time,
during each test, for all systems to be in equilibrium and
second, that none of the condensation coils were permitted to
become cold enough for the condensated moisture to freeze.

Dew Point Temperature Control

Normally, dew point levels were obtained by controlling
Sthe temperature of the evaporator coils at the desired level.
This was. usually accomplished by manual adjustment of the
evaporator pressure regulating valves, however, for several
of the lower dew.points, it was necessary to manipulate the
dampers of the recirculating duct to attain the desired levels.

,Dry Bulb Temperature Control

Air within the climatic control chamber was heated by a
steam coil equipped for automatic dry bulb temperature control.
The essential components of this control system were as follows:
motorized proportional control steam valve, electronic propor-
tional controller with reset and rate action and thermopile
temperature sensing element.

Air Flow Rate Control

As indicated in-Figure 1, two centrifugal blowers with
backwardly inclined impeller blades were used to obtain the
desired air velocity through the test section of the climatic
control chamber. It has been stated earlier that the secondary
(10 hp) blower was used primarily to facilitate ease of con-
trolling air velocities over the heating and cooling coils,
however it did furnish varying amounts of air (depending on dew
point level) for the test section. A motorized damper was
installed in the discharge duct of each blower to regulate the




21


,- -... .. nY 2L .,- ^^^.^ ^ ,-^^^ ^ -^B .,^^ ^ .-^ -- -.. --: --, ^ ^-- ^^.t -^ ^ = .. . .









PROCEDURE


Air velocity, dry bulb temperature, dew point temperature,
water temperature and rate of precipitation were factors tested
at different levels to measure their independent effect on rate
of evaporation. Levels at which these factors were tested is
given in Table 1. The influence of these factors on evaporation
was considered in terms of losses from water droplets (spray)
and from water intercepted by plants.

All tests were conducted in the climatic control chamber
that has been described in the "Methods and Experimental
Facilities" section of this report. Evaporation rates are
expressed as a -percentage of the discharge rate of the water
droplet generator (precipitation rate) and presented in
graphical form.

TABLE 1. The factors Tested and Levels of Treatment.

Water Droplets (Spray)


Factor


Air velocity, mph
Dry bulb temperature, F
Dew point temperature, F
Water temperature, F
Precipitation rate, iph


Level of treatment

2, 3, 4, 5, 6
75, 80, 85, 90, 95,
50, 55, 60, 65, 70,
75, 80, 85, 90, 95,
5.4


(nominal)


100
75
100


Plant Intercepted Water


Level of treatment


Air velocity, mph
Dry bulb temperature, F
Dew point temperature, F*
Water temperature, F
Precipitation rate, iph


2, 3, 4, 5, 6
75, 80, 85, 90,
60, 65, 70, 75
82
0.15, .50, 1.0,


Factor


2.0, 5.4








discharge rate for each blower. Unless prohibited by a
test requiring a low dew point treatment level, the damper
for the secondary blower was completely open at all times.
The damper for the primary blower would then be adjusted to
attain the desired air velocities through the test section.
Air velocities through the test section could be controlled
at levels up to 6 miles per hour.

Instrumentation

Instrumentation systems were required for dry bulb air
temperature control and measurement, dew point temperature
measurement and air velocity measurement.

An adjustable zero-adjustable range, proportioning band
potentiometric controller with reset and rate action was
used in conjunction with a motorized proportioning steam
valve to sense the dry bulb air temperature and regulate the
rate of steam flow into the air heating coil. The sensing
element for-the controller was a 5 junction thermopile
located at the approximate centroid of the cross section of
the test section of the chamber and about 3 feet up the air
stream with respect to the water droplet generator. Dry
bulb temperature measurements were made with a dual-element
quartz thermometer. Both elements were located at the
approximate centroid of the cross section of the test section
with one element being located about 3 feet up the air stream
and the other about 3 feet down the air stream with respect
to the water droplet generator. The thermometer elements were
connected to a digital read-out indicating the nearest 0.001
degree Celsius.

Dew point temperature measurements were made at two
locations. One was in the air stream at approximately the
same location as that of.the up-stream dry bulb air tempera-
ture sensing element and the other was outside the climate
control chamber. A direct reading dew point indicator equipped
with dual miniaturized "Heated Salt" thermistorized probes
was used to measure dew point temperatures. Meter readout
was scaled so that dew point temperatures could be read to
the nearest 0.1 F.

Air flow rate measurements were made in a section of the
return air duct of the climatic control chamber as indicated in
Figure 1. All the air was channeled through three 21 inch
diameter pipes, each equipped with a calibrated annular ring
type velocity probe. An electronic pressure meter was used
to measure the pressure output of the probes. Readout accuracy
of the electronic pressure meter was to the nearest 0.001 mm
Hg. Based on manufacturers claims for accuracy for the two
primary components of the air flow rate measuring system, it
is believed that air velocity measurements are accurate to
within 2.0 percent of the values presented in this report.













DISCUSSION AND RESULTS


All the data given in this report were taken under con-
trolled conditions in an environmental control chamber. The
exact duplication of field conditions for sprinkler irrigation
systems was sacrificed for conditions which could be adequately
controlled and described.

Therefore, the results of these tests should not be taken
as directly applicable to field conditions for sprinkler irri--
gation, but as a basis for making decisions concerning the
design and operation of sprinkler irrigation systems and the
long range advantages and limitations of sprinkler irrigation.

The data taken were for two separate sources of evapora-
tion, that from the spray and that from the plant intercepted
water. Since the configuration of the water applicator is
quite different from most field conditions, the spray losses
require considerable interpretation before applying to field
conditions. However, that from the plant intercepted water,
which also included some spray losses, should be-closely related
to field conditions if the variations due to different crop
configurations are taken into account.

Evaporation is directly proportional to the difference
between the saturation vapor pressure corresponding to the
temperature of the water surface and the vapor pressure of
the air (8). Therefore, the mean temperature of the water
surface directly affects the evaporation rate.

In most of the previous evaporation.studies, evaporation
rate has been reported as a function of air quality only with-
out regard to the initial water temperature or application
rate. One term commonly used in evaporation studies is vapor
pressure deficit, which is the difference between the satura-
tion vapor pressure of the air and the actual vapor pressure
of the air. When considering evaporation of water droplets in
air, the vapor pressure deficit is the vapor pressure difference
between the air and the droplet only for a mean water tempera-
ture equal to the air temperature. Another similar term which
has been reported as being directly proportional to evaporation
rate is wet bulb depression. These terms, of course, are useful
and very practical since they do not envolve the mean water
temperature, which is hard to determine. However, the evapora-
tion rate should not be expected to be directly proportional to
these terms.













0




DEW POINT, F 52
VELOCITY, mph 4
APPLICATION, iph 5.4
S DRY BULB, F 9.5


I I I I I I


100


INITIAL WATER TEMPERATURE, F


Figure 8. Evaporation of water droplets as a function of
initial water temperature. Dashed lines are not an
extrapolation of experimental results and are presented
only to support discussion.


DEW POINT, F
Oj/ APPLICATION,
WATER, F
DRY BULB, F


55
iph 5.4
82
95


0 1 2 3 4 5 6
0 1 2 3 4 5 6


SAIR VELOCITY, mph

Figure 9. Evaporation of water droplets as a function of air
velocity


1.2


1.0


0.8


0.6


0.4


0.2


0


70


0.8


0.6


0.4


0.2


~88I~sr~rasr- I- -- I i I -1-- -1- -- - A~--


L
c







Since the independent air quality variables such as dew
point, dry bulb, wet bulb and relative humidity are not
) directly proportional to the actual water vapor pressure
difference; one should not expect them to be directly pro-
portional to the evaporation rate. However, in many cases,
for the ranges indicated, an approximately linear relation-
ship does exist, but it should not necessarily be expected
to hold for other values of the independent variables.

The results of the tests for spray evaporation losses
are consistent with those of other investigators (5, 13, 33)
in that the losses are very small compared to the total amount
applied and also small compared to evaporation losses from
the plant and soil surface. The evaporation loss from drop-
lets expressed as a percentage of the amount applied ranged
from 0.20% to 1.13%, while that from the plant intercepted
water ranged from 3.5% to 60.3%. The effect of each inde-
pendent variable on evaporation, as indicated by Table 1, will
be discussed separately. Theoretical reasoning as well as
recorded data, was used in discussing the characteristics of
the curves that follow.


Evaporation from Water Droplets


Initial Water Temperature (Figure 8)--Figure 8 indicates
1 the percentage evaporation as a function of initial water
temperature. When the temperature of the droplet is equal to
the dew point temperature of the air, no evaporation will
occur. However, for an initial droplet temperature equal to
the dew point (52 F) or even lower, heat from the air will
be transferred to the droplet as it moves through the air
and evaporation will occur. The droplet will be heated until
the wet bulb temperature (67.7 F) is reached, provided the
exposure time is sufficient. There will be an abrupt change
in the slope of the curve where the initial water temperature
is equal to the wet bulb temperature with the slope immediately
above the wet bulb temperature being greater than that immedi-
ately below it. The evaporation rate will increase at an
increasing rate up to the dry bulb -air -temperature (95 F) at
which point another abrupt change will occur with the slope
immediately above 95 F being less than that immediately below
it. The evaporation rate increases at an increasing rate since
the saturation water vapor pressure increases at an increasing
rate with respect to water temperature.

The abrupt changes in the slope of the curve occur due
to the change in the relative amount of heat which is used
for evaporation in comparison with that used as sensible heat.
Below the wet bulb temperature some of the heat transferred
from the air must go to increase the temperature of the droplet;




25


:*^~ar^~md&~,am,t",^A-2!a^.\fi& ^s~~i--t3^s^.^~ j?`_~;_;~"







between the wet bulb temperature and the dry bulb temperature
heat for evaporation comes from both the air and the droplet;
b and above the dry bulb temperature heat is transferred from
the droplet to the air.

In past studies of evaporation the initial water temper-
ature has not been considered in most cases. However, as
can be seen from these results, the initial water temperature
is an important factor even though the droplets rapidly approach
the wet bulb temperature of the air. For field conditions
where the droplet exposure time is greater than that for the
laboratory tests, the effect of initial water temperature on
evaporation is reduced.

Air Velocity (Figure 9)--The effect of air velocity on
evaporation appears to be more closely related to the movement
of high-moisture-content air from the general vicinity of the
droplets rather than to the increase of the relative velocity
between the air and the droplet. This is substantuated by
results presented in Figure 9 where evaporation is directly
proportional to air velocity rather than proportional to a
lower power of the air velocity as would have been the case
if the change in relative velocity was the only contributing
factor (19).

Dry Bulb Temperature (Figure 10)--Since a change in dry
bulb temperature, in itself, does not affect the vapor pres-
sure of the air, the effect of dry bulb temperature on evapor-
ation is related only through the rate at which the droplet
temperature changes and the equilibrium temperature (wet bulb)
attained by the droplet. As the dry bulb increases the wet
bulb increases at a slightly decreasing rate. Thus the mean
droplet temperature during flight also increases at a slightly
decreasing rate, causing evaporation to have a similar rela-
tionship. There is an abrupt change in the slope of the curve
where the air temperature is equal to the initial droplet
temperature (82 F). The slope immediately below the droplet
temperature (82 F) is less than the slope immediately above
it. There is another abrupt change in the slope.where the air
temperature is equal to the dew point. Note that the evapora-
tion at this point is not zero since the droplet will heat the
air; thus allowing further moisture transfer to the air.

Dew Point (Figure ll)--As the dew point of the air increases,
both the vapor pressure and the wet bulb temperature increase
at increasing rates. These two occurrences have an opposing
effect on the evaporation rate and it appears from the curve of
Figure 11 that the relationship is linear. There will be an
abrupt change in the slope where the dew point equals the
initial water temperature (82 F). The dew point at which zero
net evaporation occurs will depend upon the time of exposure
for the droplet. For a dew point above the initial water
temperature and below the dry bulb, the droplet will gain
moisture from the air until the droplet is heated to the dew
point. At this point evaporation will begin when the droplet
is heated above the dew point.


27










0.8 VELOCITY, mph 4
APPLICATION, iph 5.4
. WATER, F 82
0.6 DRY BULB, F 95


E- 0.4


<0.2


0 L_____.____L___ ._.____
50 60 70 80 90

DEW POINT TEMPERATURE, F


Figure 11. Evaporation of water droplets as a function of
Dew point temperature








P 20 DEW POINT, F 67
APPLICATION, iph 5.4
WATER, F 82
DRY BULB, F 95



10




0 I
0- 1 2 3 4 5 6

AIR VELOCITY, mph

Figure 12.. Evaporation of plant intercepted water as a
function of air velocity,





29


~:~'*asl~.m~rr~a;~8~I-~~*LL~.~~S7UL~.~




















DEW POINT, F,
VELOCITY, ,nph
APPLICATION, iph
WATER, F


54
6
5.4
82


60 70 80 90 100
0 60 70 80 90 I00


DRY BULB TEMPERATURE, F.

Figure 10. Evaporation of water droplets as a function of dry bulb'temperature
Dashed lines are not an extrapolation of experimental results and
are presented only to support discussion.


1.0


0.8



0.6



0.4


0.2



0







Evaporation from Plant Intercepted Water


The curves for evaporation (Figures 12-15) from plant
intercepted water appear to be similar in shape to those
for evaporation from the spray, with the main difference
being the magnitude of the evaporations. It is more diffi-
cult to determine the slope of the curves for the plant
intercepted water because heat is transferred between the
water and the plant as well as between the air and the water.
The curves should still possess abrupt changes in slope
corresponding to the dew point and initial water temperature
on the appropriate curves.

Because of the much longer exposure time for the plant
intercepted water, the effect of initial water temperature
should be much less than for the spray. This means that all
of the curves for plant intercepted water would have been
changed only slightly if a different water temperature had been
used.

As has been discussed in the review of literature, all
of the evaporation from plant intercepted water should not
be considered as a loss charged to irrigation since some evapo-
transpiration would have occurred without irrigation. The
evapotranspiration rate was measured before the plants were
wet, for 95 F dry bulb temperature, 60 F dew point and 4 miles
per hour and was found to be 30 grams per minute or 0.066
inches per hour. This indicates that, for most of the tests,
more than 90% of the evaporation would be considered a loss.

It should be noted that solar radiation which was elimi-
nated in these tests, would have increased the amount of evapor-
ation.

In the following discussion of each variable, only the
important differences between the curves for evaporation from
plant intercepted water and for spray evaporation will be given.

Air Velocity (Figure 12)-Percent evaporation appears to
be directly proportional to air velocity for the range of
values used in this test. The discussion in the section on
spray evaporation with respect to air velocity is applicable
here.

Application Rate (Figure 13)--This curve indicates that
the percent evaporation increases very rapidly for application
rates below 1 inch per hour. Since all of the water that was
discharged was not intercepted by the plants, it is believed
that most of the intercepted water is evaporated for applica-
tion rates below 0.1 inch per hour. It should be noted that
this test was for a particular crop configuration and that
these values would vary according to the plant surface area
and the kind of plant.


IIIIIII~Y~L-~ -11I^~-~-____--~I~~ILX. .IIE~i~C-~YI^114sYI-~-ILU~PIYI~-~i~-I~













60 -




DEW POINT, F 63
50 VELOCITY, mph 4
WATER, F 82
DRY BULB, F 95




0 40



M .

0 30





20





10 -





0
0 I -
0 1 2 3 4 5 6

WATER APPLICATION RATE, iph

Figure 13. Evaporation of intercepted water as a function of appli-
cation rate,- .








31







Dry Bulb Temperature (Figure 14)--Here again, as was dis-
cussed for spray losses, a change in dry bulb temperature does
1 not affect the vapor pressure of the air but only the rate
at which the water changes temperature and the equilibrium
temperature attained. But in the case of plant intercepted
water the equilibrium temperature is not the wet bulb temper-
ature, but slightly higher, because heat is conducted from
the plant.

Dew Point (Figure 15)--The curve for evaporation from
plant intercepted water indicates a definite curvature with
the evaporation decreasing at an increasing rate with respect
to dew point while the curve for spray evaporation was approx-
imately linear.


Interpretation of Results


The values indicated for spray losses are not directly
applicable to field conditions for sprinkler irrigation systems
since they were obtained from uniform size droplets (3mm)
falling a distance of 8 feet with a zero initial velocity.
The factors which would significantly contribute to a different
value for field sprinkler systems are the droplet size, time
of exposure and relative velocity between droplet and air.

In addition to correcting for these factors, one must
measure the climatic conditions in the immediate vicinity of
the spray, such as was done by Mather (24) and referred to in
the review of literature section of this report.

In order to show how the results of these tests might be
applied to field conditions, consider a sprinkler with a
7/32 inch diameter nozzle operating at 40 psig on a 10 foot
riser. Assume the climatic conditions are 95F dry bulb, 54 F
dew point and 4 mph wind. The water leaves the nozzle at
82 F.

According to studies by Frost and Schwalen (13) the
average size droplet under these conditions would be approxi-
mately 3mm in diameter. Thus no correction is needed for
droplet size. It is realized that the use of the average size
droplet does not give the correct value for the total surface
area since the area is proportional to the square of the diam-
eter. However, it is a good approximation for a properly
designed and operated sprinkler where a very small percent of
the distributed water would be in the form of a mist.

If the total exposed surface area per volume is different
than that for 3 mm diameter droplets, the percent evaporation
can be considered inversely proportional to the diameter (D)
1 of the droplets (evaporation is proportional to D2 while total
amount applied is proportional to Dd).



32

- *,,,s .^^'^,.is- .^;', ,.1-












DEW POINT, F 60
VELOCITY, mph 3.81
APPLICATION, iph 5.4
WATER, F 82


20


15


10


5


0


DRY BULB TEMPERATURE, F


Figure 14. Evaporation of plant intercepted water
function of dry bulb temperature


VELOCITY, mph
APPLICATION, iph
WATER, F
DRY BULB, F


as a


4
5.4
82
95


i I__ _~___ ~ rL ____LJ~L L_


100


DEW POINT TEMPERATURE, F


Figure 15. Evaporation of plant intercepted water as a
* function of dew point temperature-









33


-


' H

U
0


0
P


,I IP J I~-(-~ I I sl-


E-I


0
H


PI
. f


5 I



06
. 6C


~ppBnroi~-u~n --~-P u~-rr~ *-araa-rr--__Jna


)







If a large portion of the water is in the form of small
droplets significantly different in size from the average,
the average size droplet should not be used to calculate the
total surface area.

The time of exposure for the droplets, for the laboratory
tests and for the sprinkler being considered was estimated by
assuming that the force due to air resistance is proportional
to the velocity. The constants were evaluated from the results
reported by Green (16) on the evaluation of air resistance
to freely falling drops of water. The time of exposure for the
laboratory tests was determined as 0.80 seconds while that for
the sprinkler was 2.00 seconds.

The average relative velocity was calculated to be 10.3 fps
for the laboratory tests and 45 fps for the sprinkler. These
velocities were determined through the use of equations of
motion similar to those described by Green (16). Although nor-
mal wind velocities did affect the relative velocities for
the laboratory tests, they do not significantly affect relative
velocities for sprinklers due to the higher velocities of the
droplets.

The effect of relative velocity on evaporation can be
determined from equation [4] as approximately proportional
to the square root of the relative velocity. Thus considering
the relative velocities used in this example, the evaporation
~ for the sprinkler would be 2.1 times that for the laboratory
tests.

The percent evaporation is not directly proportional to
the exposure time for the droplet since the temperature of the
droplet is approaching the wet bulb temperature of the air
(Figure 16). However, as shown in Figure 17 the percent evapor-
ation is almost directly proportional to exposure time for this
example. This is a typical relationship as long as the dif-
ference between the initial water temperature and the wet bulb
temperature is not large compared to the difference between
the wet bulb temperature and the dew point temperature. If this
is not the case, the curve becomes more non-linear.

The curve in Figure 16 was generated by the equation:


tdroplet= (tdi tb) e + tb [5]


where

tdroplet = temperature of droplet at time 8, F

tdi = initial temperature of droplet, F

twb = wet bulb temperature of the air, F



34


iB ,H!^ rrt eas Bgil i%,B1a..33Isfe^^^""






2J/ uj-vl r Jii. r 4
VELOCITY, mph 4
WATER, F 82
DRY BULB, F 95
WET BULB, F 68.7
NOZZLE DIA., in. 7/32
PRESSURE, psi 40
NOZZLE ANGLE, deg. 2.0
RISER, ft.10


I


0.4


0.8


1.2


1.6


EXPOSURE TIME, SECONDS


Figure 16.







2.5


2.0


1.5


1.0


0.5


0


Effect of exposure time on water droplet temperature


0.4 0.8 1.2 1.6


2.0


EXPOSURE TIME, SECONDS

Figure 17. Exposure time of water droplets as a function of
evaporation

I/ Values are applicable for figures 16 and 17.


2.0


-- --- --- uacrs~snaer~n~ae8


DEWMrx nTm PO


r- A







A = a constant for this example, but a function of
droplet size and relative velocity.

6 = time of exposure, sec.

Equation [5] was derived by assuming that the droplet tempera-
ture can be described by the following differential equation:


dtdroplet (t [6
d 9 = A dt t [6]
d A droplett wb

with conditions: tdroplet = tdi when =


droplet wb when

The constant "A" was evaluated through the use of equation [31
(review of literature) in conjunction with laboratory tests
for percent evaporation. A limited number of measurements
were made on the initial and final temperatures of the water
droplets, to check equation [3]. "A" was determined to be
0.447 for the laboratory tests and 2.1 (0.447) for the sprink-
ler. The factor 2.1 was determined from the ratio of the
relative velocities.

The curve in Figure 17 was generated by the following equa-
Stion:
B' -AB
m =0.146 ( 1 e ) + Be (0.011 t P ) [71
A wb a

where m,= total evaporation expressed as percentage for a drop-
let.. exposure time of e.

B = constant for this example, but a function of droplet
size and relative velocity.

A = the constant in equation [6]

Pa = water vapor pressure of the air, in. Hg

Equation [7] was derived assuming that the water mass
transfer rate to air could be described by the following dif-
ferential equation:

dm ( B (P lt Pa) B (0.011 troplet P) [8]
do droplet a droplet a

This equation assumes a linear relationship between the water
vapor pressure of the droplet and the droplet temperature which
is a "poor fit" but gives reasonable accuracy for the range of
' temperatures used in this example. For higher accuracy a dif-
ferent model should be used, for example, a polynominal expres-
sion. The constant "B" was evaluated from test results and
~36


-r9 -- ~~L-ii~a~s~Z1~---. -..--~"i~inTLr~a"- ---- ---u~"a`m~`~ira8--~a~rrPs~-h-~88-~~o~







the assumed relationship between relative velocity and
evaporation and was found to have a value of 3.0

This example indicates that a value of 2.5% evaporation
should be used for a sprinkler in comparison to 0.52% obtained
from laboratory tests for the same climatic conditions A
factor of 5 could be used as a "rough" value for most of the
tests.

The application of laboratory results for plant inter-
cepted water should be directly applicable to field condi-
tions if the climatic conditions are measured in the vicinity
of the sprinklers and the plant surface area and configuration
are taken into account.


CONCLUSIONS


This study has resulted in the following conclusions:

1) Rate of application is the most significant factor influ-
encing evaporation losses where a large proportion of the
applied water is intercepted by vegetative material. It
should be optimized with respect to economical system
design and limited by maximum infiltration rate.

2) Evaporation losses from water droplets while in transit
in air should not exceed 5% of the total water application
under typical climatic conditions in Florida. The amount
is relatively insignificant when compared to the larger
losses that can occur after the water droplets have been
intercepted by plant surfaces.

3) The effect of the climatic factors of wind, air tempera-
ture and air dew point on evaporation losses from irriga-
tion are approximately linear within the ranges tested in
this study.

4) The effect of initial water temperature on evaporation
losses from irrigation does not appear to be significant
in the realm of general irrigation practice. The contri-
bution of this factor to evaporation losses is approximately
linear for climatic conditions and natural water tempera-
tures prevailing in Florida.


_I ___P~_PU~__~____~__~_P~,I______I~~~---l












ACKNOWLEDGMENTS


Dr. I. J. Ross, Associate Professor, Department of

Agricultural Engineering, University of Kentucky (for-

merly Department of Agricultural Engineering, University

of Florida) served as a leader on the project during the

planning phase of the study.

Mr. J. H. Weldon, Machinist-Mechanic, Department of

Agricultural Engineering, University of Florida, served as

the project refrigeration technician for the construction

and data collecting phases of the project.

The authors hereby express their sincere appreciation

to both of these gentlemen for their contribution to the

project.


























38


~p~S~-~ u.-Ti.. r-, r ;










LITERATURE CITED


1. Baird, C.D. Measurement of Water Evaporation Rates Utilizing
an Electronic Condensation Hygrometer. Unpublished M.S.
Thesis. University of Florida, Gainesville, Florida.
1967.

2. Burgy, R.H. and C.R. Pomeroy. Interception Losses in Grassy
Vegetation. Trans. Am. Geophys. Union. 39:1095-1100.
1958.

3. Cannell, Glen H. Irrigation Efficiency as it Influences
Water Requirements of Crops. Special Publication,
American Society of Agricultural Engineers. SP-SW-0162.
pp. 47-56. 1962.

4. Christiansen, J.E. Estimating Evaporation and Evapotranspir-
ation from Climatic Data. Paper presented at Annual
Meeting, Rocky Mountain Section, American Society of
Agricultural Engineers. Fort Collins, Colorado. April
23, 1966.

5. Christiansen, J.E. Irrigation by Sprinkling. California
Agricultural Experiment Station, Berkley. Bulletin 670.
1942.

6. Clark, O.R. Interception of Rainfall by Prairie Grasses,
Weeds and Certain Crop Plants. Ecological Mimeographs.
10: 243-277. 1940.

7. Cunningham, R.T., J.L. Brann, Jr. and G.A. Fleming. Factors
Affecting the Evaporation of Water from Droplets in
Airblast Spraying. Journal of Economic Entomology.
55: 192-199. 1962.

8. Dalton, J. Experimental Essays on the Constitution of Mixed
Gasses; on the Force of Steam or Vapor from Waters and
Other Liquids in Different Temperatures, both in a Tor-
ricelliam of Gasses by Heat. Mem. Manchester Lit. and Phil.
Sc. 5:535-602. 1798.

9. Davis, John R. Efficiency Factors in Sprinkler System Design.
Proceedings Sprinkler Irrigation Association. 100 Vermont
Ave. N.W., Washington, D.C. 1963.

10. Fortier, Samuel. Use of Water in Irrigation. New York: McGraw-
Hill Book Company. 1915.

S11. Frost, K.R. Factors Affecting Evapotranspiration Losses During
Sprinkling. Transactions of the ASAE. 6(4) 282-283, 287.
1963.


39

'.*-. 4',r. !. M-f^ ^i^ >W^4







12. and H.C. Schwalen. Evapotranspiration During
Sprinkler Irrigation. Transactions of the ASAE. 3(1)
) 18-20, 24. 1960.

13. and H.C. Schwalen. Sprinkler Evaporation
Losses. Agricultural Engineering. 36(8): 526-528. 1955.

14. George, J.T. Evaporation from Irrigation Sprinkler Sprays
as Determined by an Electrical Conductivity Method.
Unpublished M.S. Thesis. University of California,
Davis, California. 1957.

15. Gray, Alfred S. Sprinkler Irrigation Handbook. Rain Bird
Sprinkler Manufacturing Corporation. Glendora, Cali-
fornia. 7th Edition. 1961

16. Green, Robert L. Evaluation of Air Resistance to Freely
Falling Drops of Water. Agricultural Engineering.
33(5): 286. 1952.

17. A Photographic Technique for Measuring the
Sizes and Velocities of Water Drops from Irrigation
Sprinklers. Agricultural Engineering. 33(9):563-568.
1952.

18. Hamilton, F.B. and J.F. Schrunk. Sprinkler vs. Gravity Irri-
gation A Basis for Choice of the Best System. Agricul-
tural Engineering. 34(4): 246-250. 1953.

19. Ingebo, R.D. Vaporization Rates and Drag Coefficients for
Isooctane Sprays in Turbulent Air Streams. National
Advisory Committee on Aeronautics TN 3265. pp 1-39. 1954.

20. Keen, Bernard A. The Evaporation of Water from Soil. Journal
of Agricultural Science. 6:456-475. 1914.

21. Kraus, J. H. Analysis of Sprinkler Irrigation Application
Efficiency. Unpublished M.S. Thesis, University of
California, Davis, California, 1961.

22. Application Efficiency of Sprinkler Irriga-
tion and its Effect on Microclimate. Transactions of
ASAE. 9(5): 642-645. 1966.

23. Leeper, G.W. Thornthwaite's Climatic Formula. Journal
Austrailian Institute Agricultural Science. 16: 2-6.
1950.

24. Mather, J.R. An Investigation of Evaporation from Irrigation
Sprays. Agricultural Engineering. 31(11) 345-348. 1950.

25. McMillan, W.D. and R.H. Burgy. Interception Losses from Grass.
SJournal Geophysical Research. 65: 8. 1960.


"tlyj~p~-~ ~-~-"*"-"ir"i;a~LI~i~gej~~- ------- ~s~a~i~aaf~aars~a~i~








26. Mutchler, C.K. and W.C. Moldenhauser. Applicator for a
Laboratory Rainfall Simulator. Transactions of the
ASAE. 6(3): 220-222. 1963.

27. Paul, H.A. and R.H. Burgy. Interception Losses from Small
Trees. Memorandum, Department of Irrigation, University
of California, Davis, California 1961.

28. Penman, H.L. Natural Evaporation from Open Water, Bare
Soil and Grass. Royal Society of Agriculture. 193:
120-145. 1948.

29. Peters, D.B. Relative Magnatude of Evaporation and Tran-
spiration. Agronomy Journal. 52: 536-538. 1960.

30. Somerhalder, B.A. Comparing Efficiencies in Irrigation
Water Application. Agricultural Engineering. 39(3):
156-159. 1958.

31. Sternberg, Y.M. Day and Night Sprinkler Irrigation-Analysis
of Spray and Evapotranspiration Losses. Unpublished
M.S. Thesis. University of California, Davis, Cali-
fornia. 1962.

32. Weaver, H.A. and R.W. Pearson. Influence of Nitrogen Ferti-
lization and Plant Population on Evapotranspiration by
" -Sudan Grass. Soil Science. 81: 443-451. 1956.

33. Wiser, E.H., J. van Schilfgaarde and T.U. Wilson. Evapo-
transpiration Concepts for Evaluating Sprinkler Irri-
gation Losses. Transactions of the ASAE. 4(1): 128-
130, 134. 1961.

34. Woodward, G.O. Sprinkler Irrigation. Washington, D.C.:
Darby Printing Company. 1959.




Full Text

PAGE 1

EVAPORATION LOSSES IN SPRINKLER IRRIGATION by J.M. Myers Professor, (Agricultural Engineer) C.D. Baird Research Instructor, (Agricultural Engineer) R.E. Choate Professor, (Agricultural Engineer) PUBLICAT10N NO. 12 of the Florida Water Resources Research Center RESEARCH PROJECT TECHNICAL COMPLETION REPORT OWRR Project Number A-003-FLA Annual Allotment Agreement Numbers 14-01-0001-580 (1965) 14-01-0001-779 (1966) 14-01-0001-903 (1967) 14-D1-0001-1077 (1968) Report Submitted: December 31, 1970 The work. upon which this report is based was supported-in part by funds provided by the United States of the Interior, Office of Water Resources Research as Authorized under the Water Resources Research Act of 1964.

PAGE 2

TABLE OF CONTENTS ABSTRACT SUMMARY INTRODUCTION REVIEW OF LITERATURE ... .. ; Page 1 2 3 4 METHODS AND EXPERIMENTAL FACILITIES .'. 10 Climatic Control Chamber 11 Air Straighteners 14 Test Section of Chamber 14 Water Droplet Generator Air Conditioning and Heating ... 19 Dew Poi:nt Temperature Control 21 Dry Bulb Temperature 21 Air Flow Rate Control 21 Instrumentation 22 PROCEDURE 23 DISCUSSION AND RESULTS 24 Evaporation by Water Droplets 25 Evaporation of Plant Intercepted Water 30 Interpretation of Results 32 CONCLUSIONS 37 ACKNOWLEDGMENTS . . 38 LITERATURE CITED 39

PAGE 3

ABSTRACT EVAPORATION LOSSES IN SPRINKLER IRRIGATION water conservation, distribution of chemicals through irrigation water and the increasing popularity of lowapplication rate irrigation systems are all important factors pointing up the need for more precision in irrigation management which in turn is dependent upon accurate estimates of expected evaporation losses. Data has been obtained to predict the independent effect of water application rate, air (wind) velocity, water temperature and dry bulb and dew point temperature of the ambient air on evaporation losses by water droplets and water droplets in combination with plant intercepted water. By far the most influential factor on tion losses is the rate of application. Results indicate evaporation losses are about 60% for low application rates (0.15 iph) with climatic conditions typical of Florida and when plant foliage is present to intercept most of the applied water. Evaporation losses by water droplets in motion is relatively insignificant in comparison to losses from extensive wetted surfaces afforded by dense vegetation. It is unlikely that evaporation by water droplets in transit could amount to more than 5% of a water application. The independ. ent effect of \'later temperature and several important climatic factors on evaporation losses were determined and are presented in graphic form in this report. Myers, J.H., C.D. Baird and R.E. Choate EVAPORATION LOSSES IN SPRINKLER IRRIGATION Completion Report of the Office of Water Resources Research, Department of Interior, December, 1970, Washington, D.C. 20240 KEYWORDS: irrigation/ evaporation losses/ water droplets/ wind velocity/ water temperature/ dew point/ air temperature/ application rate/ intercepted water.

PAGE 4

< ( SUMMARY The objectives of these studies were to evaluate independently the climatic factors of air temperature, dew point temperature and wind velocity, and the physical factors of water temperature and application rate in terms of evaporation losses for sprinkler irrigation. Evaporation losses-were separated into two sources, that from the spray (water droplets) and that from the wetted surface of plant material. Supporting tests were conducted under controlled conditions in an environmental control chamber built in the laboratory of ,the Department of Agricultural Engineering at the University of Florida. The results of the study are not directly applicable to field conditions f\.r sprinkler irrigation but the data in this report are interpreted ,so that it supplies a basis for making decisions concerning design ana operational of sprinkler_ The results of tests on evaporation losses for spray are consistent with those of several other investigators in that the losses are very small when compared to the total amount of water applied and also small when compared to, evaporation losses from plant surfaces. Spray evaporation losses, expressed as a percentage of the amount applied, ranged from 0.20% to 1.13% while that'from the plant intercepted water ranged from 3.5% to 60.3%. Based on laboratory tests and in consideration of adjustments for droplet size, ,time of exposure and relative velocity, between droplets and air, it is estimated that evaporation from this source shoUld not exceed 5% of the amount applied under typical field conditions in 'florida. In past studies of evaporation the initial water temperature was not considered in most cases. 'However, the laboratory tests on this factor indicate that it is important even though the droplets approach the wet bulb temperature very rapidly. For field conditions where the droplet exposure time will be greater than that for the laboratory the effect of initial water temperature on evaporation will be reduced. The influence of air, (wind) velocity on evaporation from droplets appears to be more closely related to the movement of high-moisture-content-air from the general vicinity of the droplets rather than to the increase of the relative velocity between the air and the droplet. The effect of air temperature on evaporation losses is strictly related to the rate at which the droplet temperature changes and the temperature (wet bulb) which is reached by the droplet. As the dew point of the air increases, both vapor pressure and wet bulb temperature of the air increases at increasing rates. The two occurrances have an opposing effect on the evaporation rate and it appears that the relationship is almost linear. 2

PAGE 5

The relationships of the effect of air velocity, dry bulb temperature and dew point to evaporation losses from plant surfaces is similar to those found for water droplets, however, the order of magnitude of evaporation from plant surfaces is many times greater. Important factors contributing to these larger evaporation rates are the larger wetted areas and longer exposure time for plant water when compared to water droplets during transit. Evaporation losses from plant surfaces are primarily a function of the rate of application. Evaporation losses in terms of percent of the total application can vary from 10% for application rates of 5 iphto more than 60%for application rates of 0.15 iph for typical Florida climatic conditions. Publications that have resulted from this project thus far are: Baird; C.D. of Water Evaporation Rates utilizing an Electrolytic Condensation Hygrometer. Unpublished.M.S. Thesis, University of Florlda, Gainesville, Florida. 1967. Baird, C.D., J.M. Myers and I.J. Ross. Precision Measurement of Dew Point Changes with Electrolytic Condensation Hygrometer. Transactions of the ASAE. 12(6): 849-853. -_1969. INTRODUCTION Water losses by evaporation from-sprinkler irrig-ation can vary from practically nothing to more than half the volume of water delivered to the sprinkler nozzle. It is believed that many users of irrigation have only a general of the magnitude of evaporation losses. In the management of irrigation, overall application efficiencies, i. e. the relative proportion of the water that is removed from the source and placed in the soil for crop use, of 70 to 80 percent are in standard usage in Florida. criteria for estimating irrigation efficiencies may not have affected significantly the economics of irrigation in the past, but, with anticipated new technological advancements and the increased awareness of the necessity for water conservation, the demand for more precise information on irrigation efficiency and evaporation losses will be required for the years ahead. The potential is promising for effective and low cost application of chemicals through irrigation for plant growth regulation and insect and disease control. Evaporation from 3

PAGE 6

the foliage is a needed value in calculating the amount of chemical residues remaining on the plants after the cessation of the irrigation application. The concentration of chemical solutions at the moment of application is dependent upon evaporation losses_from the water droplets as they move through the air. When plant foliages are to salt residues resulting from evaporation of saLine irrigationcwater, the rate of evaporation with respect to rate'of application becomes one of the important factors Eo consider in adjusting to thisproblerri. At times, evaporation from irrigation spray is desirable as a medium for cooling plants and the surrounding air. Certainly there is a-trend toward greater control of the micro-environment for-growing agricuH:ural -Moreknowledge about water evaporation losses associated with sprinkler irrigation is a these situations. -: ,-0 -sourse I any water that is removed from for irrigation purposes and does not reach the intended application reflects. unfavorably upon operating costs and-is not, in. concert with the of water resource' conservatiori. .--_ -its source zone of irrigation philosophy dew point temperature and wind velocity factors that, evaporation fosses"while irrigating. .-Water temperature, water droplet size, -cvelocity of water droplet, time duration that the droplet is, in : tr2l.llsi t between nozzle and the interception-point_and interception characteristics are other factors playing partin this phenomenon. It was the object of the study reported herein to independently evaluate these factors in terms of their influence on evaporation losses -: -.: -2 --::-y-. -e ? --= ':' -:--: .-' .. -="," -:--? ? .,--: -=: -:---0 _Irrigation. has ebeen practiced in the United States for mor-e than -1:00 __ years -_Fortier (-10) stressed the importance for irrigatiorjefficiency as :early as 19l5 when experiments show that for every three -galToris -of water' --diverted from -natural streams one a useful purpose in nourishing plant life." -. -. --. -... --' -: "arid -practices -have-advanced to the poirif-fhat:wateY application -efficiency:isprimarily controlled 'QY :the :am6uritof evaporatiori losses ... Water-application effic may De .defined --as :e-,-ratio of the quanti ty of water put::_ -into the root zone and utilized by growing crops-to the quantity delivered to the field (34). ---. -. ,_' Interest in evaporation --and tion is not of -(8Yiri1798 showed that the rate of evaporation was-proportional to the difference between the 4

PAGE 7

" water vapor pressure at the evaporating surface and in the atmosphere. Essentially all vapor transport formulas since that take this principle into account. According to Frost (11) the operational factors which may influence the losses during sprinkling are droplet size, application rate,crop, crop height, soil moisture retention and water temperature. He also lists the following climatological factors as influencing evaporation losses during sprinkling: vapor pressure deficit, wind velocity.and cloud Evaporation spray losses by sprinklers have been studied by Frost and Schwalen (13). A test plot was set up for collecting the discharge from sprinklers by using collecting containers from which the total volume of water reaching the ground surface corild be computed. Thedischarge from the sprinkler nozzles was measured through a calibrated meter and the' spray loss was determined by the difference between the metered discharge and the computed amount of water reaching the ground surface. Using these tests as a basis a nomograph was developed which showed the relationship of relative hu.rnidity, air temperature, nozzle diameter, nozzle pressure and wind velocity to evaporation losses. The spray losses computed by using this nomograph include wind drift losses for small droplets that were blown out of the collecting area. Since this nomograph was computed for a single sprinkler and because wind losses might not be actual losses in a large area, Frost has suggested that a value approximately 25% of those computed from the nomograph could be used for a solid set system. Under extreme condidions this nomograph shows evaporation losses as high' ,. as 20% of studies with a single lateral by Krause (2) and with two laterals by Sternberg (31) at Davis California show that 'Frost and Schwalen's nomograph. may result in low estimations of spray losses. Kraus' data show losses up to 20% higher than the nomograph and Sternberg's studies show losses up to three times greater than the nomograph. It should be noted that one would expect the greatest loss from the single lateral. In order to be consistent with Frost and Schwalen's interpretation of their nomograph, Kraus' and Sternberg's results should have shown less evaporation loss. On the other hand, Christiansen (5) in his work with sprinkler irrigation in California studied direct evaporation -loss from the spraT and conclUded that this loss should be less than 2%. The Sprinkler Irrigation Association (SIA) Proceedings (9) point out other apparent conflicts in the results of evaporation studies which are perplexing to the engineer. One of these is the amount of evapotranspiration during 5

PAGE 8

sprinkling. Sternberg (31) reported it was negligible for grass. On the other hand, Frost and others have found that evapotranspiration -is approximately equal for sprinkling and nonsprinkling periods. SIA has also pointed out that many have concluded that the combination of evaporation, drift losses and interception by vegetation do not significantly reduce evapotranspiration from normal dry leaf values and that losses from wet leaves are equal to losses from dry leaves. Hence, interception of water by closely crops is not a loss. Paul-and Burgy (27) on the other hand found thai interception evaporation losses might approach 60% of the gross interception for widely spaced plants. Wiser (33) tested the that evaporation loss during sprinkling is approximately the same as evaporation from a free water surface under similar meteorological conditions, and found it to be true. He attempted to do this in field tests at Oxford, North Carolina, where he accounted for all the water applied except that los due to evaporation, and took the difference to be equal to evaporation. Evidently there was no crop, so this evaporation was for spray losses, and soil surface evaporation but not evaporation and transpiration In order to test his hypothesis, he compared -his results-with that o other investigators who had developed equations for estimation of evapoiation from free water surfaces. An equation by Leeper (23) showed a good overall relation to the test results. E = 0.0207 (ea -e d ) X where Er= evaporation, inches per day e = saturation vapor pressure, millibars a e d = water vapor of the air, millibars [1] X = relative duration of hours of actual daylight/12 Weaver and Pearson (32) a similar equation the wind velocity [2] -W"-=wind velocity at a four foot level-iriiniles--perliotir-----Wiser also compared these results with some equations by Penman (28) which were more complicated and harder to use. Most'designers of sprinkler irrigation systems are using a water application efficiency of approximately 70% which includes all water losses (1,15,34). Cannell (3) gave a 6 r-'

PAGE 9

summary of water application efficiencies obtained from sprinkler systems as reported by several investigators. These efficiencies range from 26 to 84% with a mean value of 55% Thelack of uniformity in irrigation efficiency is partly due to disregard of rate of application. The use of any uniform figure for efficiency as a design criteria assumes that the total loss is proportional to the application amount but depends on no other factor. Published data do not bear this out. Several investigators, Christiansen (4), Mather (24), Hamrnilton and Schrunk (18) and Somerhalder (30) have pointed out that the efficiency is increased higher application rates as long as the ilfiltration rate of /' the soil is not exceeded. For example, Mather got approximately the same rate of evaporation loss from two systems although one had a water application rate of 4 times the other. Christiansen (5) developed an indirect method of estimating evaporation loss from the spray through the use of thermodynamic principals. Evaporation of water requires heat. Three sources of heat are available for evaporating water from a spray: .(1) heat from the water, (2) heat absorbed from the air, and (3) radiant heat [principally from the sun). If all the heat carne from the water it would require a temperature drop about 10.5 F to evaporate 1% of the water. When the water is coolex than the air, which is normally the case in the daytime, the water will absorb heat from the air and the temperature drop will be less than 10.5 F for a loss of 1%. Absorption of radiant heat will increase the evaporation for the same temperature change, however, thi$ has been shown to be negligible in most cases. When the initial water temperature is the same as the wet bulb temperature of the air an equilibrium temperature exists, in which case, -all the heat required for evaporation comes from the air and the water remains at a constant temperature. When the initial temperature of the water is lower than the wet bulb temperature, the temperature of the water will increase even though some evaporation still takes place. The evaporation would be zero, however, if the water temperature was at the dew point; and if it were lower, condensation would occur and there would be a gain rather than a loss of watei. Neglecting radiant heat, Christiansen developed the following expression for the evapora-tion loss from the spray. r .[_Pw -Pa P P v7 a E = 100 C At .-. 0.00037B (t a where E = the evaporation of water from the spray expressed as a percentage of the amount discharged. [3] C = the specific heat of water, caloriea per gram per ....-: degree F 7 t'-. _W;

PAGE 10

r = the heat of vaporization, calories per gram 6t = drop in temperature of the water from the time it leaves the nozzle until it reaches the ground tw =, the mean water temperature, degrees F t = the a air temperature, degrees,F p = .the vapor. pressure at temperature t in. Hg w w' p = a the pressure of water vapor in the air, ,in. Hg B = the barometric pressure; in.Hg This equation fails to take into consideration the very smali droplets which are completely evaporated or blown away by the wind and which do not contribute to the final tempera-. ture of the wateras it reaches the ground. A study of the distribution of droplet size indicates that only a very small part of the water discharge is in the form of tiny droplets that are lostih this manner. Thus, it.is believed that this loss would not cause an appreciable error in the determination of evaporation' loss from irrigation spray .. Tests havebeeu conducted in which the change of water temperature as a result of evaporative cooling has been measured. Tests on rotating sprinklers with initial water temperature of 69.5 to 84 F and with .air temperature ranging from 75 to 101 F show decreases of water temperature from 1. to 7 F corresponding to evaporation losses of 0.23 to 0.81%. Another test with the initial water of 98.7 F and an air temperature about, 105 F resulted in a temperature drop of 20.7 F, corresponding to a loss of about 2%. Christiansen concludes that evaporation loss from the -sprayis.negli'1ible iilcbmparison'with subsequent losses from wet soil and vegetation. Mather (24) made afield investigation of evaporatlon ,from .sprinklers by observing the actual increase in moisture content of the air moving through an irrigated spray area. His values were obtained from measurements of the dew point upstream and downstream of the air entering the irrigated area. He caldulatedthe evaporation loss through the use of the absolute moisture gain of the air as it' passed through the irrigated area. He either estimated the amount of moisture movement upward or assumed that it was negligible. His results show that as the distance downwind from the' irrigated area ,. incr-easedthe amount of evaporation into the 'a'ir-'become's For example, his data indicate that only within the first 40 meters is there much gain in the moisture content of the air. Thus, the percent of water lost would be minimized by making the size of the field to be irrigated as large as possible. The evaporation loss from the spray and'moist soil 8

PAGE 11

ranged from 4 to 30% of the water applied. Hovlever, the application rate in some cases was less than 0.1 of an inch per hour. From actual observations and evaporation computations, Mather suggests that from a water conservation point of view the application of water by irrigation should occur as rapidly as is economically possible. Ingebo (19), in his studies of vaporization rates of iso-octane sprays, developed a semiempirical equation for the prediction of spray losses during the initial period _after atomization at_the noz-zle. where dID de m e D K = = = = = DKlTb..t H v b..VDW U mass of water time diameter of droplet coefficient of thermal conductivity [4] -difference of temperature between the surface of the droplet and the surrounding gas atmosphere. H = heat-of vaporization of liquid v = the relative velocity between the droplet and the surrounding gas W = density of surrounding gas atmosphere U = viscosity of the surrounding gas atmosphere This equation was derived by converting and simplifying a mass-transfer equation. Peters (29) studied the relative magnitude of evaporation from soil surfaces and the transpiration by plants. He concluded that in the midwest where fr-equent summer showers occur, as much of 50% of the total water loss in a season can be accounted for by evaporation from the soil surface. He used plastic covered plots in his experiments and determined that the amount of transpiration from a particular crop is within rather narrow limits. -He pointed outthat-photosynthesis has been thought to be such a minor fracti.on of the total heat budget that it could be neglected and that the net radiation isused up principally in heating air and evaporating Fortier and Beckett (10) conducted experiments to determine evaporation losses after an irrigation -from undisturbed-and cultivated soils -at Davis, California. They found that one 9

PAGE 12

to two inches of water evaporated from a soil within three to four weeks after an irrigation, and that more than half of it occurred during the first five days. According to other experiments conducted by these men, the rate of evaporation from saturated soils is about the same as that from a free water surface or about 0.3 inches per day in the Sacramento Valley during the summer. When relatively small amounts of water are applied to exposed soils at frequent intervals by sprinkling, much of the can be lost by evaporation. In some instances, irrigation applications of about linch are made at weekly intervals to aid in the germination and starting of a crop and to prevent the drying out and crusting of the surface soil .. Most of this water may be lost directly from the soil by evaporation. It is estimated that if application rates of 0.2S to 0.5 inches per hour are used, more than 10% of the water may evaporate when it is applied during daylight hours. When crops are sprinkled part of the water is intercepted by the foliage and later evaporated without reaching the soil. Clark (6) determined the maximum interception capacity of many planb3 and it appears, from his data, that few crops can attain more than 0.1 inches of water Kraus (21) reported spray evaporation losses as a function of vapor pressure deficit and separated the losses into evaporation loss and drift loss. He measured the drift losses by detecting and measuring impressions made by droplets falling on a layer of magnesium oxide which was smoked on a glass slide. This method is applicable f6r drops from 10 to 200 microns in diameter. Total losses ranged from 3.4 to 17.0% for vapor pressure deficits of 0.123 and 0.673 in. Hg, respectively. The average drift loss was 36% of total .. Through the use of lysimeters he determined that evapotranspiration in the drift zone, as compared to a dry control area, was increased under high wind speed conditions, and was decreased under low wind speed conditions. George (14) studied spray evaporation losses by determining the salt content of the water in the lateral andin catchment bottles. Drift losses were not considered. The author reported a correlation between relative humidity and evaporation loss. METHODS AND EXPERIMENTAL FACILITIES Studtes of irrigation evaporation have been conducted in the field under natural conditions and in the laboratory. Each 10 r---didi) iF 11 ... ____ ...... 111 1 Ii! L!TiI! 'l!i!I! .dliiA I JJIIIl!.@!lili!i!il!ll!iliill'!i!HIl

PAGE 13

of these locations has advantages and disadvantages. In the field, precision must be sacrificed in controlling and measuring the properties of the surrounding atmosphere, however, adequate space is available to operate irrigation sprinklers in the conventional manner. In the laboratory it is possible to control the properties of the atmosphere during the experiment and make precise measurements, however, it is costly to provide adequate space within-an atmospheric control facility in which to operate a standard agricultural irrigation sprinkler. In either case, the experimental values that are obtained must be projected and related in order for them to provide practical information on irrigation evaporation. A climatic control chamber was used in conducting all the tests this study. Climatic Control Chamber The climatic control chamber, shown in Figure 1 in cutaway perspective view, was built specially for conductingthe experiments supporting this study. The chamber is equipped for control of a range of dry bulb temperatures, dew point Jcemperatures, and air flow rates. Also I a water droplet generator with the capacity for controlling precipitation rates and water temperatures at different levels was constructed as an integral part of the chamber. _The approximate outside dimensions of the chamber are 50 feet long, 16 feet wide and 10feet high. It was designed to minimize hea-t and vapor transfer between the surrounding atmosphere and the air inside the chamber. Typically, as shown in Figure 2, the exterior consisted of 4 inch thick panels of paper honey-comb insulation with sheet alunlinumbonded to both sides and a 3 inch layer of polyurethane foam poured in place on the inside to assure a good air and vapor seal. Holes made through the exterior for water, electric and refrigeration conduits were sealed by pouring polyurethane foam around them. Nevertheless, there WaS some air leakage through small openings around the blower shafts, door seals, etc.-However, the quantity of air transfer was determined for various operating conditions and taken into consideration for all data presented. The magnitude of the air transfer was estimated by operating the chamber with an inside dew point lower than outside and measuring the rate of moisture removal required to maintain a constant dew point temperature within the chamber. This value, along with the inside and outside dew points, was used to compute the quantity of air transfer. Air leakage under the most adverse conditions was approximately 125 cfm and this quantity was not significant except for tests where evaporation rates were low. As indicated in Figure 1, air moves in a closed circuit from the blower discharges through the air conditioning section, airflow measurement section, air straighteners, test section 11

PAGE 14

IV Ai, Flow Rote Control Domper Figure 1. C1imatic control' chamber ,,", .-? .... .? : 4 .... a ;::"c; '< I,. iii Iii

PAGE 15

. I-' W (-'" -'f) '4U 3" -OUTSIDE-. < 22 GA. ALUMINUM -----/ TYPICAL VvALL SECTION .. ; ... .. .. t, .. .. '24 .r--ALUMINUM -INSI DE...:... Figure 2. Typical wall section indioqting an'd dimensions

PAGE 16

and back to the blower intakes. The reason for using two blowers was primarily for convenience in manipulating air flow velocities through the heating and cooling coils with minirnalinterference with air flow rates through the test section. Air Straighteners Air straighteners and resistance layers were installed at both ends of the test section as shown in Figure 3. They consisted of stacks of 3 inch diameter by 12 inch long sheet metal tubes, layers of aluminum honey-comb type material and 10-mesh screen Trial and error techniques were used to attain the desired degree of uniformity of air velocity -through the test section. The technique used required that air velocity measurements, obtained with a hot wire type velocity meter, be made at each intersection of an imaginary 12 inch square grid across the test section in the vicinity of the water droplet generator. On a basis of these values,unsatisfactory air flow patterns were altered by adjusting the location, size and number of layers of screen \\Tire patches that on the leading air side of the air straighteners. The level of uniformity of air flow was considered satisfactory when none of the individual velocity measurements varied by more than 25% from the mean. Test Section of Chamber The test section is approximately 24 feet long, 8 feet high and 5.5 feet wide. Figures 3, 4 and 5 are of the climatic control chamber on which the location and relative size of the test section is indicated. Ten "viburnum" plants were placed in the test section immediately beneath the water droplet generator to provide vegetative material for tests involving evaporation losses water. Individual plants were approximately 3 feet high and 2 1/2 feet in diameter and conformed generally to an ellipsoidal shape. In elevation, shown schematically in Figure 5, they were located at three levels so as to fully occupy the volumetric space beneath the generator. On each plant there were approximately 500 each with a surface area of about three square inches. Leaf density appeared to be about the same-as that found in a mature citrus grove. During test, it appeared that 80 -90% of the droplets were intercepted by the plants. Water Droplet Generator A special apparatus was developed to generate water droplets that could be used to simulate irrigation spray. Figure 6 shows

PAGE 17

I-' !J1 PRIMARY BLOWER SECONDARY BLOWER AIR RETURN DUC-r .LAIR STRAIGHTENERS -----WATER DROPl..EYliflr "']"1 OBSERVATION GENERATOR 11'11r WINDOW I TION I h 1 1""' .. SECTION THRU CLIMATIC CONTROL CHAMBER Figure 3. Section through climatic control qhamber indicating "relative position of air straighteners and test section

PAGE 18

I-' 0'\ ELECTRICAL PANEL BOXES ,"'=---... 3 SECONDARY' I' ..... y-I %OMPRESSORSY ) I /" I I --I" CONTROL"-:::> AIR RETURN AIR L-r -"l..J. r-" (1" IC;" ROOM .4 .. 1. DUCT(ABOVE) I I I I }.... I' 'i I --FLOW ....... f I" L -a.l .::,; '_' AIR FLO\"', ., _. f1.'{ Y__ {f __ ._ --I -rr;---Ii! WATER DROPLET IV r T 11,?f' GENERATOR II I <.. Y I" 'AIR FLOW IPRIMAR AIW I: c. TEST SECTION .Ii =:::J4' ., I BL2ER: ','., __ ___ ::: II I, '_ I J L -=;---"'"""'f'j ...... PANEL "gJ -,.. =-""121 :;w_ .... _-"*iS"= ENTRANCE. ,CLIMATIC CONTROL CHAMBER LAYOUT .. '. Figure' 4. ,i"', ; Layout' drawing thto,ugh, the: climatia control' chamber indicating reiatiye position of the' component parts' of the chamber .'

PAGE 19

! I-' -..J ," ACCESS' : ) ; ..... I !i tAt J .. ;' ; WATER DROPLET "' AIR RETURN DUCT i OBSERVATION WINDOW, "lCONTROL ROOM <... SECTION' THRU CLIMATIC CONTROL CHAMBER AT CONTROL ROOM '" _ow>",oJi.'J. Figure-50' Section view through olimatio cOI)trol chamber at the test section indicAting the of : Ii --0

PAGE 20

"'l I-' co '. SYRINGE NEEDLES SPACED 2ux 2" ',-\:!j:'//'(, j' /'f -1F;" 1"1,7 i / r I' :'!'(/'T '-C( .,;,,7/ .r' Y'"j> f ? ;<''j' /,:'/.S ,/f/ "" ../ //./' .. .. C:-<1" /T/ 4' /' l' g/ 'r." /"" '{://c;, ""t ',/ ",/ "l -, '1' ,,-,-. ',' ... '<-.... '/,',. -/ /' Of 'f';:>.,/?>i::'/,:0,-"{ / ,r", j,:. r.1:
PAGE 21

, / a view of this apparatus. The reservoir with disposable type syringe needles projecting through the bottom, was recessed into the ceiling of the test section of the climatic control chamber so that the tips of the needles were flush with the ceiling. It was constructed of plexiglass panels fastened into a fabricated aluminum framework. Four hundred and three needles spaced on two inch centers in a square pattern were required to dispense droplets uniformly over an area 62 inches, wide and 26 inches long. When in operation the reservoir was vented to the atmosphere through a stand pipe. The rate of flow was controlled by adjusting the rpm of the paristaltic type pump until the desired water pressure head was obtained over the needles. Number 20 gage X 1 inch syringe needles were used for all tests: Preliminary tests had revealed that this size needle produced drorlets that were approximately 3 rom in diameter which is also approximately the same average diameter as water droplets produced by many irrigation sprinklers (13,17). Flow rates equivalent to precipitation depths of 0.1 to 5.4 inches per hour could be obtained by adjusting the water pressure head between 0.1 and 6.0 inches. In order to have instant shut off of flow from the needles it was necessary to install sole.noid valves in. the standpipe and inlet water lines. The valves were electrical Ii wired in series wi th the pUInp motor and thus were open or closed when the pump was on or off. It was necessary for the entire water droplet generating system to be purged of air in order to obtain sudden shut off of flow from the needles. that was not was collected in a pan recessedinto the floor of the test section of the chamber. ',A trap wasinst:.alled in the pan drain pipeline so tha't 'the depth of water in the pan was maintained at a.constant level of about three inches above the bottom. In order to minimize evaporation from the watercbllected in the pan, a'layer of type I hydraulic 3/8 inch iri depth, was maintained over the water surface during all tests. A small electrical resistance type emersion water heater, equipped with rheostat, was used to maintain the water tempera ture in the reservoir at the desired temperature. Air Conditioning and Heating Air leaving the test section of the climatic control chamber is divided so that part of it goes through the air conditioning-heating system and the remainder is recirculated. The quality of air passing through the air conditioning-heating system is dehumidified, cooled or heated to a level so that when it is mixed with the recirculated air, the two will combine to produce air with the desired physical properties for a particular test. Figures 1 and 7 show the relative location of 19

PAGE 22

tv o CONDo D' !I PRESS.REG. J I ......... ,' > I EV:P. ., '., EVAP. t .' PR ESS. '1 i' I EVAP. >= 3 ,3""'j" .... ;;'i" PROPORTIONING CONDENSATE .. VALVE MOTOR DRAIN t STEM-l MANUAL VALVE HEATING flirlES INTO COIL THE RETURN STEAM) SECTION OF THE CHAMBER LOUVERED DAMPER RECIRCULATING DUCT TO MAIN SECTION OF CHAMBER DEHUMIDIFYING COOLING ANDiHEATING, COMPONENTS (Located i.n the return section of the CHAMBER) I : I I Figure 7. Schematic indicating lciyout 6.f components of the dehumidifying, cooling and heating sec:tion of the climatic 'c,ontrol ,chamber I

PAGE 23

the components of the air heating, cooling and dehumidification system as well as the distribution and direction of air flow. If the dew point of the air coming in contact with the water droplets is less than the temperature of the water droplets, some of the water is evaporated from the droplets and becomes water vapor in the air stream. In order to keep the dew point of the air within the chamber from increasing it becomes necessary to remove water from the chamber at the same rate that it is evaporating. Water is removed from the chamber as condensation on refrigeration coils. This condensation is collected and weight measurements made with respect to time to determine the rate. Since the only significant source of moisture added to the system comes from the water droplets, the rate of condensation is also the rate of evaporation. This measurement is the primary criteria for evaluating tieatment responses-presented in this report. Two precautionary measures had to be taken to assure accuracy. First, instrumentation had to be monitored to assure that the chamber, had been operated for a sufficient length of time, during each test, for all systems to be in equilibrium and second, that none of the condensation coils were permitted to become cold enough for the condensated moisture to freeze. Dew Point Temperature Control Normally, dew point levels were obtained by controlling the 'temperature of the evaporator coils at the desired level. This was usually accomplished by manual adjustment of the evaporator pressure regulating valves, however, for several of the lower dew points, it necessary to manipulate the dampers of the recirculating duct to attain the desired levels Dry Bulb Temperature Control Air within the climatic control chamber was heated by a steam coil' equipped for automatic' dry bUlb 'femperature control The essential components of this control' system were as follows: motor'ized proportional control steam valve, electronic proportional controller with reset and rate action and thermopile temperature sensing element. Air Flow Rate Control As indicated in.Figure 1, two centrifugal blowers with backwardly inclined impeller blades were used to obtain the desired air velocity through the test section of the climatic control chamber. It has been stated earlier that the secondary (10 hp) blower was used primarily to facilitate ease of con trolling air velocities over the heating and cooling coils, however it did furnish varying amounts of air (depending on dew point level) for the test section. A motorized damper was installed in the discharge duct of each blower to regulate the 21

PAGE 24

. '; PROCEDURE Air velocity, dry bulb temperature, dew point temperature, water temperature and rate of precipitation were factors. tested at different levels to measure their independent effect on rate of evaporation. Levels at which these factors were tested is given in Table 1. The influence of these factors on evaporation was considered in terms of losses from water droplets (spray) and by plants. All tests were conducted in the climatic control chamber that has been described in the "Methods and Experimental Facilities" section of this report. Evaporation rates are .. 'expressed as 'a 'percentage of the discharge rate of the water droplet generator (precipitation rate) and presented in graphical form. TABLE 1. The factors Tested and Levels of Treatment. Water Droplets (Spray) Factor Level of treatment (nominal) Air velocity, mph 2, 3, 4, 5, 6 Dry bulb temperature; F 75, '80', 90, 95, .. 100 Dew point temperature, F 50, 55, 60, 65, 70, 75 Water temperature, F 75, 80, 85, 90, 95, 100 .. Precipitation rate, iph 5.4 ---...... '--. ."----. '-Plant Intercepted Water Factor Level of treatment Air velocity, mph 2, 3, 4, 5, 6 Dry bulb temperature, F 75, 80, 85, 90, 95 Dew point temperature, F 60, 65, 70, 75 Water temperature, F 82 Precipitation rate, iph 0.15, .50, 1.0, 2.0, 5.4 23 -..

PAGE 25

discharge rate for each blower. Unless prohibited by a test requiring a low dew point treatment level, the damper for the secondary blower was completely open at all times. The damper for the primary blower would then be adjusted to attain the desired air velocities through the test section. Air velocities through the test section could be controlled at levels up to 6 miles per hour. Instrumentation Instrumentation systems were.required for dry bulb air temperature control and measurement, dew point temperature measurement and air velocity measurement. An adjustable zero-adjustable range, proportioning band potentiometric controller with reset and rate action was used in conjunction with a motorized proportioning steam valve to sense the dry bulb air temperature and iegulate the rate of steam flow into the air heating coil. The sensing element for-thecdhtrollerwas a 5 junction thermopile located at the "approximate centroid "of the cross section of the test section of the chamber and about 3 feet up the air stream with respect to the water droplet generator. Dry bulb temperature measurements were made with a dual-element quartz thermometer. Both elements were located at the approximate centroid of the cross section of the test with one element being located about 3 feet up the air stream and the other about 3 feet down the air stream with respect to the water droplet generator. The thermometer elements were connected to a digital read-out indicating the nearest 0.001 degree Celsius. Dew point temperature measurements ,,"vere made at two locations. One was in the air stream at approximately the same location as that of the up-stream dry bulb air temperature sensing element and the other was outside the climate control chamber. A direct reading dew point indicator equipped_ with dual miniaturized "Heated Salt" thermistorized probes was used to measure dew point temperatures. Meter readout was scaled so that dew point temperatures could be read to the nearest 0.1 F. Air low rate measurements were made in a section of the return air duct of the climatic control chamber as indicated in Figure 1. All the air was channeled through three 21 inch diameter pipes, each equipped with a calibrated annular ring type velocity probe. An electronic pressure meter was used to measure the pressure output of the probes. Readout accuracy of the electronic pressure meter was to the nearest 0.001 rom Hg. Based on manufacturers claims for accuracy for the two primary components of the air flow rate measuring system, it is believed that air velocity measurements are accurate to within 2.0 percent of the values presented in this report. 22

PAGE 26

DISCUSSION AND RESULTS All the data given in this report were taken under con,trolled conditions in an environmental control chamber. The exact duplication of field conditions for sprinkler irrigation systems was sacrificed for conditions which could be adequately controlled and described. Therefore; the results of these tests should not betaken as directly applicable to field .conditions for sprinkler irrigation, but as a basis for making decisions concerning the design and operation of sprinkler irrigation systems' and the lo?g range and limitations' of sprinkler The dat'a taken were '-for two separate sources of evap'ora';':;." tion, that from the spray and that from the plant intercepted water. Since the configuration of the water applicator is quite different from most field conditions, the spray losses require considerable interpretation before applying to field -conditions. However, that from the plant intercepted water, which also included some spray losses, should beclosely related to field conditions if the variations due to different crop are taken into account. Evaporation is directly proportional to the difference between the saturation vapor pressure corresponding to the temperature of the water surface and.,the yapo:c,pres/?ure of the air (8). Therefore, the mean temperature of the water surface directly a.ffects the evaporation rate. In most of the previous evaporation. studies, evaporation rate has been reported as a function 'of cdr qualrty -only wi th-out regard to the initial water temperature or application rate.' One term commonly used in evaporation studies is vapor pressure deficit, which is the between the satura-tion vapor pressure of the air and the actual vapor pressure of the air. When considering evaporation of water droplets in air, the vapor pressure defJ.cit is the vapor pressure difference between the air and the droplet only for a mean water temperature equal to the air temperature. Another similar term which has been reported as being'directly proportional to evaporation rate is wet bulb depression. These terms, of course, are useful and very practical since they do not envolve the mean water v temperature, which is hard to determine. However, the evaporation rate should not be expected to be directly proportional to these terms. 24

PAGE 27

.. ,. \ -,' -. '" '.. 1.2 1.0 E-t z 0.8 ril 0 ril III 0.6 .. Z 0 j H 0.,4 8 0 III 6.2 .,.' r:4 "':' ., .,. 70 .. ,_. -'" DEW POINT, -F VELOCITY, mph APPLICATION.,. iph DRY BULB, F INITIAL WATER TEMPERATURE, F 52 4 5.4 9.5 Figure 8. Evaporation of water droplets as a function of initial water temperature. Dashed lines are not an extrapolation of experimental results and are presented only to support discussion. 8 0.8 l z .. r:4 U r:4 p.6 III ... Z 0 H 8 S{ 0 0.2 P; .. r:4 .. 0 0 AIR VELOCITY I mph Figure 9. .Evaporation of water droplets as a function of air velocity 26 ,--!

PAGE 28

" Since the independent air quality variables such as dew point, dry bulb, wet bulb and relative humidity are not directly proportional to the actual water vapor pressure difference; one should not expect them to be directly proportional to the evaporation rate. However, in many cases, for the ranges indicated, an approximately linear relationship does exist, but it should not necessarily be expected to hold for other values of the independent variables. The--results of the tests for spray evaporation losses are consistent with those of other investigators (5, 13, 33) in that the losses are very small compared to the total amount applied and also small compared to evaporation losses from the plant and soil surface. The evaporation loss from droplets expressed as a percentage_of the amount applied ranged from 0.20% to 1.13%, while that from the plant intercepted water ranged from 3.5% to 60.3%. The effect of each independent variable _on evaporation; as indicated by Table 1, will be discussed separately. Theoretical reasoning, as well as' recorded data, was used in discussing the characte'ristics of the curves -.. Initial Water Temperature' (F"igure 8)--Fig-q.re 8 "indicates the percentage evaporation as a function of initial water temperature'. When the temperature of the droplet is equal to the dew point temperature of the air, no evaporation will occur. However for an' initial droplet temperature equal_ to the dew point (52 F) or even lower, heat from the air willbe transferred to the droplet as it movesthrough theair_,_. and evaporation will The droplet will be heated until the wet bulb temperature (67.7 F) is reached, provided the exposure time. is sufficient. There will be an abruptchange .. in the slope of the curve where theinitial..water temperature is equal to the wet 'bulbtemperature wit.h the slope immediately above the wet bulb temperature being greater than that immediately below it. The evaporation rate will increase at an increasing rate up to the dry bulb air -temperature' (95 F) at which point another abrupt change will occur with the slope immediately above 95 F being less than that immediately below it. The evaporation rate increases at an increasing rate since the saturation water vapor pressure increases at an increasing rate with respect to water temperature. The abrupt changes in the' slope of the curve occur due to the change in the relative amount of heat which is used for evaporation in comparison with that used as sensible heat. Below the wet bulb temperature some of the heat transferred from the air must. go to increase the temperature of the droplet; 25

PAGE 29

between the wet bulb temperature and the dry bulb temperatureheat for evaporation comes from both the air and the droplet; and above the dry bulb temperature heat is transferred from the droplet to the air. In past studies of evaporation the initial water temperature has not been considered in most cases. However, as can be seen from these results, the initial water temperature is an important factor even though the droplets rapidly approach .the wet bulb temperature of theair. For field conditions where the droplet exposure time is greater than that for the laboratory tests, the effect of inltial water temperature on evaporation is reduced. Air Velocity (Figure 9)--The effect of air velocity on evaporation appears to be more closely related to the movement of high-moisture-content air from the general vicinity of the droplets rather than to the increase of the relative velocity between the air and the droplet. This is substantuated by results presented in Figure 9 where evaporation is directly proportional to air velocity rather than proportional to a lower power of the air velocity as would have been the case if-the'change in relative velocity was the only contributing factor (19). ,--Dry' Bulb Temperature (Figure lO)--Since a change in dry bulb temperature, in itself, does not affect the vapor pressure of theair-, the effect of dry' bulb temperature on evaporation is related only through the rate at which the droplet temperature changes and the equilibrium temperature (wet bulb) attained by the' droplet. As the dry bulb increases the wet bulb increases 'at a slightly decreasing rate. Thus the mean droplet temperature during flight also increases at a slightly decreasing rate, causing-evaporatlon to have a similar rela tionship'. There is an abrupt change in the slope of the curve where the air temperature is equal to the temperature (82 F). The slope-immediately below the droplet temperature (82 F) is less than theslope immediately above it. There is another abrupt change in the slope ,where the air temperature is equal to the dew point. Note that the evaporation at this point is not zero since the droplet will heat the air; thus allowing further moisture transfer to the air. Dew Point (Figure'll)--As the dew point of the air increases, both the vapor pressure and the wet bulb temperature increase at increasing rates. These two occurranceshave an opposing effect on the evaporation rate and it appears from the curve of Figure 11 that the relationship is linear. There will be an abrllpt change in the slope where the dew point equals the initial water temperature (82 F). The dew point at which zero net evaporation occurs will depend upon the time of exposure for the droplet. For a dew point above the water temperature and below the dry bulb, the droplet will gain moisture from the air until the droplet is heated to the dew point. At this point evaporation will begin when the droplet is heated above the dew point. -27

PAGE 30

, -70 VELOCITY, mph APPLICATION, iph WATER, F DRY BULB, F 80 DEW POINT TEMPERATURE, F 90 45.4 82 95 t-Figure 11. Evaporation of water droplets as a function of Dew point temperature: _0 ?'-"... Z o : .... : ... 20 10 o P-! Fl! -:> -r::J o .. .. :: .. DEW POINT, F WATER, F DRY BULB, F l' 2 3 4 .67 iph 5.4' 82 .. 5 6_ AIR VELOCITY, mph .. --.' Figure 12 Evaporation of plant intercepted water as a function of air velocity29 ro ........................................................................ ...

PAGE 31

1 t-.> co 1.0 E-t Z r:r:l 0.8 o p:; 01 .. Z 0.6 o H E-I o 0.4 !> r:r:l 0.2 a .-, ,,/ ....... .".;,/ 50 "t ,'> J DEW POINT, F 54 : .. VELOCITY, ,mph" 6 APPLICATION, iph J 5.4' WATER, F 82 <:> .. ........ 0 ......... ......... '......... ,. '/ ,." .-I, 60 70 100 :' BULB. TEMPERATURE,F, Fig1.1re 10. of droplets as a of dry, bulb 'temperature,: lines not ,an ,are presented only to support discussion.

PAGE 32

Evaporation from Plant Intercepted Water The curves for evaporation (Figures 12-15) from plant intercepted water appear to be simllar shape to those. for evaporation from the spray, with the main difference being the magnitude of the evaporations. It is more diffi cult to determine the slope of the curves for the plant intercepted water because heat is transferred between the -water-and-the plant-as well as between the air and the water The curves should still possess abrupt changes in slope corresponding to the dew point and initial water temperature on the appropriate curves. Because of the much longer exposure time for the plant intercepted water, the effect of initial water temperature should be much less than for the spray. This means that all of the curves for plant intercepted water would have been Changed only slightly if a different water temperature had beeri used. As has been discussed in the review of literature, all of the eyaporation from plant intercepted water should not be considered as a loss charged to irrigation since some evapotranspiration would have occurred without irrigation. The evapotranspiration rate was measured before the plants were "wet, for 95 F dry bulb ,temperature, 60 F dew-point and 4 miles per hour and was found to be 30 grams per minute or 0.066 inches per hour. This indicates that, for most of the tests, more than 90% of the evaporation would be considered a loss "It should be noted that solar radiation which was eliminated in these tests, would have increased the amount of evapor-" at"ion. In the following discussion of each variable, only the important differences between the curves. for evaporation from planfintercepted water and for spray evaporation will be. given. Air Veloci ty (Figure evaporation appears to be directly proportional to air for the range of values used in this test. The discuss{on in the section on spray evaporation with respect to air velocity is applicable here. Application Rat"e (Figure 13) --This curve indicates that the percent evaporation increases very rapid1y for application rates below 1 inch per hour. Since all of the water that was discharged was not intercepted by the plants, it is believed that most of the intercepted water is evaporated for applicationrates below 0.1 inch per hour. It should be noted that this test was for a particular crop configuration and that these values would vary according to the" plant surface area and the kind of plant. 30 .,.-_.

PAGE 33

.. .. .. 60 50 8 30 H' 8 o 20 10 o o 1, .. ... o :2 ", DEW POINT I F :., VELOC ITY I mph WATER, F ,DRY BULB, F ...... ... .... 3 ,4 63 4 82 95 > :. '.. .': ", --.-:----'.' -' --. 5 WATER APPLICATION RATE, iph I .. 6 Figure 13. Evaporation of intercepted water as a function of app1iI catl.on

PAGE 34

Dry Bulb Temperature (Figure l4)--Here again, as was discussed for spray losses, a change in dry bulb temperature does not' affect the vapor pressure of the air but only the rate at which the water changes temperature and the equilibrium temperature attained. But in the case of plant intercepted water the temperature is not the wet bulb temperature, but slightly higher, because heat is conducted from the plant.' ,Dew-Point (Figure 15) --The curve for evaporation from plant intercepted water indicates a definite curvature with the evaporation decreasing at an increasing rate with respect to dew point while the cUrve for spray evaporation was approx. imately linear. Interpretation of Results The values indicated for spray losses are not directly applicable to field cond,itions for sprinkler irrigation systems since they were obtained from uniform size droplets (3mm) falling a distance of 8 feet with a zero initial velocity. The factors which would significantly contribute to a different value for field sprinkler systems are the droplet size, time of exposure and relative velocity between droplet and air. t In addition to correcting for these factors, one must measure the climatic conditions in the immediate. vicinity of the spray, such as was done by Mather (24) and referred to in th.e review of literature section of this-report. In order to show how the results of these tests might be -' applied to field conditions, consider a sprinkler witha 7/32 inch diameter nozzle operating at 40 psig on a 10 foot riser. Assume the climatic conditions are 9SF dry bulb, 54 F v dew point and 4 mph wind. The water leaves the nozzle at 82 Accord-ing to studies by Frost and Schwalen (13) the average size droplet under these conditions would be approximately 3mm in "diameter. Thus no correction is needed for droplet size. It is realized that the use of the average size droplet does not give the correct value for the total sUrface area since the area is proportional to the square of the diam-eter. However, it is a good approximation for a properly designed and operated sprinkler where a very small percent of the distributed water would be in the form of a mist. If the total exposed surface area per volume is different than that for 3 mm diameter droplets, the percent evaporation can be considered inversely proportional to the diameter (D) of the droplets (evaporation is to D2 while total amount applied is proportional to D ). 32 II

PAGE 35

--. .. ...... '.' 8 20 z 0 P::. 15 .. .. Z 0 10 H 8 0 '5 ;.' 0 55 : ... .. --' DEW POINT, F VELOCITY, mph APPLICATION, iph WATER, F '-: 75 60 3.81 5.4 82 85 DRY BULB TEMPERATURE, F .-.. .. 95 '. ,Figure 14 Evaporation of plant intercepted water as a' function of dry bulb.temperature ..... .' --' ... --' :.. : -..!.. -. ... ITY, mph APPLICATION, iph WATER, F DRY'BULB, F 100 DEW POINT TEMPERATURE, F 4 50.4 82 95 Figure 15. Evaporation of plant intercepted water as a function of dew point temperature:.:: ./" 33 .. .:-, P.,' ___________________ .......... .. ___ .. G.-

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, If a large portion of the water is in the form of small droplets significantly different in size from the average, the average size droplet should not be used to calculate the total surface area. The time of exposure for the droplets, for the laboratory tests and for the sprinkler being considered was estimated by assuming that the force due to air is proportional to the velocity. The constants were evaluated from the results, reported-by Green (16) on the evaluation of air resistance to freely falling drops of water. The time of exposure for the laboratory tests was determined as 0.80 seconds while that for the sprinkler was 2.00 seconds. The average .relative velocity was calculated to be 10.3 fps for the laboratory tests and 45 fps for the sprinkler. These velocities were determined through the use of equations of motion similar to those described by Green (16). Although normal wind velocities did affect the relative velocities for the laboratory tests, they do not significantly affect relative velocities for sprinklers due to the higher veloci ties of the droplets. The effect of relative velocity on evaporation can be determined from equation [4] as approximately proportional tO,the square root of the relative velocity Thus considering the relative velocities used in this example, the evaporation for the sprinkler would be 2.1 that for the laboratory tests. The percent evaporation is not directly proportional to the exposure time for the droplet:sincethe temperature oI'the droplet is approaching the wet bulb temperature of the air (Figure 16). However, as shown in Figure 17 the percent evap6r;,: ation is almost directly proportional to exposure time for this example. This is a typical relationship as long as the difference between the initial water temperature and the wet bulb temper.ature is not large compared to ,the difference between the wet bulb temperature and the dew' point temperature. If this is not the case, the curve becomes more non-linear. -The curve in 16 generated by the equation: t (tdi t wb ) -A6 + t wb = -e droplet where t droplet = temperature of droplet, at time tdi = initial temperature of droplet, F t wb = wet bulb temperature of the air, F e F [5] T-

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, 82 1/ DEW POINT, F 54 VELOCITY, mph 4 80 WATER, F 82 P-i DRY BULBi F. 95 .. WET BULB, F 68.7 78 .1-NOZZLE DIA., in. 7/32 b PRESSURE, psi 40 8 NOZZLE ANGLE, deg. 2.0 76 RISER,. ft. Pol 8 74 .. 8 H Pol 72 0 p:; Q p:; 70 ::: .... rLJ. 8 68 0 0.4 0.8 1.2 1.6 2.0 EXPOSURE TIME, SECONDS -" Fi-gure 16. Effect of exposure time on water droplet temperature '" Z .0 H o Pol 2.5 2.0 1.5 <"-1.0 0.5 0.4 0.8 1.6 EXPOSURE TIME, SECONDS Figure 17. Exposure time of water droplets as a function of evaporation 1/ Values are applicable for figures 16 and 17. 35 ,"''''''.* ____ ..... ____________________________ .. _______ 1IIi7

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A = a constant for this example, but a function of droplet size and relative velocity. e-time of exposure, sec. Equation [5J was derived by assuming that the droplet tempera tUre can be described by the following differential dtdroplet" = de with conditions: A (tdroplet twb) [6] td 1 t = td when e = co rop e "1 td 1 t = 't b" when e = co rop e w The constant "A" was evaluated through the use of equation [3] (review of literature) in conjunction with laboratory tests orpercent evaporation. A limited number of measurements were made on the initial and final temperatures of the water droplets, to check equation [3]. "A" was determined to be 0.447 for the laboratory tests and 2.1 (0.447) for the sprinkler. The factor 2.1 was determined from the ratio of the relative velocities. The curve in Figure 17 was generated by the following equa tion: B-Ae m = 0.1461\ ( 1 -e ) + Be (O.Oll_twb Pa) [7] where m.= total evaporation expressed as percentage for a drop let.exposure time of e. B = constant for this example, but a function of droplet size and relative velocity. A = the constant in equaion [6] P a = water vapor pressure of the air, in. Equation [7] was derived assuming that the water mass transfer rate to air could be described by the following differential.equation: ddme = B (p P ) ::: B (0. 011 td 1 t P) [8 ] droplet a rop e a This equation assumes a linear relationship between the water vapor pressure of the droplet and the droplet temperature which is a "poor fit" but gives reasonable accuracy for the range-of temperatures used in this example. For higher accuracy a different model should be used, for example, a polynominal expression. The constant "B" was evaluated from test results and"

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ACKNOWLEDGMENTS Dr. I. J. Ross, Associate Professor, Department of Agricultural Engineering, University of Kentucky (formerly Department of Agricultural Engineering, University of Florida) served as a leader on the project during the planning phase of the study. Mr. J. H. Weldon, Machinist-Mechanic, Department of Agricultural Engineering, University of Florida, served as the project refrigeration technician for the construction and data collecting phases of the project. The authors hereby express their sincere appreciation to both of these gentlemen for their contribution to the project. 38 /

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the assumed relationship between relative velocity and evaporation and was found to have a value of 3.0 This example indicates that a value of 2.5% evaporation should be used for a sprinkler in comparison to 0.52% obtained from laboratory tests for the same climatic conditions "A factor of 5 could be used as a nroughn value for most ui the tests. The application of laboratory results for plant intercepted water should be directly applicable to field conditions if the climatic conditions are measured in the vicinity of the sprinklers and the plant surface area and are taken into account. CONCLUSIONS This study has resulted in the conclusions: 1) Rate of application is the most significant factor influencing evaporation losses where a large proportion of the applied water is intercepted by vegetative material. It should be optimized with respect to economical system design and limited by maximum infiltration rate. 2) Evaporation losses from water droplets while in transit in air should not exceed 5% of the total water application under typical conditions in Florida. The amount is relatively insignificant when compared to the larger losses that can occur after the water droplets have" been intercepted by plant surfaces. 3) effect of the climatic factors of wind, air tempera
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LITERATURE CITED 1. Baird, C.D. Measurement of Water Evaporation Rates Utilizing an Electronic Condensation Hygrometer. Unpublished M.S. Thesis. University of Florida, Gainesville, Florida. 1967. 2. Burgy, R.H. and C.R. Pomeroy. Interception Losses in Grassy Vegetation. Trans. Am. Geophys. Union. 39:1095-1100. -1958. 3. Cannell, Gle!l H. Irrigation Efficiency as it Influences Water Requirements of Crops. Special Publication, American of Agricultural-Engineers. pp. 47-56.1962. 4. Christiansen, J.E. Estimating-Evaporation and Evapotranspiration from Climatic Data. Paper presented at Annual Meeting, Rocky Mountain Section, American-Society of Agricultural Engineers. Fort Collins, Colorado. April 5. 23, 1966. Christiansen, J.E. Irrigation by Sprinkling Agricultural Experiment Station, Berkley. 1942. California Bulletin 670. 6. Clark, O. R. Interception of Rainfall by Prairie Gras$es,_ Weeds and Certain Crop Plants. Ecological Mimeographs. 10: 243-277.1940.>7. Cunningham, R.T., J.L. Brann, Jr. and G.A. Fleming. Factors Affecting the Evaporation of Water from Droplets in Airblast Spraying. Journal of Economic Entomology. -55: 192-199. 1962. 8. Dalton, J. Experimental Essays on the Constitution of Mixed Gasses; on the Force of Steam or Vapor from Waters' and Other Liquids in Different Temperatures, both in a Torricelliam of Gasses by Heat. Mem. Manchester Lit., and Phil. Sc. 5:535-602. 1798. 9. Davis, John R. Efficiency Factors in Sprinkler System Design. Proceedings Sprinkler Irrigation Association. 100 Vermont Ave. N.W., Washington, D.C. 1963. 10. Fortier, Samuel. Use of Water in Irrigation. New York: McGraw Hill' Book Company. 1915. 11. Frost, K.R. Factors Affecting Evapotranspiration Losses During Sprinkling. Transactions of the ASAE. 6(4) 282-283, 287. 1963. 39

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12. 13. ____ and H.C. Schwalen. Evapotranspiration During Sprinkler Irrigation. Transactions of the ASAE. 3(1) 18-20, 24. 1960. __ _____ and H. C. Sch't'lalen. Sprinkler Evaporation Losses. Agricultural Engineering. 36(8): 526-528. 1955. 14. George, J.T. Evaporation from Irrigation Sprinkler Sprays as Determined by an Electrical Conductivity Method. Unpublished M.S.-Thesis. University of California, Davis, California. 1957. 15. Gray, Alfred S. Sprinkler Irrigation Handbook. Rain Bird Sprinkler Manufacturing Corporation. Glendora, California. 7th Edition. 1961 16. Green, Robert L. Evaluation of Air Resistance to Freely Falling Drops of Water. Agricultural Engineering. 33 (5): 286. 1952. 17. A Photographic Technique for Measuring the Sizes and Velocities of Water Drops from Irrigation Sprinklers. Agricultural Engineering. 33(9}:563-568. 1952. .-"--. -----.----"'. 18. Hamilton, F.B. and J.F. Schrunk. Sprinkler vs. Gravity Irrigation -A Basis for Choice of the Best System. Agricul tural Engineering. 34(4): 246-250. 1953. 19. Ingebo, R.D. Vaporization Rates and Drag Coefficients for Isooctane SErays in Turbulent AirStreams. National Advisory Committee on Aeronautics TN 3265. pp 1-39. 1954. 20. Keen, Bernard A. The Evaporation of Water from Soil. Journal of Agricultural Science. 6:456-475. 1914. 21. Kraus, J. H. Analysis of Sprinkler Irrlgation Ap-pifcation "Efficiency. Unpublished M.S. Thesis, University of California, Davis, California, 1961. 22. Application Effi9iency of Sprinkler Irriga-tion and its Effect on Microclimate. Transactions of ASAE. 9(5): 642-645. 1966. 23. Leeper, G.W. Thornthwaite's Climatic Formula." Journal Austrailian Institute Agricultural Science. 16: 2-6. 1950. 24. Mather, J.R. Sprays. An Investigation of Evaporation from Irrigation Agricultural Engineering. 31(11) 345-348. 1950. 25. McMillan, W.D. and R.H. Burgy. Interception Losses from Grass. Journal GeoEhysical Research. 65: 8. 1960. 40 ]

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.. .. 26. Mutchler, C.K. and W.C. Moldenhauser. Applicator for a Laboratory Rainfall Simulator. Transactions of the ASAE. 6(3): 220-222. 1963. 27. Paul, R.A. and R.R. Burgy. Interception Losses from Small Trees. Memorandum, Department of Irrigation, University of California, Davis, California 1961. 28. Penman,R.L., Natural Evaporation from Open Water, Bare Soil and Grass. Royal Society of Agriculture. 193: 120-145. 1948. 29. Peters, D.B. Relative Magnatude of Evaporation and Transpiration. Agronomy Journal. 52: 536-538. 1960. 30. Somerhalder, B.A. Comparing Efficiencies in Irrigation Water Application. Agricultural Engineering. 39(3): 156-15-9. 1958. 31. Sternberg, Y.M. Day and Night Sprinkler Irrigation-Analysis 'of Spray and Evapotranspiration Losses. Unpublished M.S. Thesis. University of California,Davis, California. 1962. 32. Weaver, R.A. and R.W. Pearson. Influence of Nitrogen' Fertilization and Plant Population on Evap6transpirationby ,-.'" Sudan Grass. Soil Science. 81: 443-451. 1956. '33. Wiser, E.R., J. van Schilfgaarde and T.U. Wilson. Evapotranspiration Concepts for Evaluating Sprinkler Irrigation Losses. Transactions of the ASAE. 4 (1): 128., .. : '130, 134. 1961. 34. Woodward, G.O. Sprinkler Irrigation. Washington, D.C.: Darby Printing Company. 1959. 41