6000-L180

TEXAS AfiRICULTURAL EXPERIMENT STATION

A. B. CONNER, DIRECTOR -
COLLEGE STATION, BRAZOS COUNTY, TEXAS

BULLETIN NO. 484 NOVEMBER, 1933

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DIVISION OF AGRONOMY

Rate of Water Evaporation
in Texas

 

I AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS
T. O. WALTON, President

STATION STAFFT

Administration :
A. B. Conner, M. S., Director
R. E. Karper, M. S., Vice-Director
Clarice Mixson, B. A., Secretary
M. P. Holleman, Chief Clerk
J. K. Francklow, Asst. Chief Clerk
Chester Higgs, Executive Assistant
Howard Berry, B. S., Technical Asst.
Chemistry:
. S. Fraps, Ph. D., Chief; State Chemist
E. Asbury, M. S., Chemist
F. Fudge, Ph. D., Chemist
C. Carlyle, M. S., Asst. Chemist
. L. Ogier, B. S., Asst. Chemist
A_. J. Sterges, M. S., Asst. Chemist
Ray Treichler, M. S., Asst. Chemist
W. H. Walker, Asst. Chemist
Velma Graham, Asst. Chemist
Jeanne F. DeMottier, Asst. Chemist
R. L. Schwartz, B. S., Asst. Chemist
C. M. Pounders, B. S., Asst. Chemist
Horticulture :
S. H. Yarnell, Sc. D., Chief
Range Animal Husbandry:
J. M. Jones, A. M., Chief
B. L. Warwick, Ph. D., Breeding Investiga.
S. P. Davis, Wool Grader
J. H. Jones, B. S., Animal Hush.
Entomology:
F. L. Thomas, Ph. D., Chief; State
Entomologist
H. J. Reinhard, B. S., Entomologist
K. Fletcher, Ph. D., Entomologist
. L. Owen, Jr., M. S., Entomologist
N. Roney, M. S., Entomologist
C. Gaines, Jr., M. S., Entomologist
E. Jones, M. S., Entomologist
. F. Bibby, B. S., Entomologist
. W. Dunnam, Ph. D., Entomologist
"R. W. Moreland, B. S., Asst. Entomologist
C. E. Heard, B. S., Chief Inspector
C. J. Burgin, B. S., Foulbrood Inspector
Agronomy:
E. B. Reynolds, Ph. D., Chief
R. E. Karper, M. S., Agronomist
P. C. Mangelsdorf, Sc. D., Agronomist
D. T. Killough, M. S., Agronomist
Publications :
A. D. Jackson, Chief

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Veterinary Science:
‘M. Francis, D. V. M., Chief
H. Schmidt, D. V. M., Veterinarian
"F. P. Mathews, D.V.M., M.S., Veterinarian
J. B. Mims, D. V. M., Asst. Veterinarian
Plant Pathology and Physiology:
J. J. Taubenhaus, Ph. D., Chief
W. N. Ezekiel, Ph. D., Plant Pathologist
Farm and Ranch Economics:
L. P. Gabbard, M. S., Chief
W. E. Paulson, Ph. D., Marketing
C. A. Bonnen, M. S., Farm Management
:t"W. R. Nisbet, B. S., Ranch Management
**A. C. Magee, M. S., Ranch Management
Rural Home Research:
Jessie Whitacre, Ph. D., Chief
Mary Anna Grimes, M. S., Textiles
Sylvia Cover, Ph.D., Foods
Soil Survey:
"W. T. Carter, B. S., Chief
E. H. Templin, B. S., Soil Surveyor
A. H. Bean, B. S., Soil Surveyor
R. M. Marshall, B. S., Soil Surveyor
Botany:
V. L. Cory, M. S., Acting Chief
Swine Husbandry:
Fred Hale, M. S., Chief
Dairy Husbandry:
O. C. Copeland, M. S., Dairy Husbandman
Poultry Husbandry:
R. M. Sherwood, M. S., Chief
J. R. Couch, B.S., Asst. Poultry Husbandman
Agricultural Engineering:
H. P. Smith, M. S., Chief
Main Station Farm:
G. T. McNess, Superintendent
Apiculture (San Antonio):
H. B. Parks, B. S., Chief
A. H. Alex, B. S., Queen Breeder
Feed Control Service:
F. D. Fuller, M. S., Chief
James Sullivan, Asst. Chief
. D. Pearce, Secretary
H. Rogers, Feed Inspector
. L. Kirkland, B. S., Feed Inspector
. D. Reynolds, Jr., Feed Inspector
. A. Moore, Feed Inspector
. J. Wilson, B. S., Feed Inspector
H. G. Wickes, D. V. M., Feed Inspector

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SUBSTATIONS

No. 1, Beeville, Bee County:

R. A. Hall, B. S., Superintendent
No. 2, Lindale, Smith County:

P. R. Johnson, M. S., Superintendent

No. 9, Balmorhea, Reeves County:
J. J. Bayles, B. S., Superintendent
No. 10, College Station, Brazos County:
R. M. Sherwood, M. S., In Charge

"B. H. Hendrickson, B. S., Sci. in Soil Erosion L. J. McCall, Farm Superintendent

"R. W. Baird, M. S., Assoc. Agr. Engineer
No. 3, Angleton, Brazoria County:

R. H. Stansel, M. S., Superintendent

H. M. Reed, M. S., Horticulturist
No. 4, Beaumont, Jefferson County:

R. H. Wyche, B. S., Superintendent
"H. M. Beachell, B. S., Junior Agronomist
No. 5, Temple, Bell County:

Henry Dunlavy, M. S., Superintendent

C. H. Rogers, Ph. D., Plant Pathologist

H. E. Rea, B. S., Agronomist

S. E. Wolff, M. S., Botanist
**H. V. Geib, Ph. D., Sci. in Soil Erosion
“H. O. Hill, B. S., Junior Civil Engineer
No. 6, Denton, Denton County:

P. B. Dunkle, B. S., Superintendent
"I. M. Atkins, B. S., Junior Agronomist
No. 7, Spur, Dickens County:

R. E. Dickson, B. S., Superintendent

B. C. Langley, M. S., Agronomist
No. 8, Lubbock, Lubbock County:

D. L. Jones, Superintendent

Frank Gaines, Irrig. and Forest Nurs.

No. 11, Nacogdoches, Nacogdoches County:
H. F. Morris, M. S., Superintendent

"No. 12, Chillicothe, Hardeman County:

"J. R. Quinby, B. S., Superintendent

"J. C. Stephens, M. A., Asst. Agronomist

No. 14, Sonora, Sutton-Edwards Counties:

. H. Dameron, B. S., Superintendent
I. B. Boughton, D. V. M., Veterinarian
W. T. Hardy, D. V. M., Veterinarian
O. L. Carpenter, Shepherd

"O. G. Babcock, B. S., Asst. Entomologist

No. 15, Weslaco, Hidalgo County:

W. H. Friend, B. S., Superintendent
S. W. Clark, B. S., Entomologist
W. J. Bach, M. S., Plant Pathologist
J. F. Wood, B. S., Horticulturist

No. 16, Iowa Park, Wichita County:

C. H. McDowell, B. S., Superintendent
L. E. Brooks, B. S., Horticulturist

No. 19, Winterhaven, Dimmit County:

E. Mortensen, B. S., Superintendent

"L. R. Hawthorn, M. S., Horticulturist

Members of Teaching Staff Carrying Cooperative Projects on the Station:

G. W. Adriance, Ph. D., Horticulture

S. W. Bilsing, Ph. D., Entomology

D. Scoates, A. E., Agricultural Engineering
A. K. Mackey, M. S., Animal Husbandry
R. G. Reeves, Ph. D., Biology

*Dean, School of Veterinary Medicine.

J. S. Mogford, M. S., Agronomy

F. R. Brison, M. S., Horticulture

W. R. Horlacher, Ph. D., Genetics

J. H. Knox, M. S., Animal Husbandry

A. L. Darnell, M. A., Dairy Husbandry

‘i-As of November 1, 1933

“In cooperation with U. S. Department of Agriculture.
IIn cooperation with Texas Extension Service.

Losses of water through evaporation from reservoirs is an
important consideration in the planning for an adequate supply
for domestic, municipal, or irrigation purposes and the rate of
such losses from the soil has a marked influence upon crop adapta-
tion and farming practices in a particular region. Measurements
of the evaporation from a free water surface have, for a number of
years, been collected by the Texas Agricultural Experiment Station
at Substations in the various regions of the State and these, to-
gether with similar records from U. S. Field Stations located in
Texas, and from U. S. Weather Bureau records at Austin and
Dilley, are reported in this Bulletin. In all, the monthly'and
annual evaporation, in inches, is given for 21 points within the
State, fairly well distributed from the Upper Panhandle to the Rio
Grande Valley and from the more humid regions of the Gulf Coast
to the drier Trans-Pecos Region in the west.

Evaporation generally increases gradually from the eastern
to the western part of the State. Totalannual evaporation from
a free water surface ranges from 45 to 55 inches in the eastern
part of the State, from 55 to 65 inches in the central part, and
from 65 to 75 inches in the western part, or a range of four to
six feet per annum. Rainfall, on the other hand, varies in exactly
the opposite direction, ranging from 52.81 inches at Beaumont, in
the Gulf Coast Region, to 13.75 inches at Balmorhea, in the Trans-
Pécos Region. The temperature, atmospheric humidity, wind move-
ment, and rainfall all have a close relationship to evaporation,
and, together with the altitude and geographical location, largely
determine the extent of losses through evaporation which may be
expected in any given region of the State.

When conditions are highly favorable to rapid evaporation, a loss
of from one-half to three-fourths of an inch of water may occur
in a 24-hour period. In the eastern and southeastern parts of the
State the annual rainfall is equal to or slightly more than the
annual evaporation but in the drier regions the total annual evapo-
ration from a free water surface may amount to as much as four
or five times the total rainfall for the year.

Total annual loss of water through evaporation, when all avail-
able measurements are considered, averages 61.65 inches in Texas,
approximately two-thirds of which occurs during the six warm
months, April to September, inclusive.

CONTENTS

Page
Introduction s. s s s . s s s s s s s _ s s s s . . s s s s s  ................................ ss 5
Location of Observations  .........................  ______________________________ .s 5
Type of Equipment Used _____________________________________________________________________________________ s- 6
Recording of_ Observations  . s . . s s s s s _ . . . . s s s . s s s s s s s s s . s s s . . . s ss 7
Evaporation Records ____________________________________________________________________________________________ ss '7
Tabulation of Evaporation, by Months and Years, at:
Nacogdoches _____________________________________________________________________ s ss _____________________ _s 12
Angleton ____ ss  
Beaumont _____ __ s ________________________________________________________ ss13
Troup (Lindale) . ______ s. A sssssssssssss  ____________ ssl3
College Station _______________________________________________________________________________________________ ss14
Denton _s s .... ss  oooo 774 ssssssssssssssssss ss14
Winter Haven ssss     ...... . s sssssssssssssssssssssss _s21
Beeville sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss ss15
Temple   - s ss sssssssssssssssssssssss s15
Weslaco s  ssssssssssssssssssssssssssssssssssssssssssssss s.21
Lubbock _____ ss -.  __________________________________ ss16
Iowa Park sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss ss16
Hill’s Ranch . ___________________________________________  ___________________________________________________ ss 17
San Antonio ..................................................................................  ___________ ._ 17
Spur  ______ __ s_-18
Chillicothe sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss s_18
Balmorhea ............. s.  A s-  s ........... _.19
Dalhart ss s- ' s s- .................................. _.2O
Amarillo ________________________________________________________ s- s _______ ss20
Dilley . sssss ss sssfs2l
Big Spring s.  .__.s20
Acknowledgments s __________________________________ ss26

s Summary  sssssssss  ssssssssss .26

BULLETIN NO. 484 NOVEMBER, 1933

RATE OF WATER EVAPORATION IN TEXAS
R. E. KARPER

Evaporation records at points representative of the various regions of
the State are of value not only in connection with agricultural development
of these regions but are also useful to municipalities, engineers, and in-
dustrial development. The loss of water through evaporation from storage
reservoirs is an important factor in reducing the quantity available for
domestic supply or irrigation. A knowledge of evaporation losses is
important in the adequate planning of reservoirs for domestic, municipal,
or irrigation purposes, flood control, and other reclamation projects.

The tremendous influence of evaporation upon crop production in climatic
regions such as prevail in Texas, is quite evident and is practically as
important as precipitation. Evaporation is an integration of many com-
ponents of the weather, such as temperature, Wind movement, precipitation,
and relative humidity, all of which, to a greater or lesser degree, determine
thet total evaporation losses from the soil. Such losses are intimately
associated with agricultural production and with farming practices, such
as choosing the right crop to grow, time and method of seed-bed prepara-
tion, planting, and cultivation, as well as rotation, fertilizer, and irriga-
tion practices. Soil moisture available for germination of seed and crop
production is affected by evaporation to a large degree and this influence
must be considered in determining the depth to cover the seed, spacing

l between plants, and rate of seeding. Loss of water from the soil through

evaporation varies for different soils and soil types, but accurate and
rapid determinations of such losses are not possible; however, records of
evaporation from a free water surface or an evaporation tank in general
should resemble evaporation losses from the soil in the different regions of
the State.

The Texas Agricultural Experiment Station has, for a number of years,
kept daily records of the evaporation, as well as other meteorological
observations, at the Substations located in the different agricultural
regions of the State. Numerous requests for information on this subject
have prompted us to make the record of these observations available in this
Bulletin.

LOCATION OF OBSERVATIONS

Evaporation records from the Texas Substations, for the most part, con-

stitute the data reported in this Bulletin, and, in addition, similar records

lfrom the several other points at which they are available have been
" included; Through the courtesy of the Division of Genetics and Biophysics,

Dry-Land Agriculture, and Western Irrigation Agriculture, of the United
States Department of Agriculture, it has been possible to include evapora-
tion records froln the Dry-Land Field Stations at Big Spring and Dalhart,
from the Western Irrigation Agriculture Field Station at San Antonio,

l and from the Amarillo Field Station from 1907 to 1919. While the

records for Dalhart, Amarillo, and Big Spring cover only the six months’

 growing season, April to September, inclusive, this is the usual period of

 

6 BULLETIN NO. 484, TEXAS AGRICULTURAL EXPERIMENT STATION

maximum evaporation losses and the mean annual can be closely ap-
proximated by calculation from other records for the entire year in the

same general region of the State. The records for Austin and Dilley are

taken from the United States Weather Bureau records.

The location of the points of observation cover, for the most part,
the important agricultural and geographical regions of Texas, extending
from the Upper Panhandle to the Rio Grande Valley and from the more

Table 1. Location of stations recording observations on evaporation

North i West lElevation,

Locatlon i County i Reglon Latitude [Longitude feet
Nawgdoches Nacogdoches East Texas Timber 31 ° 35’ 91 ° 40’ 271
Angleton Brazoria Central Coast Prairie 29 ° 9’ 94 ° 24’ 23
Beaumont Jefferson Eastern Coast Prairie 30° 94° 28
Troup Smith East Texas Timber 32 ° 20’ 95 ° 17’ 484
College Station Brazos East Texas Timber 3 1 ° 39’ 96 ° 16’ 308
Denton Denton Grand Prairie 33° 10' 97 ° 10’ 600
Winter Haven Dimmit Western Rio Grande Plain 2 9 ° 40’ 99 ° 50’ 650
Beeville Bee Rio Grande Plain 28°32’ 97°38’ 240
Temple Bell Blackland Prairie 31°15’ 97°37’ 740
Weslaco Hidalgo Lower Rio Grande Valley 26°9’ 97°57’ 80
Lubbock Lubbock Central High Plains 33°37’ 101°45’ 3106
Iowa Park Wichita Rolling Plains 33°55’ 98°39’ 978
Austin Travis Edwards Plateau 30 ° 17 " 97 ° 44’ 502
San Antonio Bexar Rio Grande Plain 29°9’ 98°21’ 539
Spur Dickens Rolling Plains 33°30’ 100°47' 2274
Chillicothe Hardeman Rolling Plains 34° 99° 1406
Balmorhea Reeves Mountains and Basins 31 ° 103 ° 32 00
Dalhart Dallam Northern High Plains 36°4’ 102°31’ 3978
Amarillo Potter High Plains i 35°13’ 101°50" 3676
Dilley Frio Western Rio Grande Plain 28 ° 42’ 99 ° 1 0’ 569
Big Spring Howard Southern High Plains 32 ° 1 5’ 1 01 ° 23’ 2500

humid regions of the Gulf Coast to the drier Trans-Pecos Region in the
west. Table 1 shows the location, region, latitude, longitude, and elevation
of the points of observation covered in these records.

TYPE OF EQUIPMENT USED

Equipment"‘for evaporation observations, which is standard for all the
Substations, consists of an evaporation tank six feet in diameter and
two feet deep, with still-well attached, a micrometer evaporation gage,
and a water supply piped to the tank (Fig. 1.) Within the same fenced
weather enclosure is housed other standard U. S. Weather Bureau equip-
ment consisting of rain gage, maximum and minumum thermometers,
whirling psychrometer, and anemometer. The weather yards are located
as uniformly as possible, at the various Stations, away from trees,
buildings, and other obstructions, and instruments installed in regulation
shelters. The anemometer is mounted on a stand about five feet from
the ground. Evaporation tanks are given full exposure to the sun. The
earlier records at Substations and Field Stations were practically all
obtained from eight-foot tanks. In a few cases tanks of-this size are
still in use but at most of the Stations they have been replaced, and as
they are renewed, tanks six feet in diameter are installed.

Evaporation tanks are round and constructed of ZO-gage galvanized
iron. The upper edge of the tank is reinforced by a piece of angle iron,

RATE OF WATER EVAPORATION IN TEXAS g 7

the top of Which comes about one-fourth of an inch below the top edge
of the tank, thus providing reinforcement on the outside of the tank. The
top edge of the tank is kept in line and not dented. About one inch above
the bottom of the tank is installed a flange coupling on both the inside
and outside, riveted and soldered, giving a water-tight thread connection for
a one-half inch galvanized iron pipe extending from the outside of the
tank to approximately its middle and open at each end to admit the
placing of a still-Well on the outside of the tank and giving connection
with the water in the tank through this small pipe. A brass still-well,

a with cover, is connected parallel with the side wall of the tank. Tanks

are sunk in the ground so that four inches of the rim projects above the
surface, and are set level. Installation in the ground provides a more
even temperature of the water and the 4-inch rim above the ground level
protects the tank somewhat from splashing rain, drifting snow, and blow-

ing dust and trash.

Fig. 1. Sunken exaporation tank, still-well, and micrometer gauge, the equipment

ed in recording evaporation from a free water surface.
' Evaporation records at Hill’s Ranch, near Austin, and at Dilley have been
itained by the U. S. Weather Bureau from their standard type of pan
talled above ground. Water in this type of installation is more readily
 ected by the temperature of the air and the rate of evaporation is
mewhat higher than Where sunken tanks are employed.

 

BULLETIN NO. 484, TEXAS AGRICULTURAL EXPERIMENT STATION

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RATE OF WATER EVAPORATION IN TEXAS

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20 BULLETIN NO. 484, TEXAS AGRICULTURAL EXPERIMENT STATION

Table 18. Monthly evaporation, in inches, at Dalhart,* April to September, inclusive, 1908-1932

   

Year April May June July Aug. ‘ Sept. Total
I ” I
1908 5.920 10.918 12.072 i 9.182 9.888 7.950 55.930
1909 8.526 9.901 10.890 11.685 10.565 7.835 59.402
1910 8.540 8.183 12.020 11.626 8.822 8.441 57.632
1911 7.562 9.900 12.373 9.707 10.899 8.769 59.210
1912 . 8.214 10.237 8.475 11.103 9.131 6.750 53.910
1913 7.687 10.063 8.714 12.697 10.771 6.338 56.270
1914 6.535 7.813 10.256 8.837 9.059 8.227 50.727
1915 5.777 8.075 8.740 - 9.262 7.919 6.853 46.626
1916 6.597 11.088 10.367 11.022 9.597 7.136 55.807
1917 8.561 9.074 11.697 10.997 9.607 7.460 57.396
1918 6.371 10.640 9.157 11.229 10.854 6.318 V 54.569
1919 5.583 6.994 7.482 ’ 9.865 8.291 6.933 45.148
1920 7.000 8.050 8.737 7.877 7.622 6.954 46.240
1921 6.826 7.425 6.912 7.655 7.351 8.304 44.473
1922 6.397 7.955 9.091 9.582 9.914 6.807 49.746
1923 6.161 7.472“ 7.430 8.140 8.337 5.817 43.357
1924 7.307 7.465 9.407 _ 9.089 9.077 6.387 48.732
1925 7.389 7.041 9.721 9.043 6.602 5.573 45.369
1926 4.618 6.314 6.960 8.503 9.269 6.953 42.617
1927 7.266 10.727 9.529 9.081 6.636 5.174 48.413
1928 6.610 - 7.236 7.546 7.545 7.213 7.430 43.580
1929 7.301 6.427 8.285 9.836 8.180 6.291 46.320
1930 7.524 9.273 9.967 11.162 9.358 8.067 55.351
1931 5.193 7.355 10.620 10.528 _ 7.731 8.153 49.580
1932 7.320 8.340 7.925 10.414 9.855 6.747 50.601
Mean 6.911 8.559 9.375 9.827 8.902 7.107 50.680
Table 19. Monthly evaporation, in inches, at Amarillo*, April to September, inclusive, 1907-1919
Year April May I June July Aug. ' Sept. I Total
1907 6.355, 8.050 9.575 10.669 i 9.390 7.911 51.950
1908 7.318 9.284 10.378 8.075 8.575 6.776 50.406
1909 8.130 10.020 10.337 9.964 9.658 8.421 56.530
1910 8.494 8.032 12.011 12.167 8.806 9.092 58.602
1911 7.360 10.100 11.482 7.472 8.895 7.268 52.577
1912 7.050 9.895 8.986 10.947 9.490 6.493 52.861
1913 7.736 9.812 7.065 12.734 10.453 5.994 53.794
1914 6.695 6.739 10.124 8.747 8.927 8.041 49.273
1915 ' 4.584 6.920 8.833 9.310 7.299 5.984 42.930
1916 5.955 10.286 10.661 11.656 10.206 7.665 56.429
1917 7.681 7.623 12.475 12.430 8.571 5.961 54.741
1918 6.991 11.014 10.128 10.736 10.339 7.376 56.584
1919 4.652 5.026 7.086 9.470 8.419 6.137 40.790
Mean 6.846 8.677 9.934 10.337 9.156 7.163 52.113
Table 20. Monthly evaporation, in inches, at Big Spring*, April to September, inclusive, 1916-1932
Year April May June I July Aug. Sept. Total
1916 6.610 10.838 13.291 10.435 9.243 7.714 58.131
1917 10.008 10.871 13.376 13.159 12.251 8.126 a 67.791
1918 9.667 11.325 10.622 13.239 12.642 8.429 65.924
1919 6.988 8.345 I 8.032 10.272 10.579 6.672 50.888
1920 8.745 8.825 9.348 12.479 6.985 7.059 53.441
1921 9.213 9.110 9.129 11.507 12.839 8.815 60.613
1922 8.481 8.460 8.794 12.930 11.671 9.692 60.028
1923 5.950 8.852 9.790 10.374 10.326 6.927 52.219
1924 7.178 8.183 11.562 10.870 11.089 7.427 56.309
1925 4.438 8.167 11.237 11.872 8.100 6.420 50.234
1926 5.398 8.292 10.148 9.981 e 10.107 7.665 51.591
1927 8.584 11.712 10.369 10.153 12.115 7.058 ‘ 59.991
1928 8.365 7.437 10.962 9.708 7.170 6.944 50.586
1929 7.334 7.220 I 10.693 9.277 | 10.098 6.861 51.483
1930 6.061 8.119 8.712 11.651 10.225 9.181 53.949
1931 5.609 7.554 9.470 _ 9.549 9.551 9.210 50.943
1932 7.496 6.511 7.896 9.734 8.034 3.946 43.617
Mean 7.419 8.813 10.202 11.011 10.178 7.538 55.161

*Records from U. S. Field Station.

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RATE OF WATER EVAPORATION IN TEXAS

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22 BULLETIN NO. 484, TEXAS AGRICULTURAL EXPERIMENT STATION
RECORDING THE OBSERVATIONS

Readings of the evaporation are usually taken at 7 a.m. and 6 p.m.
daily, and when only one daily observation is made it is in the evening read-
ing. The micrometer-point gage is graduated to read to one-thousandth
of an inch and all readings are recorded to three decimals and the
actual gage readings entered in the record. The evaporation for the past
24-hour period is the difference between the last previous gage reading
and the current one. If rain intervenes, the amount of rainfall is added
to the current reading and from the total the previous gage reading is
subtracted. In attempting to secure evaporation records for the twelve
months’ period it may often happen, at some of the stations, that the tank
remains frozen over for a considerable period. In such cases the word
“frozen” is entered in the record for those days and when the tank thaws

out the total accumulated evaporation for the period since the last reading’

is recorded Daily observations are recorded in the daily meteorological
record book and at the end of the month are transferred to monthly blanks
provided for this purpose.

Tanks are refilled, at a regular observation time, frequently enough to
keep the fluctuations within a narrow range and provide a comparatively
uniform depth of water in the tank. Water in the tank is kept clean and
occasionally the water is all removed and the tank thoroughly cleaned.

EVAPORATION RECORDS

On account of their close association with and influence upon evapora-
tion, other meteorological observations, such as maximum, minimum, and
mean temperatures, precipitation, wind run, and relative humidity, are
included in Table 2. Observations on these latter phenomena are shown
as the mean monthly and annual for the whole period for which records
are available, which is, in most cases, longer than that covered by the
records on evaporation. With few exceptions, however, all these meteoro-
logical data cover a sufficiently long period of time to furnish a reliable
mean which records, accumulating over additional years, will, of course,
change slightly but not materially.

Tables 3 to 21 are a complete record of the evaporation, by months and
years, since the beginning of evaporation readings at the various Stations.
These tables are numbered and arranged in ascending order of the mean
annual evaporation, Nacogdoches, which has the lowest mean annual
evaporation, in inches, appearing in Table 3. Evaporation records at many
of the Stations were begun in 1915 to 1917, while at San Antonio, Dalhart,
and Amarillo, the records were begun in 1907 and 1908 and are presented
in these tables, including the year 1932. At Iowa Park records have been
kept only since 1926. At Chillicothe records for the six months’ grow-
ing period, April to September, inclusive, are complete from 1912 to 1932,
but for the other months of the year are only partially complete, except
since 1923. Readings shown for Balmorhea (Table 17) were recorded at

. . t.......-L.......,.r..___..¢.~.. -1... __.._..__.._.._.1_-........a._.u._.1.-;=ki-m:.m

RATE OF WATER EVAPORATION IN TEXAS 23

Pecos from 1916 to 1922. The Substation was moved to Balmorhea and
records were not resumed until 1926, causing a break of four years in
this record.

At Dalhart, Amarillo, and Big Spring, readings are available only for the
six months’ growing season, April to September, inclusive. An ap-
proximation of the mean annual evaporation, in inches, can be calculated for
these points by assuming a similar percentage relationship between the
evaporation for the six months and the whole year for these points as
that existing at Lubbock,‘ Where 72.16 per cent of the yearly total evapora-
tion occurs from April to September, inclusive. Keeping of evaporation
records at Amarillo ceased in 1919.

Records of evaporation are available for only four years, 1929-32, at
Dilley; for only two years, 1931-32, at Winter Haven, and for only one
year, 1932, at Weslaco, in the Lower Rio Grande Valley. These short-
time records are inadequate to furnish a reliable reading of the average
evaporation that may be expected but the data for the three points are
combined and presented in Table 21.

The detailed data covering the inches of evaporation from a free Water
surface, and shown in Tables 3 to 21, by months and years, need little
discussion. Total inches of evaporation for each year, together with the
average inches monthly and annually, are also given in these tables, so far
as the available records will permit.

A summarization of certain climatic factors as they prevail at each of the
points where evaporation readings have been recorded is given in Table
22. This includes the eleyation, mean temperature, average relative
humidity, miles of wind run, and rainfall, all of which undoubtedly have a
bearing upon the water lost from the soil or from a free water surface
through evaporation. Stations are arranged in this table in order of
total inches of evaporation per annum, Nacogdoches, which has the least
evaporation, appearing first. With certain exceptions, it will be seen that
the order of the Stations listed moves from the eastern to the Western
part of the State, the evaporation increasing markedly in the higher and
drier regions. Total annual evaporation ranges from 45 to 55 inches in
the eastern part of the State, from 55 to 65 inches in the central part,
and from 65 to 75 inches in the western part (Fig. 2.) From a glance at
this table it appears that the Stations located in the lower elevations
generally have the lowest annual evaporation. This is probably not due
so much to the elevation as to the fact that at the higher elevations
the humidity is lower, the wind run greater, and the precipitation less, all
of which are favorable to a more rapid loss of water through evaporation.
The average relative humidity per cent figure, in the third column of the
summary table, is a measure of the degree of saturation of the atmosphere.
When the relative humidity and evaporation for these 21 points in Texas are
compared, a close relationship is found between a dry atmosphere and a
high evaporation. The actual correlation coefficient, as calculated be-
tween humidity and evaporation at these points, is —.82;|:.05, showing that

24 BULLETIIYANO. 484, TEXAS AGRlCUhTURAL EXPERIMENT STATION

at a point where the humidity is lOW the rate of evaporation can be expected
to be high.

Precipitation at the Stations included in these records ranges from
52.81 inches at Beaumont, in the Gulf Coast Region, to 13.75 inches at

l0

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Jan. Feb. Mc/z. A/on May June Ju/y. Auy. Sepf Oc7‘. Nov. Dec.

Fig. 2. Mean monthly evaporation curves for six stations in Texas showing the
high summer rate. Total annual evaporation ranges from 44.5 inches at Nacogdoches
to 70.6 inches at Balmorhea.

Balmorhea, in the Trans-Pecos Region. In general, the change in rate
of evaporation in moving across the State from east to west is almost
directly inverse to the change in precipitation. A high negative correla~
tion, —.87i.04, is found, then, between rainfall and evaporation, but
many other factors, aside from precipitation, are operating to cause an

increase in evaporation in moving toward the western part of the State."

Other regional characteristics, such as dryness of the atmosphere and
greater wind movement, are also favorable to high evaporation. Monthly
wind run, in miles, increases in this same Westerly direction across the
State and a positive correlation of 491.11 is found between total monthly
wind run and total annual evaporation when these readings at the 21
Stations are considered.

Temperature is an important factor affecting the rate of evaporation from
a free water surface or from the soil. From the mean temperature and

RATE OF WATER EVAPORATION IN TEXAS 25

evaporation figures for the various Stations given in Table 22 there seems
to be no very marked or consistent relationship between them. The highest
mean annual temperatures occur at the Stations in the eastern and
southern parts of the State, whereas, the greatest evaporation occurs in
the western part, where the mean temperature is lower. Other factors
less favorable to high evaporation, such as higher humidity and lower wind
movement, are operating in opposition to the higher mean temperature in
the eastern part of the State. The general climatic features as found at
Lubbock compared “Yith those found at Spur and Chillicothe, are not
greatly different except that a higher altitude and a somewhat lower mean
temperature are found at Lubbock than at the other two points. Slightly
lower humidity and greater wind movement at Lubbock should favor a
higher annual evaporation but the higher temperature prevailing below the
Caprock Escarpment and at the lower elevations of Spur and Chillicothe
are probably responsible, in the main, for the higher rate of evaporation
found at these two points. An example of the marked influence of high
temperature upon the rate of evaporation can be seen in a comparison of
the monthly and annual evaporation record from Dilly and Winter Haven for
the years 1931 and 1932. These two Stations are in the same general
region of the State and located only about 40 miles apart. At Winter
Haven the usual sunken tank, like that at all the other Substations re-
ported in this Bulletin, is used, while at Dilley records are obtained from
the above-ground type of pan in use by the U. S. Weather Bureau. As
the water in the latter type of pan is more completely exposed to the

air, its temperature fluctuates more rapidly and follows the air temperature
more closely than does that of the water in the sunken tanks. The mean
annual air temperatures at Dilley and at Winter Haven are not signifi-
cantly different, yet the evaporation for these two years was 10 to 15
inches greater at Dilley than at Winter Haven, probably due very largely
to the difference in the type of pan used and the influence of temperature
of the water upon the rate of evaporation. From this comparison it
appears that evaporation records from the sunken tank will more nearly
approach the evaporation from reservoirs or natural bodies of water, which
fluctuate even less rapidly in relation to air temperatures.

The extreme daily maximum evaporation, in inches, occurring at each of
the points of observation throughout the period covered by the records
is shown in Table 22. The maximum total water loss from evaporation
for any one day varied from .440 inch at Beaumont to .845 inch at
Iowa Park. Ordinarily, when conditions are highly favorable for a large
water loss through evaporation, a loss of from one-half to three-fourths
of an inch in a 24-hour period may be expected in Texas. From an
examination of other weather data for the days when such extreme
maximum evaporation occurs it is found that the temperature and wind
movement are usually high and the atmosphere dry, as shown by a low
relative humidity. Abnormally high evaporation occurs during the spring
or summer months and during dry years or periods.

26 BULLETIN NO. 484, TEXAS AGRICULTURAL EXPERIMENT STATION

Extreme total evaporation, in inches, including all the points in the State
from which we have a record for the whole twelve months’ period, has varied
from 38.97 inches at Nacogdoches in 1922 to 83.78 inches at Pecos in 1917.
The average annual evaporation in Texas ranges from 44.5 inches at Nacog-
doches to above 70 inches at some of the western Stations, or a water loss
of from slightly less than four feet to over six feet per annum. The
ratio of precipitation to evaporation, which is a direct comparison of the

‘total annual precipitation with the total annual evaporation for the

various Stations and regions of the State, is also shown in Table 22. At
some of the Stations in the eastern and southeastern parts of the State
the annual rainfall is equal to, or slightly more, than the annual evapora-
tion, while in the drier regions of the State the total annual evaporation
greatly overbalances the precipitation and may be four or five times as
much as the total annual rainfall. When all of the evaporation records
available are considered from all parts of the State it is found that the
average annual evaporation for Texas is 61.65 inches and that approximately
two-thirds (68.7%) of this occurs during the summer months, April to
September, and one-third (31.3%) during the remaining months of the
year (Fig. 2). .
ACKNOWLEDGEMENTS

Because of the close association between the results of field experiments
and the weather, meteorological records have been maintained for many
years at State Substations and Federal Field Stations. To the continued,
painstaking recording of these observations by the superintendents of the
Stations, who have, in most cases, been the observers, we are indebted for
these long-time records. Such records embody, in the aggregate, many
thousands of observations and calculations made in addition to more
pressing duties and with no hope of reward except for the facts recorded.
Mention of all the individuals responsible for these evaporation data is pre-
cluded but their records have made this compilation possible. Grateful ack-
nowledgment is also made to Mr. Chester Higgs, Executive Assistant, for
carefully and painstakingly computing and checking these records for

accuracy.
SUMMARY

Losses of water through evaporation have an important bearing upon crop
production and farming practices as well as upon the economic and ade-
quate planning of reservoirs and similar engineering projects. The evapo-
ration from afree water surface, by months and years, is shown from 21
observation points scattered throughout the State. ' .

The total evaporation in Texas amounts to about four to six feet per
year and ranges from 45 to 55 inches in the eastern part, from 55 to 65

inches in the central part, and from 65 to 75 inches in the western part.'

Extreme maximum evaporation in any 24-hour period ranged from .440
inch (at Beaumont) to .845 inch (at Iowa ark). A water loss of one-
half to three-fourths of an inch per day may occur when the temperature
and wind movement are unusually high and the relative humidity low.

RATE OF WATER EVAPORATION IN TEXAS 27

The extreme total minimum evaporation for any one year was 38.97
inches, recorded at Nacogdoches in 1922, and the maximum extreme was
83.78 inches, recorded at Pecos in 1917. Inches of annual rainfall and
evaporation are practically equal in the eastern and southeastern parts
of the State but in the western part evaporation greatly overbalances
precipitation and may be three to five times as much per year. ‘

The total average annual loss of water through evaporation from a free
water surface in Texas is 61.65 inches, of which approximately two-thirds
occurs during the six warm months of the year, April to September,
inclusive.