TC 824 C2 A2 no. 113
LIBRARY
UNIVERSITY OF CALIFORNIA
LIBRARY UNIVERSITY OF CALIFORNIA
THE RESOURCES AGENCY OF CALIFORNIA partment of Water Resources
BULLETIN No. 113
VEGETATIVE
WATER USE STUDIES
1954-1960
Interim Report
UNIVERSITY or
LIdRARY
AUGUST 1963
HUGO FISHER
Adminisfrafor The Resources Agency of California
EDMUND G. BROWN
Governor State of California
WILLIAM E. WARNE
Director
Department of Water Resources
stale ol California THE RESOURCES AGENCY Of CALIEORNIA
Department of Water Resources
BULLETIN No. 113
VEGETATIVE
WATER USE STUDIES
1954-1960
Interim Report
AUGUST 1963
HUGO FISHER EDMUND G. BROWN WILLIAM E. WARNE
Adminisirafor Governor Director
The Resources Agency of California State of California Department of Water Resources
LIBRARY
UNlVKR.<;iTV OR PAT TTrnPlMTX
ERRATA
Bulletin No. 113, '•Vep:etatlve V/ater Use Studies, 195^ - 196O" Plate 1, Agrocllmatlc Stations No. h, 57. 61, 75, 93, 101:
For "Inactive - i960" read "Active - 196C" Plate 2, For "Evapotransperometer" read "Evapotranspirometer" Page 58, line 10, For "Figures E and F" read "Figures A and B' Page 66, line 21 , For "Figures A and B" read "Figures E and F'
TABLE OF CONTENTS
Page
lET'lER OF TRANSMITTAL vii
ORGANIZATION, DEPARTMEJIT OF WATER RESOURCES viii
ORGANIZATION, CALIFORNIA WATER COMMISSION ix
ACKNOWLEDGMENT x
CHAPTER I. INTRODUCTION 1
Need for Vegetative Water Use Studies 1
Authorization 2
Objective • 3
Scope of Present Program and Report • h
CHAPTER II. AGROCLIMATIC MONITORING PROGRAM 7
Instrumentation at Agroclimatic Stations • 8
Atmometers 8
Evaporation Pans 9
Agroclimatic Data Analysis 9
CHAPTER III. EVAPOTRANSPIRATION MEASUREMENT 21
Measurement of Data Related to Evapotranspiration 22
Criteria for Selection of Plots 2i|
Evapotranspiration Measurement Technique and Discussion of
Development and Current Methods 25
Field Plot Sampling Neutron Scattering Technique 29
Pittville Neutron Probe Moisture Depletion
Measurements 31
Arvin Neutron Probe Moisture Depletion Measurements . , 3li
iii
TABLE OF CONTENTS (continued)
Page
Evapotranspirometer Measiirements 36
Alturas-Dorris Ranch Evapotranspirometer Measxirments .... 37
Coleville Evapotranspirometer MeasTirements ......... 39
Davis Evapotranspirometer Keasurments UO
Evapotranspiration Data Summary iiO
CHAPTER IV. CORRELATION OF EVAPOTRANSPIRATION DATA
WITH AGROCLIMATIC DATA 51
Evapotranspiration and Climatic Data ............ 52
Evapotranspiration and Plant Conditions 53
Evapotranspiration and Soil Moisture ....• 5U
Other Factors Affecting Evapotranspiration 55
Determination of Coefficients 55
Grass and Pasture Coefficients 57
Alfalfa Coefficients 58
Cotton Coefficients 63
Application of Coefficients and Evaporation Data to
Estimation of Evapotranspiration , 67
CHAPTER V. SUMMARY, CONCLUSIONS MD RECOMMENDATIONS . . 71
Suininary 71
Conclusions • 7h
Recommendations .... .... .....•.•• 75
Appendix A. Supplemental Agroclimatic and Evapo- transpiration Data 77
iv
TABIE OF CONTENTS (continued)
TABLES
Nimber
1 Mean Monthly Evaporation From Standard
U. S. Weather Bureau Evaporation Pans 12
2 Mean Monthly Evaporation Difference Between
Livingston Spherical Black and White Atmometers. . 12
3 Monthly Evaporation From Standard U, S. Weather
Bureau Evaporation Pans in Order of Decreasing
Magnitude For Irrigated Pasture and Dryland
Stations 16
k Monthly Evaporation Difference Between Livingston Spherical Black and VJhite Atmometers in Order of Decreasing Magnitude For Irrigated Pasture and Dryland Stations 19
5 Summary of Measurements of Evapotranspiration and
Related Data U2
6 Pan and Atmometer Coefficients for Pasture and Grass 59
7 Pan and Atmometer Coefficients for Alfalfa 6U
8 Pan and Atmometer Coefficients for Cotton 68
9 Comparison of Seasonal Consumptive Use of Alfalfa,
Pasture, and Cotton Based on Bulletin No. 2
Growing Season 69
FIGURES
Page
Atmometer Assembly ., 10
Typical Agroclimatic Stations 11
Platforms Used to Minimize Crop Damage and Soil
Compaction 32
h Access Tube Design 33
5 Evapotranspirometer Designs « Ul
Number 1 2 3
TABLE OF CONTENTS (continued)
PLATES (Bo\ind at End of Bulletin)
Number
1 General Location of Agroclimatic Stations
2 General Location of Evapotranspiration Stations
3 Comparison of Evapotranspiration Curves of Different
Crops Grown at the Same Location on the Same Soil Series
h Variation of Pan and Atmometer Coefficients for
Individual Periods of Measurements
5 Comparison of Pan and Atmometer Coefficients for
Cotton, Alfalfa and Grass
6 Jlelationship Between Pan and Atmometer Coefficients
For Alfalfa and Ground Cover
EDMUND G. BROWN GOVERNOR OF CALIFORNIA
HUGO FISHER
ADMINISTRATOR
RESOURCES AGENCY
THE RESOURCES AGENCY OF CALIFORNIA
DEPARTMENT OF WATER RESOURCES
1120 N. STREET, SACRAMENTO
June 20, 1963
Honorable Edmund G. Brovm, Governor and Members of the Legislature of the State of California
Gentlemen:
I have the honor to transmit herewith Bulletin No. 113, "Interim Report on Vegetative Water Use Studies, 19514-1960," of the Department of V/ater Resources, dated May 1963. This report describes techniques and approaches ■which have evolved, and summarizes data on vegetative con- sumptive use or evapotranspiration. Interrelationships between these data are set forth, together with evapotran- spiration values for some crops in Central and Northern California agricultural areas. This is a continuing study with many conclusions yet to be reached.
Data pertaining to evapotranspiration, irrigation requirements, and agricultural hydrology are basic to most water resource development studies. With the continued growth of the State, necessitating more complex and costly water development facilities, there is increasing need for more accurate water use data. Such data will enable de- veloped surface and ground water resources to be used effectively, and will facilitate design and operation of land drainage systems.
The studies reported herein were initiated in 195U as part of the Northeastern Counties Investigation. A con- tinuing Vegetative Water Use Studies Program was established, and the studies were broadened, sls a result of Senate Bill Ii3li; 1959 Legislative Session. Specific authorization for these studies is set forth in Section 226(e) of the Water Code.
Sincerely yours ^
' ' T)t rpct.OT
STATE OF CALIFORNIA
THE RESOURCES AGENCY OF CALIFORNIA
DEPARTMENT OF WATER RESOURCES
EDMUND G. BROWN, Governor
HUGO FISHER, Administrator, The Resources Agency of California
VJILLIAM E. WARNE, Director, Department of VJater Resources
ALFRED R. GOLZE, Chief Engineer
Division of Reso\irces Planning
William L. Berry, Division Engineer Albert J. Dolcini, Chief, Planning Management Branch
Technical studies were conducted and the bulletin was prepared under the supervision of
John W. Shannon
Reginald E. Merrill Norman MacGillivray
Water Utilization Staff Specialist Assisted by
Associate Land and Water Use Analyst Assistant Land and Water Use Analyst
Field data were collected under the supervision of
Jack H. Lawrence Senior Land and Water Use Analyst
Assisted by
John Kono Assistant Land and Water Use Analyst
Andrew Lee Junior Land and Water Use Analyst
Arthur deRutte Assistant Land and Water Use Analyst
Patrick Duval Assistant Land and Water Use Analyst
Robert Bowman Assistant Land and Water Use Analyst
Victor Uhlik Assistant Land and Water Use Analyst
Darrell Nichols Assistant Land and Water Use Analyst
Zene Bohrer Assistant Land and Water Use Analyst
viii
CALIFORNIA WATER CCMMISSION
RALPH M. BRODY, Chairman, Fresno WILLIAM H. JF^INGS, Vice Chairman, La Mesa
JOHN W. BRYANT, Riverside JOHN P. BUNKER, Gustine
IRA J. CHRISMAN, Visalia JOHN J. KING, Petaluma
EDWIN KOSTER, Grass Valley NORRIS POULSON, La Jolla
MARION R. WALKER, Ventura
0-
WILLIAM M. CARAH Executive Secretary
GEORGE B. GLEASON Principal Engineer
ACKNOVJLEDGElffiOT
The Department of VJater Resources wishes to express appreciation to many organizations and individuals who have assisted the department in the Vegetative Water Use Program. I-lany private farm operators have provided use of their property and equipment, as well as time. The list is too numerous to completely enumerate; however, the Frick Farms at Arvin, Roland Hutchings at Pittville, and the U. S. Fish and Wildlife Service (formerly Dorris Ranch) at Alturas have been particularly helpful.
A very considerable amount of technical guidance has been given by the Irrigation Department of the University of California at Davis. The University Agricultural Extension has given assistajice in the search for plot sites.
The assistance and collaboration provided by the U. S. Forest Service, the Agricultural Research Service and the Soil Conservation Service of the U. S. Department of Agriculturej the California Division of Forestry; and the Agricultural Commissioner's Office, to mention a few, are likewise gratefully acknowledged.
CHAPTER I, INTRODUCTION
This report presents data on vegetative consumptive use of water, or evapotransplratlon, together with certain Interrela- tionships with agricultural climatic factors Influencing such use. The findings summarized cover the period 195^-1960, and represent a large quantity of Individual measurements of evapotransplratlon and related agricultural climatic data. The measurements of evapo- transplratlon represent scores of soil samples, neutron probe read- ings, and evapotranspirK)meter measurements of Irrigated alfalfa, pasture, plums, cotton, and grass crops. Agricultural climatic or agrocllmatlc data are likewise summarized from a large number of measurements of evaporation from pans and atmometers. Certain other agrocllmatlc data, such as measurements of solar radiation and rela- tive humidity, were collected at a few stations. These data have not been analyzed as yet, and will be reported in later publications,
Need for Vegetative Water Use Studies Historically, irrigated agriculture has been the largest user of our developed water resources. This condition probably will continue indefinitely. The Department of Water Resources, hereinafter referred to as the department, and its predecessor agencies, have made many measurements of water deliveries for agri- cultural uses with regard to water right adjudication. However, for broad planning purposes the department has relied largely upon
empirical methods for estimating seasonal values of evapotransplra- tion or consumptive use for various crops. State Water Resources Board Bulletin No, 2, "Water Utilization and Requirements of Cali- fornia, 1955/' has been the primary source for such estimates.
As more complex and costly water development facilities are contemplated, more accurate values for irrigation requirements and evapotranspiration v/ill be needed. The location and sizing of reservoirs, distribution systems, and final disposal or drainage systems are dependent upon accurate estimates of at least monthly values of irrigation requirements and evapotranspiration for various kinds of vegetation. Accurate irrigation requirements and evapo- transpiration values are also important in planning for the con- junctive operation of ground water reservoirs, the reclamation of salt-affected lands, and in the maintenance of a favorable salt balance within agricultural soils. Moreover, as water costs rise, more accurate knowledge of evapotranspiration rates will become of increasing importance in order to achieve greater efficiencies in irrigation practices.
Authorization Estimates of evapotranspiration and irrigation require- ments have long been a part of water development investigations, as conducted by the department and its predecessor agencies. The preS' ent program, designed to provide more accurate data on rates of evapotranspiration, was initiated in July 195^ and broadened in 1959. pursuant to Senate Bill 43^, 1959 Legislative Session. Specific authorization for conducting these studies is set forth
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in Section 226 (e) of the Water Code, which states that the depart- ment may "Conduct investigations of the rate of use of water for various purposes and considering various soil conditions."
Objective The overall objective of the vegetative water use studies is to Investigate and establish a means whereby the department can accurately determine long-term monthly and seasonal irrigation re- quirements and evapotranspiration for the principal crops grown within the various agricultural zones throughout California. To accomplish this broad objective, the vegetative water use studies are divided into three principal programs; namely, agroclimatic monitoring, evapotranspiration measurement and correlation, and irrigation requirement determination. The first two of these pro- grams are designed to accomplish the following primary objectives: first, to collect agroclimatic data in major agricultural areas to provide a means of dividing the State into agroclimatic zones of potential water use, and for estimating evapotranspiration within those zones; and second, to test, on a statewide basis, certain procedures suggested by fundamental research by the University of California and other agencies, regarding correlation of evapotranspi- ration with various types of agroclimatic data. The objective of the third program is to correlate measured values of total applied water with evapotranspiration. These data will make possible the calculation of other pertinent water use information, such as ir- rigation efficiencies and drainage requirements. Very little has been accomplished on the third program to date,
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Scope of Present Program and Report
To accomplish the foregoing objectives. It Is necessary to measure evapotransplratlon for various crops within the major agricultural zones of the State, and to measure various climatic, plant, and soil factors which Influence evapotransplratlon. To date, accurate measurements of evaporation have been made of only a few crops within certain of the major agricultural service areas of the State, because of financial and personnel limitations. Ad- ditional installations will be required to provide complete evalua- tion of all major agricultural zones and the principal crops grown within California.
In order to maximize the utility of the data provided by the relatively few evapotransplratlon measurement stations, a cor- relative program has been carried on to relate evapotransplratlon to evaporation indices. Theoretically, coefficients derived by comparing evapotransplratlon to evaporation from pans or atmometers can be used to make reliable estimates of evapotransplratlon within any agrocllmatic zone where evaporation data are available. Basic research on such relationships is being conducted by the University of California as a part of the vegetative water use program.
The agrocllmatic monitoring program, described fully in Chapter II, is designed to collect the basic agrocllmatic data necessary to make reliable estimates of evapotransplratlon within each agrocllmatic zone. Chapter III discusses evapotransplratlon measurements and the collection of data relative to plant condi- tions, soil moisture, and other factors which may affect evapo- transplratlon rates. The criteria, methods, and Instrumentation
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used in the measurements are described generally, and the data col- lected through i960 are summarized. Since the initiation of this program in 195^^ improvements and standardizations within the pro- gram have vastly Improved the quality of the data collected, such that one hesitates to compare data collected In 196O with earlier years of records. Consequently, Judgment was exercised in sum- marizing certain of the earlier data.
In Chapter IV, measured evapotranspiration rates described in Chapter III are correlated with pan and atmometer evaporation data which were collected concurrently at the evapotranspiration plots. The pan and atmometer coefficients, so derived, are then applied to the agroclimatic data to estimate evapotranspiration for a few crops throughout much of the northern part of the State. While comparisons are made with the values published In Bulletin No, 2, it is not the Intent of this report to imply a question as to the accuracy of previous values used by the department. In- stead, this report Is Intended to Indicate some of the problems involved In the collection and analysis of the data and, to the extent of the data collected, to show tentative values that may be used for the determination of water requirements for certain crops.
A great deal of the basic research fundamental to this study was conducted by the University of California at Davis, both prior to and since the initiation of this program. The continuing counsel and guidance provided by various members of the University of California have been of Invaluable assistance In the develop- ment of these studies.
CHAPTER II. AGROCLIMATIC MONITORING PROGRAM
As stated In Chapter I, the objective of the agrocllmatlc monitoring program is to collect and analyze climatological data throughout the various agricultural water service areas within the State. The analyses of these data will accomplish two purposes. First, they will enable segregation and delineation of zones or areas with similar evaporation potentials. Secondly, these data will provide a basis for estimating evapotranspiration rates of various crops within those zones. This can be accomplished by utilizing coefficients which relate measured crop evapotranspira- tion (to be discussed in Chapter III) to agrocllmatlc data. The program of correlating measured evapotranspiration to various evaporative indices, such as evaporation pans and atmometers, is discussed in Chapter IV.
To date, agrocllmatlc stations have been established at typical locations within certain of the major inland agricultural areas in the central and northern portions of the State. The data collected and summarized in this report comprise weekly measure- ments of evaporation from U, S. Weather Bureau Standard Class A pans, and differences of evaporation between Livingston black and white atmometers. Measurement of solar radiation, air temperature, and humidity was made at a few locations. These data, however, are not included in this report, as research regarding their re- lationships to evapotranspiration and methods of analysis are still in the process of development.
As of i960, the program Included 52 stations, although a total of 112 stations have been operated for various periods of
time. Many of the original stations have been discontinued be- cause of unfavorable site conditions or other causes. The location and status of each station are shown on Plate 1, entitled "General Locations of Agro climatic Stations, 1954-60." A more detailed de- scription of each of the agroclimatlc stations is presented in Table A-1 of Appendix A.
Instrumentation at Agroclimatlc Stations Two types of equipment were utilized to measure evapora- tion potential; the Livingston spherical atmometer, and the U. S, VJeather Bureau Standard Class A evaporation pan. U. S. Forest Service precipitation gages, approximately 8 Inches in diameter and 10,5 inches in height, were installed at all agroclimatlc stations at the same elevation above ground as prescribed for a standard U, S, Weather Bureau nonrecording rain gage. Following is a description of evaporation equipment in use and methods of installation.
A tmo meters
A Livingston spherical atmometer is a specialized instru- ment used for measurement of evaporation. The atmometer is a hol- low porous porcelain sphere 5 centimeters in diameter. In a typical assembly the sphere is mounted on a 1-gallon water supply bottle by means of a small-diameter glass tube. The sphere and connecting tube ar'e filled with distilled water, with the lower end of the tube extending nearly to the bottom of the reservoir bottle. Thus, there is a continuous water system from the reservoir bottle to the outer surface of the porous sphere, where evaporation takes place.
Evaporation Is determined by measuring the amount of water re- quired to refill the reservoir bottle to a reference mark. A typical atmometer assembly Is shown on Figure 1,
Atmometers are operated as pairs consistlnc of one white and one black sphere set 15 Inches apart and ^4 Inches above ground surface. Prior to 1958^ many installations had only a single pair of atmometers; however, since that time three or more pairs of atmometers have been Installed at each of the sta- tions included in the monitoring program.
Evaporation Pans
U, S. Weather Bureau Standard Class A evaporation pans were adopted in the agroclimatic program in 1937 and installed at certain of the stations. The pans were installed in accordance with the procedure prescribed in "instructions for Climatological Observers," Circular B. Tenth Edition, Revised October 1955, U. S. Department of Commerce.
All stations included in the Agroclimatic Monitoring Program are periodically inspected to ascertain that equipment is correctly installed and properly exposed. Complete records for all stations are available in the files of the department. Typical agroclimatic station installations are shown in Figure 2.
Agroclimatic Data Analysis Summaries of the agroclimatic data collected during the period from July 195^ through December 196O are shown in Tables 1 and 2. Table 1 shov;s the means of monthly evaporation from standard
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IBLACK PORCELAIN SPHERE!
PORCELAIN STEM
LIVINGbl^,N ATMOMbiEr
RUBBER STOPPER
IGLASS TUBE]
P\'^' VtNT TUBEI
RUBBER STOPPER!
jU'I^TlN plusI
Figure I, ATMOMETER ASSEMBLY
•..ir%A:jsl-^.> ,fi^. . >-#■
station Located
in Irrigated Pasture
near Lodi
">
*
S -T
Station Located
in Dryland
Environment
near Redding
.s'^-..^- >'*-A.*':- •• • "i-^-'" '-;f^' 'i:-:
• 1 t^ li*
^-U i
?k'..>:
f
Station Located in Non- irrigated Alfalfa near Adin, Modoc County
FIGURE 2. TYPICAL AGROCLIMATIC STATIONS
|
TABLE 1 |
|||||||||||||||
|
MEAH MOKTHLY EVAPORAHOB |
FROM |
STANDARD |
|||||||||||||
|
U. |
S. WEATHER BUREAU EVAPORATION PASS |
||||||||||||||
|
(In |
|||||||||||||||
|
Years |
May |
||||||||||||||
|
Qivlronment and area |
: Station |
of Reconl |
Sept. Total |
||||||||||||
|
Mar. |
Apr. |
May |
June |
July |
Aug. |
Sept. |
Ort. |
■ Nov. |
Dec-. |
||||||
|
Pasture |
|||||||||||||||
|
KLamath-Trinlty Mt. Valleya |
2 |
1959-60 |
6.95 |
8.76 |
11.06 |
8.1.1. |
6.18 |
41.39 |
|||||||
|
Sacramentolttver Basin Mountain VaUeys |
9 |
1957-60 |
1.1.8 |
3.25 |
5.10 |
6.16 |
7.69 |
b.96 |
8.78 |
6.16 |
3.86 |
1.66 |
0.66 |
37.75 |
|
|
Sacramento RLver Basin Fbothllls |
1957-60 |
1.52 |
2.29 |
3.56 |
5.19 |
6.10 |
9.15 |
10.66 |
9.27 |
6.1.4 |
5.00 |
2.20 |
1.52 |
41.82 |
|
|
Sacramento River Basin Valley Floor |
12 |
1958-60 |
1.65 |
2.1.9 |
1..01. |
5.1.8 |
7.26 |
10.28 |
10.73 |
9.18 |
6.87 |
5.34 |
2.58 |
1.74 |
44.32 |
|
Son Joaquin River Basin Valley Floor |
11 |
1959-60 |
1.67 |
1..19 |
6.08 |
8.84 |
10.60 |
10.55 |
9.08 |
6.76 |
5.14 |
1.92 |
1.30 |
tv.u |
|
|
Tiili^T^ T«kp Rflflin Vnllpy Plnnr |
6 |
1958-60 |
1.79 |
h.li |
5.76 |
8.77 |
9.71. |
9.36 |
6.00 |
4.24 |
1.96 |
||||
|
lassen-Alplne Mountain Valleys |
6 |
1957-60 |
6.30 |
8.91 |
lo.grr |
9.81 |
6.85 |
4.35 |
- |
42.84 |
|||||
|
Dryland |
SacramentoRlver Basin Mountain Valleys SacramentoRlver Basin Foothills SacramentoRlver Basin Valley Floor
1958-60 — 1.20 2.99
7 1958-60 1.42 2.75
9 1958-60 1.26 2.48
5.98 5.95 10.02 12.03 11.06 7.41 6.52 8.69 13.36 15.04 12.46 10.27 6.52 8.95 13.25 14.03 11.87 9.45
2.09 - 46.47 3.89 2.94 59.82 3.19 1.90 57.55
(in milliliters)
|
: Number : of : Stations |
Years Record |
HOirrHS |
S^t. total |
||||||||||
|
Environment and area |
Jan. |
Feb. |
Mar. |
Apr. |
May |
.June Ju}y Aug. |
Sejt. |
Oct. |
Nov. |
Dec. |
SacramentoRlver Basin Mountain Valleys Sacramento River Basin Foothills Sacramento River BasinValley Floor San Joaquin RLver Basin Valley Floor Tulare Lake Basin Valley Floor Lassen-Alpine Mountain Valleys
-Trinity Mountain Valleys 3
Sacramento River Basin Mountain Valleys 11
Sacramento River Basin Valley Floor 1?
San Joaquin River Basin Valley Floor I3
Tulare laJte Basin Valley Floor 9
Klamath- Trinity Mountain Valleys Sacramento River Basin Mountain Valleys Sacramento River Basin Foothills Sacramento River Basin Valley Floor lassen-Alpine Mountain Valleys
MisceUaneous
River
sin Veilley Floor
|
1955-60 1958-60 1958-60 1959-60 1958-60 1955-60 |
292 324 374 |
|
1955 1955-59 1955,58-60 1958-60 1958-60 |
it 402 |
|
1954-60 1954-60 1959-60 1954-60 1955-56 s, 1958-59 |
in |
|
1954-55, 57, 4 60 |
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|
494 |
572 |
619 |
|
491 |
588 |
6l4 |
|
529 |
569 |
580 |
|
520 |
572 |
580 |
|
460 |
545 |
572 |
|
550 |
558 |
|
|
486 |
537 |
566 |
|
539 |
580 |
618 |
470 548 571 582 548 462 538 589 617 563
|
521 |
584 |
546 |
413 |
|
|
536 |
569 |
540 |
408 |
310 |
|
7?6 |
658 |
593 |
458 |
388 |
|
588 |
655 |
573 |
465 |
366 |
|
582 |
535 |
2765
?703 2754
2510 2523 ?753 2?92
U, S, Weather Bureau pans. Table 2 indicates the mean monthly difference of evaporation between Livingston spherical black and white atmometers.
At the Initiation of the proo'ram in 195^j little was known of the effects of the immediate ground cover environment on evaporation from atmometers and pans. Furthermore, little consideration had ever been given to the effects on evaporation rates of surrounding land areas or cleanliness of pans at sta- tions having apparently similar immediate environmental conditions. In analyzing the data it became apparent that certain of these factors are extremely Important.
In the initial tabulations of evaporation data, great differences were noted between adjacent stations having dissimilar environmental conditions. A tabulation on the basis of station environment shows this to be especially true for evaporation pans, as may be noted in Table 1, For example. Table 1 indicates that the May through September total of the mean monthly evaporation from pans located on dry-farmed rangelands was more than 25 per- cent greater than evaporation from pans situated on irrigated pasture. This difference became increasingly greater during the summer months. The higher and increasingly greater evaporation on dry-farmed rangelands resulted from the greater availability of energy in surrounding dryland areas, and the increase of ad- vectlve heating that results as the drylands exhaust moisture carried over from wintertime precipitation during the summer.
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An interesting fact determined from studies at the Bakersfield station was that cleanliness, or presence of algae growth, had little effect upon evaporation rates from evaporation pans. During an l8-month period starting in January 1959, three pans were maintained in the same environment and were treated in an identical manner, except that algae was permitted to grow in one pan while the other two were cleaned frequently. The dif- ference of evaporation vms small, with only 3 percent greater evaporation in the pan where algae was allowed to grow.
In an evaporation investigation carried on by A, A. Youn in Southern California during the period from 1935 to 1939, inclu- sive, a study was conducted to determine the effect of pan color upon evaporation. He found differences varying from approximately 17 percent less to 7 percent more than from a standard U. S, Weathe Bureau pan. It is of Interest to note that evaporation from a dark green colored pan was 2.5 percent greater than that from the standard U, S, Weather Bureau pan. The presence and growth of algae appear to give similar results.
The difference in evaporation between black and white atmometers, as shown in Table 2, appears to be affected less by environmental conditions than are pans. This indicates a differenc in response bet^^reen pans and atmometers to various climatic condi- tions. This will be discussed further 3n Chapter IV.
Monthly evaporation data from pans and atmometers for each year and for each station are set forth in Tables A-2 and A-3, respectively, of Appendix A, The data are segregated by area and by environment.
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The area designations set forth in this report are arbitrary andj in general, principally geographical subdivisions. When additional years of data become available, these area break- downs must be reconsidered. Analysis of the records of individual stations to date indicates as much variability in evaporation be- tween adjacent stations, within any one area, as between areas. This variability is shoim in Tables 3 and 4, in which all of the stations located on irrigated pasture in 1959 and 196O were arranged in order of decreasing evaporation rate by month. The same was done for the 1959 and 196O dryland stations. On the basis of these data. It is concluded that no definite segregation of the stations into areas of uniform evaporation Is possible.
A general pattern has been discerned with certain of the stations tending to be high and others low. There are indications that, for stations having similar environments immediately sur- rounding the site, adjacent dryland areas exert climatic influences and affect evaporation rates at the station site.
This factor is being given further consideration In rela- tion to the agrocllmatlc stations currently in operation. Efforts are being made to standardize conditions where pan and atmometer data are collected. Insofar as possible, large, well-irrigated pastures providing nearly 100 percent ground cover are being se- lected as sites for agrocllmatlc stations. As data are obtained under similar environmental conditions, more conclusive compari- sons may be made. It may be fo\jnd that there are small differences In monthly evaporative rates between different agricultural areas of the State, and that the length of growing season is the most im- portant factor affecting seasonal evapotranspiration in Inland areas.
-15-
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19
CHAPTER III. EVAPOTRANSPIRATION MEASUREMENT
The objective of the evapotransplration measurement and correlation program Is to provide a more accurate basis for pre- dicting evapotransplration for the major crops In the various agri- cultural areas of the State. This is to be accomplished through measurements of evapotransplration of various crops at several in- land locations having different climatic conditions, and correlating with the evaporative demand, as measured by evaporation pans and atmometers. This chapter discusses the techniques and procedures utilized in the measurement of evapotransplration, and changes that have occurred during the development of the study. In Chapter IV the correlation of the evapotransplration with pan and atmometer evaporation data will be discussed and analyzed.
The principal evapotransplration stations are located near Bakersfield in the southern San Joaquin Valley and near Al- turas and Fall River Mills in mountain valleys of the Sacramento River Basin. Plate 2, entitled "General Location of Evapotranspl- ration Stations, 1955-1960," shows the location, type, and status of each station. More detailed information is given in Tables A-4 and A-5 of Appendix A, Measurements were made primarily on alfalfa and grass, which are grown universally throughout the State. As plant and soil moisture conditions affect evapotransplration rates, evaluation of these factors is also an essential part of evapo- transplration measurement.
-21-
Measurement of Data Related to Evapotranspiratlon
Correlative pan and atmometer evaporation measurements were made at agroclimatic stations established near the evapotran- spiratlon measuring stations. These stations are listed in Table ^ of Appendix A, Detailed information regarding the agroclimatic sta tions, and pan and atmometer data, are given in Tables A-1, A-2, and A-3 of that appendix.
At Arvin (in Kern County) the pan and atmometer data were initially collected at stations (Arvin Jewett #1 and #2) located in an irrigated alfalfa field near the evapotranspiratlon station, or soil moisture depletion plots. In June 1959^ a new station (Arvin-Frick) was established in an irrigated grass environment. All of the soil moisture depletion plots were within 1 mile of this station.
Only atmometer data were collected at the Pittville AA plot site (in eastern Shasta County) during 1959. This agroclimat station is identified as Pittville 1 S. Pan data were collected within an irrigated pasture site, designated as Fall River Mills 4 Nw, during 1959:» and until June I960, but due to unfavorable operational procedures at this site in 1959^ the pan data were not used in this report.
In June I960, an agroclimatic station vms established
at a location within an irrigated pasture 8 miles west of the
Pittville AA plot. This station is identified as Glenburn DWR.
Comparison of atmometer evaporation measurements at the Pittville
1 S and the Glenburn DWR stations showed that the difference in
evaporation between the black and white atmometers is very close
for these two locations.
-22-
At the Arvln and Glenburn sites, three sets of nev; at- mometers were Installed at the berlnning of each season, and each month one pair was replaced with a new pair. At Arvln, three pans were operated, and the evaporation, which v;as nearly the same, was avera,";ed.
Data on percent of r;round cover were collected to deter- mine effects of varying cover on evapotranspiratlon rates. The term "percent ground cover," as used in this report, refers to the percentage of ground surface covered by a canopy of living foliage as viewed looking downward from directly above the crop. During the first years, 1955-1937, few records were kept of percent ground cover. However, from 1958 through 1960 it was standard procedure to measure crop height, estimate percent ground cover, and record both.
When most of the moisture which plants can readily ex- tract from within the root zone has been used, crop growth is slowed and evapotranspiratlon rates may also be correspondingly affected. To estimate available soil moisture at the test plots, samples were taken and laboratory measurements of the moisture content of the soil were made, utilizing the pressure plate mem- brane technique with pressures varying from 0.1 to 15 atmospheres. T ensiometers, instruments ^^^hich can be used to measure availability of soil moisture for crop utilization, were installed at some plots. Calculations of available soil moisture in the root zone were based on the difference between moisture profiles determined from field measurements and moisture profiles representing the moisture level below which crops cannot readily extract moisture.
■23-
Criteria for Selection of Plots In the selection of plots for the measurement of evapo- transplration^, certain physical conditions are recognized as es- sential to collecting valid data. Experience over the years has emphasized the Importance of certain necessary conditions. Un- fortunately, the most ideal plot conditions are difficult to find, and considerable time and effort have been expended over the years selecting the most favorable sites. However, this is not to imply that evapotranspiration rates would necessarily be different under different conditions. The following criteria indicate the condi- tions under which good measurements of evapotranspiration repre- sentative of field conditions can be obtained. After good measurements have been obtained under these conditions, the studie; should be broadened to include some of the adverse soil and other conditions which might affect evapotranspiration.
1. Measurement sites should be located 200 feet inside the edge of the irrigated field to avoid accentuated border effect;
2. Generally, the land should be of smooth topography.
3. Since the sites are located on private lands, it is necessary to have freedom of access and cooperation of the landowner or manager,
4. The soil should be deep, well drained, productive, and unaffected by salinity. The soil preferably should also be medium textured, as very fine or very coarse textures have un- favorable soil-moisture relationships. The soil profile should not be stratified to such an extent as to impede moisture flow or create sampling pi'-oblems.
• 24-
5. The lrrln:ated field should be located in typical lrri,e^ated areas, not on the frlnee of irrigated areas.
6. There should be an adequate supply of irrigation v/ater. It is highly desirable that there be possibilities for controllinr; and measurin,"; the amount of vvater applied to the test plot.
7. Except for measurements which are made by evapo- transplrometers, no water table should exist within or near the root zone of the crop,
Evapotranspiration Measurement Techniques and Discussion of Development and Current Methods
The tools and techniques used in this study to measure
evapotranspiration fall into two general categories. One is field
plot sampling, and the other is evapotranspirometer measurements.
Field Plot Sampling - Gravimetric Method
Periodic measurement of soil moisture provides a means of determining total change of water content within a selected portion of the soil profile. Evapotranspiration may be determined from data on soil moisture change and precipitation. Soil moisture must be sampled or measured each time at or near the same location in each plot, with several locations being situated in each plot. Moreover, the moisture determinations must be made at least twice following wetting of the soil by any heavy irrigation or heavy precipitation. To obtain satisfactory results, it is necessary that sufficient time lapse be permitted following thorough wetting of the soil (usually several days) before making the first moisture
•25-
determination in a cycle of measurements. Otherwise, moisture moving out of the sampled profile would be incorrectly included as evapotranspiration in the soil moisture depletion measurement.
During the growing season, the general procedure was to sample approximately every seven days, except as modified by ir- rigation, harvest, or other cultural (farming) operations. Durin: the nongrowing season, measurements were made less frequently be- cause of the lower rates of v/ater use.
At the initiation of the evapotranspiration measuring program in 1955, the gravimetric technique was accepted as the best method available, and was the first technique employed in the studies reported here. Moisture content of soil samples was determined by weight change resulting from moisture loss during oven drying. Soil samples were taken by means of a soil tube, in 1-foot increments of depth, from the soil surface to a depth of 7 or 9 feet. As the soil tube is difficult to handle at depth below 9 feet, sampling below that depth was attempted only in special cases where knowledge of the substratum conditions was desired.
The initial evapotranspiration measurements were made in the mountain valley areas in the northern and northeastern part of the State, and in the northern Sacramento Valley. The objectiv at that time was to determine the irrigation requirements of only those areas. Plots in the mountain valleys were located on typica irrigated parcels of land. The irrigated lands in this area exist as narrow and isolated "oases" separated by large areas of native vegetation.
-26-
From three to eight core holes were made per sampling. This number did not prove to be adequate because of Inherent variability of the soils.
During analysis of data collected during the 195:? season, it was determined that observation holes should have been main- tained at all plots to determine if water table conditions existed. Through observation holes on a few of the plots, and examination of soil samples taken from the lower profiles, it was found that water tables did exist on some plots where they were not expected. When a water table is present in or near the root zone, there is a probability that the crop will utilize some of this source of moisture. It is impossible to determine this amount.
The greatest problem, however, was that irrigation in some cases added too much water, and in other cases was too in- frequent or too little. As previously mentioned, when too much water is applied, downward moisture movement continues for a con- siderable length of time. A series of field moisture measurements will include both moisture movement, or change, due to plant ex- traction and evaporation. If too little water is applied, the soil moisture may become critically short, and crop growth may be affected. If the soils become very dry, the evapotranspira- tion rate may also be affected.
For the next several seasons, work was concentrated on fewer plots, and more detailed observations were made of crop growth, presence of water tables, and other conditions. As the need for irrigation control became recognized as being critical
-27-
to obtain adequate evapotransplratlon data from soil moisture de- pletion measurements, attempts to modify Irrigation were initiated.
It was observed that weekly visits to plot sites ad- verseley affected the crop cover and soil conditions by trampling the crop. To overcome these undesirable effects, a portable sampling platform v/as built in 1956 to sample one of the plots. This was the forerunner of platforms which were used later with neutron probes.
In 1957^ the water use studies were expanded to other areas of the State, Alfalfa fields were sampled in Stanislaus and Kings Counties. These plots were abandoned because data ob- tained from them were not considered reliable for estimating evapo- transplratlon because of excessive moisture movement that resulted from overirrigation at the Stanislaus County plots and unfavorable soil conditions at the plots in Kings County.
In 1958, one man was stationed in Kern County following a reconnaissance for plot sites. Plots of alfalfa, grapes, and plums were sampled. Problems of two kinds were encountered. On plots receiving lesser quantities of irrigation, the crops ex- tracted moisture from below the zone sampled, while on plots re- ceiving very frequent irrigations, considerable moisture movement occurred between sampling.
No further gravimetric samples were taken following the adoption of neutron scattering equipment in the spring; of 1959. While complete detailed records were kept and calculations made for each of the gravimetric sampling sites, the results of these measurements are not Included in this report,
-28-
Field Plot Sampling - Neutron Scattering Technique
A recently developed method to obtain ttie soil moisture data, referred to as the neutron scattering technique. Is based upon the principle that high energy or "fast" neutrons are mod- erated, or "slowed down," in soils almost exclusively by hydrogen atoms contained in soil moisture. The instrument consists of a source of "fast" neutrons, a detector tube which Is sensitive only to "slow" neutrons, and a slow neutron counter. Both source and detector are combined in a cylindrical probe 1.5 Inches in diameter by l4 Inches long. The probe is lowered Into the soil through a small-diameter, cased hole to the desired depth, sus- pended by its electrical cable. The cable is connected to the counting device which counts pulses produced by slow neutrons returning to the detector. Since the "fast" neutron output of the source is essentially constant, the count recorded in a fixed time period may be used with a suitable calibration to determine the moisture content In the soil surrounding the probe.
The neutron scattering technique has certain advantages over the gravimetric technique. In addition to the ease of making deeper measurements, the neutron measurements take less time, re- peatedly represent approximately the same soil mass, and are gen- erally more precise than gravimetric measurements. Measurement of the same soil mass is particularly important, since soil moisture distribution and extraction patterns appear to be nonuniform. It must be noted, however, that overirrlgation and resulting moisture movement remain a problem with this method. Also, for greater
.29-
accuracy, measurements of the soil surface layer, to a depth of about 1 foot, require a different calibration than the measure- ments at greater depth below the soil surface. Determination of a suitable calibration is under study by the department and other agencies at this time. It is believed at the present that the er^ ror of measuring the losses of water from the soil surface is not large, considering the total water use from the entire profile, in the case of deeper rooted crops.
Inherent variabilities, such as found in physical measure ments of any natural phenomenon, occur in soil moisture depletion measurements. Generally, although affecting any given measurement, such variations tend to be compensating and, over a period of time, such as a year, tend to cancel out.
Two neutron scattering devices were acquired in 1958^ shortly after this equipment became commercially available. The neutron equipment was used for determination of soil moisture in all field plots since early spring of 1959. The same criteria used for selection of gravimetric sampling plots were followed in establishing the plots sampled with the neutron probe.
Effort was made to keep the crop in the plot area gen- erally typical of the normal conditions of the entire field. Light weight, portable sampling platforms with working areas of 15 to 30 square feet were fabricated in 1959 to carry the neutron scattering equipment. These also served as portable working platforms. They have been particularly advantageous in facilitating the field work and in avoiding trampling and injury to the alfalfa and grass crops
■30-
and compaction of the soils. Three types of portable sampling platforms used at Fall River Mills and Bakersfield are shovm in Figure 3.
To provide neutron probe access into the soil, thin- walled aluminum tubes 20 feet in length with removable l8-inch extensions at the top were permanently installed flush with the soil surface. Stoppers were placed in the tube at the surface and immediately below the extension to exclude foreign material from the tubes. In this way, the tubes did not extend above the ground to Interfere with tillage and crop cultural operations. When tillage operations damaged the upper extensions, they were simply and easily replaced. The access tube design is shown in Figure 4.
Plttvllle Neutron Probe Moisture Depletion Measurements. The Plttvllle site is located at an elevation of about 3^340 feet, in the northeastern intermountaln region, at a latitude of 4l de- grees. Selection of the neutron measurement site was preceded by four years of gravimetric sampling in the Fall River Valley and other mountain valleys in the northeastern area. The Plttvllle 1 S site was sampled using the gravimetric technique in 1956, 1937 i and 1958. This prior experience indicated that the Plttvllle al- falfa field possessed the desirable combination of soil and irri.- gatlon conditions for a moisture depletion plot. Topographically, the site is gently sloping with small swales. There is a small ridge 6OO feet north of the plot site, which is about 100 feet higher than the plot. The land at the plot site slopes 3 percent
•31-
Platform Developed in Early Stage of Program for Obtaining Soil Cores
Small, Wheeled
Platform Used to
Measure Soil
Moisture Depletion
by the Neutron
Scattering Technique
Aluminum Platform Used with the Neutron Scattering Equipment
FIGURE 3. PLATFORMS USED TO MINIMIZE CROP DAMAGE AND SOIL COMPACTION
^
V/
1
i
No.9 Rubber Stopper
18"- |5/8" Aluminum Tubing (Top Flush With Soil Surface]
2"-l'/2" Plastic Sleeve To Join Tubing (Heated To Install)
No.8 Rubber Stopper (With Hook For Removing)
20' -I Vs" Aluminum Tubing Bottom End Capped
Figure 4, ACCESS TUBE DESIGN
to the south-southwest, and the 30 acres of alfalfa In the field are surrounded by small irrigated fields, dry- farmed grain, and native vegetation. Prevailing winds in the area are from the west
Initially, three rows of five access tubes were installec 73 feet apart, with the tubes spaced in the rows 15 feet apart. In September 1959^ four more tybes were installed in one of the rov;s, and the other two rows abandoned, reducing the plot to nine tubes. This enabled the plot to be irrigated in two days, rather than the three to four days required for the sprinklers to pass over the original three rows of access tubes.
Irrigation water is applied by a portable sprinkler system, using full circle (360 degrees) rotating sprinklers. The sprinklers sometimes stuck in one position, and irrigation applica tion, as a result, was not uniform enough to determine applied wat< from pumping records. This plot was subjected to somewhat deficit irrigation, which left a dry zone generally below a depth of 8 fee' For this reason, the soil moisture measurements can be used with confidence as estimate of evapotranspiration.
Neutron moisture depletion measurements were made during 1959 and 196O at another alfalfa site 3 miles west of the Pittvill«' plot. Due to apparent excessive moisture movement, however, the rt suits of these measurements are not included in the report.
Arvin Neutron Probe Moisture Depletion Measurements. These measurement sites are in the southern San Joaquin Valley, near the 35 degree latitude, located at an elevation of about hkO feet. The plot sites are on broad, smooth, recently formed fans
-34-
from the outwash of the Sierra Nevada Ranre at the southern end of the valley. The land slopes to the southwest at the plot area at about 30 feet per mile (0.6 percent).
Irrigation in the area is supplied from deep wells lifting water several hundred feet. All of the Arvin plots are located on Hesperia fine, sandy loam. This soil has no apparent clay or cemented layers. Moisture drainage is good, Noncontinuous silt layers and pockets of silt of varying thickness are found from 3 feet down to 22 feet below the surface. Plot sites were located where the least amount of silt layers are found. Sur- rounding the sampling areas were irrigated orchards, vineyards, alfalfa, cotton, and other crops. The irrigated area extends 20 miles to the north, 15 miles to the east, kO miles to the south, and 60 miles to the west.
Four crops, cotton, alfalfa, plum orchard, and fescue grass, were sampled. All sites were irrigated by furrow or border methods. In order to obtain reasonably precise data, more than 20 sampling tubes were installed on the cotton and alfalfa. Six tubes each were installed on the plums and grass plots for ex- ploratory purposes, the intent being to determine moisture extrac- tion patterns.
The plum orchard is planted on a 24-foot square pattern. Water is applied to five or six straight furrows running in one direction. Results of the neutron probe measurements indicate that the extraction of moisture is greatest from the furrov/ area near the trees, intermediate from the middle furrows, and least from the soil in the tree rov;s. Extraction was noted to a depth of l6 feet. Depth of extraction probably depends largely on Irrigation practices,
-35-
On the grass plot, the moisture was extracted primarily from the upper 2 or 3 feet. With such a large portion of the total water use from such shallov/ depths, the inherent uncertainty of surface neutron probe measurements assumes greater importance. It has been concluded that the neutron scattering technique is not well- suited for measuring evapo trans pi rat ion of grasses due to their shallow moisture extraction patterns and frequent irrigations. Plan have been made to use evapotranspirometers on this crop.
On the alfalfa plot, ample tubes were sampled to obtain a good estimate of moisture depletion.
On the cotton plot, three sets of seven tubes each were placed at the upper, middle, and lower ends of the 440- foot furrow runs. The tubes were placed diagonally, crossing the rows, such that the tubes were located in the plant row, and in the furrow bottoms and furrow shoulders. The number of tubes was adequate to determine moisture change with good precision.
Cotton is not normally overirrigated, which is an advantag in soil moisture depletion studies, since soil moisture movement is not as much a problem in data interpretation as with most other crop Portable water meters were used to measure the water applied to the cotton. These measurements confirmed the seasonal depletion record obtained from the neutron probe measurements.
Evapo transpirometer Measurements
Evapotranspirometers, sometimes referred to as lysimeters, are instruments designed for the measurement of evapotranspiration. They can be of various shapes, sizes, and designs. Essentially,
•36-
they are devices which enable the evaluation of the moisture recime of a confined soil mass, of known dimensions, in which a crop is grown. Moisture changes of the crop- soil system are determined by periodic or continuous welBhing, or by volumetric determination of water displaced, added, and/or removed from the system.
When used for the determination of field evapotranspiration. it is particularly important that the tanks be installed in such a manner that their presence does not modify the environment of the measured crop. Although this technique appears to be an excellent method for precise measurement of crop water use, certain factors, such as the artificial restriction of crop rooting and possible modification of soil heat transfer, have yet to be completely evalua- ted. Research on these factors is presently being conducted by the University of California.
The use of evapot ran spirometers in the field was not com- mon in California at the Initiation of this program, although tanks had been used in the 1920's and 1930' s. Because soil moisture de- pletion studies are not adapted to crops frequently irrigated or having high water tables, small evapotranspirometers were installed to provide a reliable measure of evapotranspiration under those conditions .
Alturas-Dorris Ranch Evapotranspirometer Measurements. In 1956, two small evapotranspirometers were Installed near Alturas to measure evapotranspiration from high water table pasture. The plot site was In an irrigated meadow pasture containing high moisture favoring grasses, legumes, and broad-leafed plants found in improved
■37-
irrigated pasture mixes and in native mountain meadows. The pas- ture was grazed nearly continuously by cattle, and was usually short but fully covered the ground. Typical percentages of green growin-: leaf surfaces were as follows: In April, 40 percent, increasing to 100 percent by the end of the month; May through September, 100 per- cent; October, 100 percent, decreasing to 50 percent by the end of the month. Cover of green foliage varies between zero and 40 per- cent during the winter, depending to a large extent upon the severit of the winter. In milder v/inters, some green live shoots survive, while in severe winters the foliage is completely Inactive, and the green color is gone.
The evapotranspirometer site v;a5 enclosed by a barbed- wire fence forming a 25- by 75- foot rectangle. Inside the fenced area the grass was mowed several times during the season to main- tain approximately a 5-inch height. Two cylind-^ical steel evapo- transplrometers, 36 Inches in diameter and 30 Inches deep, were installed in the soil within a fenced area, one at each end. Also, inside the plot were a hygrothermograph and evaporation pan, at- mometers, phyheliometer, and a precipitation gage.
V/ater was supplied to the evapotranspirometers by means of a steady, small flow, at a rate calculated to exceed evapotran- spiratlon. It took approximately one week to utilize the water from a cylindrical supply tank 5 feet deep and l8 Inches in diameter. A discharge tube was attached to the evapotranspirometer 6 inches be- low the ground surface, and the excess water not consumed in the transpirometer spilled into a buried sump tank, where it was measurei
The numerous mechanical problems encountered during the first 2.5 years rendered the collected data of questionable validity
-38-
Therefore, these data have not been used for this report. The data collected In 19[?9 and i960 are considered representative of evapotransplration from hlp;h water table meadov; pasture, and are reported herein.
Coleville Evapotransplrometer Measurements. In 1957, data on high water table meadow pasture were collected from an evapotransplrometer tank near Coleville in the Lassen-Alpine area. The measurement site was located at the eastern ed^e of the State, at a latitude of about 38° 30", at an elevation of 5,100 feet. The site was similar to the Alturas-Dorris Ranch site in vegetation and in Irrigation methods. The field was subject to long irrigations by wild flooding. The v;ater level at this site varied from O-16 inches below the ground surface, and was usually about 8 inches below the ground surface. A cylindrical steel evapotransplrometer, 36 inches in diameter by 3 feet deep, was installed. Water was supplied to the evapotransplrometer from a supply tank floated on the water table surrounding the evapotransplrometer. With this system the water table inside the evapotransplrometer was kept at essentially the same level as that in the field. Moisture utilised by the plants was constantly replaced from the supply tank. The level of the supply tank v;as recorded on a Stevens v/ater stage re- corder. The field water table level was also measured on a separate recorder. By integration of the two charts, the rate of evapotranspl- ration was determined*
The topography in the area is smooth, with a 2 percent northerly slope. Data were collected at this site for one season in connection with an investigation of water use in watersheds in the eastern Sierra Nevada.
-39-
Figure 5 shows diagraminetlcally the functioning of the Alturas-Dorris Ranch and Coleville evapotranspirometers .
Davis Evapotranspirometer Measurements. In 1958, three sinall evapotransplrometers 2 feet In diameter v;ere installed at Davis in cooperation v/ith the Department of Irrigation of the Uni- versity of California. The purpose v;as to determine how well thej small tanks would compare v;ith a large 20-foot diameter tank, whic was installed "by the university in 1958. Over a 10-month period,, the mean evapotransplration from the 2- foot evapotransplrometers differed less than 5 percent from the 20- foot evapotranspiromete: One reason for this favorable comparison is that both kinds of tanks were located in the same field environment having a con- tinuous, uniform crop height and cover in and around the tanks. The data from the 2-foot evapotransplrometers are presented in thi report.
Evapotransplration Data Summary Summaries of evapotransplration for measured and esti- mated periods are tabulated in Table 5, v/lth corresponding measure ments of pan and atmometer evaporation. Evapotransplration for missing periods v;as usually estimated as the pix)duct of approprlat pan or atmometer coefficients, and pan or atmometer evaporation data collected during these periods, plus calculated increments fc surface evaporation follov/ing irrigation. Monthly evapotranspirat.d totals have been computed and are also presented in Table 5- A detailed tabulation of evapotransplration and related data are pre- sented in Tables A-6 and A-7 of Appendix A, for the approximatelv v/eekly measurement schedule. Variability of soil moisture values
-40-
pDropping Orifice Clock 11 III Recorder
High Water Table Meadow Grassland
Recorder
NOTE- Intake and outflow columns approximate 2" holes inside tank
EVAPOTRANSPIROMETER 36"CIRCULAR TANK
OUTFLOW TANK
ALTURAS- DORRIS RANCH EVAPOTRANSPIROMETER
High Water Table Meadow Grassland
1 I I I I / / I I I I I i I I 11/ /L
lllllll I I II
Water Table
COLEVILLE EVAPOTRANSPIROMETER
Figure 5, EVAPOTRANSPIROMETER DESIGN
TABLE 5
SUMMARY OP MEASUREMENTS OF EVAPOTRANSPIRATION AND RELATED DATA
Year : Month
Evapotransplratlon
Pan eT&poratlon
Meas- : Cstl- : Accum. :Monthly : Eaoh lAcoum. : Monthly ured : mated : totals : est. t period :total8 : est.
Atmometer avaporat
Eaoh :Aooum. :Mon period : totals : e»
Saeramento River Mountain Valley Pasture - Alturas - Dorrls Ranoh
Maj
June
July Aug. Sept.
Oct.
i960 Apr. May Jun*
July Aug.
Sept. Oct.
U/7 - 1+/30 3/31- V30
V30- 5/31
5/31- 6/30
6/2 - 6/30
6/30- 7/31 7/31- 8/31 8/31- 9/30
8/31- 9/22 9/30-11/2
V7
6/2
V8 - 5/1 5/1 - 5/31 5/31- 6/7 6/7 - 6/14 6/14- 6/21 6/21- 6/28 5/31- 6/30 6/28- 8/1 6/30- 7/31 8/1 - 8/31 7/31- 8/31 8/31- 9/30 9/30-10/31 9/30-10/3 10/31-11/21 11/21-12/1 10/31-11/30
12/1 -12/31
11/30-12/31
4.03
5.99 8.95 (8.33)
10.45 9.04
4.90
(3.78)
3.02
.11/2 46.38
. 9/22 31.60
2.33 4.61
10.33 8.99
6.01
3.56 (0.47)
0.17 0.75
1.96
1.96
|
4.03 |
5.15 |
'♦.35 |
'*.35 |
5.54 |
||
|
10.02 |
5.99 |
6.09 |
10.44 |
6.09 |
||
|
18.97 |
8.95 |
7.94 |
18.38 |
7.9'+ |
510 |
510 5 |
|
29.42 |
10.45 |
9.83 |
28.21 |
9.83 |
645 |
1,155 6 |
|
38.46 |
9.04 |
8.65 |
36.86 |
8.65 |
547 |
1,702 5 |
|
43.36 |
4.90 |
5.U1 |
42.27 |
5M |
315 |
2,017 |
|
46.38 |
2.85 |
3.80 46.07 |
46.07 |
3.59 |
2,017 |
|
|
2.33 |
2.99 |
3.50 |
3.50 |
'+.53 |
||
|
6.94 |
4.78 |
5.71 |
9.21 |
5.90 |
||
|
8.90 |
1.96 |
|||||
|
10.31 |
1.73 |
12.90 |
129 |
129 |
||
|
12.27 |
1.96 |
116 |
||||
|
13.89 |
6.56 |
1.98 |
16.84 |
8.00 |
140 |
385 |
|
24.22 |
9.6I |
9.45 |
26.29 |
8.82 |
608 |
993 |
|
33.21 |
9.31 |
8.02 |
34.31 |
8.31 |
48o |
1,473 |
|
39.22 |
6.01 |
5.91 |
40.22 |
5.91 |
421 |
1,894 u |
|
42.78 |
3.56 |
3.81 |
44.03 |
3.81 |
43 |
1,934 |
|
43.49 |
0.76 |
|||||
|
43.66 |
0.51 |
0.38 |
1*5.17 |
0.76 |
||
|
44,4l |
0.78 |
0.62 |
^5.79 |
0.65 |
4/6 6/7
•12/31 •10/3
39.78 28.83
1,821
-42-
TABLE 3 (oontlnuad)
SUMMARY OF MEASUREMENTS OF EVAFOTRANSHIRATION AND REUTED DATA (Continued)
|
. |
; ; |
Evapot ranspl rat 1 on |
Pan evapo |
ration : Atraometer evaporation |
||||
|
Meas- |
Estl- : |
Acoua. |
: Monthly |
Eaoh |
:Acoum. |
: Monthly : Each :Accum. : Monthly |
||
|
Y.ar : Month |
: Period : |
ured |
■lated : |
total. |
: est. |
period |
: totals |
: est. : period : totals : est. |
|
Saeramento |
Uver Basin Valley Floo |
|||||||
|
Pasture - Davis Caopball |
||||||||
|
1959 Jan. |
12/31- 2/2 12/31- 1/31 |
I.U2 |
1.42 |
1.15 |
2.03 |
2.03 |
1.65 |
|
|
Fab. |
2/2 - 2/27 1/31- 2/28 |
2.27 |
3.69 |
2.56 |
2.18 |
4.21 |
2.65 |
|
|
Mar. |
2/27- Vi ?/28- 3/31 |
4.1+5 |
8.14 |
4.31 |
6.57 |
10.78 |
6.32 |
|
|
Apr. |
Vi - '+/16 |
2.51 |
10.65 |
4.26 |
15.04 |
|||
|
Vl6- 4/30 |
1.82 |
12.47 |
2.23 |
17.27 |
||||
|
3/31- V30 |
4.45 |
7.55 |
||||||
|
May |
U/30- 5/14 |
2.87 |
15.34 |
4.34 |
21.61 |
|||
|
5/14- 5/21 |
1.56 |
16.90 |
2.56 |
24.17 |
||||
|
5/21- 5/28 |
1.06 |
17.96 |
1.91 |
26.08 |
||||
|
V30- 5/31 |
6.05 |
9.75 |
||||||
|
June |
5/28- 6/8 |
2.19 |
20.15 |
3.67 |
29.75 |
|||
|
6/8 - 6/15 |
1.67 |
21.82 |
2.23 |
31.98 |
||||
|
6/15- 6/30 |
4.08 |
25.90 |
5.72 |
37.70 |
||||
|
5/31- 6/30 |
7.38 |
11.02 |
||||||
|
July |
6/30- 7/29 6/30- 7/31 |
8.11 |
34.01 |
8.74 |
10.74 |
48.44 |
11.15 |
|
|
Au«. |
7/29- 8/31 7/31- 8/31 |
7.65 |
41.66 |
7.02 |
9.88 |
58.32 |
9.13 |
|
|
S.pt. |
8/31- 9/3 |
0.76 |
42.42 |
0.59 |
58.91 |
|||
|
9/3 -10/2 |
5.63 |
48.05 |
8.23 |
67.14 |
||||
|
8/31- 9/30 |
5.97 |
8.14 |
||||||
|
Oot. |
10/2 -11/2 9/30-10/31 |
4.26 |
52.31 |
4.56 |
6.66 |
73.80 |
7.11 |
|
|
Not. |
11/2 -12/5 10/31-11/30 |
2.12 |
54.43 |
1.92 |
4.19 |
77.99 |
3.53 |
|
|
Dae. |
12/5 -12/31 11/30-12/31 |
0.88 |
55.31 |
(1.25) |
1.68 |
79.67 |
2.65 |
|
|
TOTALS "/31/58-12/3a/55 |
44.50 |
64.90 |
||||||
|
i960 Jan. |
12/31- 1/30 12/31- 1/31 |
0.84 |
0.84 |
0.88 |
1.48 |
1.48 |
1.63 |
|
|
Feb. |
1/30- 2/26 1/31- 2/29 |
1.53 |
2.37 |
1.78 |
2.86 |
4.36 |
3.08 |
■^3-
TABLE 5 (eontinued)
SUMMARY OF MEASUREMENTS OF EVAPOTRANSPIRATION AND REUTED DATA (Continued)
TOTALS 5/27- 9/23 28.52 6/10- 9/23 25.50
Saoramento River Basin Mountain Valleys Alfalfa - Fall River Mills - Plot AA
1959 Mar. 3/17- U/8 1.92 1.92
3/17- 3/31
Apr. 4/8 - '4/23 3.86 5.78
V23- V30 1.34 7.12
3/31. I+/30
May V30- 5/6 1.33 8.45
5/6 - 5/28 U.2I+ 12.69
'+/3"- 5/31
Juno 5/28- 7/2 8.48 21.17
5/31- 6/30
July 7/2 - 7/6 1.27 22.44
7/6 - 7/27 6.41 28.85
7/27- 7/31 0.95 29.80 6/30- 7/31
31.7'+
1,977
6.76 6.86
9.o4
715
715
86 801 463 1»264 70 1,33'*
|
. |
Evapotransplratlon |
Pan evapo |
ration |
Atoiomtter evaporatH |
|||||||
|
Meas- |
Esti- |
Acoum. |
: Monthly |
Eaoh |
:Accua. |
: Monthly |
Each |
: Ac cum. |
:Moir |
||
|
Year : Month |
: Period : |
ured |
mated |
totals |
: est. |
period |
: totals |
: est. |
period |
: totals |
: ar |
|
i960 Mar. |
2/26- 3/31 2/29- 3/31 |
3.35 |
5.72 |
3.06 |
4.21 |
8.57 |
3.86 |
||||
|
Apr. |
3/31- V29 4/18- 4/29 3/31- 4/30 |
4.85 (1.73) |
10.57 |
5.27 |
5.65 |
14.22 |
6.11 |
138 |
138 |
||
|
May |
4/29- 6/1 4/30- 5/31 |
7.50 |
18.07 |
7.10 |
9.20 |
23.42 |
8.74 |
570 |
708 |
||
|
June |
6/1 - 7/1 5/31- 6/30 |
5.87 |
23.94 |
5.87 |
12.02 |
35.44 |
12.02 |
607 |
1,315 |
||
|
July |
7/1 - 7/19 |
4.25 |
28.19 |
6.44 |
41.88 |
390 |
1,705 |
||||
|
TOTALS |
12/31- TM 4/18- 7/19 |
28.19 19.35 |
41.88 |
1,705 |
|||||||
|
Lassen - Alpine Mountain Valleys |
|||||||||||
|
Pasture - Colevlll. - 2E |
|||||||||||
|
1957 May |
5/27- 6/3 |
1.34 |
1.34 |
1.38 |
1.38 |
||||||
|
June |
6/3 - 6/30 5/31- 6/30 6/10- 6/30 |
6.91 (5.23) |
8.25 |
7.53 |
7.31 |
8.69 |
7.95 |
434 |
434 |
||
|
July |
6/30- 8/1 |
9.12 |
17.37 |
9.12 |
9.33 |
18.02 |
9.33 |
601 |
1,035 |
6c |
|
|
Aug. |
8/1 - 8/31 |
7.76 |
25.13 |
7.76 |
9.09 |
27.11 |
9.09 |
583 |
1,618 |
5f |
|
|
Sept. |
8/31- 9/23 |
3.39 |
26.52 |
4.63 |
31. 7H |
359 |
1,977 |
-44-
TABLE 5 (oontlnued)
SUMMARY OP MEASUREMENTS OF EVAPOTRANSHIRATION AND RELATED DATA (Continued)
|
t : : : |
:vapotran«piratio |
n |
Pan evaporation |
I Atraometer evaporation |
|||||||
|
Meas- |
Esti- |
Aooum. |
: Monthly |
Eaoh |
:AceuB. |
: Monthly |
: Eaoh |
:Acoum. |
: Monthly |
||
|
Y«ar : Month |
: Period i |
ured |
mated |
totals |
: est. |
period |
I totals |
: est. |
: period |
: totals |
: est. |
|
1?5> *"«• |
7/31- 8/3 8/3 - 8/14 8/1I4. 8/31 7/31- 8/31 |
1.81 |
0.86 3.58 |
30.66 32.47 36.05 |
6.25 |
59 232 355 |
1,393 1.625 1,980 |
646 |
|||
|
S.pt. |
8/31- 9/3 9/3 - 9/15 9/15- 9/30 8/31- 9/30 |
3.'+9 |
0.92 2.51 |
36.97 40.46 42.97 |
6.92 |
65 207 209 |
2,045 2,252 2,461 |
481 |
|||
|
TOTALS |
3/17- 9/30 5/28. 9/30 |
30.21 20.19 |
1,617 |
||||||||
|
Sacramento River Basin Mountain Valleys |
|||||||||||
|
Alfalfa - Pall River Mills |
- Plot |
AA |
|||||||||
|
i960 Mar. |
3/10- VI9 3/10- 3/31 |
3.71 |
3.71 |
1.90 |
5.91 |
5.91 |
3.80 |
||||
|
Apr. |
V19- 5/11 3/31- H/30 |
2.39 |
6.10 |
2.83 |
3.82 |
9.73 |
4.65 |
||||
|
May |
5/11- 5/19 5/19- 6/3 4/30. 5/31 |
3.31 |
2.37 |
8.47 11.78 |
6.42 |
2.25 2.91 |
11.98 14.89 |
6.67 |
|||
|
June |
6/3 - 6/24 6/10- 6/24 |
3.29 (1.21) |
15.07 |
6.20 |
21.09 |
275 |
|||||
|
6/24- 6/30 |
1.28 |
16.35 |
1.64 |
22.73 |
121 |
396 |
|||||
|
5/31- 6/30 |
5.57 |
8.69 |
588 |
||||||||
|
July |
6/30- 7/?5 |
7.85 |
24.20 |
8.61 |
31.3'+ |
492 |
888 |
||||
|
7/25- 8/1 |
0.92 |
25.12 |
2.09 |
33.43 |
114 |
1,002 |
|||||
|
6/30- 7/31 |
8.40 |
10.40 |
584 |
||||||||
|
Aug. |
8/1 - 8/5 |
1.06 |
26.18 |
1.33 |
34.76 |
80 |
1,082 |
||||
|
8/5 - 8/26 |
6.30 |
32.48 |
6.41 |
41.17 |
375 |
1,457 |
|||||
|
8/26- 9/1 |
1.52 |
34.00 |
1.46 |
42.63 |
97 |
1,554 |
|||||
|
7/31- 8/31 |
8.88 |
9.20 |
550 |
||||||||
|
Sept. |
9/1 - 9/28 8/31- 9/30 |
4.55 |
38.55 |
5.10 |
5.41 |
48.04 |
5.85 |
425 |
1,979 |
452 |
|
|
Oot. |
9/28-11/8 9/30-10/31 |
4.82 |
'♦3.37 |
3.80 |
5.05 |
53.09 |
3.97 |
3/10-11/8 36.22 6/10- 9/28 19.91
44.32
1,567
-45-
|
TABLE 5 (oontinued) |
|||||||||||
|
SUMMARY OF |
MEASUREMENTS OF EVAPOTRANSPIRATION |
||||||||||
|
AND |
RELATED |
DATA (Continued) |
|||||||||
|
: |
Evapotranspirati |
on |
^n evaporation |
Atmometer evaporat! 1 |
|||||||
|
: |
Meas- |
Eati- |
: Accuo. |
: Monthly |
Each |
:AccuB. |
: Monthly |
Each |
: Acoura. |
:Mon1 |
|
|
Year : Month |
I Period |
ured |
: mated |
: totals |
: est. |
period |
2 totals |
: est. |
: period |
« totals |
: edi |
|
Tulare Lake |
Basin Valley Floor |
||||||||||
|
Alfalfa - Arvln - Plot CC |
|||||||||||
|
1959 Mar. |
3/13- 3/27 |
1.07 |
1.07 |
2.28 |
2.28 |
186 |
186 |
||||
|
3/27- V3 |
1.06 |
2.13 |
1.35 |
3.63 |
106 |
292 |
|||||
|
- 3/31 |
4.53 |
3: |
|||||||||
|
Apr. |
V3 - W21 |
2.54 |
U.67 |
4.16 |
7.79 |
293 |
585 |
||||
|
V21- V28 |
1.16 |
5.83 |
1.77 |
9.56 |
100 |
685 |
|||||
|
3/31- 4/30 |
4.49 |
7.00 |
4; |
||||||||
|
May |
V28. ^/Ik |
2.3U |
8.17 |
3.82 |
13.38 |
244 |
929 |
||||
|
5/14- 5/25 |
1.31 |
9.48 |
2.96 |
16.34 |
172 |
1,101 |
|||||
|
5/25- 6/1 |
I.3U |
10.82 |
2.58 |
18.92 |
123 |
1,224 |
|||||
|
V30- 5/31 |
4.54 |
8.69 |
4: |
||||||||
|
June |
6/1 - 6/9 |
2.06 |
12.88 |
2.45 |
21.37 |
148 |
1,372 |
||||
|
6/9 - 6/15 |
0.93 |
13.81 |
1.81 |
23.18 |
112 |
1,484 |
|||||
|
6/15- 6/22 |
1.00 |
11+.81 |
2.13 |
25.31 |
142 |
1,626 |
|||||
|
6/22- 6/29 |
1.45 |
16.26 |
2.22 |
132 |
1,758 |
||||||
|
5/31- 6/30 |
5.80 |
9.06 |
5: |
||||||||
|
July |
6/29- 7/3 |
1.09 |
17.35 |
1.58 |
29.11 |
88 |
1,846 |
||||
|
7/3 - 7/8 |
0.90 |
18.25 |
1.54 |
30.65 |
102 |
1,948 |
|||||
|
7/8 - 7/17 |
1.20 |
19.45 |
2.71 |
33.36 |
170 |
2,118 |
|||||
|
7/17- 7/22 |
1.10 |
20.55 |
1.37 |
34.73 |
84 |
2,202 |
|||||
|
7/22. 7/29 |
1.70 |
22.25 |
2.30 |
37.03 |
126 |
2,328 |
|||||
|
6/30- 7/31 |
6.34 |
9.95 |
51 |
||||||||
|
Aug. |
7/29- 8/8 |
2.38 |
24.63 |
2.83 |
39.86 |
168 |
2,496 |
||||
|
8/8 - 8/13 |
0.39 |
25.02 |
1.67 |
41.53 |
102 |
2,598 |
|||||
|
8/13- 8/27 |
2.56 |
27.58 |
3.50 |
45.03 |
220 |
2,8l8 |
|||||
|
7/31- 8/31 |
6.07 |
8.67 |
5' |
||||||||
|
Sept. |
8/27- 9/15 |
3.86 |
31.44 |
3.96 |
48.99 |
312 |
3,130 |
||||
|
9/15- 9/22 |
1.30 |
32.74 |
1.40 |
50.39 |
106 |
3,236 |
|||||
|
9/22-10/2 |
I.7U |
34,48 |
2.11 |
52.50 |
158 |
3,394 |
|||||
|
8/31- 9/30 |
5.27 |
5.93 |
4; |
||||||||
|
Oct. |
10/2 -10/9 |
0.90 |
35.38 |
1.04 |
53.54 |
96 |
3,490 |
||||
|
10/9 -10/21 |
1.29 |
36.67 |
1.72 |
55.26 |
172 |
3,662 |
|||||
|
10/21-11/3 |
i.oU |
37.71 |
1.65 |
56.91 |
135 |
3,797 |
|||||
|
9/30-10/31 |
3.22 |
4.49 |
4; |
||||||||
|
Nov. |
11/3 - 12/2 10/31-11/30 |
2.52 |
40.23 |
2.76 |
2.49 |
59.40 |
2.69 |
||||
|
Deo. |
12/2 - 1/5 11/30-12/31 |
2.01 |
42.24 |
1.87 |
1.76 |
61.16 |
1.68 |
||||
|
TOTALS |
3/13- 1/5 3/13-10/21 |
23.78 19.25 |
- |
34.28 |
2,069 |
||||||
|
-46- |
TABLE 5 (oontlnuvd)
SUMMARY OF MEASUhEMENTS OP EVAPOTRANSPIRATION AND REUTED DATA (Contlnuad)
|
: |
= |
Evapotraneplratlor |
^an evaporation |
Atmooeter evaporation |
|||||||||
|
Meas- |
: Estl- |
Aoouii. : |
Monthly |
Eaoh |
:Accuiii. |
: Monthly |
Eaoh |
:Accua. |
: Monthly |
||||
|
Y>ar |
■.Month |
: Perl |
od |
ured |
: mated |
totale : |
est. |
period |
: totals |
: est. |
period |
: totals |
: est. |
|
i960 |
Mv |
5/12- |
5/31 |
2.98 |
2.98 |
5.58 |
5.58 |
354 |
354 |
||||
|
June |
5/31- |
6/2U |
6.69 |
9.67 |
8.24 |
13.82 |
541 |
895 |
|||||
|
6/21*- |
7/1 |
1.01 |
10.68 |
1.93 |
15.75 |
138 |
1,033 |
||||||
|
5/31- |
6/30 |
7.67 |
9.98 |
670 |
|||||||||
|
July |
7/1 - |
7/8 |
1.44 |
12.12 |
2.27 |
18.02 |
151 |
1,184 |
|||||
|
7/8- |
8/1 |
4.56 |
16.68 |
7.13 |
25.15 |
488 |
1.672 |
||||||
|
6/30- |
7/31 |
6.00 |
9.39 |
639 |
|||||||||
|
Aug. |
8/1 - |
8/10 |
1.36 |
18. OU |
2.53 |
27.68 |
180 |
1,852 |
|||||
|
8/10- |
9/1 |
5.27 |
23.31 |
5.56 |
33.24 |
402 |
2,254 |
||||||
|
7/31- |
6/31 |
6.63 |
8.09 |
582 |
|||||||||
|
Sept. |
9/1- |
9/16 |
1.67 |
2U.98 |
3.19 |
36.43 |
251 |
2,505 |
|||||
|
9/16- |
9/22 |
1.58 |
26.56 |
1.19 |
37.62 |
98 |
2,603 |
||||||
|
9/22- |
9/29 |
0.72 |
27.28 |
1.U1+ |
39.06 |
108 |
2,711 |
||||||
|
8/31- |
9/30 |
4.10 |
6.13 |
480 |
|||||||||
|
Oct. |
9/29-10/27 9/30.10/31 |
2.42 |
29.70 |
2.59 |
3.85 |
42.91 |
4.08 |
341 |
2,052 |
372 |
|||
|
Nov. |
10/27-11/18 |
0.87 |
30.57 |
1.60 |
44.51 |
179 |
3,231 |
||||||
|
TOTALS |
5/12-11/18 |
18.69 |
26.84 |
2,000 |
Tulare Lake Basin Valley Floor Cotton - Arvin - Plot CD
1959 MbJ
4/30- 5/8 5/8 - 5/21 5/21- 6/3 4/30- 5/31 6/3 - 6/16 6/16- 6/23 6/23- 6/30 5/31- 6/30 6/30- 7/7 7/7 - 7/15 7/15- 7/28 6/30. 7/31 7/28- 8/4 8/4 . 8/11 8/11- 8/18 8/18- 8/25 8/25- 9/2 7/31- 8/31 9/2 - 9/24 8/31- 9/30
2.67
'4.55
1.38
3.85
|
0.13 |
0.13 |
|
0.45 |
|
|
1.51+ |
1.99 |
|
3.87 |
|
|
2.51 |
6.38 |
|
9.05 |
|
|
11.46 |
|
|
2.79 |
14.25 |
|
18.80 |
|
|
21.11 |
|
|
1.86 |
22.97 |
|
24.35 |
|
|
1.50 |
25.85 |
|
28.07 |
1.58
7.47
10.67
7.76
5.10
1.68 1.68 3.88 5.56 3.76 9.32
4.19 13.51
2.16 15.67
2.11 17.78
2.43 20.21
2.56 22.77
4.02 26.79
2.23 29.02
2.15 31.17
1.91 33.08
1.63 3**. 71
2,23 36. 9t
4.18 41.12
8.69
9.06
|
117 |
117 |
|
217 |
334 |
|
210 |
544 |
|
252 |
796 |
|
153 |
949 |
|
126 |
1,075 |
|
138 |
1,213 |
|
159 |
1,372 |
|
236 |
1,608 |
|
126 |
1,731+ |
|
131 |
1,865 |
|
125 |
1,990 |
|
106 |
2,096 |
|
146 |
2,242 |
8.67
5.93
338 2,580
498
571
582
548
473
-47-
|
TABLE 5 ( continued ) |
|||||||||||
|
SUl^MARY OF |
MEASUREMENTS OP EVAPOTRANSPIRATION |
||||||||||
|
AND |
RELATED DATA (Continued) |
||||||||||
|
: |
: : : t |
Evapotranspiration : |
Pan evaporation |
Atmometer evaporatl( |
|||||||
|
Meas- |
Esti- |
: Aoeum. |
: Monthly : |
Eaoh |
:Acouiii. |
: Monthly |
Each |
lAccuni. |
: Monti |
||
|
Y«ar : Month |
: Period , |
ured |
mated |
> totals |
: est. : |
period |
: totals |
: est. |
period |
: totals |
: est. |
|
1959 Oct. |
9/2U.IO/I9 |
3.66 |
35.60 |
4.21 |
45.33 |
372 |
2,952 |
||||
|
Defoliated |
|||||||||||
|
10/19-ll/U |
0.36 |
35.96 |
2.01 |
47.34 |
163 |
3,115 |
|||||
|
9/30.10/31 |
2.95 |
4.49 |
Ul |
||||||||
|
Not. |
iiA -12/17 10/31-11/30 |
0.33 |
36.29 |
0,19 |
3.21 |
50.55 |
2,69 |
1 |
|||
|
TOTALS |
V8 -12/17 5/8 -llA |
25.96 25.63 |
36.61 |
2,239 |
1 |
||||||
|
Tulare Lake |
Basin Valley Floor |
||||||||||
|
Cotton - Arvln - Plot CP |
|||||||||||
|
i960 Mar. |
3/18- 3/23 |
0.50 |
0.50 |
0.85 |
0.65 |
||||||
|
Planted U/6 |
|||||||||||
|
Apr. |
3/23- 5/6 3/31- V30 |
1.46 |
1.96 |
0.83 |
8.40 |
9.25 |
5.82 |
634 |
634 |
43 |
|
|
Ma^ |
5/6 - 6/9 V30- 5/31 |
0.37 |
2.33 |
0.31 |
10.39 |
19.64 |
8.72 |
666 |
1,300 |
57 |
|
|
June |
6/9 - 6/15 |
0.84 |
3.17 |
2.03 |
21.67 |
131 |
1,431 |
||||
|
6/15- 6/20 |
1.81 |
4.98 |
1.82 |
23.49 |
124 |
1,555 |
|||||
|
6/20- 6/30 |
2.41 |
7.39 |
3.20 |
26.69 |
226 |
1,781 |
|||||
|
5/31- 6/30 |
5.31 |
9.98 |
67 |
||||||||
|
July |
6/30- 7/7 |
1.94 |
9.33 |
2.13 |
28.82 |
138 |
1,919 |
||||
|
7/7 - 7/15 |
2.63 |
11.96 |
2.54 |
31.36 |
179 |
2,098 |
|||||
|
7/15- 7/28 |
4.15 |
16.11 |
3.59 |
3't.95 |
249 |
2,347 |
|||||
|
6/30- 7/31 |
10.14 |
9.39 |
63 |
||||||||
|
Aug. |
7/28- 8/9 |
4.25 |
20.36 |
3,44 |
38.39 |
234 |
2,581 |
||||
|
8/9 - 8/19 |
3.28 |
23.64 |
2.62 |
41.01 |
198 |
2,779 |
|||||
|
8/19- 8/23 |
1.43 |
25.07 |
1.34 |
'+2.35 |
86 |
2,865 |
|||||
|
8/23- 8/31 |
1.29 |
26.36 |
1.71 |
44.06 |
127 |
2,992 |
|||||
|
7/31- 8/31 |
8.87 |
8.09 |
56 |
||||||||
|
Sept. |
8/31- 9/21 8/31- 9/30 |
■+.35 |
3O071 |
5.04 |
4.61 |
48.67 |
6.13 |
346 |
3,338 |
4e |
|
|
Oct. |
9/21-10/14 |
1.79 |
32.50 |
3.96 |
52.63 |
318 |
3,656 |
||||
|
Defoliated -IO/I9 |
|||||||||||
|
9/30-10/31 |
1.03 |
4.08 |
37 |
||||||||
|
Nov. |
10/14-11/22 10/31-11/22 |
0.94 |
33M |
1.02 |
3.^5 |
56.08 |
1.89 |
365 |
4,021 |
27 |
|
|
TOTALS |
3/23-11/22 |
22.82 |
46.09 |
3,398 |
MR.
obtained during any one period of depletion is expressed In Table A-6 under the heading "Twice the Standard Error,"
The effects of percent ground cover and, possibly, of stage of crop maturity and available soil moisture, are illustrated in Plate 3, which compares accumulated evapotransplration of dif- ferent crops. These measurements were made in the Arvln area under similar climatic conditions and on the same soil series. Differences in percent ground cover d.nd possibly crop maturity and available soil moisture cause differences in slopes of the curves shown in Plate 3. Defoliation caused the abrupt changes in evapotransplration rates reflected in the curves on cotton and plums. Alfalfa remains green at this location throughout the year, and shows little seasonal slope changes. It is of interest to note the much higher July and August rates of evapotransplration by cotton, as compared to alfalfa and plums in both 1959 and 1960. A complete explanation for this cannot be presented; however, certain of the factors affecting evapo- transplration are discussed in the following chapter.
-49-
CHAPTER IV. CORRELATION OF EVAPOTRANSPIRATION DATA WITH AGROCLIMATIC DATA
To attempt concurrently to measure evapotransplration of the many species of Irrigated crops presently grovm In Cali- fornia is impractical because of financial and manpower require- ments. Likewise impractical is the measurement of evapotransplration of a single crop at more than a few locations.
The most promising approach at this time appears to be to determine the important and measurable parameters affecting evapotransplration rates, and to correlate actual measurements of evapotransplration with those parameters . Three Important para- meters which appear independently to affect evapotransplration are climate, plant conditions, including physiological factors, and soil moisture availability. Differences in the physical and chemical properties of soils and soil fertility are not considered to directly affect evapotransplration, even though they may have indirect effects,
This chapter discusses the relationship of each of those parameters of evapotransplration, and summarizes the analysis of data collected through 1960. In this regard, basic research on factors affecting evapotransplration is being conducted by the Uni- versity of California, as an integral part of the Vegetative Water Use Program. The Agricultural Research Service is also conducting basic research in this field. The results of these research pro- grams have affected, and shall continue to Influence, the course of these studies.
■51-
Evapotransplratlon and CllTnatic Data Climate In the evapoti^ansplratlon process can be though of as a combination of evaporative elements, such as air tempera- ture, wind, dryness of the air, and solar radiation. Other fac- tors of climate, such as length of daylight, may be indirectly related to evaporation.
The energy sources for the evapotransplratlon processes are derived principally from direct solar radiation and advection or exchangeable heat from the air. The evaporative demand of the atmosphere is largely a function of those tv70 elements. However, not all of the solar radiation that falls directly on the plant or ground surface is used in evapotransplratlon. A portion is re fleeted back into the atmosphere, a portion Is utilized in heatin the air, a portion is absorbed in heating the soil, and the balan is utilized in evapotransplratlon and plant growth. It is likev^i; probable that the energy available from advection is not all uti- lized, depending upon many factors, such as vapor pressure deficis and extent of v/lnd movement. Under certain conditions, it has bei demonstrated that advective cooling, as well as advecting heating may occur.
As the moisture content of the air increases through evapoi'atlon and/or transpiration, the moisture gradient (vapor pressure gradient) between an air mass and an evaporating surface becomes less steep and retards further moisture transfer. Under field conditions, the air mass near the ground is far from stable Air movements act to mix moisture- saturated air near the evaporatn
-52-
surface with drier air from above. Wind speeds and surface rough- ness Influence the relative turbulence of the air, moving the moisture away from the evaporating surface and bringing In drier air to further the evaporation process. Thus, it is apparent that the evaporative demand of the atmosphere is determined by the inter- action of several climatic elements.
Progress is being made in determining the relationships between the aforementioned climatic factors to arrive at a quan- titative approach to estimating evapotranspiration.
Evapotransplratlon and Plant Conditions The term "evapotransplratlon" implies the sum of evapo- ration plus transpiration. In the case of plants that are actively growing and well supplied with moisture, transpiration is related and responsive to climatic conditions. Evaporation from soils, hov/- ever, is related more closely to, and limited by, the moisture con- tent of the exposed soil surface than to climatic conditions. In most irrigated areas in California, rain is sparse during the growing season and, except for areas of high water tables, soil svirfaces soon dry through evaporation following irrigation. As a result, under California irrigation conditions, transpiration is usually the larger of the two components comprising evapotransplratlon.
The primary plant parameter affecting evapotransplratlon rate appears to be the percent of ground cover. This is an im- portant consideration when determining evapotranspiration for an- nual field crops, such as sugar beets and cotton, and for other
53-
crops having variable ground cover percentages, such as alfalfa, which is cut frequently.
Crops having rapid growth rates and vigor tend to provld greater ground cover more rapidly than a slow-grov/ing crop, even of the same species. Thus, differences in growth rate may affect evapotranspiration rates through the direct mechanism of percent of ground cover, although other physiological factors, such as stage of maturity or growth, may also affect evapotranspiration.
Evapotranspiration and Soil Moisture Research findings relative to the effect of variations of available soil moisture upon evapotranspiration and plant grov/t are varied.
The amount of soil moisture available above the permaner wilting point does not seem to affect the evapotranspiration rate of crops, according to many research reports. Other research has indicated that maximum growth rates are obtained only under condi- tions of high moisture availability, and that growth rates and yields are retarded as soil moisture availability decreases. Thes concepts differ from other research investigations which have indi cated a close relationship between evapotranspiration and plant growth. These concepts are of particular Importance in conslderir if evapotranspiration rates are affected by lov; soil moisture leves vjhlch appear to affect growth rates, such as occur v;hen irrigatior is deliberately withheld from grapes and cotton to change their fruiting characteristics.
Besides intentional withholding of irrigation, there are also occasions of drought due to insufficient irrigation water
__54-
supplies. Due to the foregoing reasons, crops are frequently sub- jected to drought for periods of time varying from a few hours up to several weeks .
Therefore, In the studies reported here, an evaluation was made of the effect of available moisture upon evapotransplratlon,
Available moisture was determined for the principal root zone for each crop from selected neutron probe soil moisture data. In the case of the alfalfa, a perennial crop, a single zone from 0-12 feet was used for the entire study. For cotton, an annual crop, the zone was increased from the 0-1-foot depth to the 0-11- foot depth as the crop grew and the root system developed and ex- panded. The results of the evaluations are discussed further under the sections on crop coefficients.
Other Factors Affecting Evapotransplratlon Soil fertility and other physical factors of the soil, such as texture, structure, salinity, and even color, affect the growth rate of a crop. Soil properties, such as texture, struc- ture, and salinity may also affect, to some degree, moisture move- ment and utilization. These factors have an undetermined, and probably much lesser, effect on evapotransplratlon than drought, climate, and plant conditions.
Determination of Coefficients Results of various research projects have indicated that the processes of evapotransplratlon and evaporation are both responsive to the same factors. As will be discussed in ensuing
-55-
paragraphs, a definite relationship exists between evapotranspi- ration and rates of evaporation from pans or atmometers . This relationship is considered fundamental to estimating evapotranspi- ratlon for other crops and in other agricultural areas throughout the State.
The ratio of evapotranspiration (ET) to evaporation from an evaporation pan (Ep) is referred to as a "pan coefficient" (ET/Ep) ; in like m.anner, the ratio of evapoti-anspiration to net atmometer evaporation^ or the difference of evaporation from a black and white Livingston Spherical Atmometer (Eb-w), is referred to as the "atmometer coefficient" (ET/eb-w) .
Pan and/or atmometer coefficients for individual evapo- transpiration measurement periods for the various plots sampled are shown in Appendix A in Tables A-6 and A-7. A casual examina- tion of these individual periods reveals v;ide variations which v/ould appear to discount the validity of such comparisons. How- ever, a more detailed analysis of the data indicates that ce^-tain relationships do exist, and upon such relationships tentative values can be established. Certain variations of the pan and at- mometer coefficients from time to time are caused by plants re- sponding differently to evaporation influences than do pans and atmometers. Likewise, variations in the coefficients were due also to individual differences in the response of atmometers or pans to these climatic influences.
Analysis of data for each individual crop and the con- clusions drawn therefrom are discussed in the follo\\fing paragraphs
56-
Grass and Pasture CoelTlolents
Pan and atmometer coerflclents have been determined using data from gx^ass and grass-pasture evapotransplrometer tanks located In the Sacramento River Basin mountain valleys, in the Lassen-Alpine mountain area, and in the Sacramento Valley floor (Alturas, Coleville, and Davis).
Graphs of coefficients and percent of ground cover for pasture and grass, plotted against time, are presented in Figures A through E of Plate ^, entitled "Variation of Pan and Atmometer Co- efficients for Individual Periods of Measurements." Percent of ground cover is relatively constant for those crops, and wide varia- tions of the coefficients occur less than with alfalfa and cotton. During the growing period, the grass was at nearly 100 percent grovind cover in all of the evapotransplrometer tanks, as mowings did not clip the foliage short enough to cause large reductions in ground cover. While the ground was alv;ays sod- covered, the colder climate at the mountain sites caused dormancy to some de- gree during late fall, V7inter, and early spring. Approximate ground cover percentages indicated on Figures A, B, and E of Plate 4 are for the green and actively growing fraction of the foliage. At Davis, the climate is not cold enough to force the grass completely into winter dormancy. Occasionally at the Davis site, however, small areas of ground surface were exposed through- out the year, as indicated on Plate 4, Figures C and D.
High water table conditions, typical of the predominant irrigation practice in the mountain valleys, were maintained in the Alturas and Coleville tanks. There was, therefore, no
■37-
moisture shortage at the::e sites. The cvapotranspirometer tanks and ryegrass rield at Davis were frequently Irrigated, and it is probable thiat soil moisture was not limiting there. Availability of soil moisture Is assumed to have had little effect on evapo- transpiratlon rates and coefficients at any of the three sites.
Seasonal accumulated evapotransplratlon plotted against accumulated pan evaporation and, except for Davis, accumulated atmometer evaporation are shown on Plate ^, entitled "Comparison of Pan and Atmometer Coefficients for Cotton, Alfalfa, and Grass,' Figures E and F. Each curve is for an individual year, and has separate zero lines for plotting evapotransplratlon. Evaporation from pans or atmometers vras plotted using the date of June 30 as the common point on all curves. Coefficients for the period of record for both years were consistently similar for Alturas for both pan and atmometer. The pan coefficient for the period of record at Davis was likewise similar.
Coefficients from three seasons of record in the mounta: areas, combining Alturas and Coleville, are compared with coeffic:i from Davis in Table 6. Coefficients are shown for both the growii seasons assumed in Bulletin No. 2 and for the longer period for v;hlch data were obtained. The reason for the differences between the valley and mountain coefficients has not been ascertained.
Alfalfa Coefficients
Pan and atmometer coefficients have been determined froi an alfalfa plot located near Pittville in the Sacramento River Ba;t! mountain valleys, and from an alfalfa plot near Arvin in the Tula:! Lake Basin Valley floor at the southern end of the Central Valley
•58-
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One of the most notable details of the alfalfa coef- ficients determined from both areas is the variation associated with percentage of ground cover. It is important to point out that the method of collecting data on percentage of ground cover was subjective^ being based upon personal judgment, and that esti- mates by individual observers differ by perhaps 5 to 15 percent. There is, however, general agreement that following mowing the ground cover is usually reduced to 5 to 10 percent, and that grour. cover usually approaches 100 percent cover prior to mov/ing . Al- though there are exceptions due to possible experimental error and other factors, the coefficients are smaller when the ground cover is low following mowing, and become larger as the ground cover increases. Plate 4, Figures F, G, H, and I, illustrate these relationships between coefficients and percent of ground cover, plotted against time.
A more direct comparison of pan and atmometer coefficien: with percent of ground cover is sho\m in Plate 6, entitled "Rela- tionship Between Pan and Atmometer Coefficients for Alfalfa and Ground Cover." Figure A shows atmometer coefficients, and Figure shows pan coefficients. The data for both figures were the same utilized in Plate 4. As indicated in Plate 6, the Pittville co- efficients appear to be higher than the Arvin coefficients. Two linear regression lines have been fitted to the data. However, it may be that additional data will indicate a somewhat curvi- linear relation. It seems reasonable to assume that coefficients at 100 percent of ground cover would not be proportionally higher than coefficients at 80 percent of gi'ound cover, which, for prac- tical purposes, also provide nearly complete shade, except near noonday.
-60-
Since the soil at both plot sites was deep, and alfalfa is a perennial crop, moisture in the 0-12-foot zone was used to estimate available soil moisture.
The lowest moistures occurred at the Pittville plot, where on several occasions the available moisture was reduced to less than 2 inches in the 12 feet, or less than 0.2 inch of moisture per foot of soil, on the average. When this condition occurred, the upper portion of the profile was usually relatively drier than the deeper soil. On several of these occasions, crop growth at the Pittville plot was slow, and considerable flower blooms and dark blue-green leaf color associated with moisture deficiency appeared. As indicated on Figures F and G of Plate 4, low available soil moisture may account for some of the smaller coefficients noted prior to mowing. The Arvin plot, in contrast, was very well supplied with soil moisture. As shown on Figures H and I, the available moisture at Arvin ranged above 1 and up to 2 inches per foot during the measurement periods.
If evapotranspa ration were reduced by low available soil moisture, the pan and atmometer coefficients would be smaller. This does not appear to be the case for the Pittville plot, al- though several of the coefficients just prior to mowing are smaller than would be expected, considering the percent of ground cover. Overall, the pan and atmometer coefficients of the Pittville data are as high, if not higher, than the Arvin coefficients, regard- less of the lower soil moistures at Pittville.
Since coefficients from the Pittville and Arvin plots show monthly variations reflecting mowing schedules, farm practices,
-61-
and, perhaps, effects of plant (growth environments. It is deemed ■, best to compare seasonal rather than monthly coefficients.
Seasonal coefficients have been determined for periods when evapotranspiration measurements were made using data shown in Table 5^ and are summarized here as follows:
Seasonal Alfalfa Coefficients Determined From Measured Periods Only
Pan Coefficient Atmometer Coefficient
1939 i960
0.0125 0.0127
0.0093 0.0093
In order to take into account the possibility that the sampling periods could be biased and not representative of groundl cover conditions, and also to include estimated evaporation incre ments following irrigations, estimates of evapotranspiration were made for the irrigation periods. These estimates fill in the misii records. Seasonal coefficients determined from these data are su- marized as follows:
Seasonal Alfalfa Coefficients, Including Estimated Data
Pan Coefficient Atmometer Coefficient
|
1959 |
i960 |
|
|
Pittville |
nr |
0.82 |
|
Arvin |
0.69 |
0.70 |
|
1959 |
i960 |
1959 |
i960 |
|
|
Pittville |
nr |
0.82 |
0.0123 |
0.0125 |
|
Arvin |
0.69 |
0.69 |
0.0099 |
0.0095 |
The close similarity between the coefficients determine from the measured periods as compared with the total seasonal period of record, including estimated periods, indicates that the
-62-
measured periods are not biased, and the seasonal coefficients appear to be reasonable. Curves of seasonal accumulated evapo- transplratlon versus evaporation are shovm on Plate 5, Figures C and D. As noted previously, each curve on Plate ^j is plotted for an individual year, with separate zero lines for indication of evapotranspiration. Evaporation from pan and atmometers was plotted, using the date of June 30 as the common point on all curves.
The pan and atmometer coefficients derived after com- bining the two years of record at the Pittville AA plot are shown in Table 7 , and are compared with coefficients derived in the same manner at the Arvin CC plot. For purposes of comparison, average coefficients were determined not only for the period of record, but also for the growing season, as shown in Bulletin No. 2.
By any method of determining seasonal coefficients, the Pittville pan coefficient is approximately 17 percent higher than the Ai'-vin coefficient, and the Pittville atmometer coefficient is approximately 27 percent higher than the Arvin coefficient. Whether the difference is due primarily to basic climatic differences be- tween the two areas, v/hlch affect different plant and evaporation response, or due to experimental error, is not knovm at this time.
Cotton Coefficients
Pan and atmometer coefficients for cotton for each period of measurement during 1939 and 196O are shown in Figures J and K of Plate 4. Also shov/n are estimates of the percent of ground cover, available moisture, and other factors affecting plant growth and water use. There is a rather close agreement betv/een the two years
■63-
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-64-
in regard to the general pattern of plant growth and the relation- ships of the coefficient with the various factors affecting water use. The soil moisture observations are believed to be of reliable quality, particularly for the i960 data, where a dry soil zone v;as maintained at depth below the root zone, assuring that no deep per- colation of Irrigation water occurred.
Coefficients for individual period of measurements show a pattern of progressive increase from the lov; early season value to a peak in July, and 'then a progressive decrease to the year's end. The differences in coefficients during the early season emphasized the direct relationship between the evapotranspiration and percent of ground cover. The decreasing pattern of coefficients after July reflects the integrated effect of decreasing ground cover, physiological aging of the plants, and availability of soil moisture.
It is of interest that, although ground cover on these plots reached 80 and 95 percent, the maximum coefficients were reached at a ground cover of about 60 percent. This corresponds in time to the boll setting. It is believed that physiological factors may have had an influence on the transpiration rate at this stage of plant development. Physiological factors are be- lieved to have caused similar effects in other crops. With small grains, for example, peak water use rates are reported to occur at the heading stage.
Late-season use of water by cotton is dependent to some extent upon the amount of moisture available prior to natural or
-65-
Ind-uced defoliation. The plants will generally use all available moisture within the root zone. The amount of use is a function of the amount of moisture available. This is to say that the avail ability of soil moisture is often the limiting factor in the late- season evapo transpiration. This also may account^ in part, for the August-September coefficients being lower than the July coefficieni
Early- or late-season precipitation, although a part of evapotranspiration and reflected in the pan and atmometer coeffic- ients, is, quite often, not a beneficial source of moisture to the plants. Early-season precipitation is evaporated from the soil su: face with little gainful effect upon plant grov/th. Late-season precipitation is either evaporated from the soil or vegetative sup- face, and/or transpired by the plant without contributing substanti,, ly to the plant cultural requirements. Thus, pan and atmometer coefficients for early and late season must be applied with cautic: and only after a thorough evaluation of rainfall amount, frequency and pattern, as well as knowledge of the late- season availability of soil moisture.
Based on the information summarized in Table 5^ monthly pan and atmometer coefficients for the two years of record have been determined, and are shown on Figures A and B on Plate b. There is, in general, rather close agreement between the monthly pan or atmometer coefficients for both 1959 and i960. There are also several indications that evapotranspiration for cotton some- times exceeds evaporation from pans. The July pan coefficients for 1959 and i960 were respectively I.07 and I.08, which indicates that evapotranspiration exceeds evaporation from the free-water
-66-
surface. It is believed that the crop surface roughness may, thi'ouch greater air niixln,":, be one of the influencing factors.
Average monthly pan and atmometer coefficients for cotton for the tv;o years of record are presented in Table 8. For purposes of comparin;c v/ith Bulletin 2 estimates, an average coefficient for tlie Tulare Lake Basin Valley Floor Hydrographic Units was determined for the grov/in;- season used in Bulletin 2. For the period from May through October, the active growing season, the average pan coefficient is 0.68, and the atmometer coefficient is 0.0098. The monthly coefficients for the period from June through September are considered to be primarily the effect of climatic evaporative demand and crop conditions, and are not sub- ject to the influence of early-or late-season nonbeneflcial uses.
Application of Coefficients and Evaporation Data to Estimation of Evapotranspiration
Using the average pan or atmometer evaporation observed in each area, as shown in Tables 2 and 3 in Chapter II, and ap- plying the appropriate pan or atmometer coefficients as described jn Tables 6, 7j and 8, estimates of monthly consumptive use values viere made for several crops. These monthly estimates are summarized in Table 9, and are compared v;it.j values utilized in Bulletin 2, "Water Utilization and Requirements in California," published by the department in 195b • To make the comparison with Bulletin 2 values valid, the grov/ing seasons used in Bulletin 2 were used in all calculations for Tables 6 through 9- In general, the esti- mates based upon the pan and atmometer coefficients are approximately equal to or ;;reater than the Bulletin 2 values. This is also true where measured values of consumptive use are available. This, in
TABLE 8 PAN AND ATOOMETER COEFFICIENTS FOR COTTON
Pan Coefficients
Month
Tulare Lake Basin Valley Floor (Arvin (CD) 1959 Arvin (CF) 19 60)
No. Days
of Record :
ET/Ep»
Atmoraeter Coefficients
Tulare Lake Basin Valley Flo" (Arvin (CD) 1959 Arvin (CF) I96O)
No. Days of Record
ET/Eb-w»
|
January |
■— |
~ |
|
February |
- |
— |
|
March |
a |
0.75 |
|
April |
30 |
O.lli |
|
May |
62 |
0.11 |
|
June |
60 |
0.67 |
|
July |
62 |
1.08 |
|
August |
62 |
0.99 |
|
September |
60 |
0.8U |
|
October |
62 |
0.U6 |
|
November |
60 |
0.26 |
|
December |
62 |
0.15 |
|
30 |
0.0019 |
|
62 |
0.0018 |
|
60 |
0.0103 |
|
62 |
0.0170 |
|
62 |
0.011i7 |
|
60 |
0.0106 |
|
62 |
0.0051 |
|
60 |
0.0023 |
|
Average Coefficient for Growing Seasonl/ Period of Record |
398 |
0.68a/ |
398 |
0.0098*/ |
2/ For growing season periods used in Bulletin No. 2, Tulare Lake Basin Valley Fm
hydrographic units. •/ April-October ♦ ET/Ep - Pan Coefficient, ET/Eb-w - Atmometer Coefficient
-68-
|
c o -p o |
O (1) 0) E ■H ? '^ sis o |
|
(0 05 CO 1 |
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|
3 |
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|
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s
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1 |
|
|
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|
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|
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Itself, does not prove that the estimates based upon pan and at- mometer coefficients are more accurate. Additional supporting data shall be required to confirm this possibility.
Examination of the data in Table 9 indicates that esti- mates of consumptive use can be made with equal confidence, on th( basis of either pan or atmometer data. It should be emphasized also that the consumptive use values must be determined for the actual period of active plant grov/th. The actual grov^ing season for most crops in the various areas of the State still remains to be determined. Furthermore, a careful analysis of precipitation pattern, frequency, and amounts must be made for both growing and nongrov^ing seasons, to determine the effectiveness of this moistui source toward meeting the water demand of the various crops.
-70-
CHAPTER V. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
This chapter presents a concise summary of the ve,?;etatlve water use studies, the conclusions drawn therefrom, and rocotninendatlons v/lth regard to the future lines of study.
Summary
Precise Jcnowi edj-e of the total seasonal as well as the distribution pattern of water use throughout the year is basic to the plannlnc> desi;;n, and operation of comprehensive water development projects. In developing this essential knov/ledge, the department has been engaged in studies directed tov;ai'd determination of evapotranspiration. During the period from 193^1 to i960, these studies v/ere limited to certain geo- graphic regions of northern and central California.
Accurate measurement of evapotranspiration is so complex and costly that practical considerations limit collec- tion of these data to relatively fev7 locations. Recent re- search work by various groups throughout the world has pointed out certain fundamental relationships between the evapotranspira- tion process and climatic factors. Transpiring crops respond to the same energy sources as evaporation devices. The response of crops, however, is modified by physical and physiological characte':'istlcs. Under any given climatic condition, factors such as availability of soil moisture, percent of vegetative gi'-ound cover, and physiological development control the rate of ovapoti'anspiratlon.
-71-
The approach taken in these studies has been to study at a few locations the relationship between measured evapotranspiration, under specified crop conditions, and certain climatic indices. Concurrently with the measurement and corre- lation of evapotranspiration and climatic factors, a netv;ork of asroclimatic stations was established and observed throughout the several major inland agricultural areas in northern and central California Having determined evaporation at these stations, esti- mates of evapotranspiration can be extrapolated into these areas by using the relationship between evapotranspiration and evapora- tion data measured at the key evapotranspiration stations.
In the early years of these studies, available knowl- edge on climatic station environmental requirements v/as very meager. However, as data v/ere collected and analyzed, the im- portance of certain environmental effects became apparent. Stations were relocated to sites where they were surrounded by an extensive area of vigorous, low-growing vegetation at full ground cover. Large, well -managed pastures best meet these requirements. At such sites, the confounding effects of micro- environment differences are minimized.
The techniques used for the determination of evapo- transpiration were the best available methods for the task, at the time they were employed Hov/ever, as the study progressed, techniques were modified to take advantage of new and better tools as they became available. The initial soil moisture measureme.its to determine evapotranspiration xvere made by the
-72-
gravimetric technique. The development and refinement of the neutron scattering technique offered promise of a far superior method of making soil moisture determinations. For this reason, this new equipment was adopted shortly after it became commercially available.
Small evapotranspirometer tanks of various designs were installed and used where high-v;ater table conditions pro- hibited the use of soil moisture depletion techniques, and were later installed on sites where no high water tables existed. The success of these devices has encouraged the extension of this method to other close growing crops.
Estimates of evapotranspiration were made for all areas studied, using pan and atmometer coefficients and evapora- tion data collected as part of the agroclimatic program. These estimates were compared to Bulletin 2 consumptive use values, using the Bulletin 2 growing seasons. In many cases, the esti- mates obtained by using the evaporation correlation technique were higher than were the Bulletin 2 values.
Data collected at the evapotranspiration field plots indicate that the actual periods of active growth are considerably longer than those assumed in the determination of Bulletin 2 values. On a yearly basis, the estimates sho\m in this report may show even a greater variance with Bulletin 2 values.
As the estimated values presented in this report are based upon only two years of record, they should be used with considerable caution. However, the evaporation correlation
-73-
technique appears to promise a reasonable means of estimatlnf with precision heretofore unknovm, evapotransplratlon rates I crops In the various geographic areas of California.
Conclusions
1. Correlation of evaporation with evapotransplratlf appears to promise a reasonable means of estimating evapotranS' plratlon within the various agricultural area of the State.
2. Reasonable estimates may be obtained by using either pan, or atmometer coefficients.
3. Pan and atmometer coefficients are strongly in- fluenced by percent of ground cover, particularly for ground cover percentage less than (60^o) sixty percent.
^1-. Estimated values presented In this report are based upon only two years of record, and so should be used with considerable judgment.
5. On the basis of the agroclimatic data collected, no definite segregation of the State into areas of uniform evaporation is possible at present. Inland areas appear to ha more uniform evaporation rates than expected, although effect of microenvlronment cause large differences of evaporation be- tween individual measurement sites.
6. It may be found that the len,^;th of growing seaso: is the most Important factor affecting seasonal evapotransplra tlon in inland areas.
■ 74-
Recommendations On the basis of the collection and analysis of the data on vegetative v;ater use^ as presented in this report^ and on the conclusions drav;n therefrom, it is recommended that:
1. The evapotranspiration studies at the present sites be continued until sufficient data can be collected to provide reasonable estimates of evapotranspiration under the range of climatic conditions vjhich can occur at these locations.
2. Additional sites for evapotranspiration measure- ments be established in locations having different climatic conditions than those now being measured to determine variability of evapotranspiration coefficients ( i.e.. Delta area, coastal area^ and desert areas).
3. The scope of the present program be expanded to include measurements of applied water under different irrigation practices and lengths of grov.'ing seasons for major crops within the various agricultural zones of the State. This would provide the basic information needed to determine irrigation efficiencies, drainage requirements, and, with the unit evapotranspiration values, to determine total irrigation water requirements.
-75-
APPENDIX A
Supplemental Agroclimatic and iilvapotranspiration Data
-77-
TABLE OP CONTENTS
Number Pap;e
A-1 Agrocllmatic Stations, Location and General
Information 79
A-2 Monthly Evaporation From Standard U, S. V.'eather
Bureau Evaporation Pans „ , 85
A-3 Monthly Evaporation Differences Between
Livingston Spherical Black and V/hlte Atmometers . 90
A-4 Location of Evapotransplratlon Measuring
Stations , 98
A-5 General Information Relative to Evapotransplratlon
Measuring Stations , . , „ IOC
A-6 Neutron Probe Measurements of Evapotransplratlon and Related Data for Several Irrigated Crops, 1959 and i960 107
A-7 Evapotransplrometer Measurements and Related Data for Hlgh-VJater Table Pasture and Irrigated Ryegrass II3
-78-
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