TC 824 LIBRARY UNIVERSITY OF CALIFORNIA DAVIS r OCT 4 1961 ■Y (K CALU-DhMU BRAP* 3PY 2 STATE OF CALIFORNIA DEPARTMENT OF WATER RESOURCES DIVISION OF RESOURCES PLANNING Bulletin No. 73 EVAPORATION FROM WATER SURFACES IN CALIFORNIA - r CAuli-ClniA DAVIS EDMUND G. BROWN Governor OCTOBER, 1959 HARVEY O. BANKS Director of Water Resources STATE OF CALIFORNIA DEPARTMENT OE \VATER RESOURCES DIVISION OF RESOURCES PLANNING Bulletin No. 73 EVAPORATION FROM WATER SURFACES IN CALIFORNIA EDMUND G. BROWN MC-,::^^^9m HARVEY O. BANKS Governor \4S '^'^^^n Director of Water Resources OCTOBER, 1959 LIBRARY UNIVERSITY OF CALIFORNIA DAVIS TABLE OF CONTENTS LETTER OF TRANSMITTAL CHAPTER I. INTRODUCTION Page ORGANIZATION, UNITED STATES DEPARTTffiNT OF AGRICULTURE vi ORGANIZATION, DEPARTMENT OF WATER RESOURCES vii ACKNOWLEDGMENT viii FOREWORD ix CHAPTER II. TYPES OF EVAPORATION PANS 7 I The Weather Bureau Pan 6 The Bureau of Plant Industry Pan 9 m The Square Ground Pan 10 The Square Floating Pan 10 W The Los Angeles County Flood Control District Pan . , 11 The Screened Pan 12 Meteorologic Equipment 13 CHAPTER III. EVAPORATION INVESTIGATIONS 16 Relation of Water Temperature to Evaporation .... 17 The Relation of Air Temperature to Evaporation ... 19 The Relation of Altitude to Evaporation 20 Mt. 'Whitney Study 21 The Salton Sea Investigation 25 Estimates of Probable Evaporation From Salton Sea 30 TABLE OF CONTENTS (continued) Page CHAPTER III, EVAPORATION INVESTIGATIONS (continued) Investigations of Evaporation From Small Water Areas 36 Denver, Colorado, Investigations 3S Fort Collins, Colorado, Investigations 40 Southern California Investigation 40 Fullerton Evaporation Station 40 Lake Elsinore Evaporation Station 41 CHAPTER IV. EVAPORATION COEFFICIENTS 43 The Weather Bureau Pan Coefficient 44 The Six-foot Diameter Ground Pan Coefficient .... 46 The Square Ground Pan Coefficient 46 The Square Floating Pan Coefficient 43 The Screened Pan Coefficient 51 CHAPTER V. EVAPORATION FROM LARGE WATER AREAS 54 CHAPTER VI o SUMMARY 5^ LITERATURE CITED 66 APPENDIXES A (Compilation of data, to be published at a later date) B Evaporation in the San Francisco Bay and Sacramento- San Joaquin Delta Areas 70 11 TABLE OF CONTENTS (continued) TABLES Table No. Page 1 Evaporation and Temperatures at Locations on the East Slope of Mt. V/hitney, California . , 23 2 Evaporation at Salt Creek Bridge, Salton Sea, Riverside County, California ... 2^ 3 Temperature at Salt Creek Bridge (Tower No. 1, Salton Sea Investigation), Riverside County, oaj_ix ornxa .....o...... ••••.. ^y Ur Evaporation at Indio, Riverside County, California 31 5 Evaporation at Mecca, Riverside County, California . o . 32 6 Evaporation at Brawley, Imperial County, California 33 7 Evaporation at Mammoth, Imperial County, California .„.. ............. . 34 8 Average Computed (Lake) Evaporation From Salton Sea, California, 1909-10 to 1918-19 ..... 36 9 Computed (Lake) Evaporation From Salton Sea, California, 1907-08 37 10 Mean Monthly Evaporation and Reduction Co- efficients for a Screened Pan and a Weather Bureau Pan at the Fullerton and Lake Elsinore Evaporation Stations . 45 11 Comparison of Evaporation From Square Ground and Floating Pans at Various Lakes and Reservoirs in California . 50 12 Comparison of Evaporation Coefficients for 3x3 Foot Square Land and Floating Pans for Reducing Pan Evaporation to Equivalent Evapo- ration From Larger Water Areas 51 13 Evaporation Coefficients as Determined by Various Investigations in V/estern States ... 53 111 TABLES (continued) Table No. Page 14 Evaporation Computed for a Few Lakes in California and Nevada 55 15 Computed Mean Annual Evaporation for Several . Lakes in California 56 16 Annual Evaporation From a Group of Weather Bureau Pans in the South Coastal Basin With Evaporation Indices Based on a 21-year Period of Record at the Riverside Citrus Station . . 64 17 Annual Evaporation From a Group of V/eather Bureau Pans in the San Joaquin and Sacramento Valleys With Evaporation Indices Based on a 19-year Period of Record at the College of Agriculture at Davis, California 65 I I FIGURES Figure No, b 1-a Relation of Evaporation From a Ground Pan to Mean Water Temperature IS 1-b Relation of Evaporation From a Weather Bureau Pan to Maximum Water Temperatures iS 2 Temperature-loop — The Relation of Mean Air Temperature to Mean Evaporation From a 12-foot Pan . . . o . . c 20 3 Relation of Altitude to Evaporation on the East Slope of Mt. Whitney 22 IV UNITED STATUS DEPARTMENT OF AGRICULTURE AGRICULTURAL RESEARCH SERVICE SOIL AND WATER CONSERVATION RESEARCH DIVISION P. O. BOX 758 FT. COLUINS, COLORADO Wc:;tern Soil and Ucter Management Research Branch May 1, 1959 Mr . Harvey . Banks , Director Department of Water Resources State of California Sacramento 5 , California Daar Mr. Banks: There is submitted herev/ith a report entitled "Evaporation from IJater Surfaces in California." It includes the records from 1881 to IS'54 published by the State Division of Water Resources in Bulletins Nos . 54, 54-A and 54-B entitled "Evaporation from Water Surfaces in California" and records for the period 1954 to 1958. The investigations on which this report is based were supported by and the report was prepared under cooperative agreement betv;een the Department of V/ator Resources of the State of California and the Agricultural Research Service of the United States Department of Agriculture. Respectfully submitte Omer J . itzlley Chief of Branch ORGANIZATION UNITED STATES DEPARTMENT OF AGRICULTURE Agricultural Research Service Byron T. Shaw, Administrator Soil and V/ater Conservation Research Division Cecil H. V/adleigh, Director Western Soil and Water Management Research branch Omer J. Kelley, Chief This bulletin was prepared by: Harry F. Blaney, Irrigation Engineer and Dean C. Muckel, Irrigation Engineer VI ORGANIZATION DEPAKTi4EMT OF WATER RESOURCES HARVEY 0. BANKS RALPH M. BRODY JAIffiS F. ViTlIGHT WILLI Alvi L. BERRY Director of Water Resources Deputy Director Deputy Director Chief, Division of Resources Planning Liaison with the United States Department of Agriculture Agricultural Research Service has been the responsibility of CARL B. MEYER Chief, Special Activities Branch PAUL L. BARNES PORTER A. TO'.'-'NER ISABEL C. NE33LER Chief, Division of Administration Chief Counsel Coordinator of Reports Vll ACKNOWLEDGMENT The assistance of engineers and water organizations throughout California is gratefully acknowledged. The inclusion of evaporation records and corresponding data in this bulletin represents a direct contribution from the many individuals and groups interested in the conservation of water. Assistance and advice given in preparing the many tabulations is especially appreciated. Vlll FOREWORD For many years, the State Department of Water Resources, and its predecessor agencies, have cooperated with the United States Department of Agriculture in study and research on the subject of evaporation from water surfaces in California. The results of this research have been published in a series of bulletins. The first of these bulletins contains general explanatory textual matter. The latter two contain extensive tables of evaporation, wind, and temperature data for various stations located in California and adjacent states. The bulletins previously published are: Division of Water Resources Bulletin No. 54, "Evaporation From Water Surfaces in California, A Summary of Pan Records and Coefficients, 1881 to 1946", 1947. Division of V/ater Resources Bulletin No. 54-A, "Evaporation From Water Surfaces in California, Basic Data", 1948. Division of V/ater Resources Bulletin No. 54-B, "Evaporation From Water Surfaces in California, 1946 to 1954", 1955. ■ The present publication. Department of Water Resources Bulletin No. 73, includes most of the text of the original Bulletin No, 54. Minor editorial changes have been made in order to include recent material and to simplify the identification of subdivisions of the various agencies involved. Since the original bulletin is out of print, this will provide a means of preserving and effecting wider dissemination of the valuable background material. Appendix B of the present publication presents the results of a recent special study of evaporation in the San Francisco Bay and Sacramento-San Joaquin Delta regions. ix Appendix A of Bulletin No. 73 will be published as quickly as available data can be collected and checked, and will include basic data for evaporation stations throughout California. Since Bulletin 54-B is no longer available, the tabular data will cover the period since 1946, preserving the continuity of records previ- ously published. Future bulletins in the 73-series will be published from time to time and will include data collected in the interim between dates of publication, together with complete records of stations not previously published. I k CHAPTER I INTRODUCTION Much of the irrigated agriculture of the west has been made possible by the impounding of flood v/aters. Storage dams conserve a water supply that otherwise might be wasted, help to prevent floods, and make possible the production of power. Reservoirs replenished by snow-fed streams flowing out of high mountains receive a more uni- form v/ater supply than those located in lower areas where snowfall is small and run-off is deficient. Reservoir replenishment in the higher areas occurs during late spring and early summer. Along the secondary streams of the lower mountains run-off quickly approaches a peak and as rapidly diminishes into periods of minimum stream flow. In such areas reservoirs must be designed for a carry-over supply from wet years for use during years of water deficiency. Under such con- ditions an extensive system of storage reservoirs may be the only means of maintaining an adequate water supply. Both conditions prevail in California, water being plenti- ful in the north and generally deficient in the southern portion of the State. Wherever water is scarce, losses are closely scrutinized. For this reason, evaporation from water surfaces is a subject that has been given considerable attention. Experimental studies have been conducted by the United States Department of Agriculture in cooperation with the California Department of Water Resources and its predecessors. Evaporation measurements have been recorded at many places by state and private organizations and by departments of the Federal Government including the Forest Service, Bureau of Re- clamation, Bureau of Plant Industry, and the Agricultural Research Service. •fi- Evaporation is the natural process of changing water into vapor. Dry air has a greater capacity for absorbing moisture than moist axr; hence, evaporation increases under conditions of low humidity. It increases with high temperatures and decreases v/ith low temperatures. V/ind increases evaporation from small water surfaces by replacing moist air over the water with drier air moving in from a distance. From large water areas dry winds increase evaporation for limited distances from the windward shore, but for the central area and toward the leeward shore evaporation remains fairly constant because the moving air has little additional capacity for moisture. In general, relatively low evaporation occurs in coastal areas and at high elevations, and high evaporation occurs in places v/here high temperature, low humidity, or strong winds prevail. Evaporation varies from day to day and from year to year according to the weather conditions at each locality. Differences in evaporation up to 50 or 100 per cent have been determined for localities separated by only a few miles. Evaporation measurements, therefore, should be made at each reservoir where records are desired. Attempts to use records obtained elsewhere may lead to error. Studies of evaporation from storage reservoirs indicate that for long periods of deficient stream-flow, reservoirs may yield, for useful purposes, as little as 50 per cent of the total water supply, the balance being lost by evaporation through years of carry- over storage. This being so, reservoirs are not always designed for the maximum quantity of water a stream will deliver over a period of years, as smaller reservoirs having less evaporating surface and smaller losses may yield in a similar period as much Water as could have been obtained from the larger storage. On streams of more uni- form flow a reservoir will be more completely replenished each year -2- and evaporation will be limited to a smaller percentage of the total water supply. In some places replenishment occurs only during winter and spring months, whereas evaporation continues throughout the year. ' Under such conditions annual evaporation sometimes exceeds annual re- plenishment . The topography of the State, with its high mountains and narrow valleys, encourages the construction of storage dams which now number over 800 of all types. The aggregate storage capacity is nearly 12 million acre-feet. One hundred of these have individual capacities of 10,000 acre-feet or more. An estimate of evaporation from reservoirs is difficult to obtain, as the aggregate surface area is unknown. Reservoir evaporation in California varies according to location from three to five feet in depth annually, which when ap- plied to the total surface area of all reservoirs, undoubtedly amounts to an impressive total. Evaporation losses are of importance as an element affect- ing the net water supply available for irrigation of crops, for pro- duction of power, and for municipal and industrial uses. Except in unusual instances they cannot be measured directly because of unknown elements of supply and loss of water entering or leaving the reservoir. Thus, recourse is necessary to research studies for determination of the relationships existing between evaporation from small containers, which is measurable, and from large bodies of water for which direct measurements are impossible. Of the items in the hydrologic equation, precipitation is measured over a wide network of stations throughout the Nation, stream- flow is recorded, and both sets of data are set forth in government publications. Evaporation records are less extensive and few are made available by publications. For the most part government agencies -3 - have confined evaporation measurements to ran investigations and the collection of data has been left principally to private organizations that are interested in the conservation and use of water. It is the purnose of this report to overcome the lack of published evaporation recoras in California through compilation of such existing data as are obtainable from publications and private and public files. Search has disclosed many records not heretofore available for public use. The total of some 250 evar^oration records throughout the State v'ill be helpful in desip^ninc- new reservoirs and estimating evaporation losses 1 rom others. In the northern portion of the btate water is more plenti- ful than in the south and less interest has been shov/n in collection of evaporation records, particularly in the northeastern counties ano along the coast as far south as Santa Barbara County. Very few records exist in these areas. In the Sacramento and San Joaquin Valleys evaporation measurements have been made in various localities by government and private agencies. The first such measurement to be recorded was by the State Engineer at Kingsburg from 1881 to 1885- (1^), (31). In mountain areas tributary to the Central Valley some records are available but they are not as numerous as might be ex- pected, v/ith the advent of the Central Valley Project and the con- struction of Shasta and Friant Dams, the lack of adequate evaporation data has been recognized and nlans have been made by the Bureau of Reclamation for installation of a network of evaporation stations throughout the area. A considerable percentage of all evaporation measurements within the State has been made in Los Angeles County where great sums have been spent for importation of water from out- side sources, and in San Diego County, where a small water supply Numbers in parenthesis refer to literature cited -4- and a large population growth have required constructioii of an ex- tensive reservoir system. Because of the size of the State and the differences in altitude and climate, depths of evaporation vary greatly in different localities. The greatest differences occur in the south where evapo- ration in the Mojave and Colorado desert regions may be two to three times the depth that occurs along the coast. This difference in evaporation is caused by differences in temperature and humidity. The desert effect is noted at borderline stations where winds alter- nately blow from the desert and from the coast. At Beaumont, in San Gorgonio Pass, dry fall winds from the desert sometimes increase evaporation to twice that occurring in a nearby area not so affected. P Evaporation has been defined as "the process by which water passes from a liquid or a solid state to a vapor" (2). Usually, evaporation is recorded from small evaporation pans by hook-gage measurement, although occasionally volumetric measurements are used. Allowance is made for rain falling in the pan which is treated in computations as so much water added, the net result being the actual depth of evaporation for the period of measurement. Evaporation from large water areas may be computed by applying the proper coef- ficient to pan records. It also may be computed as the residual factor in the summation of the items of inflow and outflow including bank storage, rain on the water area, and changes in the elevation of the water surface. This total is sometimes referred to as "gross" evaporation, as it is the actual loss from lake or reservoir. Gross evaporation minus the rainfall is called "net" evaporation, a term intended to indicate the net loss in storage resulting from evapora- tion losses and precipitation gains. Net evaporation may be a minus quantity. Gross evaporation is always positive. -5- An evaporation coefficient is defined as the ratio for con- version of evaporation from a given volume of water to equivalent evaporation from another volume of water, differing in depth or area. It is useful for the conversion of known evaporation from a small water area, such as an evaporation pan, to equivalent evaporation from a larger area, as a lake or reservoir. It may be used for the reduction of a normally high rate of evaporation from a small pan to equivalent evaporation from a large pan or from one type of pan to another of different characteristics. Later in this report a tabu- lation shows all known coefficients for the principal evaporation pans as determined by experiments of the former Research Division of Irrigation and Water Conservation, Soil Conservation Service (now Agricultural Research Service), U. S. Department of Agriculture. ( -6- CHAPTER II TYPES OF EVAPORATION PANS The importance of evaporation has been recognized by engi- neers as an item in the water supply of a region, but there has been no organized effort to obtain widespread records from a single standard type evaporation pan. Neither has there been planned coverage of a region by evaporation stations to obtain a comparable group of records that would show the extent of evaporation losses from water surfaces under different conditions of topography, climate, altitude or lati- tude. Consequently, a haphazard group of records has been accumulated by various organizations throughout the western states that have only relative values to each other since different types and sizes of evaporation pans were used in obtaining them. The principal pans used in obtaining these records are the Weather Bureau Class A pan, the Bureau of Plant Industry pan, a square floating pan sometimes called the United States Geological Survey pan, and a corresponding land pan of the same size sometimes designated as the Colorado pan. These are used under many conditions in several states. In Los Angeles County there also is a group of about 25 ground pans used by the Los Angeles County Flood Control District, for which records of 10 to 15 years are available representing both valley and mountain areas. Ground pans of various diameters have been used in experi- mental studies and their records are valuable in showing the effect of size of pan on the depth of evaporation loss. Since 1936 the Department of Agriculture has experimented with a screen covered pan designed to reduce the evaporation approximately to the depth of loss from a larger body of water. -7- The Weather Bureau Fan The Weather Bureau pan first came into use in the western states about 1916 and its records are the most numerous of any single I type of pan now used. As a result they are valuable for comparative ■ study. Because of the extent of the water requirements and the need ' for ivater storage in most sections of the state, the Weather Bureau pan has been used extensively in California and about 50 of its records have been collected from publications and public and private files for tabulation in this report. The "Weather Bureau pan is four feet in diameter, 10 inches deep, made of 22-gage galvanized iron, and set on 2 x 4-inch timbers that permit circulation of air beneath the pan. A stilling well in the pan permits measurement, by hook gage, of water evaporated. Depth of water in the pan should not be less than seven inches nor more than eight inches (36) although these limitations often are difficult to meet and many times water surfaces have been too high or too low. Since it is exposed above ground and receives the full effect of sun and wind, water in the Weather Bureau pan warms up rapidly in the morning and cools rapidly after sundown. During the daytime it has a high rate of loss that exceeds that of any other evaporation pan in common use. Although it is set above ground where it is relatively free from drifting sand or rolling weeds it is not easy to keep the water clean. At certain temperatures growths of algae accumulate to form a scum on the water surface. Copper sulphate kills the algae but it should not be used, as the copper replaces the galvanizing and forms rust spots that eventually become leaks. A more satisfactory method is to use any one of a niiraber of bleaching liquids containing a small percentage of sodiura-hypochlorite. These liquids may be obtained at any grocery store. Within a few minutes the chlorine kills the algae and clarifies -S- the water. Experience has demonstrated that it is harmless to the galvanized surface. Infrequently the Weather Bureau pan has been placed on a raft floating on the surface of a lake or reservoir or used as a floating pan partly submerged in water. Few Weather Bureau pan records are published regularly, but a small group are included in the monthly U. 3. Weather Bureau Climatological Data (41). Usu- ally air temperature and rainfall records may be c "tained from the same publication so that fairly complete meteorologic data are often available for use with the evaporation records. The Bureau of Plant Industry Pan This pan has been used by the Bureau of Plant Industry, United States Department of Agriculture, at its numerous plant ex- periment stations throughout the West. The first records were made about 1907. Records for a majority of the stations prior to 1934 have been published in issues of the ^^ionthly Weather Review (21) (22). As compared with the 50 Weather Bureau pans in California only five Bureau of Plant Industry pans appear to have been used at one time or another in the State. These were located at the Biggs Rice Station, Butte County; U. S. Cotton Field Station at Shafter, Kern County; the U. S. Date Garden, Indio, Riverside County; the U, S, Yuma Field Station, near Bard, Imperial County, all being operated by the Bureau of Plant Industry. An experimental pan of the same type was at the Division of Irrigation Experiment Station, Fullerton, Orange County. These pans were made of 22-gage galvanized iron six feet in diameter, 24 inches deep, set 20 inches in the ground with the water surface in the pan at ground level (36). Changes in v/ater level in such pans should approximate one inch. Measurement is made with a hookgage in an outside stilling well. A rain gage, anemometer, maximum and minimum thermometers in a shelter, and a psychrometer are standard equipment at each Bureau of Plant Industry Station, Because this pan is set in the ground and contains a greater volume of water than the Weather Bureau pan, its water temperatures are cooler during the day and warmer during the night. Consequently, evaporation is i lower than from the more exposed pan, 1 The Square Ground Pan The square ground pan, sometimes called the Colorado pan, was first used at the Colorado Agricultural Experiment Station about 1890 and with a few exceptions has since been in continuous use. It appears to have the longest record of any evaporation pan known. This ran is made of l8.-gage galvanized iron, three feet square, usu- ally IB inches deep, and set 14 inches in the ground with the water surface held at ground level o Water surface fluctuation should not exceed one inch. Measurements are made by hook gage in a stilling well on the inside wall of the pan (36), The evaporation loss is less than from the Weather Bureau pan because it is protected by surrounding soil but mere than from the Bureau of Plant Industry six-foot pan because of its smaller size. About 37 of these pans have been used in California, The Square Floating Pan This is sometimes known as the United States Geological Survey pan, but according to a letter to Rohwer (36) from the former Chief Hydraulic Engineer of the U, S, Geological Survey, the survey has no official floating pan. It is the same type and size as the square ground pan. This pan is made of l8-gage galvanized iron and is sometimes supported by two metal cylinders so placed that the surface of the water in the pan coincides with the surface of the reservoir. Diagonal perforated diaphragms, extending from corner to corner, reduce surge, although many of the floating pans used in California reservoirs do not have them. The pans are partly pro- tected from wave action by surrounding rafts that may be either square or triangular and they may be attached to rafts either flexibly by chains or fastened solidly to the raft timbers. I the pan is thus supported the metal cylinders are unnecessary. Depth of evaporation is determined by cup measurement to bring the water level up to a fixed index point in the center of the pan. Advantages of the floating pan are that because it is partly submerged, temperatures of the water in the pan and in the reservoir are almost identical; they change slowly and are more uniform than temperatures in the IVeather Bureau pan. As the pan is located off- shore, it is subject to the same conditions of wind, humidity and temperature that control reservoir evaporation. The main disadvantage of the floating pan is loss of record by splashing of water into or out of the pan in time of storm. It is not always possible to know when this occurs, and there is little doubt that many evaporation records for floating pans are erroneous. The Los Angeles County Flood Control District Pan The pan commonly used by the Los Angeles County Flood Con- trol District is two feet in diameter, three feet deep, and set in the ground with three inches of the rim exposed. A brass rod pointed at the upper end and set in a block of concrete on the bottom of the pan is the index point for the water level which normally is at the level of the ground surface. Depth of water evaporated is computed from cup measurements which restore the water level to the height -11- of the index point. Prior to Sertember, 1937, the index point in the Flood Control pan at the Baldwin Park Station, near Los Angeles, was at a level about one inch above ground; after this date it was lowered to the level of the ground surface. Because of the change : the Flood Control pan record is divided in two periods of approxi- mately equal length, showing 10 inches greater annual evaporation for the period of higher water surface. The reasc s for the hip:her evaporation are readily apparent: First, the water surface is closer to the top of the pan where it has greater exposure to wind; second, with the water surface higher than the surrounding ground, the heat of the sun shining on the exposed side of the pan between the ground and the water surface is transmitted to the water within, with re- sultant increased evaporation. This example emphasizes the value of maintaining water levels in ground pans at or below the level of the ground surface. The Screened Pan The screened pan has been used experimentally by the Depart- ment of Agriculture at the Fullerton Evaporation Station and else- i where in order to study the effect on evaporation of shading the water surface (49). The pan was of the same size as the Flood Con- ■ trol pan, two feet in diameter by three feet deep, set in the ground 2.75 feet. Water levels were maintained at ground level and measure- ments of water evaporated were made with a hook gage in an outside stilling well. The screen was made of galvanized hardware cloth with one-fourth inch mesh and suspended horizontally in the pan mid- way between the rim and the water surface. Tests were also made with a screen of six meshes per inch, but the annual evaporation result- ing from use of the finer mesh was only slightly less than from the -1 ?- more open screen. Experiments with the screened pan were undertaken for the purpose of finding one that would have the same annual evapo- ration as a larger water surface. If evaporation could be reduced to an amount equivalent to that from a lake or reservoir no reduction factor would be necessary for estimating reservoir evaporation from pan measurements. Experiments with the screened pan under different climatic conditions in Southern California indicat dthat a coefficient of nearly unity could be obtained. Following these experiments, the Los Angeles County Flood Control District adopted the one-forth inch screen for use with all Flood Control District pans. This could be accomplished without difficulty as both sets of nans were of the same dimensions. Meteorologic Equipment Since evaporation varies with the atmospheric changes there should be at each major evaporation station a set of instruments for recording meteorologic data including wind movement, maximum and minimum air and water temperatures, humidity, and precipitation. The anemometer for recording v/ind movement should be set at the north- west corner of the 2 x 4's supporting the V/eather Bureau pan where it will not throw shadows on the water. The anemometer cups should be six inches above the rim of the pan. It is best to employ a standard height for the cups as the velocity of the wind increases with distance above ground. At a number of stations in Southern California, the anemometer is placed about seven feet above ground, thereby setting up a different standard of wind velocity than that obtained from the lower instruments. Thermometers of the recording maximum and minimum Weather Bureau type should be kept in a standard thermometer shelter about -13- five feet above ground , with the opening on the north side to prevent the sun from shining on the instruments when the door is open. Float- ing maximum and minimum thermometers supported by corks or by stop- pered test tubes are suitable for registering water temperatures in evaporation pans. Mean temperatures, both for air and for water, are taken as the average of the maximum and minimum recordings. Air temperatures are sometimes recorded on the chart r a seven-day thermograph but they are less accurate than thermometer readings. In this case the mean temperatures may be taken as the average of the sum of the izemperatures shown for each of the two-hour periods throughout the 24 hours. It also may be determined by means of a planimeter but the two-hour average method is simpler. Humidity may be determined from the temperatures of the wet and dry bulbs of either the sling or whirling type of psychrometer , or from the recording charts cf a hygrothermograph or a hair hygro- meter. The psychrometer gives the most accurate results. If either of the recording instruments is used it should be kept in the shelter ■..'ith the thermometers. The standard eight-inch V/eather Bureau type rain gage should be installed at all stations where evaporation is measured, as the depth of rain falling in the pan must be known in computing the true depth of evaporation. The station should be enclosed in a tight mesh wire fence for protection of equipment and to keep out intruders, A gate in the fence should be kept locked. Reference is made to Circular L, Instrument Division of the V/eather Bureau (24) for instructions as to size of fence necessary for enclosure of the station equipment and its location within the fence, (The 12-inch X 15-foot area shovvT?. in Circular L is the minimum size that will hold the necessary equipment,) It is the opinion of the author -14- that the close proximity of the 4 x 4-inch fence posts to the evapo- ration pan permits undesirable shadows to pass over the water surface. They also create wind eddies over the pan and at the anemometer. As few posts as possible should be used and they should be kept as far as is feasible from the pano Four posts of boiler tubin/?; or two- inch pipe set 16 feet apart at the corners, with horizontal tubular bracing at the level of the top strand of wire, ma'? a strong and satisfactory fence that throws few shadows and creates a minimum of wind disturbance. Wherever possible the evaporation station should be located on open, level ground » free from shade and obstructions to wind. In the preparation of the tabulations in this report there have come to light some records obtained from evaporation pans located in the vicinity of shrubs or trees which, while small in the beginning, grew each year until in the course of time the grown shrubbery shaded the water in the pan or blanketed it from the wind 3 resulting in a gradual reduction of the evaporation » The future growth of vege- tation or the possibility of future building construction that will have an influence on the evaporation should be considered in select- ing the site of an evaporation station. -15- CHAPTER III EVAPORATION INVESTIGATIONS In order to make use of existing evaporation records and obtain the greatest benefit from them, the relationship between evapo- ration from various small standard pans and from larger bodies of water has long been of interest to engineers. In ■''jlietin 54~A and in this bulletin there are tabulated a large number of evaporation data recorded by various v/ater organizations throughout the State. In the present- chapter are brief descriptions of evaporation experi- ments carried on, not only in California but in other areas where different climatic conditions prevail. To a considerable degree the experimental studies have been made for the purpose of arriving at factors or coefficients showing the monthly and annual relationships existing betv/een evaporation from small artificial water surfaces contained in metal tanks or pans and the larger water areas such as those of large tanks or lakes and reservoirs. In most cases the accuracy of coefficients obtained by investigation has had little opportunity for proof, but where such opportunity has occurred there has been good correlation. It is generally assumed that agreement is reasonably good under all conditions of water storage. The principal error in this assumption lies m the favorable conditions under which the coef- ficient is determ.ined as related to the less favorable conditions where pan evaporation is measured at the lake or reservoir. "Usually an experimental station is in an open, level location where there are no immediate obstructions to divert the wind from the pan or create wind eddies over its water surface. Because of rough topo- graphy at many reservoirs the evaporation pan is sometimes placed -16- on top of the dam and thus above the reservoir surface which fluctu- ates widely with the seascr, or at, some distance from it at either a higher or a lower level o Its location is determined by topography or the convenience of ths operator. Evaporation froir a pan situated in an area where hillside slopes, brush and tall trees offer obstruction to v/inds cannot be expected to agree with evaporation from a similar ' ai located on a float at the water surface, or on an island where wind movement over the water surface is uninterrupted. Moreover, greater humidity exists close to the water surface m a reservoir than at a distance or at another elevation. For these reasons, land pans at reservoirs frequently are not in the best locations for estimating reservoir evaporation. In theory, floating pans, partly submerged, would have evaporation losses more nearly commensurate with actual reservoir evaporation were it not f'jr their unreliability caused by water splashing into or cut of the pan during times of storm. A number of floating pans are used in California, they would be more numerous but for this tendency tovrard unreliability. Relation o f ^ Vifater Temperatu re to Evaporation At various times in the past 40 years evaporation studies have been carried on by the Agricultural Research Service or its predecessors. All but two of these studies were in cooperation with the State of California, The first was in connection with investi- gations of evaporation in irrigation and water requirements of crops in the years 1903-05 (14) « Among other studies, the relation of temperature of the water to evaporation was established by means of heated v;ater in evaporation pans. Average daily water temperatures were obtained at four stations during the summers of 190'->.-05 for "17" comparison with average daily evaporation. The results indicated in Fig. 1-a, shov/ that evaporation increases with water temoeratures, but as other factors were involved it may be expected that for the same average water temperature evaporation varies for different localities. Thus, in Fig. 1-a, the line representing Berkeley conditions shows less evaporation for the same average temperature because of the higher humidity of the coastal area than that shown for ' oldiers Camp near Lone Pine, which is removed from the coastal influence. -18- A different form of curve was obtained during studies at Baldwin Park, Los Angeles County, through plotting maximum daily water temperatures against daily evaporation from a Weather iiureau pan (46). The points on the curve, shown in Figure 1-b, are weighted averages of many observations, so that the diffusion of points is confined to a narrow range. The curved line is fitted to the points by observation. The tendency of the curve to approach the horizontal at its upper limits is an indication of heat dissipation resulting from the process of evaporation. The Relation of Air Temperature to Evaporation K Although temperature is one of the principal factors caus- ing evaporation, it is not the only one. The differences between air and water temperatures, wind, and humidity, together with the length of the day (which differs with the seasons), all combine to control the evaporation rate. The relation of evaporation to air temperature plots as a temperature-loop instead of a straight line. The longer the period of record the greater is the opportunity for securing a smooth curve with all points falling in regular order. The temperature-loop in Figure 2 shows the relation betv/een monthly evaporation from a 12-foot diameter ground pan and mean monthly air temperature at the evaporation station near Fullerton. Wot all points fall directly on the curve, as other factors are involved. The temperature-loop plots in two parts, each representative of a different period of the year. For the same mean monthly air temper- ature, evaporation from a shallow pan is greater in the first half of the year than during later months. For example, for a mean monthly temperature of 65 aegrees the average monthly evaporation in Figure 2 is approximately 6.3 inches in early summer as compared -19- with 4.3 inches for the same length of time in the Jeptember-October period. For a deep lake or reservoir the temperature-loop is re- versed, since the heat stored at depth in the water returns Lo the surface in the late summer or fall, where it causes increased evapo- ration. 75 — / . AJig. ro — Sop../ L. / J. 'y 3 O 01 R'% / / 1/ c> June E « Oct. t / / o / / c o 60 / / / o May 2 55 - D N ac. °/ >v./o / / 1/ /J rll / o/ Mor ch — Fig. I o' /Feb Jon. Temperature loop • tho relation ivapo rotlon from 12- t. pan . 1 2 3 4 S « > 7 e Monttily evaporation-Inches Fhe Relation of Altitude to Evaporation With other conditions unchanged, evaporation would increase with elevation as the rarefied atmosphere at higher levels offers less obstruction to the water molecules that escape from a freely-exposed -20- water surface. Highei' elevations, however, ai-e characte:»'ized by- lower temoeratures and changes in other climatic factors that more than offset the effect of decrease in barometric pressure. The net result is a decrease in evaporation that is more or less proportional to the decrease in temperature. Few attempts have been made to determine this relationship; - the wt . V/hitney study is the only one known to have been undertaken in California. Mt. 'jVhitney Study , An early attempt was made in 1905 by Frank Adams, then of the Office of Experiment Stations, J. S. Depart- ment of Agriculture, in cooperation with the State of California (14) to determine the effect of altitude on evaporation from a water _ surface by measuring the depth of water vaporized from a series of pans set in the ground at different elevations on the eastern slope of Mt, 'iftfhitney. Each pan was 22 inches in diameter. Besides the evaporation pans the equipment at each station consisted of a rain gage, maximum and minimum thermometers, hook gage and sling psy- chrometer. The period of measurement was limited to 20 days. The positions of the stations, located between Lone Pine and Mt . Whitney, were selected with care but did not possess altogether uniform conditions as regards the surrounding topography and ground cover. _ Observations were conducted at the following places be- w tween elevations 4,51$ and the top of Mt. Whitney at 14,502 feet: Station Kiev. ,f eet Soldiers Camp , , 4,515 Junction South Fork and Lone Fine Creeks . . . 7,125 Hunters Camp .,.,....,.,....., 8,370 Lone Pine Lake , 10,000 Mexican Camp , , 12,000 Summit Mt , IvTiitney. .... ,......., . 14,502 -21- I 5.000- 12,500 10,0 00- 7,500 5,000- Fig. 3 Relation of oltlluda to evoporotlon on tha «act siopo of Mt. Whjtniy "Temperature 'F 45 50 55 60 65 70 75 80 I o Summit of 9 Mt. Whitney - Elev. 14,502 ft. 0.13 .14 .15 .16 17 .13 19 20 21 22 -23 Daily evaporation in feet From examination of the Mt. Whitney topographic map it ap- pears the evaporation stations probably followed Lone Pine Creek approximately. The lower station at Soldiers Camp appears to have been in the moderately rough country three to four miles west of Lone Fine. The .junction of South Fork and Lone Pine Creeks probably was in steep country rising sharply above the creek on both north and south slopes, and from there on to the top of the mountain the slopes appear to be rough and steep. Under such conditions it is probable that pan exposure varied with respect to sun, wind, and temperature. -22- Copies of the original data are unavailable, but from a chart of the results prepared by Carl Rohwer, Table 1 and Fip;ure 3 have been nre- pared showing probable daily evaporation and temperatures that are in conformity with curves plotted at earlier times when the data must have been at hand, TABLE 1 Evaporation and Temperatures at Locations on the East Slope of Mt . V/hitney, California Elevation , feet Mean daily evaporation, foot Mean daily temperature, °F. 4,515 7,125 S,370 10 , 000 12,000 14,502 0.223 .170 .147 .136 .134 .140 82 82 74 58 49 48 Plotted evaporation and temperatures show a close relation- ship to each other. The curves are nearly parallel except at ele- vations 4,515 and 7,125 feet where there was little or no change in temperature regardless of altitude. Evaporation decreased uniformly from elevation 4,515 to 8,370 feet and more rapidly from there on up to 12,000 feet. The evaporation pan on the summit of Mt . Whitney, in contrast with those on the eastern slope, was exposed to winds from all directions and shows a slightly higher rate of evaporation than that from the two pans below. It is doubtful if the curve should pass through the summit point, and for this reason the dot- ted line misses it at the left. Above 7,000 feet the temperature decreases at a nearly uniform rate up to 10,000 feet, more rapidly to 12,000 feet, and from this point to the summit the temr^erature -23- difference appears to be only one degree. This study was made for the purpose of determining the effect of altitude on evaporation, but the results are inconclusive as they show a decrease in evapo- ration to be more closely related to change in temperature than to change in barometric pressure. Under certain conditions, hov/ever, evaporation may increase at higher elevations. This relationship is best shown from records of the Los An^rceles County Flood Control District pans obtained from sea level to elevations of 3,000 to 4,000 feet in the San Gabriel Mountains. The Flood Control pans are all of the same size, so that a direct comparison is possible. Al- though other factors than elevation affect the evaporation, it has been found in Los Angeles and San Diego Counties that evaporation increases with elevation. Also evaporation increases with distance from the ocean, as the higher mountain areas are the farthest from the coast. In the areas involved, the lov;er altitudes, being closer to the ocean, have higher humidities than those at a distance or at greater elevations. In the lower levels fogs are not uncommon. They may be dense local ground fogs or high fogs; in either case they obscure the sun and cool the atmosphere. Thus, evaporation is lowered. The higher elevations, being at some distance from the ocean, are less affected and usually are entirely above the fog belt; thus evaporation is increased. In some instances, particularly for stations situated in summit areas betv;een the ocean and desert regions, dry winds from the desert contribute further to increase the evaporation. The relationship of evaporation to altitude in Los Angeles County is shovm roughly in Figure 4-a. The same data have been used in Figure U-h to show the general relationship be- ^.•fi.-^n evaporation and distance of the evapor-ition pans from the \'./ic Ocean. Both charts show higher evanoration as both elevation -24- and distance from the ocean increase. Cuyamaca Reservoir in San Diego County, at elevation 4,600 feet near the summit between San Diego and the Imperial Valley, is thus affected. Evaporation at Cuyamaca Reservoir is greater than at several other reservoirs located at lower elevations nearer the ocean. ft The Salton Sea Investigation Prior to 1907 there had been little interest in evaporation investigations made wholly for the purpose of studying evaporation —laws and developing formulas applicable to western arid and semiarid conditions. In 190? the U. S. Weather Bureau undertook some pre- liminary studies at Reno, Nevada (3) in order to determine the means of approach and the type of equipment necessary for further studies then contemplated in the Salton Sea desert area of Southern Cali- fornia. In 1908 preliminary studies were undertaken in the area surrounding Salton Sea, and in 1909-10 the Salton Sea investigation was in progress. The purpose of the investigation was the study of natural laws affecting evaporation from water surfaces and the development of a general formula embracing all the conditions in- volved. As far as can be determined, however, the results were inconclusive and published reports on the investigation are fragmen- tary. The program included study of air and water temperatures, v;ind movemeQt at different levels, vapor pressure, evaporation from '^ans of different sizes and at difl'erent elevations with regard to ■.he surface level. The main portion of the study was undertaken at ^alt Creek Bridge over an arm of Salton Sea, but supplementary studies were carried on concurrently at Indio and Mecca in the Coachella Valley to the north of Salton Sea, at Brawley in the -25- Imperial Valley, and at Mammoth in the desert area southeast of the sea. Salt Creek Bridge was on the Southern Pacific Railroad on the eastern shore at Salton Sea. This large body of water, which had an area of approximately 42$ square miles at the time of the investi- :-ation, lies below sea level in a desert region of extremely high summer temperatures. The water surface fluctuated according to in- flow from the few streams in the vicinity, principally Nev/ and Alamo Rivers, which carried surplus water from the Imperial Valley; also from rainfall on the water area and by loss of water from evaporation, San Felipe Creek and Whitewater River flow into the sea following storms, but published records for the period of investigation are not available. Being below sea level there was no outflow, and since the bottom of the sea is presumed to be composed of tight materials, seepage losses may be considered negligible. Evaporation was measured from pans located at towers erected on land and offshore. Tower No., 1 was 1,500 feet inland from Salt Creek Bridge, on a mesa 30 to 40 feet above the sea. Five two-foot diameter pans each about 10 inches deep were observed. Pan Mo. 1 was at the bottom of the tower and four similar pans were at 10-foot intervals on staging to a height of 40 feet. Anemometers accompanied each pan, but the records of wind movement at all levels do not appear to be available. Tower No. 2 was 500 feet offshore in 2$ feet of water. Pans at this tower were four feet in diameter with Fan No. 1 suspended above the water as close as the waves would permit, with other pans at 10-foot intervals to a height of 45 feet. Tower I'lo. 3 was offshore and was used for special experiments which are not here discussed. Tower No. 4 was about 7,500 feet offshore in y5 feet of v;ater with four-foot circular pans placed as at Tower I -26- No. 2. In addition, several land pans were located in line between the sea and Tov\'er No. 1 to determine the effect of distance from a water surface on the evaporation. Data for these pans are not available. At each of the four supplementary stations six-foot pans were placed on boards at the ground level and tv/o-foot pans on towers 10 feet above ground. There appears to have been no effort to have all pans exactly true to dimensions of diameter and depth. Nominally, the pans were described as tv;o, four and six feet in diameter and 10 inches deep. Actually these dimensions were not maintained in I construction as apparently it was not understood by the investi- gators that uniformity in size was of any importance. At the be- ginning of the investigation it apparently was believed that evaporation would be the same from pans similarily exposed regardless of size. During the course of the work it developed that such an assumption was in error. Diameters of the two-foot pans varied from 23 to 26 inches, which was sufficient to affect evaporation rates slightly. Diameters of the four-foot pans were more uniform, but their depths ciffered from 9.4 to 10.4 inches, a variation that probably would have less influence on evaporation loss than variations in diameter. The six-foot pans varied in diameter from 70.0 to 73.9 inches and in derth from 9.1 to 9.4 inches instead of the prescribed 10 inches. No corrections appear to h'ive been made in any of the evaporation records on account of discrepancies in size. Some of the results of the study at Salt Creek Bridge uring narts of 1909 and 1910 are presented in Table 2. Mean temper- atures are shown in Table 3. Only data for pans at the top and the bottom of the towers are shown, and these are not for a complete 12- m. ' .^h period. There is, however, a complete set of evaporation data -27- r CO CO ■n O LfN 0) c c Ih • to (^ O OvJ to [>- to oo o o o o O-^-^O -4- ON rH ^o^ ■p ■p j:: — ;- • ••••• • • • rH rH o to C^ i^ ur\ (^ C^vO vO - to CO <^tM c r-\ r-i t-\>-i o '0 O Cm ^Cv3 • O ^ tO o > CO > s -p • • • • O o .--o • •a ^ X) IS t. 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H.CU ON o CO £) o -28_ for the top and bottom of the three tov/ers for the last six months of 1909 (4) and from them certain conclusions can be drawn: In all cases, evaporation is considerably higher at the top of the towers than at lower levels regardless of whether the pans were over land or water. This resulted from greater wind movement and lower humidity in vertical sections. Evaporation at Towers 2 and 4 was nearly identical for each elevation above the water surface, indicating probable uniform moisture conditions of the air at these levels re- gardless of distance from the shore. This could be expected, since prevailing winds passed over several miles of water surface before it- reached the towers. TABLE 3 Temperature at Salt Creek Bridge (Tower No. 1, Salton Sea Investigation), Riverside County, California Month Mean air temperature degrees F,-^ Month Mean air- temperature-, degrees F, 1909 June July August September October November December 1910 ss January 92 February 90 March ^5 April 72 May 60 52 43 56 66 74 82 -'■Records obtained from original notes of the investigation ■ Differences in evaporation at Towers 1 and 2 are attributed to two factors: Tower 1, at which the higher evaporation occurred, o 1,500 feet inland, in the desert, where temperatures were higher tnan over the water, and the drier air had a greater capacity for -ture. Also, these pans were smaller than at Tower 2, Both 1 tors indicate increased evaporation at the land station. I -29- The evaporation station at Indio was in an alfalfa patch which was irrigated and cut as necessary. The effect of the tall grass surrounding the pan was to decrease wind movement at the water surface and have a lowering effect on the evaporation. However, the yearly total amounted to 119 inches from a 6-foot diameter pan (Table 4), which is a reflection of the high temperatures and long evaporating season in the Indio region. Evaporation from the 2-foot pan at the top of the 10-foot tower totaled 200 inches, an increase that should be expected because of the opportunity for greater wind travel and because of the smaller size of the pan. Evaporation measured at Mecca, Brawley and Mammoth during a 12-month period is shown in Tables 5 to 7. The data were taken directly from the United States Weather Bureau Abstract of Data No. 4 (4) in which the year was not specified. There is some reason to believe that the tabu- lation was made up of broken records obtained during 1909-10. The high rates of evaporation indicated for these localities show the effect of desert temperature and humidity. Estimates of Probable Evaporation F ro m Salton Sea . Salt on Sea was formed in 1905 as a result of a break in the banks of the Colorado River which poured into the Salton Basin for a period of nearly two years, eventually forming a body of water some 15 miles wide by 45 miles long. From this area there is no outlet, as the bottom of the Salton Sea lies at a depth of 273.5 feet below sea level. Into it drain the flood waters of a large mountain and desert region through the channels of Vi/hitewater River, San Felipe Creek and Mammoth Wash. Surplus water from the Imperial Irrigation District flows into the sea through Alamo and New Rivers. Flood v/aters enter unmeasured, but for many years the irrigation district has kent records of drainage inflow. 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CM x: p c o s: CO u^ -J- CVJ to (>i vO rH O O-O H • ••••• LTN CC C^ to r^CV C3n u^m rH rH rH P CO 0) E 0) d) XI P> O to P,P :3 3 QJ u •-:) < '/5 O S XI e O 0) Q ^tOvO rH O rH ^OtlO rH O-CV -^ rH rH C\i CV CV O O O LTN [>. S -4-u^tx3 cv l/^^0 >. >1 fl U 03 03 ;:3 x: rH 3 U o •H (U C XI Ih f- >^ c 03 d) 03 P 03 3 -oPjh ^^ <«:s:*^ o CO r-\ OS P O Eh (0 tn CO Q) P, P. 03 03 S • <+H CU o x: p> c o c •H -H P 03 03 P -H (0 Jh P 'C3 03 03 a. O -H tn rH t-i 03 •H O o! Pi ch o o p CO 03 Cm •H CJ 0) 03 C (D Sh X) (D x: 0) p > 3 03 o x: CO o (U p X f-l 03 P 03 OJ G •-3 O 03 C 03 •-3 Q) Xi +J • ON PO 03 O x; H -p c CO -H 03 03 0) +J a 03 p.-a 03 U P Q) •H X) E P Q) 3 O XI 0) a u 03 O rH O >^ r-i rH "-O Q) ^1 •H P C Q) x: p -a a 03 p o CO •-{ to o •H rH o O T3 tU Q) !-, -O O 03 0) Q) >> f-, tu 5 x: CV -34- at several stations north and south of the sea from which it is possible to estimate the depth of rain falling on the water surface. Stage heights also are recorded by the Imperial Irrigation District. Data are available from which estimates of water surface areas may- be computed for different water stages. With these records it be- comes possible to estimate roughly the probable evaporation from Salton Sea for periods of moderate to normal precipitation and of low flow in unmeasured streams. In other years there probably would be sufficient contribution to the sea to affect the accuracy of the estimated evaporation. These years should not be included in evapo- ration tabulations. Using such data a few engineers have estimated evaporation for Salton Sea, the results being in general agreement. Unpublished figures prepared by the Salton Sea investigators show computed evapo- ration for the 10-year period 1909--10 to 191^.-19 to be 6^,76 inches annually as shown in Table S. In estimating these values the in- flow from Alamo and Nev; Rivers was arbitrarily taken as 277,000 acre-feet annually. The records do not show that any inflov; was considered from such streams as San Felipe Creek, V/hitewater RiA^er or the numerous flood washes that enter from the east. Robson (34) estimated total evaporation for the six-year period April 1, 1907 to April 1, 1913, to be 65.34 inches, on basis of the following data: Loss in elevation of Salton Sea 26oj.O feet Total rainfall on lake surface 1,3^ feet Total run-off into Salton Sea 1.25 feet Discharge from Alamo and New Rivers (estimated) 4.19 feet I Total 32.92 feet Yearly average 65.84 inches -35- , oince the dischar.^zie from Alamo and i\iev/ Rivers h-aa to be estimated and there was no record of inflow from ^vhibewater River and dan Felipe Creek it is probable that the total and average are too lov;. TABLE 8 Average Computed (Lake) Evanoration From Salton Sea, California, 1909-10 to 1918-19 (Source: Unpublished estimates by Salton Sea investigators, U.S. '«i'eather iiureau) Year Evaporation in inches Year ; 1-0^^-10 1. '10-11 1-11-12 lvlJ-13 191J-14 72.76 66.57 64.22 65.99 68.27 1914-15 1915-16 1916-17 1917-18 1918-19 Average Evaporation in inches 84.69 65.23 53.17 69.73 76.96 d8,76 ■ Probable evaporation shown in Table 9 was computed by Grunsky for 1907-08 (l6) in the same manner. This table appears to be subject to some adjustment, possibly because of the effect of wind in changing water surface elevations for some months. In recent years the rising v/ater level in Salton Sea has resulted in encroachment on adjoining lands, causing some concern to the owners. The flat lands at the southern end of the sea are most affected as a small rise in the sea level here covers a v.'ide expanse. In addition, the outlets of drainage channels are being submerged by rising waters. Investigations of Evaporation From Small Water Areas Evaporation studies have been carried on for a number of years at Denver and Fort Collins, Colorado, and at Fullerton and other -36- Southern California localities. The results obtained have been of general benefit to engineers through discussion of evaporation funda- mentals, the development of evaporation formulas, and in establishing the values of evaporation coefficients for the reduction of pan evapo- ration to equivalent evaporation from larger water areas. The difficulty of direct determination of evaporation from large water areas results from the general impossibility of obtaining a complete inventory of all the waters entering and leaving the reservoir. In isolated instances where the only change in water levels is through dissipation of moisture into the atmosphere, evaporation may be measured directly with staff gages. Occasionally, opportunities exist for computing evaporation from records of inflow, outflow, bank storage, precipitation on the water surface, and changes in water surface levels. In such cases evaporation is the residual item in the water supply. Both conditions are predicated on the assumption that seepage from the bottom of the reservoir is negligible. TABLE 9 Computed (Lake) Evaporation From Salton Sea, California, 1907-08 (16) : Evaporation Evaporation Year and month : in inches Year and month in inches 1907 1907-continued April 5.16 October 6.84 i-iay 8.52 November 6.48 June 8.8a December 4.20 July 8.28 August 6.36 1908 September 11.16 January February March 2.16 2.64 3.00 ^ ^ X ^ Total 73.68 -37 - Usually evaporation is measured from small water surfaces in standard evanoration pans. Such measurements may be reduced to lake or reservoir equivalents through use of conversion factors or coef- ficients derived experimentally for the type and size of pan from which the records at the reservoirs are obtained. In actual practice a number of types and sizes of evaporation pans are in common use. The Weather Bureau pan is set above the ground surface where it is exposed to the sun's rays and the svveep of the wind, both of which increase the evaporation. Both circular and square pans set in the ground with only a few inches of rim exposed are partly insulated by the surrounding soil so that there is a tendency toward a more uni- form water temperature and a lower evaporation than in the exposed Weather Bureau pan. The ratio of the wetted perimeter of the pan to the water area is likewise a factor in increasing the evaporation, as water evaporates at a more rapid rate when in contact with the warm metal that forms the boundary of the water surface. The rim effect varies inversely according to the diameter of the pan, the greatest relative effect being on pans having the smallest diameters. The ratio of pan circumference to area of the water surface is — which d is equal to a value of four for a one-foot diameter pan as compared with a value of 0.333 for the 12-foot pan. Thus, the rim effect is 12 times greater per unit area for the small pan than for the larger one. The capillary rise of moisture on the inside of the pan, in- creased by a slight wave action, creates a wetted area from which evaporation occurs at a higher rate than from the horizontal water surface. Denver, Colorado, Investigations . Determination of evapo- ration coefficients for a variety of pans has been one of the long time objectives of the Department of Agriculture. The first of these -33- studies was undertaken at Denver, Colorado, where an outdoor evapo- ration laboratory was established in 1915 for studying, from an engi- neering point of view, problems connected with the utilization of water in irrigation. A general lack of information regarding specific conclusions that would be useful to the engineering profession prompted the investigators to undertake the following studies: (a) Variation in the amount of evaporation from pans of varying sizes ; (b) Variation in the amount of evaporation from pans of varying depths; (c) Comparison of the amount of evaporation from flowing and still water; (d) Comparison of the results obtained from different types of so-called standard evaporation pans; (e) Comparison of the evaporation amounts from round pans and square pans of small size; (f) An extension of the results of experimental pans to larger water areas. Measurements were made during 1916 and 1917 from a series of circular ground pans of diameters from 1 to 12 feet, each three feet deep, set in the ground 2.75 feet. Other types included a Bureau of Plant Industry pan, a Colorado type square pan, a V»eather Bureau pan and a floating pan. Coefficients were determined as a relation of the evaporation from the various pans to evaporation from the 12-foot diameter pan. Because of climatic conditions resulting from the high altitude at Denver it was not possible to carry on evaporation measurements during winter months and the coefficients are necessarily based on approximately eight months of record. On completion of the season of 1916 a progress report pre- sented a partial list of evaporation coefficients for the principal pans studied (37). During the following year, 1917, measurements -39- were continued and a summary of the coefficients obtained during the investigation was published (3^). Fort Collins, Colorado, Investigations . A second investiga- tion was undertaken at the Colorado Agricultural Experiment Station, in which the objectives were "determination of factors causing the derivation of the general law under which these factors operate and the evaluation of the relation between evaporation as it takes place from various types of standard evaporation tanks and as it is found to occur from a larger water surface." Evaporation from a V/eather Bureau pan, a three-foot square ground pan and a three-foot square floating pan was compared with the loss from an 85-foot diameter reservoir seven feet in depth. Measurements were begun in September, 1926, and continued through the open-water season of 1927 and 192^. This study was also limited to a period of approximately eight months each season, Although determination of an evaporation formula was the principal objective, the data permitted establishment of coefficients relative to evaporation from the 85-foot reservoir. Descriptions of the ex- periment and conclusions arrived at were published in 1931 (35). Southern California Investigation . This investigation dealt with evaporation losses from various types of evaporation pans in a coastal region where freezing was not a factor and measurements were possible throughout the year. It established relationships of such losses for monthly and annual periods continuously from 1935 to 1939, inclusive, at a central evaporation station at Fullerton, Orange County, about 10 miles from the coast, and from 1939 to 1941 at Lake Elsinore, Riverside County, about 25 miles inland. Fullerton Evaporation Station . At this station the mean annual temperature during the period of investigation was 60 degrees and the relative humidity was 68 per cent, with high thin fogs a -40- common occurrence during summer months. Wind velocity, 20 inches above ground, averaged 2.8 miles per hour. Rainfall, varying from 11 to 23 inches annually during the periods November to April, averaged ft ^5.75 inches per season. The principal study was determination of pan coefficients relative to evaporation from a 12-foot diameter pan, three feet deep, set 2.75 feet in the ground, with the water surface coincident with the ground surface. Previous experiences with a 12-foot pan and an 8$-foot diameter reservoir by Sleight (37) and Rohwer (35) had led to the general conclusion that for diameters greater than 12 feet the size of the pan had little effect on evaporation. It is believed, however, as a result of the author's studies, that evaporation from a 12-foot pan is not the absolute minimum that would be obtained from a larger pan, but that the difference is immaterial in view of other discrepancies often appearing in evaporation measurements. For comparison with evaporation from the 12-foot pan measure- ments v/ere made from a series of circular ground pans of similar depth, with diameters of from one to six feet. In addition there was a V^'eather Bureau pan, a square ground pan of the Colorado type, a Bureau of Plant Industry pan, screened pan and an insulated pan, from all of which evaporation measurements were obtained continuously for periods of from two to five years. Also, there was a series of small pans i from which tests were made to determine the effect of color of pans and the effect of different concentrations of salt solution on evapo- ration losses. Summaries of evaporation from the principal pans at the Fullerton Station are shovm later in this report. Lake Elsinore Evaooration Station. Lake Elsinore, with an area of about 5,500 acres, is an excellent outdoor evaporation labo- ratory. Its water supply comes from the San Jacinto River which -41- flows only during the winter and spring months; this flov/ is measured by the U. S. Geological Survey a short distance above the lake. There has been no outflow since 1916. All the evidence points toward a tight lake bottom that prevents seepage losses of any importance. Evaporation studies were undertaken for the purpose of checking some of the Fullerton station coefficients. Evaporation was measured from a Weather Bureau pan and from a screened pan and was computed for the lake from records of inflow, rainfall on the lake surface, and changes in lake levels. A considerable degree of accuracy was possible in arriving at lake evaporation throughout the long dry summer months when the only change in water surface v/as through evaporation. Meteorologic conditions at the lake were similar to those at the Fullerton station. During the period of measurement the average temperature was 64 degrees; wind movement averaged 2.0 miles per hour, alternating between land and ocean breezes. Rainfall varied from 10.96 to 24.45 inches annually. -42- CHAPTER IV EVAPORATION COEFFICIENTS The usefulness of evaporation coefficients is better under- stood when it is recognized that evaporation from small artificial water surfaces is greater than the loss for larger areas. Ground pans of equal depth but of different diameters, installed under identical conditions of temperature j wind, humidity and rainfall, have different rates of evaporation, the smaller pans having the higher losses. The relation of evaporation from a given size or type of pan to evapo- ration from a different pan or from a larger body of water is designated as a coefficient and is a ratio. It is variable according to the integrated effect of the meteorologic factors on different pans, and is usually higher in summer than in winter. Annual coefficients are less variable than monthly coefficients. Coefficients are useful for the reduction of evaporation from a small pan to that from a larger pan or from one pan to another of different characteristics. It is the common method of estimating reservoir evaporation from pan records. In general, it may be presumed that coefficients obtained as a result of the investigations in Colorado are applicable to the region of the inter-mountain states where winter temperatures are be- low freezing. Coefficients obtained in Southern California would appear to be applicable to the lower areas of the southwestern states where winters are short and mild. In California the Colorado coef- ficients probably apply to the higher mountain regions, while the Southern California coefficients are more suitable for the lower ele- vations of coastal and interior valleys. -43- The Weather Bureau Pan Coefficient A summary of evaporation records at the Fullerton station is presented later in this report. The ratio of evaporation from the 12-foot diameter pan to evaporation from the smaller pans gives the value of the coefficients that are for use under similar conditions of exposure and weather conditions. The Weather Bureau pan coefficients were consistent throughout the five-year period of investigation, the average annual value being 0.77 with variations from O.76 to 0.7^. During the three-year test at Lake Elsinore the average annual coef- ficient for the 'Weather Bureau pan, based on computed evaporation from the lake, was identical with that at the Fullerton stations, but major differences occurred in the monthly coefficients as shown in Table 10. The excellent agreement obtained through the use, as basic evaporation areas, of such dissimilar v;ater areas as a 12-foot pan and a $,500-acre lake is proof that the 12-foot pan is as large as is necessary for the computation of satisfactory evaporation coefficients. Differences in coefficients as regards pans and reservoirs are due to the capacity for heat storage in the different water volumes. In the pans much of the heat recieved from the sun during the day is lost at night. In the larger volumes of v;;;ter a portion of the heat received during the spring and early summer is used in evaporating the surface water and another portion is absorbed in warming the water to a considerable depth. Later in the season the heat in storage gradually returns to the surface where it becomes available for increasing the evaporation during the cooler fall months when pan evaporation is approaching a minimum. During the early part of the season pan evaporation exceeds lake evaporation, but in the fall and winter months evaporation from the deeper body of water may -44- OJ O C»H tn 0) o ■P x: -H C p -p Q) 03 ■H p +J O CO cn •H tH c c Cm CO o a, -H O -p o d CO CO u c Q) o o Sh a •H ^S CO ■P OQ > o w 3 Jh X) 0) 0) QJ x: Jh ■xi P o o CO C H T5 -H C '^ CO W 01 rH hA CO M m C -a: O -a Q) E-« •H c ^ P CO CO rt hJ fn C o (0 X) ao, c cd CO > T3 ty 0) c c o >. 0) p> rH 0) t, X! 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H tOvO -4-nO C^^OtO u-NU^LPvC^urNLrN-^-d-NO unqn-* > U O ■■-:* o OT ch •-i -/3 to x^ X3 H Q) 4J Sh (0 S ^ hC^H O u OrHu^O-Orr^rrA^CVONrH -^ u\tO tO CV -J-ON-d- ON ;ti o c o O 03 rH rH CV r-\ <-i r-i r-{ r-\ r-{ t>- ^ ^. X! H < c O CO 10 H rH •H 0) CO CO cfl CO +J ^ CO +J +J +J % 03 CO . 0) bDhOM hthObDoSOOOhChD o o ^ ■a; iTiQQ c:aQi::i>tHfHtHQQ QJ +J 1 ^l -H O CO) Geo cO+J4->+3 GS G u Ih o to^i^i!::c:(::>>o)c:ccrHs:;cs::c:c:>>o3 E-< d ^ rt GQ)03030303Gt^03fl30303OOO0303PrH o o > O CTJ ;-Jl L:^' ^_) cn r/T:n M tin cn ^.0 CO o o cj o 'O '0 M (X 0) 03 03 •H Jh E X) C ao. G u tn O to (0 O f~, U U -H U (D C QJ 03 O c o •H +J (0 +J CO ouquet Canyon Rese uena Vista Lake lear Lake uyamaca Reservoir 1 Capitan Reservoi enshaw Reservoir ndependence ingsburg ake Hodges ower Otay Reservoi orena Reservoir ardee Reservoir an Pablo Reservoir an Pablo Reservoir an Pablo Reservoir an Vicente Reserve weetwater Reservoi ineraaha Reservoir pper San Leandro R iXonthly records i Pans v/ere 10 inch Square concrete b 1 PQ m O O W 3: M :^ hJ t-q s: t^ CO fO ro CO CO H o H CNi r'N -50- coefficients determined by Sleight l3S) for a floating pan and by the author (i. V) for a lane pan. Sleight's floating pan was 3,400 feet from the laboratory where his other records were obtained and the different location or ohe higher humidity at the lake, could account for the higher evaporation ratio. The high coefficient for the land pan at the Fullerton station has previously been explained as the result of a higher pan rim than those reported at other evaporation stations. For square land pans similarly installed a coefficient of 0.89 is applicable for Southern California. TABLE 12 Comparison of Evaporation Coefficients for 3x3 Foot Square Land and Floating Pans for Reducing Pan Evaporation to Equivalent Evaporation From Larger Water Areas Investigator Evaporation coefficients for 3 X 3 ft. pans Land Floating Sleight Rohwer Subcommittee Hall Young (38) (35) ( 1) (17) (49) 0.79 0.89 .79 .77 .78 .80 .81 .80 1.89 l Higher coefficient is a result of a four-inch rim above the ground surface . The Screened Pan Coefficient The most efficient pan is the one for which the coefficient approaches unity; that is, the evaporation from the pan closely ap- proximates evaporation from a larger body of water. Attempts by the Department of Agriculture to produce a pan having this characteristic resulted in a screened pan designed and tested at the Fullerton station during a four-year period. This pan was two feet in diameter, three feet deep, set in the ground 2.75 feet. At this point a new -51- principle in evaporation studies v/a3 introduced in the form of a $-inch galvanized mesh screen suspended horizontally midway between the top of the pan and the average water surface. The screen reduced the interception of heat energy at the water surface during the day, reduced back radiation at night and lessened the wind effect over the water. Average annual evaporation was less than that for any other type of small pan and closely approximated the evaporation from a 12-foot ground pan. ■ The average annual coefficient for reducing evaporation from the screened pan to equivalent evaporation from the 12-foot pan was 0.98. Monthly coefficients varied considerably, being slightly above unity from March through July and tapering off to values as low as 0.81 during the colder months. At Lake Elsinore a three-year test produced identical annual coefficients (47) but viith significant differences in the monthly coefficients. Because of the greater capacity of the lake, heat stored in the water at depth earlier in the year moved upward as the surface water turned colder and sank. Thus, the surface of the lake continued v/ar-m for many weeks after the _ air temperatures began to cool and lake evaporation during fall months exceeded the loss from the evaporation pan. In consequence, the monthly coefficients were less than unity during the early part of the year and greater than unity during the later months. Results of the screened pan tests at both stations have been shown in Table 10, For a more general application, Table 13 shows evaporation coefficients for a majority of evaporation pans in common use as determined by similar investigations at Denver and Fort Collins, Colo- rado, Milford, Utah, and Fullerton, California. The agreement for the several locations was generally good although some tendency existed toward higher values at the Fullerton Station. For the -52- .;eather Bureau pan at the Fullerton coefficient was 0.77 as compared with an average of 0.70 obtained through the Colorado studies. For the Colorado-type square ground pan the coefficient was found to be 0.89 at the Fullerton station (49) as compared with 0.79 obtained at both Colorado investigations. This has been explained as being caused by the difference in rim heights at the different stations. TABLE 13 Evaporation Coefficients as Determined by Various Investigations in Western States Evaporation coe fficients V/eather: Screen : Bureau : Square : 12-ft. Location Period Authority: Bureau : (Young) : Plant : floating: diameter pan : pan : Indus- : pan : pan : try pan :U.S.G.S.: Elsinore, Calif. Annual Young 0.77, .77^ 0.98 Fullerton, Calif. Annual Young .98 0.91 1.00 Silver Lake, Calif. Annual Young & Blaney .612 East Park Reservoir, Calif. — Rohwer .69 Denver, Colo. Apr-Nov . Sleight .66 ..90 0.91 .99 Denver, Colo. Apr -Nov . Sleight .70l Denver, Colo. Apr-N ov . Blaney .691 .91 1.00 Ft. Collins, Colo. Apr-Nov . Rohwer .70 .77 1.00 Lake Hefner, Okla. Annual Kohler .09 .67^ .91 .91 Mil ford, Utah Mar-Oct . White 1.00 GeneraJ. A.3.C.E. .70 .80 Based on 12-foot pan. 2 Mojave Desert, Calif, -53- CHAPTER V EVAPORATION FROM LARGE V^ATER AREAS The number of evaporation records and coefficients available permit estimation of lake evaporation that is fairly dependable and the probable evaporation from most lakes and reservoirs is computed by this means. The most accurate data are obtained directly from staff gage measurements from closed lakes during the dry season when there is neither inflow nor outflow. A tight lake bottom is a prerequisite for this condition. Such opportunities are few. During the rainy season when streams are flowing, evaporation is computed from records of in- flow, outflow, rainfall on the lake surface and change in water levels. A few such records of monthly evaporation for California and Nevada are shown in Table 14. Mean annual evaporation is shov;n in Table 15. Buena Vista Lake is a shallow reservoir of fluctuating size covering several thousand acres about 20 miles southwest of Bakersfield, The data representing this reservoir are summarized for the period 1937 to 1945 from records of inflov;, outflov>f, rainfall on the lake surface and lake fluctuations by ';i/alter Ruppel, office of Harry L. Haehl, consulting engineer, San Francisco. The lake records were obtained by the Buena Vista V/ater Storage District. Rainfall was averaged from Vveather Bureau stations at Bakersfield, Buttonwillow and Maricopa. Evaporation from Tulare Lake has been estimated by Harding (20) for a period prior to 1916 when there was no inflow, and with the exceptions of periods of rainfall the evaporation could be measured directly from changes in lake levels as shown on staff gages. Seepage was considered to be negligible. Rainfall was taken from the Hanford records. Computed records for Buena Vista and Tulare Lakes, although -54- -J- 0) Xf > -d ctJ CO •H C O tH •H rH rt O c •H w 0) :s 0) CTJ u O -o x: o •H c •H c o •H ■P CO u o ^ ,-H ^ O P P — ax: c CO o o •H W 3 > "^ eg s CO o a)■. TD Oh 03 > ^ ^ ^ c • CO rH -> O +J ^ CO ;>>-H CfH ;^ 4J 4J — i-i (a c cn o Q o c :=i > r^cv Jsi -H O (U O — ' rH i: O rH - CO c£i -<^ Q) C • ^ ~ O +J CO C >. -H 1^ ^J (D P +J -— CO C CO O O CD w d > o cv rH CO O CD rH--' tiCt-q O rH - CO W Li-\ i:i *^ QJ fn O CD - C • q T3 >. O -P •H -H +J -H «H tn (D c P rH tn d CO O a 0) O >\0 > O O CNJ CO QJ -H rH '- •H ^ tf M rH c CO u -^ o •H " iH 0) CO -i^ c O CO - O • ^ (n >,-H +J hD+J P =H — CL) C C CO O fn -H d > O CV CO « O CD O — r-\ O rH (M d rd Fh CO P G CO - o • •H ~ >->.H P > (U C 4J +J Ch ^ 5-1 C CO CO (0 CD d > O c:,-j :^ o CD o (U O rH C\J d M XI JU 4J G O .^ 00"-0\0(\itOOOtO-^-COC\i\0 OO ON -4- ^tJO -;}•-<^OtOO^-01X)-4■too CV rH rv CV ,-^ -d-\0 ^O O- u^ -d- 1-^ O r-\ rHrH r-i t)OCo-4■ONOtx^o~;^-d■^-0-4■•to rHrHCVr^r^-^vOuAU-\r^cVrH O cv -4- ■CO^D O --d-tO O-tO 0~i> C\! CV (^ rHrHCV -4-u-\--OC^r--^Ou^(v>>cv CV ^sDOnOO-^-'OCVCV-vO-J-CV rH r-i r^r^^-D-CO O r-- r- c^ CV r-t C\t cvtJ^c^^^Ocvl/^(V■coM^u^^~- rHrHCV~d--^^tOOO^CVrH r-i >> >U X> Ui >dPOCDCD c:x)t-tH>-.CrHhDaj-)>o cocDcoacodddoJoooj o- -a 0) +j CO a rH •H CO p d to c Ui a ^ . 1 ■r^ >> doc • o 3 c 9 9 C ■a a 9 I. J o o +> c o o. o ^ • »H CO •> +> • TJ to M ^ S>§5 Sir. a) ^K 9 C O «> rH i-H —3 C t. C 9 a ^rt ^ 3 < a) -P c &:* > '^ bl 1 (0 ^ 0} 0) 1 X t V> +^ <-" a) +J .ii^ ui c > a> 9 9c 3 c c ,-, ^^ c o n a. o «< o •^ rv C +* to •vCO e ^2^^ 0) .» to t. C 9 T3 O « C C f-H U. C 3 O o o i; a. •rt o cS g5,§:5 g^5 u *» u ^ $^ X t. +> 9 9 C t. 0) 3 C +» •O C 9 C O Oj M o o » n ■f> •. O •>-( 9 ^ «, +> «H Oj o d •> o J3 > cd I" •-I t. C © -H t. ^ ,-H CM o o j:: O ® t. o • D. •rt O m 03 rH g^5 « m a: tJ •». 1 c fS X t- -P o 9 9c > to •a a 9 i. o -p c o a •-! •, c » M M 0) >> o a> Re Ang cunt vatl 20 f a) •to t. C 9 C to O 9 O o o x: iH O i-f • D. -H o O J « rH a! +» c c > -H u U •* 1 ^ X t- -P I. >, 9 9 C to -P c c *> •o a 9 c o 0) c 3 o o l-l CC (d O "H c 1 p •> to E. c 9 CO t. t- o c^ 0) ^ qj »-H CM O O X 3 > t. CM g^5 - s i CO ^ t. e 1 -»' X t. J-> ffl r-! a! 0) t, to O) > o 3 o to ^ 10 ca c <-i 9 9 C •a a 9 c o tH <^ 0) o to ST > • Oj • 10 o a! J +> rH C C 9 J c « c b c 3 c O O J3 O. T< o ® -.^ o o g^5 &. 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O ONJ- O fH iH ON ON S^ c^jU- J- no r> C^f^t-N,NO NO r-« O UNCO t^ r^r^NO r-^No * O ONCO C^ <-• O ON ON o o fsi. ONJ3- OO OA t^ C^ r-4 ITNNO CM CM CM -Zf C^ UN LTn UN UN UN NO ON O CM O^ O ON O rH ON NO -=}■ CO J- O r-t \0 t^ UN CM f^VO NO t*^NO t^NO iV C^ C^ CO C^ND C^ 2/ Station osiabllshod in oaoperctlon with the Corps of Engineers, ^ Eva^oratlon pan and ralngage installed by Leslie Salt Companj', Atmometer, hygrothennograph, and tliemionetoi'S Ins-iaLlod for this investigation. Observations by Leslie Salt Corapanyc -74. In connection Vv'ith the Corps of Engineers comprehensive investif.ation of the San Francisco Bay, it was required that evapo- ration be determined for the period October, 1922, to September, 1955, inclusive. Only two records were of sufficient length to cover this period, one at Davis and one at Newark. The Newark station was originally at Alvarado and the two records (Alvaradc and Newark) are sometimes combined to form a continuous record. Studies during this investigation showed that the two records differed and that an adjustment should be made of the Alvarado record in order to extrapolate the Newark record. This was done by accumulating the Alvarado-Newark record in reverse chronological order and plotting, A distinct break in the plotted line occurred at the time when the station was moved from Alvarado to Newark, Calculations showed that the Alvarado records should be multiplied by 1.056 to extend the Newark record properly. From the records of the two base stations (Davis and Newark extended) monthly percentages of the means were calculated and the short-time records at various other stations extrapolated on this basis. Table 2 shows the mean monthly evaporation at Davis with the per cent variation from mean for each month. Table 3 presents similar data for the Newark station. The annual evapo- ration at Davis varied from 89 to 115 per cent of the 33-year average and the annual evaporation at Newark varied from 93 to 108 per cent of the 33-year average. Somewhat greater departures from the 33 -year average will be found for individual months. In Table 4, the computed 33-year average monthly evapo- ration at several stations within the San Francisco Bay and bacramento-San Joaquin Delta areas are shown, -75- oa ^= c: = O.H. >-° xa: 1-00 y H 1 u U *J 0) c 1. d 3 ■• • c: a I c < c *] i- t. *j 0) 1. 9 a ( 01 1 c ^i c a a. >- lU (Q » xl t. L. tJ iD C *J 1. q GO L 3 ■■ • < CX-r >0 •■ > (1 ^ Li t, *J X Q > t J C a.'r tn k ^ Ih L< (J 4> c: i. a a; (. ^Vr 0. .- ID *■ jU t. t, ^ a> c c >, .. .. ^ c Cl.'H > t^ 'i u k *J 0) c H a < O-.H lU «J >] t. t, *J ■■ .- L. 1 c 03 a ' a.^ n: ►J ■u 1^ t* tJ OJ c 1. 11 •0 ■■ •■ 3 1 c j3 C1.-H

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O ^ OS rH J- ir\ US USNO rH CM p-^tv, ONC^ NO NO NO US UNJtf- k CM ONf^ OS C^ UN UN US USNO ON rH jT Q ON OS NO NO NO NO US-:f ON UN O US rH C^ J- US UN USNO NO a "ail 71 . > o 3 * o o o r> ^ U) O Z Q .80. Tq = water surface temperature, in degrees Fahrenheit Tq = air temperature, in degrees Fahrenheit The coefficients nov/ being used for the San Francisco Bay area are given in Table 6, TABLE 6. --MONTHLY COEFFICIENTS FOR REDUCTION OF EVAPORATION AS MEASURED IN A WEATHER BUREAU TYPE PAN TO EVAPORATION FROM A LAKE SURFACE, SAN FRANCISCO BAY REGION Month : Coefficient : Month : Coefficient January 0.60 July 0.80 February .70 August .80 March .70 September .75 April .75 October .75 May .75 November .70 June . 80 December .65 -81- 'L^TE TRACY Trac@ LEGENC; |T| STATIONS OPERATED FOR THIS INVESTIOATION (T) STATIONS OPERATED Br OTHERS UNITED STATES DEPARTMENT OF AGRICULTURE AGRICULTURAL RESEARCH SERVICE BERKELEY. CALIFORNIA EVAPORATION IN THE SAN FRANCISCO BAY AND SACRAMENTO-SAN JOAQUIN DELTA AREAS EVAPORATION STATIONS EQUIPPED WITH WEATHER BUREAU TYPE EVAPORATION PANS SCALt Of MILE 5 J 2 J ? !? 'LtUt THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW BOOKS REQUESTED BY ANOTHER BORROWER ARE SUBJECT TO IMMEDIATE RECALL JUL 1 7 1990 RECEIVED Nil ^9 1S9C PHYS SCI UBRAR^ LIBRARY, UNIVERSITY OF CALIFORNIA, DAVIS Hook Slip— Series 45K ?!in^c :: Cal-l^omia. Dept. of water resources. PHYSICAL SCIENCES LIBRARY Call Number: TG82U C2 A2 t n,0' ;: 3 1175 00486 0253' LIBRARY iWIVEHSlTV OF CALIF ORNU" DAVIS 240502