\- STATE OF CALIFORNIA DEPARTMENT OF PUBLIC WORKS DIVISION OF WATER RESOURCES EARL WARREN, Governor C. H. PURCELL, Director of Public Works EDWARD HYATT, State Engineer Bulletin No. 54 EVAPORATION FROM WATER SURFACES IN CALIFORNIA A SUMMARY OF PAN RECORDS AND COEFFICIENTS 1881 to 1946 1947 75021 printed in California state printing office TABLE OF CONTENTS Page LETTER OF TRANSMITTAL 5 ORGANIZATION, State Department of Public Works 6 ORGANIZATION, United States Department of Agriculture ACKNOWLEDGMENT 7 FOREWORD 8 INTRODUCTION 9 TYPES OF EVAPORATION PANS 12 The W^eather Bureau pan 12 The Bureau of Plant Industry pan 18 The square ground pan 13 The square floating pan 14 The Los Angeles County Flood Control District pan 14 The screened pan 15 Meteorologic equipment 15 EVAPORATION INVESTIGATIONS 17 Relation of water temperature to evaporation 17 The relation of air temperature to evaporation 19 The relation of altitude to evaporation 19 Mt. Whitney study 19 The Salton Sea investigation 24 Estimates of probable evaporation from Salton Sea 27 Investigations of evaporation from small water areas 30 Denver, Colorado, investigations 31 Fort Collins, Colorado, investigations 32 Southern California investigation 32 Fullerton evaporation station 32 Lake Elsinore evaporation station 33 EVAPORATION COEFFICIENTS 34 The Weather Bureau pan coefficient 34 The six-foot diameter ground pan coefficient 36 The square ground pan coefficient 36 The square floating pan coefficient 37 The screened pan coefficient 39 EVAPORATION FROM LARGE W^ATER AREAS 41 EVAPORATION PAN RECORDS 43 Collection of data 44 Tabulation of data 44 Long term mean evaporation 45 Alphabetical summary of evaporation pan records by counties 48 SUMMARY 57 LITERATURE CITED 60 2—75021 V •-> ) LIST OF TABLES Table Page 1. Evaporation and temperatures at locations on the east slope of Mt. Whitney 22 2. Evaporation at Salt Creek Bridge, Riverside County, California 26 3. Temperature at Salt Creek Bridge (Tower No. 1, Salton Sea Investigation) Riverside County, California 26 4. Evaporation at Indio, Riverside County, California 27 5. Evaporation at Mecca, Riverside County, California 28 6. Evaporation at Bravrley, Imperial County, California 28 7. Evaporation at Mammoth, Imperial County, California 29 8. Average computed (lake) evaporation from Salton Sea, 1909-10 to 1918-19 29 9. Computed (lake) evaporation from Salton Sea, 1907-08 — 30 10. Mean monthly evaporation and reduction coefficients for a screened pan and a Weather Bureau pan at the Fullerton and Lake Elsinore evaporation stations of the Division of Irrigation and Water Conservation 35 11. Comparison of evaporation from square ground and floating pans at various lakes and reservoirs in California 38 12. Comparison of evaporation coefficients for 3x3 foot square land and floating pans for reducing pan evaporation to equivalent evaporation from larger water areas 39 13. Summary of evaporation coeflScients as determined by various investigations in western states 40 14. Evaporation computed for a few lakes in California and Nevada 41 15. Annual evaporation from a group of Weather Bureau pans in the South Coastal Basin with evaporation indices based on a 21-year mean period of record at the Riverside Citrus Station 46 16. Annual evaporation from a group of Weather 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 47 17. Summary of evaporation pan records by counties (California) 49 LIST OF FIGURES Figure Page 1-a Relation of evaporation from a ground pan to mean water temperature 18 1-b Relation of evaporation from a Weather Bureau pan to maximum water tem- peratures 18 2 Temperature-loop — The relation of mean air temperature to mean evaporation from a 12-foot pan 20 3 Relation of altitude to evaporation on the east slope of Mt. Whitney 21 4-a Relation of evaporation to altitude in Los Angeles County 23 4-b Relation of evaporation to distance from Pacific Ocean-Los Angeles County 23 (4) UNITED STATES DEPARTMENT OF AGRICULTURE SOIL CONSERVATION SERVICE College Hill, Box D, Logan, Utah June 18, 1947 Mr. Edward Hyatt, State Engineer Department of Public Works, Sacramento 5, California Dear Mr. Hyatt : Transmitted herewith for publication is a coop- erative report ' ' Evaporation from Water Surfaces in California. ' ' This report, prepared b}^ Arthur A. Young, is a comprehensive pre- sentation of available data relating to evaporation from water surface in California. It includes a summary of pan records and coefficients devel- oped during the period 1881 to 1946. These data are of practical and economic importance to all water using agencies because evaporation is a basic element in the evaluation of potential water supplies, and in the conservation of water resources. The investigations upon which this report is based, and the prepara- tion of the report, were made under a cooperative agreement between the Division of Water Resources of the California State Department of Public Works and the Division of Irrigation and Water Conservation of the Soil Conservation Service, U. S. D. A. Respectfully submitted, George D. Clyde, Chief Division of Irrigation and Water Conservation (5) ORGANIZATION STATE DEPARTMENT OF PUBLIC WORKS DIVISION OF WATER RESOURCES C. H. PuRCELL Director of PuMic Works Edward Hyatt State Engineer A. D. Edmoxstox Assistant State Engineer GoRDOx Zaxder Principal Hydra id ic Engineer Assisted by : George B. Gleasox Supervising Hydraulic Engineer T. R. SiMPSox Supervising Hydraulic Engineer G. T. GuxsTOX, Administrative Assistant ORGANIZATION UNITED STATES DEPARTMENT OF AGRICULTURE SOIL CONSERVATION SERVICE H. H. Bexxett Chief of Service M. L. Nichols Chief of Besearch DIVISION OF IRRIGATION AND WATER CONSERVATION RESEARCH George D. Clyde Chief A. T. MiTCHELSox State Supervisor Harry F. Blaxey Research Project Supervisor This report was prepared by Arthur A. Youxg, Irrigation Engineer (C) ACKNOWLEDGMENT The author gratefullj^ acknowledges the assistance of engineers and water organizations throughout the State in furnishing evaporation records and corresponding data heretofore available only in office files and private reports. Permission to include these data in this bulletin represents a direct contribution to the many individuals and groups of those who are interested in the conservation of water. Assistance and advice given in preparing the many tabulations is especially appreciated. ( T ) FOREWORD This report is the first of two volumes which together will comprise all available data on the subject. Because of the bulk of the statistics and for fiscal reasons, this volume is a description of methods and apparatus used in recording evaporation, recommendations for the conduct of future ascertainments, and a summarization of the records now available. Volume 2, comprising basic tables, is not presently available to readers of Volume 1, but will be published at an early date. The records were obtained from some 250 evaporation pans located throughout the State. (8) INTRODUCTION Much of the irri£}:ated agriculture of the west has been made possible by the impounding- of flood waters. 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 uniform water supply than those located in lower areas where snowfall is small and run-off is defi- cient. 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 conditions an extensive system of storage reservoirs may be the only means of maintaining an adequate water supply. Both conditions prevail in California, water being plentiful in the north and generally deficient in the southern portion of the State. Where- ever water is scarce, losses are closely scrutinized. For this reason, evap- oration from water surfaces is a subject that has been given considerable attention. Experimental studies have been conducted by the Research Division of Irrigation and Water Conservation, Soil Conservation Serv- ice, IT. S. Department of Agriculture^ in cooperation with the California Department of Public Works, Division of Water Resources. Evapora- tion 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 Reclamation, Bureau of Plant Industry, and the Soil Conservation Service. Evaporation is the natural process of changing water into vapor. Dry air has a greater capacity for absorbing moisture than moist air ; hence, evaporation increases under conditions of low humidity. It increases with high temperatures and decreases v/ith low temperatures. Wind 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 fairlv 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 where 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 percent have been determined for localities separated by only a few miles. Evaporation measurements, therefore, should be made at each reservoir where records are desired. Attemi^ts to use records obtained elsewhere mav lead to error. 1 Formerly Division of Irrigation, Bureau of Agricultural Engineering. (9) 10 DIVISIOX OF WATER RESOURCES Studies of evaporation from storage reservoirs indicate that for long periods of deficient stream-flow, reservoirs ma}' yield, for useful purposes, as little as 50 percent 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 j^ears, 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 uniform flow a reservoir will be more completely replen- ished each year 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 replenishment. 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 ^ve feet in depth annually, which when applied to the total surface area of all reservoirs, undoubtedly amounts to an impressive total. Evaporation losses are of importance as an element affecting the net water supply available for irrigation of crops, for production 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, streamflow 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 have confined evap- oration measurements to pan 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 purpose of this report to overcome the lack of published evaporation records in California through compila- tion of such existing data as are obtainable from publications and private and public files. Search has disclosed many records not heretofore avail- able for public use. The total of some 250 evaporation records throughout the State mil be helpful in designing new reservoirs and estimating evap- oration losses from others. In the northern portion of the State water is more plentiful than in the south and less interest has been shown in collection of evaporation records, particularly^ in the northeastern counties and 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 of such measurements to be recorded was b}' the State Engineer EVAPORATION PROM WATER SURFACES IN CALIFORNIA 11 at Kingsburg from 1881 to 1885 (18), (31) .- In mountain areas tributary to the Central Valley some records are available but they are not as numerous as might be expected. With the advent of the Central Valley Project and the construction of Shasta and Friant Dams, the lack of adequate evaporation data has been recognized and plans have been made by the Bureau of Reclamation for installation of a network of evapora- tion stations throughout the area. A considerable percentage of all evap- oration measurements within the State has been made in Los Angeles County, where great sums have been spent for importation of water from outside sources, and in San Diego County, where a small water supply and a large population growth have required construction of an extensive 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 evaporation 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 differ- ences in temperature and humidity. The desert effect is noted at border- line stations where w^inds alternately 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. 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 coefficient 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 w^ater 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 evaporation losses and precipitation gains. Net evaporation may be a minus quantity. Gross evaporation always is positive. An evaporation coefficient is defined as the ratio for conversion 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 char- acteristics. Later in this report a tabulation shows all known coefficients for the principal evaporation pans as determined by experiments of the Research Division of Irrigation and Water Conservation, Soil Con- servation Service, U. S. Department of Agriculture. 2 Numbers in parenthesis refer to literature cited. TYPES OF EVAPORATION PANS The importance of evaporation long 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 topograph}^, climate, altitude or latitude. Consequently, a haphazard group of records has been accumulated by various organizations throughout the western states that have only rela- tive 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 experimental studies and their records are valuable in showing the effect of size of pan on the depth of evaporation loss. Since 1936 the Division of Irrigation and Water Conservation has experi- mented with a screen covered pan designed to reduce the evaporation approximately to the depth of loss from a larger body of water. The Weather Bureau Pan The Weather Bureau pan first came into use in the western states about 1916 and its records are the most numerous of any single 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 water 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 morn- ing 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. (12) EVAPORATION FROM WATER SURFACES IN CALIFORNIA 13 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 number of bleaching liquids containing a small percentage of sodium-hypochlorite. These liquids may be obtained at any grocery store. Within a few minutes the chlorine kills the algae and clarifies the water. Experience has demonstrated that it is harmless to the galvanized surface. Infrequently the Weather Bureau pan has been placed on a raft float- ing 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. S. Weather Bureau Climatological Data (41). Usually air temperature and rainfall records may be obtained from the same publication so that fairly com- plete 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 experiment 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 Monthty 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 Countj^, 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 water 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. Consequentlj", evaporation is lower than from the more exposed iDan. 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 pan is made of 18-gage galvanized iron, three feet square, usually 18 inches deep, and set 14 inches in the ground with the water surface held at ground level. Water surface fluctuation should not exceed one inch. Measurements are made by hook gage in a stilling well on the inside 14 DIVISION OF WATER RESOURCES wall of the pan (36) . The evaporation loss is less than from the Weather Bureau pan because it is protected by surrounding soil but more 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 18-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 protected from wave action by sur- rounding 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. If 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 Weather Bureau pan. As the pan is located off-shore, it is subject to the same con- ditions of wind, humidity and temperature that control reservoir evapo- ration. 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 Control 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 of the index point. Prior to September, 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 approximately equal length, showing 10 inches greater annual evaporation for the period of higher water surface. The reasons for the higher 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 resultant i EVAPORATION FROM WATER SURFACES IN CALIFORNIA 15 increased evaporation. This example emphasizes the value of maintain- ing 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 Division of Irrigation at the Fullerton Evaporation Station and elsewhere in order to study the effect on evaporation of shading the water surface (49). The pan was of the same size as the Flood Control pan, two feet in diameter by three feet deep, set in the ground 2.75 feet. Water levels were maintained at ground level and measurements of water evaporated were made with a hook gage in an outside stilling well. The screen was made of galvanized hardware cloth Avith one-fourth inch mesh and sus- pended horizontally in the pan midway between the rim and the water surface. Tests were also made with a screen of six meshes per inch, but the annual evaporation resulting from use of the finer mesh was only slightly less than from the more open screen. Experiments with the screened pan were undertaken for the purpose of finding one that would have the same annual evaporation as a larger water surface. If evapora- tion could be reduced to an amount equivalent to that from a lake or reservoir no reduction factor would be necessary for estimating reser- voir evaporation from pan measurements. Experiments with the screened pan under different climatic conditions in Southern California indicated that a coefficient of nearly unity could be obtained. Following these experiments, the Los Angeles County Flood Control District adopted the one-fourth inch screen for use with all Flood Control District pans. This could be accomplished without difficulty as both sets of pans were of the same dimensions. (All the Flood Control District records listed in this report were obtained before the screens were installed. ) 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 wind movement should be set at the northwest corner of the 2 X 4 's supporting the Weather 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 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. Floating 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 16 DIVISION OP WATER RESOURCES taken as the average of the maximum and minimum recordings. Air tem- peratures are sometimes recorded on the chart on a seven-day thermo- graph 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 temperatures 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. Humidit}^ may be determined from the temperatures of the wet and dry bulbs of either the sling or whirling tj^pe of psychrometer, or from the recording charts of a hygrothermograph or a hair hygrometer. The psychrometer gives the most accurate results. If either of the recording instruments is used it should be kept in the shelter with the thermometers. The standard eight-inch Weather 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. Keference is made to Circular L, Instrument Division of the Weather 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 shown in Circular L is the mini- mum size that will hold the necessary equipment.) It is the opinion of the author that the close proximity of the 4 x 4-inch fence posts to the evaporation 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 pan. Four posts of boiler tubing or two-inch pipe set 16 feet apart at the corners, with horizontal tubular bracing at the level of the top strand of wire, make 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 prepara- tion of the tabulations in this report there have come to light some records obtained from evaporation pans located in the idcinity of shrubs or trees which, while small in the beginning, grcAV each year until in the course of time the grown shrubbery shaded the water in the pan or blanketed it from the wind, resulting in a gradual reduction of the evaporation. The future growth of vegetation or the possibility of future building con- struction that will have an influence on the evaporation should be considered in selecting the site of an evaporation station. EVAPORATION INVESTIGATIONS In order to make use of existing evaporation records and obtain the greatest benefit from them, the relationship between evaporation from various small standard pans and from larger bodies of water has long been of interest to engineers. In A^olume 2 of this bulletin there are tabu- lated a large number of evaporation data recorded by various water organizations throughout the State. In the present chapter are brief descriptions of evaporation experiments carried on, not only in California but in other areas where different climatic conditions prevail. To a con- siderable degree the experimental studies have been made for the purpose of arriving at factors or coefficients showing the monthly and annual relationships existing between 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 oppor- tunity 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 in the favorable conditions under which the coefficient is determined as related to the less favorable conditions where pan evaporation is meas- ured 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 topography at many reservoirs the evaporation pan is sometimes placed on top of the dam and thus above the reservoir surface which fluctuates widely with the season, or at some distance from it at either a higher or a lower level. Its location is determined by topography or the convenience of the operator. Evaporation from a pan situated in an area where hillside slopes, brush and tall trees offer obstruction to winds cannot be expected to agree with evaporation from a similar pan located on a float at the water surface, or on an island where wind movement over the water surface is uninter- rupted. Moreover, greater humidity exists close to the water surface in 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 esti- mating reservoir evaporation. In theory, floating pans, parth^ submerged, would have evaporation losses more nearly commensurate with actual reserA^oir evaporation were it not for their unreliability caused by water splashing into or out of the pan during times of storm. A number of float- ing pans are used in California; they would be more numerous but for this tendency toward unreliability. Relation of Water Temperature to Evaporation At various times in the past 40 years evaporation studies have been carried on by the Research Division of Irrigation and Water Conserva- tion or its predecessors. All but two of these studies were in cooperation with the State Engineer of California. The first was in connection with investigations of evaporation in irrigation and water requirements of (17) 18 DIVISION OF WATER RESOURCES 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 water in evaporation pans. Average daily water temperatures were obtained at four stations during the summers of 1904-05 for com- parison with average daily evaporation. The results indicated in Fig. 1-a, show that evaporation increases with water temperatures, 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, % E o o w > < 100 Ootly •voporotion -inch Fig. l-b Rtiation of avoporotion from o Wtather Bureau pon to maximum wot«r ttmp«rature» .15 .20 .2 5 .3 .35 .40 Doily e voporotion - i nch EVAPORATION FROM WATER SURFACES IN CALIFORNIA 19 the line representing Berkelej^ conditions shows less evaporation for the same average temperature because of the higher humidity of the coastal area than that shown for Soldiers Camp near Lone Pine, which is removed from the coastal influence. A different form of curve was obtained during studies at Baldwin Park, Los Angeles County, by the Division of Irrigation and Water Con- servation, through plotting maximum daih^ water temperatures against daily evaporation from a Weather Bureau 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 dissipa- tion resulting from the process of evaporation. The Relation of Air Temperature to Evaporation Although temperature is one of the principal factors causing evap- oration, 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 tempera- ture-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 betw^een monthly evaporation from a 12-foot diameter ground pan and mean monthly air temperature at the evaporation station near Fullerton. Not 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 tempera- ture, evaporation from a shallow pan is greater in the first half of the year than during later months. For example, for a mean monthly tempera- ture of 65 degrees the average monthly evaporation in Figure 2 is approxi- mately 6.3 inches in earh^ summer as compared with 4,3 inches for the same length of time in the September-October period. For a deep lake or reservoir the temperature-loop is reversed, since the heat stored at depth in the water returns to the surface in the late summer or fall, where it causes increased evaporation. The Relation of Altitude to Evaporation With other conditions unchanged, evaporation would increase with elevation as the rarefied atmosphere at higher levels offers less obstruc- tion to the water molecules that escape from a freeh^-exposed water surface. Higher elevations, however, are characterized by lower tempera- tures and changes in other climatic factors that more than offset the effect of decrease in barometric pressure. The net result is a decrease in evap- oration that is more or less proportional to the decrease in temperature. Few^ attempts have been made to determine this relationship ; the Mt. Whitney study is the only one know^n to have been undertaken in Cali- fornia. Mt. Whitney Study : An early attempt was made in 1905 by Frank Adams, then of the Office of Experiment Stations, U. S. Department of Agriculture, in cooperation with the State of California (14) to determine the effect of altitude on evaporation from a water surface by measuring 3—75021 20 DIVISION OF WATER RESOURCES 1 » < ^ ! 5 6 T { 3 Monthly evoporotion-incties the depth of water vaporized from a series of pans set in the ground at different elevations on the eastern slope of Mt. Whitney. Each pan was 22 inches in diameter. Besides the evaporation pans the equipment at each station consisted of a rain gage, maximum and minimum thermom- eters, hook gage and sling psj-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. EVAPORATION FROM WATER SITRPACES IN CALIFORNIA 21 l| Observations were conducted at the following places between ele- vations 4,515 and the top of Mt. Whitney at 14,502 feet : Station Elev., feet Soldiers Camp 4,515 ^ Jiinctiou South Fork and Lone Pine Creeks 7,125 m Hunters Camp 8,370 ■ Lone Pine Lake 10,000 m Mexican Camp 12,000 i^ Summit Mt. Whitney 14,502 22 DIVISION OF WATER RESOURCES From examination of the Mt. Whitney topographic map it appears the evaporation stations probably followed Lone Pine Creek approxi- mately. The lower station at Soldiers Camp appears to have been in moderately rough countrj^ three to four miles west of Lone Pine. 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 vSteep. Under such conditions it is probable that pan exposure varied with respect to sun, wind, and temperature. Copies of the original data are unavailable, but from a chart of the results prepared by Carl Rohwer, Table 1 and Figure 3 have been prepared sho^ving probable daih^ evaporation and temperatures that are in conformity with curves plotted at earlier times when the data must have been at hand. TABLE 1 Evaporaiion and Temperatures at Locations on the East Siope of Mt. Whitney, California Elevation, Mean daily Mean daily feet evaporation, foot temperature, ° F. 4,515 0.223 82 7,125 .170 82 8,370 .147 74 10,000 .136 58 12,000 .134 49 14,502 .140 48 Plotted evaporation and temperatures show a close relationship to each other. The curves are nearly parallel except at elevations 4,515 and 7,125 feet where there was little or no change in temperature regard- less 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 dotted 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 tempera- ture 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 evaporation to be more closely related to change in temperature than to change in barometric pressure. Under certain conditions, however, evaporation may increase at higher elevations. This relationship is best shown from records of the Los Angeles 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. Although 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 lower alti- tudes, being closer to the ocean, have higher humidities than those at EVAPORATION FROM WATER SURFACES IN CALIFORNIA 23 a distance or at greater elevations. In the lower levels fogs are not nncommon. 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 evapo- ration is increased. In some instances, particularly for stations situated in summit areas between the ocean and desert regions, dry winds from the desert contribute further to increase the evaporation. The relation- shop of evaporation to altitude in Los Angeles County is shown roughly kJ e a. o > UJ Fig.4-a Relotion of evaporation to altitude o / / 3000 inLosAnaeles Co. X * > Los Angeles Co. Flood Control pan record/ 2000 r "" y/o o Q/^ 1000 / / o y / o / 1 1 1 60 O 60 70 80 90 Annual evaporation in inches- 1 o / Fig.4-b / ) 80 Re'otion of evaoorotion to distance /— .. from Pacific Ocean / Los Angeles Co. / «, V 8 70 o / f " Pin o / y 50 / 1 i 20 3 4 5( D pistonce of evaporation station from Pocific Ocean - miles 24 DIVISION OF WATER RESOURCES in Figure 4-a. The same data have been used in Figure 4-b to show the general relationship between evaporation and distance of the evapora- tion pans from the Pacific Ocean. Both charts show higher evaporation as both elevation and distance from the ocean increase. Cuyamaca Reser- voir in San Diego County, at elevation 4,600 feet near the summit between San Diego and the Imperial Valley, is thus affected. Evapora- tion at Cuyamaca Reservoir is greater than at several other reservoirs located at lower elevations nearer the ocean. The Salton Sea Investigation Prior to 1907 there had been little interest in evaporation investi- gations made wholly for the purpose of studying evaporation laws and developing formulas applicable to western arid and semiarid conditions. In 1907 the U. S. Weather Bureau undertook some preliminary 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 California. 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 embrac- ing all the conditions involved. As far as can be determined, however, the results were inconclusive and published reports on the investigation are fragmentary. The program included study of air and water temperatures, wind movement at different levels, vapor pressure, evaporation from pans of different sizes and at different elevations with regard to the surface level. The main portion of the study was undertaken at Salt 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 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 425 square miles at the time of the investigation, lies below sea level in a desert region of extremely high summer tempera- tures. The water surface fluctuated according to inflow from the fcAV streams in the vicinity, principally New 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 No. 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. EVAPORATION FROM WATER SURFACES IN CALIFORNIA 25 Tower No. 2 was 500 feet offshore in 25 feet of water. Pans at this tower were four feet in diameter with Pan 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 No. 3 was offshore and was used for special experiments which are not here discussed. Tower No. 4 was about 7,500 feet offshore in 55 feet of water with four-foot circular pans placed as at Tower No. 2. In addition, several land pans were located in line between the sea and Tower 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 two-foot pans on toAvers 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 two, four and six feet in diameter and 10 inches deep. Actually these dimensions were not maintained in construction as apparently it was not understood by the investigators that uniformity in size was of any importance. At the beginning 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 differed 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 depth from 9.1 to 9.4 inches instead of the prescribed 10 inches. No corrections appear to have 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 during parts of 3909 and 1910 are presented in Table 2. Mean temperatures 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-month period. There is, however, a complete set of evaporation data for the top and bottom of the three towers 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 move- ment 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 regardless of distance from shore. This could be expected, since prevailing winds passed over several miles of water surface before it reached the towers. Differences in evaporation at Towers 1 and 2 are attributed to two factors : Tower 1, at which the higher evaporation occurred, was 1,500 feet inland, in the desert, where temperatures were higher than over the water, and the drier air had a greater capacity for moisture. Also, these pans were smaller than at Tower 2. Both factors indicate increased evaporation at the land station. The evaporation station at Indio was in an alfalfa patch which was irrigated and cut as necessary. The effect of the tall grass surrounding 26 DmSIOX OP" WATER RESOURCES 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 TABLE 2 Evaporation at Salt Creek Bridge, Salton Sea, Riverside County, Calitomia Elevation 205 feet below sea-level (4), Lat. 33'- 25' N., Long. 115= 50' W. Evaporation in inches Tower No. 1 1,500 feet inland Tower No. 2 500 feet offshore Tower No. 4 7,500 feet offshore Year and month Pan No. 1, diameter 26.8 inches Depth, 9.3 inches -A.t ground level Pan No. 5, diameter 24.2 inches Depth, 9.4 inches On tower 41.0 feet above ground Pan No. 1, diameter 48.2 inches Depth, 10.2 inches Suspended from tower 4 feet above Salton Sea Pan No. 5, diameter 48.2 inches Depth. 9.4 inches On tower 45 feet above Salton Sea Pan No. 1, diameter 48.2 inches Depth, 10.3 inches Suspended from tower 4 feet above Salton Sea Pan No. 5, diameter 48.1 inches Depth, 10.4 inches On tower 45 feet above Salton Sea 1909 Julv 22.15 IS. 50 15.50 13.19 7.49 6.42 24.96 21.21 19.47 17.20 10.05 9.55 14.77 12.53 12.40 9.20 6.21 4.67 18.63 15.03 15.18 12.21 8.13 6.97 14.03 12.19 12.08 9.24 5.96 5.25 17.98 August-- ---- - -- 15. .33 September - - 15.40 October 13.02 November . - - - _. 7.48 December - - - - 6.97 6-month total - . - . . 83.25 5.08 7.42 11.00 14.78 19.00 102.44 7.05 9.45 13.56 18.54 24.11 59.78 3.61 5.01 76.15 5.14 7.17 58.75 3.41 5.09 76.18 1910 January.. . . - - .. 4.69 February - -- ---.. 7.40 March i \prili i 1 Mayi-.. -- 1 \ 1 These data obtained from original field notes of the Salton Sea investigation. TABLE 3 Temperature at Salt Creek Bridge (Tower No. 1, Salton Sea Investigation), Riverside County, California 1909 June. July August September-. October November-. December.. Month Mean air temperature degrees F.i 88 92 90 85 72 60 52 Month Mean air temperature degrees F.i 1910 January. February March.. April May 43 56 66 74 82 1 Records obtained from original notes of the investigation. EVAPORATION^ FROM WATER SURFACES IN CALIFORNIA 27 for greater wind travel and because of the smaller size of the pan. Evapo- ration 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 tabulation 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 from Salton Sea : Salton 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 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 Whitewater River, San Felipe Creek and Mammoth Wash. Surplus water from the Imperial Irrigation District flows into the sea through Alamo and New Rivers. Flood waters enter unmeasured, but for many years the irriga- tion district has kept records of drainage inflow. Rainfall records are maintained 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 com- puted for different w^ater stages. With these records it becomes 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 TABLE 4 Evaporation a! Indio, Riverside County, California Station Location At U. S. Date Farm about 1^ miles west of Indio. Lat. 33°44' N., Long. 116° 16' W. Elevation 20 feet. Evaporation pan Description Pan No. 1 Diameter 72 inches, depth 9.2 inches, at ground level at foot of tower in alfalfa field. Pan No. 2 Diameter 24 inches, depth 9.3 inches, at top of 10-foot tower in alfalfa field. Authority for data U.S. Weather Bureau Pubhcation reference Abstract of Data No. 4 (4). Meteorologic data None. Evaporation in inches Month 1 Evaporation in inches Month 1 Pan No. 1 on ground at bottom of 10-foot tower Pan No. 2 at top of 10-foot tower Pan No. 1 on ground at bottom of 10-foot tower Pan No. 2 , at top of 10-foot tower January.. ... ... 3.18 5.08 7.50 12.05 15.84 16.11 5.52 8.83 12.09 19.17 25.13 26.69 July 16.34 13.78 12.37 8.91 5.17 3.00 27.24 February August Seotember 23 05 March .... 21 13 April . October 16.85 May . . November 9.44 June .... . December 5.25 Totals 119.33 200.39 1 The year of record Is not entirely clear, but it appears that the January to June data were recorded in 1910 and the July to December data in 1909. 28 DIVISION OF WATER RESOURCES TABLE 5 Evaporation at Mecca, Riverside County, California Station Location In Coachella Valley, north of Salton Sea. Lat. 33° 3 1 ' N., Long. 116° 02 ' W. Elevation 189 feet below sea level. Evaporation pan Description Pan No. 1 Diameter 73.9 inches, depth 9.1 inches, on ground at foot of tower. Pan No. 2 Diameter 23.0 inches, depth 9.5 inches, at top of 10-foot tower. Authority for data U.S. Weather Bureau. Publication reference Abstract of Data No. 4 (4). Meteorologic data None. Month 1 January - February March... April May June Evaporation in inches Pan No. 1 on ground at bottom of 10-foot tower 2.92 5.00 8.07 10.87 12.72 14.23 Pan No. 2 at top of 10-foot tower 5.46 8.75 11.87 16.96 21.26 21.56 Month I July August September. October.. - November. December - Totals Evaporation in inches Pan No. 1 on ground at bottom of 10-foot "tower 15.21 13.22 10.29 8.17 4.13 2.98 107.81 Pan No. 2 at top of 10-foot tower 22.59 20.41 16.86 12.43 7.15 4.65 169.95 1 The year of record is not entirely clear but it appears that the January to June data were recorded in 1910 and the July to December data in 1909. TABLE 6 Evaporation at Brawiey, Imperial County, California Station Location In Imperial Valley, south of Salton Sea. Lat. 32° 58' N., Long. 115° 32' W, Elevation 1 10 feet below sea level. Evaporation pan Description Pan No. 1 Diameter 70.5 inches, depth 9.2 inches, on ground at foot of tower. Pan No. 2 Diameter 24 inches, depth 9.5 inches, at top of 10-foot tower. -Authority for data U. S. Weather Bureau. Publication reference Abstract of Data No. 4 (4) . Meteorologic data None. Month 1 January.. February March... April May June Evaporation in inches Pan No. 1 on ground at bottom of 10-foot tower 05 00 8.00 10.74 13.79 13.68 Pan No. 2 at top of 10-foot tower 5.32 8.00 11.00 16.04 21.57 20.21 Month 1 July August September. October... November. December. Totals Evaporation in inches Pan No. 1 on ground at bottom of 10-foot tower 14.14 11.26 10.15 6.99 4.09 2.66 103.55 Pan No. 2 at top of 10-foot tower 20.96 21.18 16.30 11.58 7.01 4.57 163.74 1 The year of record is not entirely clear but it appears that the January to June data were recorded in 1910 and the July to December data in 1909. EVAPORATION FROM WATER SURFACES IN CALIFORNIA 29 TABLE 7 Evaporation at Mammoth,^ Imperial County, California Station Location On the main line of the Southern Pacific Railroad, southeast of Salton Sea. Lat. 33° 05' N., Long. 115° 13' VV. Elevation 245 feet above sea level. Evaporation pan Description Pan No. 1 -- Diameter 70.0 inches, depth 9.4 inches, on ground at foot of tower. Pan No. 2 Diameter 23.0 inches, depth 9.4 inches, at top of 10-foot tower. Authority for data U.S. Weather Bureau. Publication reference Abstract of Data No. 4 (4). Meteorologic data None. Month 2 January.. February March... April May June Evaporation in inches Pan No. 1 on ground at bottom of 10-foot tower Pan No. 2 at top of 10-foot tower 4.24 5.67 8.99 12.02 15.52 16.75 6.47 8.89 11.65 17.13 22.00 24.17 Month 2 July August September. October... November. December. Totals Evaporation in inches Pan No. 1 on ground at bottom of iO-foot tower 18.00 13.73 12.16 9.49 5.26 3.70 125.53 Pan No. 2 at top of 10-foot tower 25.68 18.15 17.04 14.72 8.08 5.12 179.10 ^ The Southern Pacific Railroad station of Mammoth is no longer on the map. It appears to have been east of Callpatria in the vicinity of Tortuga or Amos. - The year of record Is not entirely clear but it appears that the January to June data were recorded in 1910 and the July to December data In 1909. to the sea to affect the accuracy of the estimated evaporation. These years .should not be included in evaporation 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 evaporation for the 10-year period 1909-10 to 1918-19 to be 68.76 inches annually as shown in Table 8. In estimating these values the inflow from Alamo and New Rivers was arbitrarily taken as 277,000 acre-feet annually. The records do not show that anv inflow was considered from such streams as San Felipe Creek, AA^hitewater River or the numerous flood washes that enter from the east. TABLE 8 Average Computed (Lake) Evaporation From Salton Sea, Calitornia, 1909*10 to 1918-19 (Source: Unpublished estimates by Saiton Sea investigators, U. S. Weather Bureau) Year Evaporation in inches Year Evaporation in inches 1909-10 72.76 66.57 64.22 65.99 68.27 1914-15 84.69 1910-11 ... 1915-16 65.23 1911-12 1916-17... 53.17 1912-13 1917-18... . 69.73 1913-14... . 1918-19 - 76.96 Average. . 68.76 30 DIVISION OF WATER RESOURCES Robson (34) estimated total evaporation for the six-year period April 1, 1907, to April 1, 1913, to be 65.84 inches, on basis of the follow- ing data : Loss in elevation of Salton Sea 26.10 feet Total rainfall on lake surface 1.38 feet Total run-off into Salton Sea 1.25 feet Discharge from Alamo and New Rivers (estimated) 4.19 feet Total 32.92 feet Yearly average 65.84 inches Since the discharge from Alamo and Nevr Kivers had to be estimated and there was no record of inflow from Whitewater River and San Felipe Creek it is probable that the total and average are too low. Probable evaporation shown in Table 9 was computed by Grunsky for 1907-08 (16) in the same manner. This table appears to be subject to some adjustment, possibly because of the effect of wind in changing Avater surface elevations for some months. TABLE 9 Computed (Lake) Evaporation From Salton Sea, California, 1907-08 (16) Year and month Evaporation in inches 1907 April May June July August September 5.16 8.52 8.88 8.28 6.36 11.16 Year and month Evaporation in inches 1907— Continued October November December 1908 January February March Total 6.84 6.48 4.20 2.16 2.64 3.00 73.68 In recent years the rising water 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 wide expanse. In addition, the outlets of drainage channels are being submerged bj^ rising waters. Investigations of Evaporation From Small Water Areas Evaporation studies have been carried on for a number of years by the Division of Irrigation and Water Conservation at Denver and Fort Collins, Colorado, and at Fullerton and other Southern California localities. The results obtained have been of general benefit to engineers through discussion of evaporation fundamentals, the development of evaporation formulas, and in establishing the values of evaporation coefficients for the reduction of pan evaporation to equivalent evapora- tion from larger water areas. The difficulty of direct determination of evaporation from large water areas results from the general impossi- bility of obtaining a complete inventor}^ of all the waters entering and leaving a reservoir. In isolated instances where the only change in water EVAPORATION FROM WATER SURFACES IN CALIFORNIA 31 levels is through dissipation of moisture into the atmosphere, evapora- tion may be measured directly with staff gages. Occasionally, oppor- tunities exist for computing evaporation from records of inflow, out- flow, 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. Usually evaporation is measured from small water surfaces in standard evaporation pans. Such measurements may be reduced to lake or reservoir equivalents through use of conversion factors or coefficients 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 vays and the sweep of the wind, both of which increase the evapo- ration. 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 uniform 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 like- wise 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 accord- ing to the diameter of the pan, the greatest relative effect being on pans having the smallest diameters. The ratio of pan circumference to area 4 of the water surface is — which 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, increased 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 evaporation coefficients for a variety of pans has been one of the long-time objectives of the Division of Irrigation andWater Conservation. The first of these 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 conclu- sions 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 typos of so-called standard evaporation pans ; 32 DIVISION OF WATER RESOURCES (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 Weather 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 measure- ments 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 presented a partial list of evaporation coefficients for the principal pans studied (37) . During the following year, 1917, measurements were continued and a summary of the coefficients obtained during the investigation was published (38). Fort Collins, Colorado, Investigations: A second investigation was undertaken by the Division of Irrigation at the Colorado Agricul- tural 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 Weather 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 1928. 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 experiment 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, inclu- sive, at a central evaporation station at Fullerton, Orange County, about 10 miles from the coast, and from 1939 to 1941 at Lake Elsinore, River- side 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 percent, with high thin fogs a common occur- rence during summer months. Wind velocity, 20 inches above ground, averaged 2.8 miles per hour. Rainfall, varjdng from 11 to 23 inches EVAPORATION FROM WATER SURFACES IN CALIFORNIA 33 annually during the periods November to April, averaged 15.75 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 85-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 oil 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 appear- ing in evaporation measurements. For comparison with evaporation from the 12-foot pan measure- ments were made from a series of circular ground pans of similar depth, with diameters of from one to six feet. In addition there was a Weather 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 from which tests were made to determine the effect of color of pans and the effect of different concentrations of salt solution on evaporation losses. Summaries of evaporation from the principal pans at the FuUerton Station are shown later in this report. Lake Elsinore Evaporation Station : Lake Elsinore, with an area of about 5,500 acres, is an excellent outdoor evaporation laboratory. Its water supply comes from the San Jacinto River which flows only during the winter and spring months ; this flow is measured by the U. S. Geo- logical Survey a short distance above the lake. There has been no out- flow 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 eor- siderable degree of accuracy was possible in arriving at lake evaporation throughout the long dry summer months when the only change in water surface was through evaporation. Meteorologic conditions at the lake were similar to those at the Fullerton station. During the period of measurement the average tem- perature 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. EVAPORATION COEFFICIENTS The usefulness of evaporation coefficients is better understood 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 tempera- ture, 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 evaporation 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 below freezing. Coeffi- cients obtained in Southern California Avould appear to be applicable to the lower areas of the southwestern states where winters are short and mild. In California the Colorado coefficients probably apply to the higher mountain regions, while the Southern California coefficients are more suitable for the lower elevations of coastal and interior valleys. The Weather Bureau Pan Coefficient A summar}^ of evaporation records at the Fullerton station is pre- sented 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 0.76 to 0.78. During the three-year test at Lake Elsinore the average annual coefficient 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 water areas as a 12-foot pan and a 5,500-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 received from the sun during the day is lost at night. In the larger volumes of water 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 (34) EVAPORATION FROM WATER SURFACES IN CALIFORNIA 35 to b_ o a. ro > UJ o 9> «a e o o e 'to a> ea a> k. fO V ^ a> -o > •o 9' B ra O T1 OI %-m c ■■M fQ ^1. 0) o O 3 o Q. m to CO ea "5 a cS o o a ■J a> c« > 05 CO I CO CO 05 ;:3 =3 O a o 3 'S 5fl a o o a c '3 o O a o o Weather Bureau pan, ratio (MCOOOCOOOt^Tj'OOt^COt^lO oocoocosDt^t^t^oooiasai t^ t^ o O Screened pan, ratio ■ 3-3 -o PQ cO'-it^coi^-Ht^eo00:t^lOCO«M T3 a ^jii ^c fe c fe a§.2 »Oa5COC-^ O icr^coo— hcn-— 1— 'coiONco aj , eocococoMC^.— lOscOi-Hcot^ 0001OOOOO05O3O50000 «-■ ea t: '—' ?, ca R Q, o P3 OCC>>000-HCO'<*CO (NiMCOTtiiocoir^r^cO'^coiM ca ih "t^ i~ a CO ^ g^t2-c g ^ a •- . 3 R ooiMc-O'^ocr-TtiooC'OOoc >— i(McoTt<»ccor^t^»0'^co'— 1 a o >> u ea ea 3-t5_J 3 b " «-S.ea a ^-'-S-S 3 OJ S a: 6 a ci S'C ^c >>W'^5 > w CO 00 o CO UO CO to Oi OS CO o Oi C<1 CO "5 ea CS -♦-> OI & C3 -a a ea c o c o •a C3 CJ C3 3 o. 36 DIVISION OF WATER RESOURCES 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 evapo- ration, but in the fall and winter months evaporation from the deeper body of water may equal or exceed pan evaporation. The length of this excess period depends on the depth of the lake, the amount of heat stored in the water and the time required for its return to the surface. Thus it may be expected that monthly evaporation from a deep lake or reservoir will differ from the computed values that are based on pan records and predetermined monthly evaporation coefficients. Regardless of such monthly differences the annual heat energy received at the reservoir and pan should be about the same ; hence, the annual evaporation from the reservoir should be nearly identical with the value computed by means of annual coefficients. The Six-foot Diameter Ground Pan Coefficient. Comparison was made of evaporation from two six-foot diameter pans at the Fullerton station, one being three feet in depth with a three- inch rim above ground, the other a standard Bureau of Plant Industry pan which was two feet in depth with a four-inch rim. Evaporation from the Bureau of Plant Industry pan was consistently less than that from the deeper pan, but only by a small amount each month. Since the pans were of the same diameter and received the same heat energy, the dif- ferences in evaporation must be attributed to some special pan char- acteristics such as depth of water or height of rim above the water surface. The coefficient for the deeper pan, based on five years of meas- urement, averaged 0.91 as compared with 0.94 for the Bureau of Plant Industry pan in a two-year period. Few of the six-foot pans are used in California. The Square Ground Pan Coefficient The square pan, 3x3 feet, 18 inches deep, is used in some areas both as land and floating types. The land pan usually is set 14 or 15 inches in the ground. Both land and floating pans are frequently painted black inside and outside, a condition that increases the evaporation. The annual coefficient for conversion of evaporation from the unpainted square ground pan to that from the 12-foot pan at the Fullerton station aver- aged 0.89 for the five-year period 1935 to 1939, inclusive, with annual values ranging from 0.87 to 0.90 (49). This coefficient is not in agree- ment with the Colorado values for this pan, which according to Sleight (38) and Rohwer (35) was 0.79. A comparison of the records shows dif- ferent rim heights for the two pans. Sleight (38) described the square ground pan at Denver, as three feet deep, set 2.75 feet in the ground with the water surface approximately at ground level. Rohwer's (35) pan at Fort Collins, was 18 inches deep, set in the ground with its top edge If inches above the ground level. Allowance for variation in the Avater surface in Rohwer's pan was one inch and the maximum distance below the rim was two inches. The square pan at the Fullerton station was 18 inches deep, set in the ground 14 inches so that there was a four-inch rim as compared with a three-inch rim at Denver and a If -inch rim at Fort Collins. Experiments have shown a rapid decrease in evaporation when water stands at progressively greater depths below the top of the k EVAPORATION FROM WATER SURFACES IN CALIFORNIA 37 pan. A lower rim at the Fnllerton station should result in more evapora- tion and a lower coefficient that would more nearly approach those derived through the Colorado studies. The monthly coefficients were quite uniform, showini^ a range of values from 0.87 to 0.93. This narrow range, similar to that found for the Bureau of Plant Industry pan, fails to show the seasonal trend in coefficients that has been disclosed for pans other sizes. For example, monthly coefficients for various circular ground pans are high in summer and low in winter. They follow the seasonal range of temperature. Likewise, coefficients for the Weather Bureau pan are highest during the hottest months when comparison is made with the 12-foot pan, but when evaporation from a Weather Bureau pan is compared with that from Lake Elsinore, the highest coefficients are seen to occur in the fall and the lowest in early spring (47) . This is the direct result of heat storage in the lake. The Square Floating Pan Coefficient The square floating pan in California is used primarily in San Diego County, although its use is by no means confined to that area. The longest period of record was 23 years at Henshaw Reservoir where a land pan of the same size was also in use for the same period. The floating pan is used for estimating reservoir evaporation by approxi- mating reservoir conditions as to water temperatures and exposure to wind. Although it is partly protected from waves by a surrounding float there often is uncertainty as to the exact depth of evaporation during storms when water splashes into or out of the pan. If water splashes in, too little evaporation is indicated; if it splashes out, the evaporation recorded is too high. When records from the floating pan are uncertain, measurements from a square ground pan may be sub- stituted with a reasonable degree of accuracy. To obtain the best results, water inside the land and floating pans should be kept at the same depth as the outside surface. Paint should not be applied, as it changes the rate of evaporation. It does, however, delay rusting and prolongs the life of the metal. In practice, the floating pan is subject to more wind than the land pan, with the probability that the wind movement increases the evaporation. Other differences occur because of splash. Examination of records of 19 pairs of land and floating square pans, as indicated in Table 11, shows a few great differences in mean annual evaporation. The mean ratio of evaporation from the floating pan to the ground pan for the 19 stations is 0.95, as compared with an experimental value of 1.03 obtained as one of the results of Rohwer's studies (35). The length of the record and the number of places of observation scattered through- out the State indicate average figures that might ordinarly be expected in practice. Most of the stations are at reservoirs or lakes where water in storage has opportunity to warm up by standing in the sun. The floating pans at Independence and at Kingsburg were in running water which is colder, especially at Independence where the Owens River flows out of the high mountains. This condition could readily account for the low evaporation from the floating pan as compared with the ground pan. The coefficient for converting evaporation from the three-foot square floating pan to evaporation from a 12-foot diameter pan at Denver, 38 DIVISION OP WATER RESOURCES TABLE 11 Comparison of Evaporation From Square Ground and Fioating Pans at Various Lal(es and Reservoirs in California County Length of record Mean annual evaporation Ratio of evaporation from floating pan to evap- oration from ground pan Station * Ground pan square 3x3 ft., 18 ins. deep Floating pan square 3.x3 ft., 18 ins. deep Years Inches Inches Ratio Bouquet Canyon Reservoir . Los Angeles Kern 10 1 5 7 10 23 3 4 12 19 11 14 15 8 8 2 4 9 14 86.90 67.84 41.32 69.72 73.17 63.77 283.57 59.76 358.57 57.03 74.80 54.37 53.83 44.. 30 45.36 60.30 59.64 90.00 42.79 71.25 81.11 36.48 68.30 69.38 170.06 265.62 46.23 55.06 57.44 65.29 58.89 39.00 47.84 48.23 64.42 53.23 74.23 47.18 Mean 0.82 Buena Vista Laiie .. 1.20 Clear Lake,.- _.. . . Lake .. .88 Cuyamaca Reservoir . San Diego San Diego San Diego Inyo .98 El Capitan Reservoir.. ... .95 Henshaw Reservoir . 1 10 Independence.- .78 Kingsburg . Fresno .77 Lake Hodges . San Diego San Diego San Diego Calaveras Contra Costa Contra Costa Contra Costa San Diego San Diego Inyo .94 Lower Otay Reservoir. 1.01 Morena Reservoir .... .87 Pardee Reservoir.. .. ..... 1.08 San Pablo Reservoir . . . .72 San Pablo Reservoir . . _ 1.05 San Pablo Reservoir .. . 1.06 San Vicente Reservoir . . 1.07 Sweetwater Reservoir .. .89 Tinemaha Reservoir.. . . .82 Upper San Leandro Reservoir . ... Alameda Total 1.10 179 0.95 1 Monthly records incomplete. 2 Pans were 10 inches deep. 3 Square concrete basin \nth four-inch walls. as recorded by Sleight (38) was 0.89 in 1915 to 1917. This is higher than Kohwer's value (35) of 0.77 obtained by comparison with evapo- ration from an 85-foot diameter reservoir. No studies to determine the values of coefficients for the floating pan have been made in California although the generally accepted value in practice appears to range from 0.79 to 0.83. This is in agreement with results obtained by experiment and by reservoir practice. A comparison of coefficients for square pans as determined by experiment, with those recommended in a Final Report of Subcommittee on Evaporation of the Special Committee on Irrigation Hydraulics (1) and others suggested by Hall (17) are presented in Table 12. As a result of this tabulation it appears that an average value of 0.80 may be accepted by engineers for computing reservoir evapora- tion from a three-foot square land or floating pan. Exceptions to these values are coefficients determined by Sleight (38) for a floating pan and by the author (49) for a land pan. Sleight's floating pan was 3,400 feet from the laboratory where his other records were obtained and the different location or the 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. Evaporation from water surfaces in California 39 TABLE 12 Comparison of Evaporation Coefficients for 3 x 3 Foot Square Land and Floating Pans for Reducing Pan Evaporation to Equivalent Evaporation From Larger Water Areas Investigator Sleight Rohwer Subcommittee HaU Young Evaporation coefficients for 3 X 3 ft. pans Land Floating 0.79 0.89 .79 .77 .78 .80 .81 .80 1.89 1 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 approximates evapo- ration from a larger body of water. Attempts by the Division of Irriga- tion and Water Conservation 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 principle in evaporation studies was introduced in the form of a J-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 with 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 warm 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 coeffi- cients for a majority of evaporation pans in common use as determined by similar investigations at Denver and Fort Collins, Colorado ; 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 Weather Bureau pan the Fullerton coefficient was 0.77 as compared with an average of 0.70 obtained through 40 DrV^ISION OF WxiTER RESOURCES TABLE 13 Summary of Evaporation Coefficients as Determined by Various Investigations in Western States Type or size of evaporation pan 85 ft. circular reservoir. Circular ground pans 3-ft. Diameter, 12 ft Diameter, 6 ft Diameter, 4 ft Diameter, 2ft Diameter, 1 ft deep Bureau of Plant Industry pan Colorado square pan Weather Bureau pan Two-foot diameter screened ground pan. U. S. G. S. square floating pan Evaporation coefficients established by investigations at Denver, Colorado, elevation 5,300 ft. (38). ratio 1.00 .92 .63 .94 .79 .70 .89 Ft. Collins, Colorado, elevation 5,000 ft. (35), ratio 1.00 0.79 .70 .77 Milford. Utah, elevation 5,000 ft. (43), ratio 1.00 0.67 Fullerton, California, elevation 100 ft. (49), ratio 1.00 .91 .89 .81 .66 .94 1.89 .77 .98 1 Differences in height of rim above ground account for the higher coefficient. 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. EVAPORATION FROM LARGE WATER 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 measure- ments from closed lakes during the dry season when there is neither inflow nor outflow. A tight lake bottom is a prerequisite for this condi- tion. Such opportunities are few. During the rainy season when streams are flowing, evaporation is computed from records of inflow, outflow, rainfall on the lake surface and change in water levels. A few such records for California and Nevada are shown in Table 14. 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 inflow, outflow, rainfall on the lake surface and lake fluctuations by Walter Ruppel, office of Harry L. Haehl, consulting engineer, San Francisco. The lake records were obtained by the Buena Vista Water Storage District. Rainfall was averaged from Weather 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 excep- tions of periods of rainfall the evaporation could be measured directly from changes in lake levels as shown on staff gages. Seepage was con- sidered to be negligible. Rainfall was taken from the Hanford records. TABLE 14 . Evaporation tcmpufed for a Few Lakes in California and Nevada (Evaporaiion in inches) Cahfornia Nevada Month Buena Vista Lake, Kern County, Elevation 290 ft. Tulare Lake, Kings County, Elevation 200 ft. (20) Lake Elsinore, Riverside County, Elevation 1,260 ft. Eagle Lake, Lassen County, Elevation 5,100 ft. (20) Walker Lake, Mineral County, Elevation 4,030 ft. (20) Pyramid Lake, Washoe County, Elevation 3,830 ft. (20) January.. .. 1.2 1.8 2.9 4.3 6.0 6.2 8.5 10.2 7.8 4.6 2.5 1.7 1.4 1.6 3.0 3.6 6.0 8.4 9.6 7.2 7.2 3.6 2.4 1.2 1.8 1.6 2.9 4.4 5.8 6.7 7.8 7.9 6.6 5.2 3.2 2.3 11.8 11.8 12.4 13.0 3.6 4.8 6.0 5.4 5.4 3.6 2.4 1.8 2.4 1.8 2.4 2.4 3.0 4.8 6.0 6.6 7.8 5.4 4.8 3.0 3 February 3 March 3 6 April 3 6 May. . . .. 4 2 June.. .... 4 8 July .. . 4 8 August- 48 September . 5.4 October . . 4 8 November December.. 4.2 3.6 Annual 57.7 55.2 56.2 42.0 50.4 49.8 1- Estimated. (41 ) 42 DIVISION OF WATER RESOURCES Computed records for Bueiia Vista and Tulare Lakes, althougii prepared independently and for different series of years, are in harmony with each other, each showing a mean evaporation of 55 to 58 inches annually. Lake Elsinore, Riverside County, California, is fed by the San Jacinto River. It overflows at long intervals and at times is nearly dry. The computed mean evaporation from the lake for the 26-year period 1916 to 1941, is 56.2 inches or 4.7 feet annually. Eagle Lake is in Lassen County, California, at elevation 5,100 feet. Some seepage from the lake occurs and some unmeasured inflow enters it during the early part of the year. xVt its highest stages Eagle Lake covered an area of some 30,000 acres. This area was large enough so that small discrepancies in water supply did not materially affect the accuracy- of the computed evaporation, which was only for the period of minor stream- flow. Since Eagle Lake is at a relatively high elevation, evaporation is low. averaging 3.5 feet or 42.0 inches per year. Gage heights at AValker Lake, in western Nevada, available for the years 1929 to 1934, inclusive, indicate an annual lake evaporation of 50.4 inches. Elevation of the lake was about 4,030 feet and its area about 90 square miles. Rainfall and most stream flow were measured at the north end of the lake. Some local unmeasured flow occurred, but in years of low rainfall it was insufficient to affect the computed depth of evap- oration measurably. Pvramid Lake, also in western Xevada, at the time of record had an area of about 200 square miles at altitude 3,830 feet. It received the measured flow of Truckee River and gage heights and rainfall records were available. Annual evaporation for the 7-year period 1927 to 1934, inclusive, amounted to 49.8 inches, which is practically identical with the computed evaporation from Walker Lake. EVAPORATION PAN RECORDS This report presents the first attempt to bring together all the evap- oration records available within California. It reflects the efforts of nnmerons water organizations since 1881 to determine the depths of water lost by evaporation from lakes, streams and reservoirs in valley and mountain areas. From the beginning, measurements were recorded from land and floating pans having different characteristics as to shape, size and depth, but it was not always understood that different pans had different rates of evaporation nor that pan evaporation differed from lake or reservoir evaporation. Such information developed as the number of measurements increased and interest in it was intensified as a result of Sleight's investigations at the outdoor laboratory at Denver (37) and Rohwer 's studies at Fort Collins, Colorado (35) . Results of studies by the Division of Irrigation in Southern California show a generally close agreement with those of the Colorado studies with the exception of the coefficient for the Weather Bureau pan which was determined to be 0.77 instead of the 0.70 developed for the colder climate. Evaporation records may be divided into three general groups depending upon the purpose for which the records were obtained. The first and largest group includes measurements made by engineers and water administrators for the purpose of estimating reservoir evaporation losses. For this purpose both land and floating pans were used, including the U. S. Weather Bureau pan and the three-foot square land and float- ing pans. The latter are generally used in pairs at reservoirs in San Diego County, along the Los Angeles Aqueduct, and by the East Bay Municipal Utility District. The benefits derived from using two similar pans at each reservoir lies in the opportunity of estimating missing records for the floating pan from the evaporation recorded for the land pan. The Los Angeles County Flood Control pans are used for estimating evaporation losses at approximately 25 locations in valley and mountain areas. The records date back from 10 to 15 years. All the Flood Control records here included were obtained from pans with open tops, but beginning about the first of 1946 these pans were screened in order to reduce evaporation to a depth approximately equal to the evaporation loss from a larger water surface, thus avoiding the necessity of using a reduction factor for converting pan evaporation to reservoir loss. The second group is limited to the Bureau of Plant Industry pans six feet in diameter and set in the ground with four inches of the rim exposed. This pan has been used by agriculturists in connection with investigations in plant growth relations, but the records are useful also to engineers in estimating water losses. The longest record from a Bureau of Plant Industry pan is at the U. S. Yuma Field Station near Bard, California, where records have been kept continuously since 1910. A third group includes various pans used in experimental studies. In addition to those mentioned, other types used experimentally include the circular ground pans of different diameters, insulated pans, and screened pans. (43) 44 DR^ISIOX OF WATER RESOURCES Evai^oration pan records are distributed throughout the State in inverse ratio to the water supply. In the northern mountainous areas water is plentiful and evaporation stations are few. In the high Sierra, stations are lacking as there has been little interest shown in evaporation losses. In Los Angeles and San Diego Counties where there are many storage reservoirs, water organizations need to know the depth of evap- oration losses and evaporation pan records have been obtained for nearly all reservoirs. About half of the records in the State are in these two counties. Records generally are available along the coast as far north as Santa Barbara, but farther north no records were found. Collection of Data Collection of data was divided into two parts: (1) a search for all published evaporation discussions and (2) tabulation of data at offices of numerous water organizations scattered throughout the State. Every effort v\-as made toward accuracy in the tabulation and computations, but records as found were generally accepted as correct. It was assumed that the record as released by the observing office was correct within the average limitations in such observations. Occasionally the data appeared to be incorrect, and in cases where there seemed to be no adequate explana- tion such data were discarded. If a suitable explanation appeared possible it was made a part of the tabulation. For instance, records for the Lower San Fernando reservoir showed abnormally high evaporation during spring and winter months which were also periods of high wind move- ment. The observing office attributed the high evaporation to wind, and as this conclusion had some foundation it was so noted in the tabulation. Undoubtedly, errors sometimes occurred in original observations but correction of them was bevond the abilitv of the author. All the data are reported here as obtained from the various sources and no attempt has been made to chansfe or correct anv of the figures. Tabulation of Data In the tabulation of monthW and annual evaporation a form heading was adopted to give descriptions of location and elevation of the station, type and description of the evaporation pan from which records had been obtained, the authority for the data, publication reference and meteoro- logic data. In many cases, these descriptions were obtained directly from the responsible office or the publication in which the data were set out. It was desired to have each description sufficient so tliat the station could be located with reasonable accuracy on a map of the region. Often, how- ever, no detailed description was available so that latitude and longitude had to be scaled from a map and the elevation taken from topographic maps or from published elevations of nearby towns. As a result of the necessity to obtain some of the descriptive matter in this fashion there may be some inexactness in the given latitude and longitude and elevation, but if there be any it is not believed to be important. The type of evaporation pan from which the record was obtained is shown in each table heading, together with its description. Pans set at some depth in the ground are described as ground pans, while those set on the ground surface, with the exception of the "Weather Bureau pan, are designated as surface pans. Only a few of this type have been EVAPORATION FROM WATER SURFACES IN CALIFORNIA 45 found. The authority for the data is as shown in publications when records were obtained from such sources, and in cases where the data have not previously been published the authority is listed under the name of the organization which supplied the records for this report. Meteorologic data sometimes were observed directly at the evaporation station and made available with the evaporation records. In other cases temperature records for the town nearest the station were obtained from the U. S. Weather Bureau Climatological Data (41). The source of the meteorologic data, if secured from an authority other than the one listed for each evaporation tabulation, usually is shown in the form of a foot- note or a literature reference. Each tabulation shows the annual evaporation from the pan described for each year of record, provided no monthly totals are missing. In many cases monthly totals were incomplete or may not have been recorded because of high altitudes and freezing w^eather. For these years the annual total is omitted. In computing the mean monthly evaporation at the bottom of each column the value v/as taken as the average of all the months listed in the column. The mean annual total evaporation in the lower right hand corner of each table is then equal to the sum of all the mean monthly totals taken horizontally across the bottom of the table. This total should equal the mean of all the annual totals, provided that none is missing. All tabulations have been checked to see that the sum of the mean monthly evaporation agrees with the average of all annual values. Where missing records are shown in the column of annual values the horizontal total and the vertical mean will not exactly agree. This divergence often appears throughout the evaporation tables. Long Term Mean Evaporation Evaporation occurs at a more uniform rate than either rainfall or streamflow, which may vary from the annual mean as much as 50 or 100 percent in either direction. Variations in annual evaporation seldom exceed more than 10 or 15 percent either way. During 1941 evaporation was low throughout the State as a result of a period of low temperatures. August, 1941, was the coldest of any August on record although warmer than usual in the northern and middle coastal districts. The annual precipitation for 1941 has been exceeded only twice during the past 45 years and the number of rainy and cloudy days was the greatest of record. As a result of these conditions evaporation in Southern Cali- fornia was between 85 and 90 percent of the average. The long term mean evaporation for a small group of Weather Bureau pans located at various places in Southern California is shown in Table 15. The base station for this tabulation is the Riverside Citrus Station, which has a record of 21 years of measurement. The table shows the annual index of evaporation for each station and sets up a compari- son of the mean evaporation with the 21-year calculated mean. A similar tabulation was computed for Weather Bureau pans at stations in the Central Valley based on a 19-year record of evaporation at the College of Agriculture at Davis, as shown in Table 16. In general the tabulations show only small differences between the mean of record and the cal- culated long term mean. 46 DIVISION OF WATER KESOURCES 3 3 ^ <= =^ r- C ■ 3 ro . c s o c o 3 O > O X 3 o o 3 o ^ a S - M O c 2 ?* -» " a> CM •a 9 '^ o t k'^ =^ o 5 • O ^ O '*-' Ph ^ o o 2 03 3 X 3 is • m 2 3 t> a o^ S--S 2 u^ na flj o o to _- 2 S-J ©CO) a OJ3 c3.3 fO <-• rt c3 S «a >^=^- =i c3 3 ECQ O •-s X « O c OJ a c J= ^ > " £ 5 t^ oi o O 5cc OOt^t^OOt^t^t^t^COt^t^t^t^ 05 CO lOOO— <05OC005O0CI^-^O30 0000-^020500000000505 oo C5 OCOC^1COCS1C*'0CO5 — 00-*t^COlC00(M CO oo CO CO eoo5'««<05— i05Cqioco-<*''0«^'005C'^ 0-r3 X 3 o o 3 S3 '-' a 05t^OI^OOOOCOOt-~W5'>»0-coeococococococoeos«5-^"*''«J"'^'«J<-* 050205020205050505050505020202050202050205 •a Tl Ui bi O O o C3 -«^ CO u o CJ a o E a> B o Evaporation from water surfaces in California 47 to 9> a> o a> (O •-« (S te »» e E d o ^^^ E fO tu a> t— o 3 «o so %yy 3 ^Ma ■a O LU c: (a 31 fiO c ■< < 3 o ^^ o- la « o Ol o c m o b^ «^ a> o ID O £ o IS -E >, fl n ^ HI ^ -f^ p o t-t-H C o 8 0) o • a O O fa O & > *i_i b OJ [^ O t^ 3 O o in O) -T3 r -M t/2 n rrl o c« >■ "3 CO .S -^J -tJ p C«« > 3o mO to oj ^ a is a c« 2 ^^^ C3 CD -13 O ^O a i- ^ O c« 0(M(MCO CO IM 05 05 t^ 05 05 05 05 oo 05 »-H CO ^-H 1-H O >0 CO ■— I .— 1 irq 05 00 CO CO CO --I t^ 00 oo 00 oo CO oo oo CO CO CD I— t a b - CO O c 0) a o_c H (S.s X 3 ai lU -a t~t a o C OM0»OiO'>*iOOOOC00505 >0 05 Ot^"— lOSTjHt— lOOlO-^'— IIC0005000 1—1 oo o 05 lo -H t^ CO --I o ■<*i -^ 05 oo t^ 00 r^ co t^cOI:^t^»f3CDCOCOCOCO»0>0'0>0>0 CO CO t^OO>0CD^HC0'^(MC0O»0C500M< 1 — (1 — ^1 — ^005005051— <05050505050 o cot^^HcO"— i-^icotN^— i(^ooc35r--.c-)0 >o COt— lOOt^COCOCOCO'^'OCOlOCOCO-^ r-i C00500»0OC0O05OOC0O5CS)^H>O t^cOCOCOCOCOCOkOt^cOiOiOCOCOCO CO oo CO oor^O'OooTioo50^^^05cooo OOi— <0050005r— 1050505050505 o t^*— "05cocsicoioco^-Ht-^r^cor^coco to csi 05 05C(C^)t-^00'>*cOCOcD0500COCDcO^^ "—I »0 ■^ cocoiOi— ico—HO'Coot^c^iiMt^t^r^ 05 co i^i^t-^i^cot->.t>-cot^cocococococo CO CO X 3 3 oj »-' a O-*i'-iCjcO'— ioq(Mc^'^co(M o o 05 05 CJ5 05 a cu COiOOO— i05>-H005'^COa50 - CO 3 c«.3 lOO5e>^O500lOCO'— iOOCD-^OOT}<0-»t<<>j(Mcocococooococccococo'«»<'^'^'^Tf, Oj 1 > > .= .S CS ^jC ?3 s a a 3 3Q C3 :« ^ J3 P? ded by b dcd by b of trees. "3 O ID -3 03 -s a a .s surroun surroun er cover .it? o 2 JS 3 CO o 11 P-c S a T3 S IS CO IS a c"3 o ^ CI u ^ c« S i-H > a c8 O Pi ^ ^ . i rJ O P^ ?/ V O To .2 M>1 5 o. (a > UJ ro E £ b 3 o J o =^ (M'^ia^' 00tJ< O CO "3 1C"5 CO(M CO " "5 CO ■<< [^ Tfi iC 00 »OiO'^CO"*'*'-^-<»< •>3'-*i-!t0 -H ■^i^ ^^ CO '"' CO-H d 03 ■^■^•o-* O O O O CO^O cqco CO CO CO eo c<« o o c c3 >, ^ r- 22 2 a O.IOU3 3 3 M (M cS j3 . • o o 3 ;:: 3 5 c« ^ *- '-' /.r r^
  • - o^^ 3 u,^' I- CO ^ 3 . CO S I-. m"3 !> ^ . . 5 =5 S S g=^aa .■-3 g g «2 g g § ►— ' p-^ ^w^ w e3 c3-<}<^ >-. r^ C ^-■^^ -* ^ "^ c ^ ^• "I. ^ rS eg "^ a a M C"3"3 CO CO CO CO CO CO ^, -x coco CO X CO ^ n4 ..4 ^ C ..* a a 2 s 00 cs a s o o C3 S "3. TO o 3 3 !:3C&H o o i: £:i.2 o t: t;j2 o o foOC o o g d o o a c« > 3 _o C8 O §1 03 <: o o .a_o 000 > > > <1> r> rn C^J m OJ ai«c^ Lh Ui t-i -a -3 -a ^ 3 ^ ^ 'i c> -1 o a c 3 is* :« 73 0.3 c o o— < O -3 c O taO o a a a ;^-3 o oi a> c3 c3 j!^ uTf* Jr^^rr rt -^ eqcq3qSi-:ji^2;ti5p^ cto bC 0-3 ^.SPIS o ■gcQoQ 23 >> o o o o 3 (.4 1^ Ur ki S 73 M to CO Jr o o o o 2 o o o o £ O O 03 o 0*3 "3 -3 "3 1> fc-i Ui Ci by S c« c3 03 -3 O o o > > a> o CO M o o p:3rt S '-!^-^ 3 O l> Ii 0-73 c* C3 ^ (-. C3 TO O -fj ti a 3 o u, 00 CO t^ b t^ ^ .^.^ ,^ 'o'oo'o'S'o'o >>>>>>> 11 11 11 1:4 b b ^ O O O O O C3 O CO t/3 M CO CO to CO 0000000 0-3 c3=: O r3 c3 c^ c5 rf c^ c^ a, Ph ar Pi Ph Pi P.4 e a a 3 3 3 c c3 c3 ^ c3 ^ c^ c3 TOTOTOTOTOTOTO M s 3 > a> a a cs > o X5 ii r^ IM CO -^ >« CD t^ 00 O O " cq CO-^iO ^E2:S2?i i-1»— ( 1—1 T.1T— 11— ( ^Ht-Hi-1»— 1 O >— OC0t^0005O (MCviiMeqcsevKMco f 50 DIVISION OF WATER RESOURCES OS S Pi .2 o •a o ^. O a. to E -T3 -3 O, t- •- O o o PL, i^ o .a -e.S .- a a a o a> 5 1-5 o a, « § a w o os-a .i-s "Hi o > Q. O a 2 o n & -fc3_Q C C !U_c3 Oca K-D e c 2 a rf rS rt a o o o. n a> •a oi •*> 3 a o O -H ■<*< O O "5 «5 CO CO to -H .— 1 T-H r-i IM oo o CO •*! ■* -^f CO o o lO »0 »C O '— ' (M T-H »— < 1-H CO CO CO <:OTj« CO 05 •"*! O 00-^ iCt}< "5 i-O CO CO »< T .— ( — < ^^ «# ^ -^ .— .— 1 ,— , -^ TJ< Tt< 05 00 0> C: -^ Tti "5 oo O ^O — -Hr^CO-^COCO I I I I I I > I I OlTtiCiCiM'OCOt^t^ OOOO — — COCOCO 05 02 O^ O O^ 05 ^2 o^ o^ lOO 0«5 O "0 ■>*i COiO oo OO 05 OM 1 > 2^ lO o >o UO o t^ O cc oc CO O •* IM C^ — 1 O OC' 00 »o »o in o o o 1^ r^ (^ O r^ r^ I>^t-;^t^OO oc O0_ CO CO CO CO CO CO OOiCOlOlOUOlOlO O -^ oo c^ _-+J «-^ CO 3 3 casq c o -C-C " K I- s s ^ c8 3 O O 'CO £ - CO CO i- CO CO a. 3 ^ CO m ?3 C3 C3 Pj -3 T3-3 — 3 3 3-5 O C O ^ o 5. -3' CO CO s s o 53 -CU o a SS, E,"P -^ « o o > > i go 3 3 2 bo-S a a g §0 ■ --, CO CO > o a> o ■^-a-a-TS^ £ c a c3h c3 cs 3 dj o 0)^ ;s is o a a a.js a a ^-3-OT3 3 a a o a a a 0;.-u- a 3 o — - _Si-2 a o '■^ ' cS -^^ o a a o o -_.~ g eaJS'*' ca > x: 2 o o • S ?S-3 cS o 5-^ .. ^ Ix ^ W« ta-^ ^^ * coco 03 S3 £ £ 3 3 cooi a *-*3 ea CO a> 3 o >i S3 s s .2.2 ea ea coco o o o22 ^^ ■■ - - ni ca ca ca u: c/: >>• C3 ea a a a> a> 3 3 a a > o o eaOO a S3 ea 3 JhJ O c3 ca --^ cq CO -^ "O CO CO CO CO CO CO CO r^ 00 05 o "-I 1^ > M (U SJ— ' >-o o a ^ 3 03 O . ^ b o <-H eu > o a eij ca S aa •i £3 a -1-1 a> a> u. O d ce e« > 3 3 03 W3 a (-1 bi Oj O) a c3 c« Ph ^^ I o >> OS o o O a '•+3 cS o a > 0) o u O o 03 O ■♦» m a o O 03 •^ > o a d M o 3 O m o ^ ^ o .9 c8 o a Ph d o a o CQ CO a a o n >, >. 3 a ea cS o o a> a> 3 a CIh PL, -r1 -r) a> !U cS CS u o >, >, 33 rt r< 3 a bO bO o o OB U a to eo a „CS 2J « a 3 3; — •" fe 3 W , Ui '^ ir^ o oi- O oi-O o coP-iO t^ to CO 1 r- 05 o ' tcor^oo05-^^Hcoc.O05lO«0O(MC0»0'-iO0iC-«ir.100 OOiOr^OcOO>tOa5COCOOOO»0»0JOOOO"005C^CO-«t<'«J<-^TtC>C»ftCO«O>Ot^00t^>OC0 cocOTticooo'C-Hi~^c^cooooi»0'^ooO'^"<*tooc^'>*io5050ooa2-<*< t^t^cO':OCOt>.005005l:^0»C-*>OtOt^tO ^# ^^ CO CO "^ ^* ^^ ^^ '^ ^^ "^ ^^ ^^ CO ^^ I I I I t t I ( I I I I I I I c>»a50MooTjo>oe<>-H cococococococococococococococo 0> Oi 03 Oi 05 Oi Oi Oi Oi Oi Oi Oi Oi Oi Oi lCC0»0>0-^C0t^iO^HlC>OlO-Hc0'^C0'^»'500'^>OO'OO'OlC«ra'^0i •^•^■^COCO'^'*'«*<-^-»tlCOCOCO'^'^-*COCO-^COTtt'»tiC0CO'^'^'^-^M<'^C?C0CO I I I I I I I I I I I I I I I I r I I I t I I I I 1 I I >— iCC^OiOi-H05 0iO'^O'^O^HC^000000C0^^0'0»o>oo'0'— IC30 cooor^coco— iO'>*5>OCOt--.OiOiOiOiar'C^OOOiOi .c>^ -t^ -4-3 <4^ jj ,,j a-^ a OfeO a ce a 3' C«( -*^ -*J ^J 1 CO , 5 S a a a a a a c3 cS c3 c^ "O -^ T3 "O c3 cS c3 ca CQ c^ ca ?5 a a a a cS cC c^ ca a a a a a a a a 3 3 3 3 o o o o t^ $-, U^ Ui S S E C^ C^ 75 • c c c > ^ c^ ^ J a a a a a 3 3 3 3 o o o CO a" g a" s' a " a a - a c a 3 3 o o ^ ■ **—«*-< CO **-« a s^ s ■3-3 "-3 a a' § a a a ^ a a a-^ a 3 3-t^ 3 P p S p CO ^ 3 — •- ^ s 3 a -5 1--3 a-a a a S a e"^ a 3^o 3 1-5 . W 0^ O XI ^ CO 3 c^ J«2 c5po . a a" ^ a a-^ e „- 'So .a 3 -S a - ^J CO ",t3 a"^3a c« crPQ t« a=^sa c-~'^ a o §»2 § a a aa^ to c^ ^ CO CO a 3 3 a a gCQpa .2 c« rJ'jSjs a '- aliaa a a C^ tT- -t-3 ^•*-' ^ 04 a:> t-t m a eaeaaaaaesaa g ca.SCQ.S.S 03.2.2 "CQ «« ^^-5 ^"3-3 ^^-o'S I- ^ a* a a" -a <^ d a a a a a a a a a c-^ a .« — Sao-aaaaSQ' e a a a o a a a o a'^'^ a a gajcd g § oi^t^ooooo^cocoaootDOT a a a' o o o'^-' a a o 3 3=3 O O OS a a a a a 3^^- o o o"-' cS 73 c "a >: O) a> en OI 03 a> a a a 3 O O 603 tQ- > - t>- E*. £> ^ • — ' --^ p^-= -« S &: a 03 O "^•^ -a JJ 03 il 03 a ,-g ^- I E a c3 t a> o a o o3 - .03 -a q => o >> ■- V. aO a .og|§ >-T3_ a o o > > 03 03 c« o: 13 03 O O a a -a 03 a a WW rn 1 03 03 a > P4 o o Q a c« 0) o a O o a a « Uh Pi( o > — 1^ .3 m O 0^ 03 O ^-^ P^ 3 «2 ;3 03 -C a ?? 03 tH CIS U> L.I L-i Ut K-i t-i Im o > — ^ ..^ Im u 03 03 03 03 03 03 03 03 03^^ .030303031—1 __^ _J-rC5rtp5rt j3 a^ OS =« <^ C3 C3 g ajfct'— "-^'~^'— ' g-3 a a a a a 5« I. t- ?2 o. 2. 22. a. 2 B-^J^4i^^^ ^-' a tS !S P - n o o o Q -3 - -- - - -- o o o o o U-Z2 52 2 S 2 a_ ■g o OS Sm «^ mPh « a . . CO c3 ^ oS eo a - 03 O 4^ 03 a o a >=• .- c« a a o o a a o o 03 o o o 03 J:; ^ ^ ^;::^ j; cs c« o3 c« cs os cs.-.a o o o jt; o .a 03 03 X2 a a 12; h O'-'C*<"5cOt^00O3O-HC^C0'«J< cpcocpcDcococpcococpt^t^tN-r-tS- »ocot>-oooiO-HC'5cot— oooiO'— icoco'^'Ocot^cxjoiO'— <£3coS!i2S2t t^t^t^t^t^OOOOQOCCOOOOOOOOOOOOOiOiOiOiOiOiOiOiOiOiOOOOOOOO 52 DIVISION OF WATER RESOURCES a o 3 C to o o o C4 C C4 <3> _C >> o ,^ =« o a %4-l •S £ o =« Z^ c4 rM t^ Ui K^ C a> oj o a> -O^ ^ > O O O * o o o ^^ •+^ -*-3 -*^ ^ >> >> >> b C« J} 08 C'S c3 c3 03 -+^ -frj ,*j -*-» O O O O -«-» -fJ -4^ -«-:» CO tZI CO CO c a a B & fe fe fe o o n O ^ J= ^ J= CO w a> «; _o o _o _o ►^H - - u >M o o o o C C C Q, 73 c3 :« S3 > > > > a o 2; OS 02 O (M o o 05 eo M c«3 00 ■* C5 oo o 2 a TO *^'^ OOt^O'^COlClOOSCDCOlOM— <03COCOOCiCOtDCOCOCOt^ CO»C— i0CC*' 00 ■* a> -o -a fe o " «5lCOCOOCO»OlOlOiO'f5lOOOCOOCO— ">0'^ — 'O'^ — ■^^J—■•<*< CO CO ^^ ^^ ^^ ^^ I I I I O — < Tf CQ IM ■* ■* Tf o: c: c; OS "5 •* I ■* ■«*< OS O >a O a. to IB E E > g CD a> O aj cot^-^oooourr^ooocoo-^-^ocoooc-^fin — o^cso^O OcoO'*- -a c a D. 00 C c« 00 c« . . . aco G '"'<-'" 3»c 3 (M c^ 3-3-2 - D.3.^ 333 .= -^"^ c o == '^33 (M 73 C3 o o r-^ 1^ t^ 3 3 3 .saw c-B -a 2 J3 ^ — 3 c3C -!-:> C« ?3 ^^ COQQ («^ C^« -«^ (4-1 CO C^ ,CO s a 3 3 O li O cr 3* _; c- CO W g^ CO ^ !3 3 «3 a ^ ^ ^ o o fcfeC&H B c9 a 3 cS a> 3 m ki a o '-^ c4 a O 3 _B 3 o O B a> -a: 3 3 h-5 3 c =-, .3 o o^ c a B 3 •— c« CQ c4 cflQQ aco OT Q-* "^ c« c4-C"C .3QQ0C "2 3 3 B a =3 C« SJ c« c« MCCCOOJCC 000 S S S 3 :« :« c3 5* QQQQ -3-3_0 3 c« 05 CS*'- OCO 03 B 3 B B c3 c8 c^ c8 o > S a) K* CO Ui >?r ^ ►— 1 o >- »- i: o S S rB 45 03 ^ ■T3 c9 O P5 =3 O >- 03 " ^ ■- 2 B^ oS a; O -3 3 3 O o bO 3 ■5 c9 o c« a> Oh CO to 00*3 >> >>-3 3 5 §^^ 3^ ?^ 3 o o > > t- a> OJ «« >, w aj >> >i 3 o3 — —J 3 -3 hJ =« J« O B M bO O O 03 B B c t. t- o o B E" 3 >z o<< OOOsO-HMCOTfilCCOt^OOOSO— ^C-JCO-^iOCOt— OOOSO 00<-ii-i.— i.-<.-<.-c,-i,-i^^^-(MMCaC^)C^, C 3S o O) .^ Km =3 13 o a> c t« fe ^ o j3 Oj n > o ea -4^ i-( a> o a e ^ m > Ph Ui W5 CO ^^ CO «o o o _3 c8 (U oo _Q CO a OS -S e« a >> 0) 2^ CO CO a o -i3 O N s a o aT o 5 « s ^ (M Ttl TO o M •*3 't3.':st3 a .2 S S a J3 2 .S a a a> '-I3 ■s^ fc 2 -4-3 a> & 2 > 0) •^^■^ CO a o a o .2 -° O I- s =« a a f^ a a C« a> 3 CO S aj '^ ^ ff3 -^ C ^5 o -*-3 3 '■^^ o Ji £ « S S a a o O g a a ,= a; a> o o o a — H Co ic oo o 00 00 CO CO t— 1 T— 1 1— I »— t ,-1 T-H ,-1 000050> 00 CO CO t^CM'-'OCOOOO — 00»O>nO5t^ t^iO^C^lOOOO-^COO'ClC-* 1^0 Oa --^ 05 CD CO e*< C^ 10 •*! — 1 ■<*< CJj CO 0> CO CO >o T-H r-l 1 00 CO CD ^ -!t< 00 1 02 -^ eC t^ t^ 00 r 1— 1 U5 (M CO 00 00 «5 t^ CO t^ ■* — 1 00 CO CO 0lC«D»0l0>C«5-^«0>0 00 (M -^ CO CO 05 iC t^ »0 0> 00 --H us CO CO CO 05 05 o> »C'*050>a5050505iooco(r <3l Oi o> 10 CD ■^ O I I 05 O •^ O 05 05 iocD-Hiococoeo-^kOOO>o ,;c» -;05 .;05cocd»— 050 ->*<-^-*-^ — ■^■n->*> >i >i 05 03 05 02 05 3 3 3 •^ ^ ►-5 00 Ci ■^ (M 05 o (MC<>IM(MCSI <3i O^ ^: Oi <3i o CO c>5 0500000000030000 oO'^cot^cocococDOoooocor^ lOOTtiCO*i-H 00 00 00000 1^ t^ >0 »C (M C^ Cq 1-H T-H «5 10 a c« e^" a 3 _. ^ ^^ h! c« 3 3 t- «^ (D C« J- aj . *-*^ TO 3 . o a c« a, 3 3 CQ TO c« a; a^ . 3 a -a a c« a CO CO a" CO -4J -*^ -*^ -*^ CO c« <1> 3 -O a^ a ^ 3 (L> t- 3 a ■5 •* (N a a a 2. ?3 ce -3 o a a -^-i a ca O) O 3 eat; a a T3T3 Oi 3 3 S O ® £ '- *- o OOto _ „ a c oCQ _c« _2 S 5 a" a' a^ S S C "^ Ci o jf ro t^ c^ - Ot^cOTO a a.S 3co c« C ■-a c a ^-3 TO tDO a o a'*- 3=^ 0) - cfl a IL4 Lri f-. L^ 3 3 3 3 mpamm t^ k^ fcM u ai aj oj aj ^ J3 J3 j3 -kJ -*^ -f^ -4^ 03 p3 ca ca aj 0) aj oi TOCOTOTO C a^ sic^ -StoS^^ Pt,t^TOH:5 a a a a . ca ""^ C3 03 CO ao3 a a a 3 a 3 3 a CQ amm S a 05 ae a a" as ca 3 ca c3 ^ a> a o) oj a . 53 . . -3 TO'r TOTO<~ Pc^tJE^TO a 2 tS 'cfl 03 PL, a 2 -3 c .a a Ph 3 m t^ aj -^ =3 ID TO t5 i'^-o a x.a T3 a§| a0H T3 O) aj a CJ 2 -3 a" a ^ t; 3 ti totoWto 00 50 a c o s o S a TO o > a a 03 oOO O aj c> cs'-'i-l a 3 o O c« ■3 ^, a' a 2 = o o a 3 03 a B a a a H 000000 oCQW 03 c« M -^•d-4^-4-9-4^.^^.4^.4.3 a %^ Ui Ui Ui hi h' h-S a> ai a> a> a> 333333 3 3 a^ aj i b fefefefe(iHfefe!ii:z;^TOTO o o a a c4 ca 3 oi aj >. O) pq a o e« -4-3 TO •4^ a .§ *C a o o a a - CO 3cetc:aj(ii(UQ; -3 aj bE ■3 bC •3 CQ PQ n .2.2 2 -5 "5^ a a c CO c« CQ Q03 o a; oO OiTO a> O O c« TO c« TO a 3 , O) ^ t^ C3 ca c3 :ooo o o .s.ai3i«-s« <-i>-i . . . g j-TOTOTO 3 -g t- o C4 £ 3 ^ o eocot-^ooo50— 'C^co'*! •Tt"^!}«Tt<-^Tti'^'^-^lOlO^OfcOlO t--.00O5O'— iOCD(^000> 10»010C0C0CDC0CDCDCDCDC0CD o C<1 CO -^ >/5 CO t-- t^ t^ I--. t^ I I ( 54 DIVISION OF WATER RESOURCES to 1 s 1 & 2 < 3 «*..r O 'So Direct measurement Desert exposure In desert area adjacent to Mojave River In partial shade of trees Evaporation shown is total, May to October Protected with 2-inch mesh screen Painted black Painted black At bottom of steep-walled canyon A concrete basin painted black Many monthly records are missing Formerly La Mesa Reservoir Formerly La Mesa Reservoir Painted black Painted black Painted black Refer- ence table'. «o CO eo «o e^ -^ t^ OC CO CO o o fl>Ct^t^0005C5 — C^C^JC^CQCOCOOSCJ Mean annual evaporation. J3 o C t^ OC O 05 c 1 — . CO 1 CO'^ idc^ 1 — oo COO— 5t^occcocot^ot^cooco-*-*oc5cocoo;cooe>j OCOCOOCM — COCOI^ — COCO»flOl^cOt^CCO'*COOC(MC:»ftOO;CO'^ Oit^oct^oc^ — occscoc^c: •^Ocot^ocicc«^r^c^-*io' — — co'S'C;-^' COCO-*COCCCOiOCOCOJ:^COlOCOt^CC«OlC»OCO»C>C»Ct^;CCO»«iClC-.0:0 Period of record r^ — ' c^ CO CO CO CO cq CO lO OC' 02 C5 oo o — C: CSI (M (M CO 0 — — CO(MCO-*'»> — t^t^O:»O"0CO — ocococo CMC^CO — CO— 'CO — — COCO-^-VC^C^-^COCO-^C^C-^COCOCO — •'J'lMCO'**^ c; c: 02 C-. c: o; c: 0-. c; C-. c: C-. 31 0-. C-- c; C-. C-. C-. c; C-. cr. r: c: c; C5 C-. C-. C-. c; Eleva- tion, feet o o o o «oco o CO C «0 UO O iM O — _ o_ o_ o_ o co_ t-._ »c"'-''''-<",-r rSci OOOOiCCsOOCcoOOOOCOOOOOOiCCiCOOOOOO OOCOO— •^(MtJ- — lOt^OOCCOCOCO — C-. oico-^-^acooioococo CO co_co t>._^- t^t^ t>._r- cj CO CO "S-^ ■* OO-f'TirjtN-.oco — <"— M Tirrir.^" — ""esTcJc^ — " co" co- Type and size of evaporation pan Three-foot floating i>an... U. S. Weather Bureau pan U. S. Weather Bureau i)an- Ground pan, diam. 23 ins.. Lake surface U. S. Weather Bureau pan U. S. Weather Bureau pan Ground pan, sq. 3x3ft ._ Floating pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 ft. Floating pan, sq. 3 x 3 ft U. S. Weather Bureau pan U. S. Weather Bureau j)an ( >round pan, sq. 3 x 3 f t _ Floating pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 ft Floating pan, sc]. 3 x 3 ft U. S. Weather Bureau i)an U. S. Weather Bureau pan Floating j)aii, stj. 3 x 3 ft (irouiul [)an, sq. 3 x 3 ft (jround pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 f t Floating pan, sq. 3 x 3 ft U. S. Weather Bureau pan.. Ground pan, sq. 3 x 3 ft Floating pan, sq. 3x3 ft U. S. Weather Bureau pan Ground pan, sq. 3 x 3 ft Floating pan, sq. 3 x 3 ft F'loating i)an, sq. 3 x 3 ft F'loating pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 ft Ground pan, sq. 3 x 3 ft. Ground pan, sq. 3x3 ft Floating pan, sq. 3 x 3 f t o ■^ 73 e .2 d a n o o c o 3 c 3 O O o c 3 « c ca Little Bear Valley Ontario San Bernardino Silver Lake - Trona Victorville 3 O o o c Barrett Reservoir Barrett Reservoir. Bernardo Bridge Big Lake (Henshaw Reservoir) - Bonsall Basin Ghula Vista Guyamaca Reservoir Guyamaca Reservoir Guyamaca Reservoir El Gai)itan Reservoir Henshaw Reservoir ._ Henshaw Reservoir. Henshaw Reservoir. Judson Reservoir Lake Hodges Lake Hodges... Lake Wohlford. Lower Otay Reservoir liO wer Otay Reservoir _ M i.ssion Basin. M orena Reservoir Morena Reservoir M urray Reservoir M urray Reservoir San Dieguito Reservoir Ran Dieguito Reservoir. San Vicente Reservoir San Vicente Reservoir 1^ t^ 00 O O — M CO t^ t^ t^ OC OC OC 00 •ftCCOt^OOOSO — lMC0'V>0COr^0CC5O— '(MCO-'T'OcOr^OOOSO — C>)CO aC0C0000aCQ0O3O05C50iC-. 00505Cr20COOOOOOOO — -H — — . i EVAPORATION FROM WATER SURFACES IN CALIFORNIA 55 a o J2: c o CO a _ *> §S O (3 o > "-5 o CO .S CO CO fl (S a a> a> a O •-< CM 03 ,-j _i T-H e^ CO CO CO CO CO to CO CO r^t^i-^t^t^i^oooooooooo oooooooooooooooooooooo C3 t^ CX) 05 05 O Oi in *< t^ (M CD iO 05 CO CO (M r-- OO o> ooco o >o •O lO "5 ■^ CD (M (M CO CD lO »-iOO(MU5IMOOCOC>qcDOOOO O^^CO-^t^C^C^OOC-JlOlM t^CDCV)Ot^00-H-^00-«t<"— ' ■^■^CO«5'^'<*<'«*<»C^»CCD CO "O 05 O CO CO O CO Oi »0 CD 00 00 "5 o (M Tti <— I I I I I 03 COCO CO 00 — * -^ »-H 00 Oi 05 05 "5 o o 03 05 CO 03 CO CO 03 CD ooooooooooo I I I I I I I I I I I ooooooooooo Oi 03 Oi 0> 05 Oi Oi O^ Oi Oi Oi kC >C CD I^ •»f< ■»*< ■*! -^ CO »0 9 oioSto CO IM ■* o:> 05 Oi C5 05 CO o C5 o o o o o CD OO 00 o ■^ -^ Tjl lO »o CD ^ CO •o iM Ct^05 CDCD-^-«t

    3 m o ■+^ <:« 0^ CO CO ';^ CO CO CO ■" CO "^ ( CO CO '^ CO CO CO ^ CO '^ ( m CO a a tA 03 ^ CO tn S c" c" c^ !S J3 ■^ a, o. ri o" _L i to __ CO CO CO . o n "^ a HI oofeOOoEo c c 3 3 O O lis ^ C G C'S 2 ^ "•CD. c o3 . «;5 a a S3 2 -3 -3 e c 03 c« O. O. 3 3 03 c« Ui Li 3 3 mpQ >-. Li c« c« C c« ft . o . . c« C4 ft ft 3 3 e« eS Ui Li 3 3 Li Li 03 rt 3^ (D 3 C c« ft ^^ ^^ aioi -a a 3 o oo no C3 ft s O s o -4-J 03 c ca '5. X 2 CC a -a Hi 3 .3 §■3 3^ O «- o i Cos c3 ■?! "o o > > > d OJ ^ u. Li >, fl> flj crt -4-' -*-i 03 f« O is is

  • QJ tl) u, ii li a I72 02 hJ a 3 o O 3 C 03 §^ CO a.-S 3 CO O Li o-s o > ti (V) ra K .2 S§ 3 a ^■"^ Ceo cQ Ol -fJ Li §■3 o £ o3'^i-J "^ <^ s 03 i^ O m 2 S c«^ a 03 GO a> a 3 , „ O - — - OJ =« - S - — * TO c<;o CO ft 3 . o be M bo Li Li Li OJ o o o ■■gooo ^ Li Li Li ^ Q> iJ OJ O ft ft ^ > ft ft o gj h3 H^ i-J (VI CO . . .-S Ph Oi OJ oi "; (u 0) Qi a (u Li Li Li a "33 oj cu oi ca O O ©►^ i2ig'co^:S oj a> °g 3 g c€ c€ cQ cQ C^ o o o w o a> 0^ QJ ^ c3 rt c^ c^ G G G C C 3 3 3 3 Z3 tJC bC bO bC bC rt o3 c3 c3 ^ ^q i-q h-; K^ i-:i s:3 C *-' t-" ^ O) a; 03;=; — to ^o3 c4 02 a CO cS •s >> SO) ^ CO (^ (P 09 C« -4-S -tJ CO CO c« c« ■a j3 com a ■" 5 03 o a O *a c9 c9 CO S o ft ft c« a 3 o o 03 E 3 ■* >0 CO t^ oo 03 O -H e<) CO -^ lO CD 1^ 00 03 O >— 1 CO CO -^ i-H r-H r-( -H T-i — ( Oil c o CO •^ CO CO CO 56 DIVISION OF WATER RESOURCES m I- a 3 o bS 0) bO I O ;i3 OS O O >> « S .2 'E o "To e o ea o fa E E 3 ^^ O 13 2 c 3 O c o S S aj to CO CO 'if CO -a h O O 0) »0 05 Ttl T-l 1 1 ■^ o C5 03 •»tl 05 I I CO to 05 05 OO o »o CO CO ^ o o o a n >^ . CO S C C a a .S.S c« c« _o o C3 03 a a 3 3 03 03 Qj 3 3 pqm >-< 1> a, o J= J3 -^^ +^ c« a ^^ mm 3 O a .2 c« O a 03 > 3 o o o >'^ S ^ tt> 'S > OS -*^ a> >-i u 3 O o to 03 .3 a> ^ 3 a o tc o J? '^ •A C 3 >0 03 CO 1—1 1— t 02 00 ^ 0 »0 "5 CO CO CO CO 05 t^ »0 Tfl 10 CO »C CO C5 t^ 00 CO 00 .— 1 t^ O •^ -^ o; o O ■<*< OJ ■* I I I I CO —< t^ — I O (M ^^ CO Oi Oi t^ C5 00 ■-H r^ O CO 00 cq -^ 3 c c _ c^ 03 o3 '^ a a a CO 3 3 3 i«i o3 03 o3 OJ O) Qi CO t^ »- *- .333 dj a> (u a c« c« 03 OJ Qi Oi <^mmm o . . . o ti ■ 3 3 1-= ^ > is CO 3 _aj _ttJ , .t; ca o3 3 .0>> ^ o3 c^ o3 o3 jfl S S S S "3333 II ca CO O) (M CO t^ 00 05 ■^jH ^J* ^Ji ^Ji OJ (N 03 a s o s o 03 C ,5 "a o\ 3 r— I O > a a o X3 ca a SUMMARY Additions to the irrigated acreage in tlie West since the first of the century, and the widespread demand for power, have resulted in the development of many reservoirs from which losses by evaporation are becoming increasingly important as a factor in the water supply. Con- struction programs for the future call for more and larger reservoirs and for further economy in water use. In early years when the supply was plentiful no attention was given to water losses, but as the demand increased attention became focused on them. This interest is expected to continue as water requirements are extended and the value of water increases. This report has been prepared as a foundation for estimating evaporation from present and future reservoirs throughout California. The depth of evaporation varies throughout the State from a maxi- mum in the hot desert regions to a minimum in the snowclad mountains. Along the coast it is held to a medium loss by moderate temperatures and the presence of haze or fog which often partially obscures the sun. At the lower elevations evaporation is recorded throughout the year, but in the mountains freezing prevents measurement during the winter months. Pan records vary also according to type of evaporation pan from which they are obtained. Pans exposed above the ground surface have the highest loss. Of the pans set in the ground the greatest depth of evaporation occurs from the smallest and the least depth from the largest pan. The four-foot diameter Weather Bureau pan is a favorite in many places. Evaporation from this pan has been recorded as high as 156 inches a year on the Colorado Desert, and as low as 58 inches annually along the coast. Desert winds at pans located on summit areas between coast and desert increase the normally low evaporation. An example is the record at Fern Canyon, at elevation 5,100 feet in the San Gabriel Mountains, where the recorded evaporation averages 76 inches annually or 18 inches higher than at sea level and approximately the same latitude. Fern Canyon is above the fog belt. Evaporation usually is measured from small metal pans and trans- lated to equivalent evaporation for large water areas through use of reduction coefficients that must first be determined by experiment. Evaporation recorded from pans is the true evaporation, with rain fall- ing in the pan accounted for as water added. This is the common method of calculating pan evaporation. In some cases evaporation from reservoirs is designated as ' ' gross " or " net ' ' evaporation. Gross reservoir evapora- tion is the actual depth of water lost to the atmosphere. Net evaporation is the gross evaporation minus the depth of rain falling on the water surface. Gross evaporation is always positive but net evaporation is negative in months when the true evaporation is less than the rainfall. The terms "gross" and "net" are used in connection with estimating reservoir losses but not in connection with pan evaporation. Different types and sizes of evaporation pans are in common use. The standard Weather Bureau pan is the most popular one and has the most extensive list of records, but it also has a rate of annual evaporation (57) 58 DIVISION OF WATER RESOURCES that is at least 30 percent higher than evaporation from a large water surface. The Bureau of Plant Industry pan, six feet in diameter, is set in the ground. Its rate of loss is considerably less than that from the more exposed "Weather Bureau pan. Ground and floating pans, each three feet square, are used in many places for estimating reservoir evapora- tion. Evaporation records from these pans are not identical but are close enough so that when a floating pan record is lost through wave action a substitute record may be obtained from the ground pan. The Los Angeles County Flood Control District observes some 25 evaporation pans form- ing a county network throughout valley and mountain areas. Records usually have been continuous for the past 15 years. The screened pan of the Division of Irrigation and Water Conservation was designed to reduce evaporation to an amount approximately equal to the loss for a large water area. Its principal use has been experimental, but the Los Angeles County Flood Control District, beginning about the flrst of 1946, adopted it as a standard pan. Theoretically, at the higher elevations, evaporation should increase as a result of the decreasing barometric pressures, but practically it decreases as a result of the lower temperatures. This was demonstrated by the Mt. Whitney study by Frank Adams in 1905. The Salton Sea investigations by the U. S. AYeather Bureau in 1909-10 demonstrated the difference in evaporation over land and water areas. Evaporation was greater over a land area than over a water area. On a water area as large as Salton Sea evaporation close to the windward shore is greater than at a distance offshore. As the air moving over the water absorbs moisture, evaporation decreases until it becomes nearly constant. Air moving from the water to dry land loses some of the moisture absorbed in its passage. For both land and water locations evaporation increased in vertical sections. Investigations by the Division of Irrigation and Water Conservation at Fullerton demonstrated that different rates of evaporation occurred from different types and sizes of evaporation pans. Evaporation coeffi- cients were determined that are applicable in estimating evaporation from large water areas from records of pans similarly located. Coeffi- cients used should be those developed under climatic conditions closely similar to those existing where they are to be applied. Coefficients obtained by experiment in Colorado should be useful in mountain areas where winter conditions prevent evaporation from water surfaces during winter months. California coefficients were determined at a low elevation and should be applicable for valley and mountain areas where winter condi- tions prevent evaporation from water surfaces during winter months. Coefficients for a majority of evaporation pans have been derived through comparison with evaporation from a 12-foot diameter pan simi- larly situated. At Fort Collins, Colorado, comparison was made with evaporation from an 85-foot diameter shallow reservoir. The results show little difference in the coefficients regardless of the size of the base pan. Coefficients determined in California agree generally with those obtained in Colorado with the exception of the coefficient for the ^Weather Bureau pan, which was 10 percent higher at the California station. Coefficients vary from month to month, usually being higher in summer than in winter. Annual coefficients are the mean values of all the monthly coeffi- cients. Monthly evaporation from shallow pans differs from evaporation EVAPORATION FROM WATER SURFACES IN CALIFORNIA 59 from deep bodies of water. The difference is due to amount of heat stor- age in the different volumes of water. Shallow pans hold but little heat in the water. Deep reservoirs absorb heat during the early months of the year and return it to the surface in the fall when the air is cooler. This increases the temperature of the water surface and the depth of evapora- tion at a time of year Avhen pan evaporation is decreasing. Because of this it is not likely that monthly evaporation computed by means of coefficients will agree with actual monthly reservoir evaporation. Annual evaporation should be approximately the same in either case, as both pan and reservoir are exposed to the same total amount of heat from the sun. Pan coefficients vary according to the size and exposure of the evaporation pan. The AYeather Bureau pan is the least efficient as it has the highest rate of loss and the lowest coefficient. For pans set in the ground the largest are the most efficient and have the highest coefficients. In experimental studies, coefficients for all other pans are based on evaporation from the 12-foot pan, or as in the Fort Collins study, on evaporation from the 85-foot diameter shallow reservoir. A coefficient of 1.00 has been adopted for both the 12-foot pan and the 85-foot reservoirs and all other coefficients are ratios based on this value. Evaporation from large water areas may be computed directly from records of inflow, outflow, rainfall on the water surface, and change in water levels. Such computations give results that are more or less approxi- mate, as usually all items entering into the equation cannot be evaluated. In most cases, however, v/here such computations have been attempted the results have been sufficiently accurate to provide valuable data. A large number of evaporation records are presented in this report. They include pan measurements obtained because of the interest of many organizations in many portions of the State. Records are most numerous where there is a scarcity of water and many areas where water is plenti- ful or is in small demand have not found it necessary to observe evapora- tion losses. For years cA^aporation has been recorded at many of the larger reservoirs storing water for irrigation, municipal use, or power. "Where streamflow is unreliable as a result of dry years, carry-over storage is a necessity. Under such conditions evaporation may become a higher percentage of the total replenishment than on streams where the flow is more regular. Evaporation records shown in this report are not comparable as they were obtained from several different kinds of evaporation pans. Nor do they represent actual lake or reservoir evaporation. Evaporation coefficients, determined under different climatic conditions, have been presented for evaporation pans in common use in order that engineers wishing to compute actual reservoir losses may have a basis for such estimates. No attempt has been made here to present such estimates, as it is believed that interested engineers generally will wish to make their own computations, Literafure Cited (1) American Society of Civil Engineers 1934. Standard Equipment for Evaporation Stations. Final Report of Sub- Committee on Evaporation of the Special Committee on Irrigation Hydraulics. Trans. ASCE., Vol. 99, pp. 716-18. (2) American Society of Civil Engineers, Special Committee on Irrigation Hydraulics. 1935. Letters, Symbols and Glossary for Hydraulics vrith Special Reference to Irrigation. ASCE., Manuals of Irrigation Practice — Xo. 11. 39 p. (3) Bigelow, Frank PI. 1908. The Observation of Evaporation Made at the Reservoir in Reno, Nevada, August 1 to September 15. 1907. U. S. Dept. of Agric, Weather Bureau, Mo. W. Rev. and Ann'l. Summary, Vol. 36, pp. 24-39. (4) Bigelow, Frank H. A provisional Statement Regarding the Total Amount of Evaporation by :Months at 23 Stations in the United States, 1909-10. U. S. Dept. of Agric, Weather Bureau, Climatological Division, Abstract of Data No. 4. (5) Blaney, Harry F., Taylor, C. A., and Young. A. A. 1930. Rainfall Penetration and Consumptive Use of Water in Santa Ana River Valley and Coastal Plain. Calif. Dept. of Pub. Wks., Div. of Water Resources. Bull. No. 33, 1G2 p. illus. (6) Blaney, Harry F., Taylor, Colin A., Young, A. A., and Nickle, Harry G. 1933. Water Losses Under Natural Conditions, Pt. I. Calif. Dept. of Pub. Wks., Div. of Water Resources, Bull. No. 44, 139 pp., illus. (7) California Water Project Authority 1939. Flows and Stages of San Joaquin River and Tributary Streams between Friant and Freeman Ford, May 1939. Unpublished. (8) Clapp, W. B. 1905. Report of Progress of Stream Measurements for the Calendar Year 1904. Pt. XI, The Great Basin :ind Pa'"ific Ocenn Drainages in Cali- fornia, Dept. of the Interior, U. S. Geol. Survey, W. S. P. 134, 276 p. (9) Clapp, W. B. and Hoyt, J. C. 1906. Report of Progress of Stream ]MeaPurements for the Calendar Year 1906, Pt. XIII, The Great Basin and Pacific Ocean Drainages in Cali- fornia, Dept. of the Interior, U. S. Geol. Survey, W. S. P. 177, 273 p. (10) Crawford. E. J., Chairman 1921. Report of Sub-Committee for Collection of Data on Evaporation from Reservoirs. Unpublished. In files of San Joaquin Light and Power Co., Fresno. California. (11) Cromwell, George 1923. Study of Rate of Evaporation from Water Surfaces in San Diego County, California. Unpublished. (12) Ellis, Arthur J. and Lee, Charles E. 1919. Geology and Ground AVaters of the Western Part of San Diego County, California. Dept. of the Interior, U. S. Geol. Survey, W. S. P. 446, 321 p., illus. (60) EVAPORATION FROIM WATER SURFACES IN CALIFORNIA 61 (13) Fortier, Samuel 1007. Evaporation Losses in Irrigation and Water Requirements of Crops. U. S. Dept. of Agric. Office of Exp. Sta. Bull. 177, 64 p., illus. (14) Fortier, Samuel 1907. Evaporation Losses in Irrigation, Eng. News, Vol. 58, pp. 304-307, illus, (15) Grover, Nathan C, McGlashen, H. D. and Henshaw, F. F. Surface Water Supply of the United States, Pt. XL Pacific Slope Basins in California, Dent, of the Interior, U. S. Geol. Survey, W. S. P's, 411 (1918), 441 (1918), 461 (1920) and 481 (1921). (16) (irunsky, C. E. 1908. Evaporation from Salton Sea, Engineering News, Vol. 60, pp. 163-166. (17) Hall, L. Standish 1934. Discussion of paper. Evaporation from Water Surfaces, A Symposium, Trans. A. S. C. E. Vol. 99, pp. 724-730. (IS) Hall, Wm. Ham. 1886. Physical Data and Statistics of California, Compiled in the State Engi- neering Dept. of California, 547 pp. (19) Harding, S. T. 1920. Water Resources of Kern River and Adjacent Streams and Their Utili- zation. State of Calif. Dept. of Engineering, Bull. No. 9, 209 pp., illus. (20) Harding, S. T. 1935 Evaporation from Large Water Surfaces Based on Records in California and Nevada. Nat. Res. Council, Trans. Am. Geophys. Union, Pt. II, Sec. of Hydrology, pp. 507-511. (21) Horton, Robert E. 1921. Results of Evaporation Observations. U. S. Dept. of Agric, Weather Bureau, Mo. W. Rev. Vol. 49, pp. 553-566. (22) Horton, Robert E. and Cole, John S. 1934. Compilation and Summary of Evaporation Records of the Bureau of Plant Industry, U. S. Department of Agriculture, 1921-32. U. S, Department of Agriculture, Mo. W. Rev. Vol. 62, pp. 77-89. (23) Houk, Ivan E. 1927. Evaporation on United States Reclamation Projects, Trans, ASCE Vol. 90, pp. 266-378, (24) Kadel, Benjamin C, 1919. Instructions for the Installation and Operation of Class "A" Evapora- tion Stations, U. S. Dept. of Agric, Weather Bureau, Circ. L, Instru- ment Division, W. B. No. 599, 30 pp., illus. (25) Lee, Charles H. 1912. An Intensive Studv of a Part of Owens Valley, California. Dept. of the Interior, U, S, Geol, Survey, W, S, P, 294, 135 p., illus. (26) Lee, Charles H, 1927, Discussion of Paper: Evaporation on Reclamation Projects, Trans. ASCE Vol, 90, pp, 330-343. (27) Lippincott, .loseph Barlow 1903. California Hydrography. Dept. of the Interior, Water Supply and Irri- gation Paper No. 81, 488 p. (28) Los Angeles County Flood Control District Annual Reports on Hydrologic Data. Seasons of 1931-32 to 1943-44. 62 DIVISION OF WATER RESOURCES (29) Matthew, Raymond 1931. Economic Aspects of a Salt Water Barrier, Calif. Dept. of Pub. Wks., Div. of Water Res. Bull. No. 28, 450 p., illus. (30) McGlashan. H. D. and Dean. H. J. 1913. Water Resources of California, Pt. III. Stream Measurements in the Great Basin and Pacific Coast River Basins. Dept. of the Interior, U. S. Geol. Survey, W. S. P. 300, 956 p. (31) Mead, Elwood 1901. Irrigation Investigations in California, U. S. Dept. of Agric, Office of Exp. Sta. Bull. No. 100, 411 p.. illus. (32) Muckel, Dean C, and Blaney, Harry F. 1945. Utilization of the Waters of Lower San Luis Rey Valley, San Diego County, California. U. S. Dept. of Agric, SCS Div. of Irrig. and Water Conservation. Mimeo., 144 p. (33) Post. William S. 1916. Discussion of Paper : Evaporation from Lake Conchos, Mexico, Trans, ASCE, Vol. 80, pp. 1906-1918. (34) Robson, F. T. 1913. Discussion of Paper : Irrigation and River Control in the Colorado River Delta Trans. ASCE, Vol. 76, pp. 1516-24. (35) Rohwer, Carl 1931. Evaporation from Free Water Surfaces, U. S. Dept. of Agric, Tech. Bull. 271, 96 p., illus. (36) Rohwer, Carl 1934. Evaporation from Water Surfaces, a vSymposium. Evaporation from Different Types of Pans. Trans! ASCE, Vol. 99, pp. 673-703, illus. (37) Sleight, R. B. 1917. Evaporation from the Surfaces of AVater and Riverbed Materials. Jour, Agric Res., Vol. 10, No. 5, pp. 209-262, illus. (38) Sleight, R. B. 1927. Discussion of Paper : Evaporation on United States Reclamation Proj- ects. Trans. ASCE, Vol. 90, pp. 306-316. (39) U. S. Dept. of Commerce, Bureau of the Census 1940. Sixteenth Census of the United States, 1940. Irrigation of Agricultural Lands, California, 57 p. (40) U. S. Dept. of Commerce, Weather Bureau, Climatological Data, Arizona Sec- tion. Monthly. (41) U. S. Dept. of Commerce, AVeather Bureau, Climatological Data, California Section, Monthly. (42) Van Etten, P. H. 1935. San Diego County Investigation. Calif. Dept. of Pul). AVks., Div. of AVater Res., Bull. No. 48, 252 p., with Appendices A and B. (43) AVhite, AA'alter N. 1932. \ Method of Estimating Ground AVater Supplies Based on Discharge by Plants and P^vaporation from Soil. Results of Investigations in Escalante A^allev, Utah. U. S. Dept. of the Interior, Geological Survey, AV.S.P..659, 105 p., illus. (44) Whitney, H. A. 1916. Discussion of Paper : Evaporation from Lake Conchos, Mexico. Trans. ASCE, A'ol. SO, pp. 1983-1991, EVAPORATION FROM WATER SURFACES IN CALIFORNIA 63 (45) Young, Arthur A., and Blaney, Harry F. 1942. Use of Water by Native Vegetation. Calif. Dept. of Pub. Wks., Div. of Water Resources, Bull. No. 50, 160 p., illus. (46) Young, Arthur A. 1942. Comparison of Evaporation from Weather Bureau and Other Types of p]vaporation Pans, Baldwin Park Cooperative Station, Calif. U. S. Dept. of Agric, SCS., Div. of Irrig. and Water Conservation. Unpub- lished, 226 p. (47) Young, Arthur A. 1942. Evaporation Studies at Lake Elsinore, Calif. U. S. Dept. of Agric, SCS., Div. of Irrig. and Water Conservation. Mimeo., 56 pp. (48) Young, Arthur A. 1942. Investigations of Evaporation from a Screened Pan and Development of a Coetficient for the Conversion of Pan Evaporation to Lake Evapora- tion. U. S. Dept. of Agric, SCS., Div. of Irrig. and Water Conservation. Mimeo., 160 pp. (49) Young, Arthur A. 1945. Evaporation Investigations in Southern California. U. S. Dept. of Agric, SCS., Div. of Irrig. and Water Conservation. Mimeo., 168 pp. PUBLICATIONS DIVISION OF WATER RESOURCES r 66 DIVISION OF WATER RESOURCES PUBLICATIONS OF THE DIVISION OF WATER RESOURCES DEPARTMENT OF PUBLIC WORKS STATE OF CALIFORNIA When the Department of Public Works was created in July, 1921, the State Water Commission was succeeded by the Division of Water Rights, and the Department of Engineering was succeeded by the Division of Engineering and Irrigation in all duties except those pertaining to State Architect. Both the Division of Water Rights and the Division of Engineering and Irrigation functioned until August, 1929, when they were consolidated to form the Division of Water Resources. The Water Project Authority was created by the Central Valley Project Act of 1933. STATE WATER COMMISSION ♦First Report, State Water Commission, March 24 to November 1, 1912. *Second Report, State Water Commission, November 1, 1912, to April 1, 1914. * Biennial Report, State Water Commission, Marcli 1, 1915, to December 1, 1916. *Biennial Report, State Water Commission, December 1, 1916, to September 1, 1918. ♦Biennial Report, State Water Commission, September 1, 1918, to September 1, 1920. DIVISION OF WATER RIGHTS ♦Bulletin No. 1 — Hydrographic Investigation of San Joaquin River, 1920-1923. ♦Bulletin No. 2 — Kings River Investigation, Water Master's Report, 1918-1923. ♦Bulletin No. 3 — Proceedings First Sacramento-San Joaquin River Problems Confer- ence, 1924. , Proceedings Second Sacramento-San Joaquin River Problems Confer- ence, and Water Supervisors' Report, 1924. No. 5 — San Gabriel Investigation — Basic Data, 1923-19 26. No. 6 — San Gabriel Investigation — Basic Data, 1926-1928. No 7 — San Gabriel Investigation — Analysis and Conclusions, 1929. Report, Division of W^ater Rights, 1920-1922. Report, Division of Water Rights, 1922-1924. Report, Division of Water Rights, 1924-1926. Report, Division of Water Rights, 1926-1928. DEPARTMENT OF ENGINEERING ♦Bulletin No. 4 — : ♦Bulletin Bulletin Bulletin ♦Biennial ♦Biennial Biennial Biennial ♦Bulletin ♦Bulletin Bulletin No No No ♦Bulletin No ♦Bulletin No ♦Bulletin Bulletin ♦Bulletin Bulletin ♦Biennial ♦Biennial ♦Biennial ♦Biennial ♦Biennial ♦Biennial ♦Biennial 1 — Cooperative Irrigation Investigations in California, 1912-1914. 2 — Irrigation Districts in California, 1887-1915. 3 — Investigations of Economic Duty of Water for Alfalfa in Sacramento Valley, California, 1915. 4 — Preliminary Report on Conservation and Control of Flood Waters in Coachella Valley, California, 1917. 5 — Report on the Utilization of Mojave River for Irrigation in Victor Valley, Cahfornia, 1918. No. 6 — California Irrigation District Laws, 1919 (now obsolete). No. 7 — Use of Water from Kings River, California, 1918. No. 8 — Flood Problems of the Calaveras River, 1919. No. 9 — ^Water Resources of Kern River and Adajacent Streams and Their Utilization, 1920. Report, Department of Engineering, 1907-1908. Report, Department of Engineering, 1908-1910. Report, Department of Engineering, 1910-1912. Report, Department of Engineering, 1912-1914. Report, Department of Engineering, 1914-1916. Report, Department of Engineering, 1916-1918. Report, Department of Engineering, 1918-1920. DIVISION OF WATER RESOURCES Including Reports of the Former Division of Engineering and Irrigation -California Irrigation District Laws, 1921 (now obsolete.) -Formation of Irrigation Districts, Issuance of Bonds, etc., 1922. -Water Resources of Tulare County and Their Utilization, 1922. -Water Resources of California, 1923. -Flow in California Streams, 1923. -Irrigation Requirements of California Lands, 1923. -California Irrigation District Laws, 1923 (now obsolete). -Cost of Water to Irrigators in California, 1925. -Supplemental Report on Water Resources of California, 1925. -California Irrigation District Laws, 1925 (now obsolete). -Ground Water Resources of Southern San Joaquin Valley, 1927. -Summary Report on the Water Resources of California and a Coordi- nated Plan for Their Development, 1927. -The Development of the Upper Sacramento River, containing U. S. R. S. Cooperative Report on Iron Canyon Project, 1927. ♦ Reports and Bulletins out of print. These may be borrowed by your local library from the California State Library at Sacramento, California. ♦Bulletin No. 1 ♦Bulletin No. 2 Bulletin No. 3 Bulletin No. 4 Bulletin No. 5 Bulletin No. 6 ♦Bulletin No. 7 ♦Bulletin No. 8 Bulletin No. 9 ♦Bulletin No. 10 Bulletin No. 11 Bulletin No. 12 Bulletin No. 13 EVAPORATION FROM WATER SURFACES IN CALIFORNIA 67 Bulletin No. *Bulletin No. *Bulletln No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. ♦Bulletin No. Bulletin No. ♦Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. ♦Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. Bulletin No. ♦Bulletin No. Bulletin No. Bulletin No. 14 — The Control of Floods by Reservoirs, 1928. 18 — California Irrigation District Laws, 1927, Revision. 18-A — California Irrigation District Laws, 1929 Revision. 18-B — California Irrigation District Laws, 1931 Revision. 18-C — California Irrigation District Laws, 1933 Revision. 18-D — California Irrigation District Laws, 1935 Revision. 18-E — California Irrigation District Laws, 1937 Revision. 18-F — California Irrigation District Laws, 1939 Revision. 18-G — California Irrigation District Laws, 19 41 Revision. 18-H — Water Code, Divisions 10 and 11, Irrigation District Laws 1943. 19 — Santa Ana Investigation, Flood Control and Conservation (with packet of maps), 1928. 20 — ^Kennett Reservoir Development, an Analysis of Methods and Extent of Financing by Electric Power Revenue, 1929. 21 — Irrigation Districts in California, 19 29. 21-A — Report on Irrigation Districts in California for the year 1929. 21-B — Report on Irrigation Districts in California for the year 19 30. 21-C — Report on Irrigation Districts in California for the year 19 31. 21-D — Report on Irrigation Districts in California for the year 1932. 21-E — Report on Irrigation Districts in California for the year 1933. 21-P — Report on Irrigation Districts in California for the year 19 3 4. 21-G — Report on Irrigation Districts in California for the year 1935. 21-H — Report on Irrigation Districts in California for the year 19 36. 21-1 — Report on Irrigation Districts in California for the year 1937. 21-J — Report on Irrigation Districts in California for the year 1938. 21-K — Report on Irrigation Districts in California for the year 19 39. 21-L — Report on Irrigation Districts in California for the year 1940. 21-M — Report on Irrigation Districts in California for the year 1941. 21-N — Report on Irrigation Districts in California for the year 19 42. 21-0 — Report on Irrigation Districts in California for the year 1943. 22 — Report on Salt Water Barrier (two volumes), 19 29. 23 — Report on Sacramento-San Joaquin Water Supervisor, 1924-1928. 24 — A Proposed Major Development on American River, 1929. 25 — Report to Legislature of 1931 on State Water Plan, 1930. 26 — Sacramento River Basin, 19 31. 27 — Variation and Control of Salinity in Sacramento-San Joaquin Delta and Upper San Francisco Bay, 1931. 28 — Economic Aspects of a Salt Water Barrier Below Confluence of Sac- ramento and San Joaquin Rivers, 1931. 28-A — Industrial Survey of Upper San Francisco Bay Area, 1930. 29, — San Joaquin River Basin, 1931. 31 — Santa Ana River Basin, 1930. 32 — South Coastal Basin, a Cooperative Symposium, 1930. 33 — Rainfall Penetration and Consumptive Use of Water in Santa Ana River Valley and Coastal Plain, 1930. 34 — Permissible Annual Charges for Irrigation Water in Upper San Joaquin Valley, 1930. 35 — Permissible Economic Rate of Irrigation Development in California, 1930. 36 — Cost of Irrigation Water in California, 1930. 37 — Financial and General Data Pertaining to Irrigation, Reclamation and Other Public Districts in California, 1930. 38 — Report of Kings River Water Master for the Period 1918-1930. 39 — South Coastal Basin Investigation, Records of Ground Water Levels at Wells, 1932. 39-A — Records of Ground Water Levels at Wells for the Year 1932, Sea- sonal Precipitation Records to and including 19 31-32. (Mimeo- graphed.) 39-B — Records of Ground Water Levels at Wells for the Year 1933, Pre- cipitation Records for the Season 19 32-33. (Mimeographed.) 39-C — Records of Ground Water Levels at T\"ells for the Year, 1934, Pre- cipitation Records for the Season 1933-34. (Mimeographed.) 39-D — Records of Ground Water Levels at "V\^ells for the Year 1935, Pre- cipitation Records for the Season 1934-35. (Mim.eographed. ) 39-E — Records of Ground Water Levels at Wells for the Year 1936, Pre- cipitation Records for the Season 1935-36. (Mimeographed.) 39-F — Records of Ground Water Levels at Wells for the Year 1937, Pre- cipitation Records for the Season 193G-37. (Mimeographed.) 39-G — Records of Ground Water Levels at "Wells for the Year 1938, Pre- cipitation Records for the Season 1937-38. (Mimeographed.) 39-H — Records of Ground Water Levels at Wells for the Year 1939, Pre- cipitation Records for the Season 1938-39. (Mimeographed.) 39-1 — Records of Ground Water Levels at Wells for the Year 19 40, Pre- cipitation Records for the Season 1939-40. (Mimeographed.) 39 -J — Records of Ground Water Levels at Wells for the year 1941 : includ- ing San Jacinto and Antelope Valleys from beginning of record. Precipitation Records for the Season 19 40-41. 39-K — Records of Ground Water Levels at Wells for the Year 1942, Pre- cipitation Records for the Season 1941-42. 39 -L — Records of Ground T^^ater Levels at Wells for the Year 19 43, Pre- cipitation Records for the Season 19 42-43. * Reports and Bulletins out of print. These may be borrowed by your local library from the California State Library at Sacramento, California. 68 DIVISION OF WATER RESOURCES Bulletin No. 40 — South Coastal Basin Investigation, Quality of Iri^igation Waters, 1933. ♦Bulletin No. 40-A — South Coastal Basin Investig-ation, Detailed Analyses Showing Quality of Irrigation Waters, 19 33. Bulletin No. 41 — Pit River Investigation, 1933. Bulletin No. 42 — Santa Clara Investigation, 1933. Bulletin No. 43 — Value and Cost of Water for Irrigation in Coastal Plain of Southern California, 1933. Bulletin No. 44 — Water Losses Under Natural Conditions from Wet Areas in Southern California, 1933. Bulletin No. 45 — South Coastal Basin Investigation, Geology and Ground Water Stor- age Capacity of Valley Fill, 1934. Bulletin No. 46 — Ventura County Investigation, 1933. Bulletin No. 46-A — Ventura County Investigation, Basic Data for the Period 1927 to 1932. inclusive. (Mimeographed.) Bulletin No. 47 — Mojave River Investigation, 1934. (Mimeographed.) *Bulletin No. 48 — San Diego County Investigation, 1935. (Mimeographed.) Bulletin No. 48-A — San Luis Rey River Investigation, 1936. (Mimeographed.) Bulletin No. 49 — Kaweah River — Flows, Diversions and Service Areas, 1940. Bulletin No. 50 — Use of Water by Native Vegetation, 1942. Bulletin No. 51 — Irrigation Requirements of California Crops, 1945. Bulletin No. 52 — Salinas Basin Investigation. Bulletin No. 52-A — Salinas Basin Investigation — Basic Data. Bulletin No. 52-B — Salinas Basin Investigation — Summary Report. Bulletin No. 53 — South Coastal Basin Investigation. Bulletin No. 5 4 — Evaporation from Water Surfaces in California. Biennial Report, Division of Engineering and Irrigation, 1920-1922. Biennial Report, Division of Engineering and Irrigation, 1922-1924. Biennial Report, Division of Engineering and Irrigation, 1924-1926. Biennial Report, Division of Engineering and Irrigation, 1926-1928. PAMPHLETS Dams Under Jurisdiction of the State of California, 19 41. Water Code, 19 43. Water Rights, Divisions 1, 2 and 4 of Water Code, 1943. Supervision of Dams, Division 3 of Water Code, 19 43. State Water Plan, Authorities and Boards, Division 6 of Water Code, 19,43. California Administrative Code, Title 23, Waters. Rules and Regulations Pertaining to Supervision of Dams in California, 1946. Rules, Regulations and Information Pertaining to Appropriation of Water in Cali- fornia, 1946. Rules, Regulations and Information Pertaining to Determination Rights to the Use of Water in California, 1946. Rules and Regulations Pertaining to Protests and Hearings, 1946. COOPERATIVE AND MISCELLANEOUS REPORTS ♦Report of the Conservation Commission of California, 1912. ♦Irrigation Resources of California and Their Utilization (Bull. 254, Office of Exp. U. S. D. A.), 1913. ♦Report, State Water Problems Conference, November 25, 1916. ♦Report on Pit River Basin, April, 1915. ♦Report on Lower Pit River Project, July, 1915. ♦Report on Iron Canyon Project, California, 1914. ♦Report on Iron Canyon Project, California, May, 1920. ♦Sacramento Flood Control Project (Revised Plans), 1925. Report of Commission Appointed to Investigate (Causes Leading to the Failure of St. Francis Dam, 1928. Report of the California Joint Federal-State Water Resources Commission, 1930. Conclusions and Recommendations of the Report of the California Irrigation and Reclamation Financing and Refinancing Commission, 1930. ♦Report of California Water Resources Comm.ission to the Governor of California on State Water Plan, 19 32. ♦Booklet of Information on California and the State Water Plan Prepared for United States House of Representatives' Subcommittee on Appropria- tions, 1931. ♦Bulletin on Great Central A^alley Project of State Water Plan of California Prepared for United States Senate Committee on Irrigation and Reclama- tion, 1932. WATER PROJECT AUTHORITY Bulletin No. 1 — Publicly Operated Electric Utilities in Northern California, 19 41. ♦Report on Kennett Power System of Central Valley Project, 1935. ♦Report on the Programming of Additional Electric Power Facilities to Provide for Absorption of Output of Shasta Power Plant in Northern Cali- fornia Market, 1938. The Story of the Central Valley Project of California, 1940. ♦Electric Power Features of the State Water Plan in the Great Central Vallev Basin of California, 1941. Auxiliary Electric Power Facilities Required for Central Valley Project, 1942. * Reports and Bulletins out of pi int. These may be borrowed by your local library from the California State Lil)rary at Sacramento, California. O 75021 6-47 2500 THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW BOOKS REQUESTED BY ANOTHER BORROWER ARE SUBJECT TO IMMEDIATE RECALL MAR 1 '85 BEcewtu OiG^ \99C LIBRARY, UNIVERSITY OF CALIFORNIA, DAVIS Book Slip-Series 458 111611 . wat er resour ces^ PHYSICAL SCIENCES LIBRARY j[^CS2li- 75 010( i:2 f'^ AE LIBRARY ^IW.RSITV OF CALIFORNIA 111613