Major Professor

AN ABSTRACT OF THE THESIS OF Richard James DeRyckefor theM. S. in Oceanography (Name) (Degree) (Major) Date thesis is presented/( Title AN INVESTIGATION OF EVAPORATION FROM THEOCEAN OFF TH.E OREGON COAST, AND FROM YAQUINA BAY,OREGON Redacted for Privacy Abstract approved /(Major Professor) A weather station was established on the dock of the OregonState University Marine Science Center, Yaquina Bay, O:Legon.A total of 197weather observations was made from 30 June1966to 23 Septem- ber1966,with emphasis on the determination of the rate of evaporation from an evaporation pan and from atmometers. Sources of observational error were investigated and corrections applied as necessary.The daily variation in evaporation was determined.The correlation between wind, vapor pressure, and evaporation was found.Atmometers were used to estimate the evaporation from the surface of Yaquina Bay, and the possibility of using atmometers at sea was investigated. AN INVESTIGATION OF EVAPORATION FROM THE OCEAN OFF THE OREGON COAST, AND FROM YAQUINA BAY, OREGON by RICHARD JAMES DERYCKE A THESIS submitted to OREGON STATE UNIVERSITY in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE June 1967 APPROVED: Redacted for Privacy Prolèssor of Oceanography In Charge of Major Redacted for Privacy Chairman of Department of Oceanography Redacted for Privacy Date thesis is presented1/ Typed by Marcia Ten Fyck ACKNOWLEDGEMENT I wish to thank Dr. June G. Pattullo for her help and guidance throughout this project. My appreciation also goes to Mrs. Susan J. Borden for her help with problems on some of my computer programs, and to Mr. Duane Frdman at the Marine Science Center in Newport, Oregon, for his help in running salinities and in setting up the atmometers. Special thanks goes o my wife, Dennis, for her help in data processing and in typing up the rough drafts of the thesis. TABLE OF CONTENTS Page INTRODUCTION 1 EQUIPMENT USED 4 OBSERVATIONAL PROCEDURE AND DATA ANALYSIS 11 Observational Procedure 11 Determination and Correction of ObservationalError 14 Data Analysis 18 DISCUSSION OF RESULTS 20 The Evaporation Day 20 Evaporation Correlated to Wind Speed 25 Evaporation as a Function of Height Above the SeaSurface 25 Daily Evaporation Rate 27 Estimation of Sea Surface Evaporation UsingAtmometers 27 Equation of Evaporation 31 COMPARISONS WITH OTHER STUDIES 34 Reduction of Pan Evaporation to Sea Surface Evaporation 34 Average Daily Sea Surface Evaporation 34 Equation of Evaporation 35 SUMMARY 36 BIBLIOGRAPHY 38 APPENDICES 40 LIST OF FIGURES Figure Page 1 Location of the weather station. 5 2 Arrangement of atmometer and burette. 8 3 Average hourly temperature of the air, bay surface water, evaporation pan water, and average incoming radiation (Q). 12a 4 Schematic representation of the tilt of the surface water in the pan caused by a northwest wind. 1 6 5 Error produced by the wind blowing over the water surface of the evaporation pan.Some points signify more than one observation. 16 6 Average hourly vapor pressure of pan water, bay water and air.The average hourly relative humidity is also shown. 21 7 Average hourly cloud cover, pan evaporation and wind velocity. 22 8 Normal surface air pressure (in mb) over the North- east Pacific Ocean, based on U. S. Weather Bureau Normal Weather Chart (August). 24 9 Average pan evaporation observed at various wind speeds. 24 10 Atmometer evaporation rate (E ) vs height.Rate a at5.5rn100%. 26 11 Correlation between atmometer evaporation (E) and pan evaporation (E). 28 12 Estimated sea surface evaporation as a function of the sea surface water vapor pressure (e), air vapor pressure (ea), and wind speed (kt). 32 LIST OF TABLES Table Page 1 A comparison of the constants of evaporation as determined by the author, Rohwer (1931), and Kohier (1954). 35 A1' INVESTIGATION OF EVAPORATION FROM THE OCEAN OFF THE OREGON COAST, AND FROM YAQUINA BAY, OREGON INTRODUCTION The purpose of this study has been to investigate various methods of estimating evaporation from the surface of the sea and to estimate the evaporation from the sea off the Oregon coast.It is not possible, at present, to determine directly the amount of evaporation from the sea surface, as no technique for doing this has yet been devised. Two indirect methods of determining evaporation from the sea surface have been used.One method involves the use of heat budget computations and another uses evaporation pans. The heat budget method assumes that the temperature of the ocean in the region is unchanging and that there is a balance at the sea surface between solar radiation, heat conduction, back radiation and evaporative heat transfer.Sverdrup, Johnson, and Fleming (1942) have shown how the heat budget method may be used to estimate evaporation from the sea surface.Because it is not possible to determine solar radiatior, back radiation, and heat conduction with a great deal of accuracy, evaporation determined using the heat budget method is considered somewhat inaccurate. Another method of determining evaporation from the sea is to measure evaporation (E) fron an evaporation pan at a known ele- vation (Z) above the sea surface.Sea surface evaporation is then estimated from empirical relationships.This method is used in this study. Evaporation measurements were made in an evaporation pan above the surface of Yaquina Bay, Oregon.As Yaquina Bay is only a short distance from the PacificOcean (Figure 1),it is assumed that the physical factors affecting evaporation in the bay are not very different from those of the open ocean within a fewmiles of the bay. The physical fac:tors assumed to be nearly the same are: wind velocity, air vapor pressure, sea water vapor pressure, andair turbulence. One of the major problems in such a study is that an evaporation pan cannot be expected to evaporate atthe same rate as the bay surface below it.Several attempts were made in this study to make the pan's evaporative characteristics closer approximations to the evaporative characteristics of the bay.An attempt has been made to correct pan evaporation rates for the errors introduced by havingthe pan above the bay surface. The possibility of using atmorneters for evaporation measure- rnents at sea was investigated.The atmometer evaporates water from a porous porcelain sphere and can be used to measure the evaporativity of the atmosphere. Evaporation was first determined in an evaporation pan, and, using atrnometers, sea surface evaporation was estimated. 3 Observational errors were determined as part of the study.The relationship between evaporation and environmental factors was estimated and an eqLlation was written expressing the relationship. A comparison was made between the results of this study and the results of other studies in this field. Evaporation from the sea has been studied by Jacobs (1951), Wast (as discussed by Defant, 1961), Sverdrup (1951), and many others.Most of their studies involved the use of evaporation pans or heat budget computations. Laevastu (1960) has summarized the work of many individuals on the subject of evaporation.He also developed a method for correct_ ing evaporation rates for the effect of the change of wind velocity with height above the sea surface. Rohwer (1931) and Kohier (1954) studied evaporation from evaporation pans on land.They developed equations expressing evaporation as a function of wind velocity and the difference between pan water vapor pressure and air vapor pressure.They also investigated many of the problems associated with the use of evaporation pans. Lane (1965) studied the climate and heat exchange at the air- sea interface off the Oregon coast.The factors affecting evaporation in the area were estimated as part of this study. 4 EQUIPMENT USED A small weather station was operated from 30 June, 1965 to 23 September, 1965 on the end of the dock at the Oregon State University Marine Science Laboratory near Newport, Oregon (Figure 1).This location was chosen to give the best available approximation to mar- me atmospheric conditions as they exist off the Oregon coast.It was assumed that the marine air flow was not greatly affected by the small amount of land between the weather station and the ocean. There were no significant obstructions to the air flow in the immedi- ate vicinity of the evaporation pan. The equipment used at the weather station consisted mainly of standard U. S. Weather Bureau instruments.The following is a description of the various items of equipment. (1)Evaporation pan An evaporation pan of four foot diameter was used.It was placed on a platform 5. 5 m above mean high water.A stilling well was placed in the pan.(Because the base of the stilling well was made of steel,it tended to rust very rapidly in the salt water.The stilling well base was painted twiceto prevent rusting; this effort met with a moderate degree of success. For this reason no evaporation data were gathered on the periods 7 July to 9 July and 24 July to 29 July.Even after the stilling IpA* LJ z 4 w WEATHER STATION o a o_______o naut. me NEWPORT 0 U- 0 4 YAQUINA a. BAY TOLEDO '2 Figure 1.Locatioo of the weather station. well base had been painted some rust accrued oit and on the bottom of the evaporation pan.) A hook gage calibrated in cm was used to measure the water level in the evaporation pan. It could be read with an accuracy of 0. 002 cm. (2) Rain gage A standard eight-inch non-recording rain gage was mounted near the evaporation pan.The measuring stick was calibrated in inches, and tenths. (3)Sling psychrometer A standard sling psychrometer was used to obtain the wet and dry bulb air temperatures.These were estimated to be accurate to 0.1°C.The relative humidity was determined from these readings.

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