CHAPTER V 

 EVAPORATION FROM FREE WATER SURFACES* 



In any study of water supply a knowledge of unavoidable losses 

 occurring in transmission and storage is important, and in this connec- 

 tion evaporation from free water surfaces is of primary importance. 

 The rate and amount of evaporation are dependent upon climatological 

 factors and vary with each locality in conformity with atmospheric 

 conditions. Evaporation losses from reservoirs used for storage of 

 W'ater materially reduce the quantities available for domestic, indus- 

 trial and agricultural uses. Although much of such losses can not be 

 prevented, a knowledge of their magnitude and of the factors which 

 influence evaporation is desirable for use in devising means of reducing 

 them to a minimum, in estimating the available supply, and in deter- 

 inining the economic feasibility of a project, taking into consideration 

 the evaporation losses from proposed reservoirs. For these reasons the 

 Division of Irrigation of the Bureau of Agricultural Engineering has 

 been making evaporation studies** in the AVest for many years. 



Evaporation data are also valuable in estimating consumptive use 

 of water by native vegetation growing in moist areas. Since 1928 the 

 Bureau of Agricultural Engineering has been keeping evaporation 

 records at several stations in cooperation with the State Division of 

 "Water Resources and other agencies also have been making such obser- 

 vations. Not all of these agencies use the same type of evaporation 

 pan, and results from the different types are not ahvays comparable. 

 For this reason, a cooperative key station was established at Baldwin 

 Park in 1932 for the purpose of correlating the data that are being 

 collected by the various organizations and for determining factors 

 that may be used to reduce the observations on various types of 

 evaporation pans to a comparable basis. Heretofore, very few data on 

 evaporation in southern California have been published and it is the 

 purpose of this chapter to bring together such records and make them 

 available for general use. 



BALDWIN PARK KEY STATION 



The Los Angeles County Flood Control District, the San Gabriel 

 Valley Protective Association, the Pasadena Water Department, the 

 California State Division of Water Resources, and the United States 

 Geological Survey are cooperating with the Bureau of Agi'icultural 

 Engineering, in conducting this investigation. Three types of evapora- 

 tion pans have been installed at the station : 



1. Standard Weather Bureau type of pan, 4 feet in diameter by 10 

 inches deep, set upon a wooden platform above ground. 



! 



* Prepared by Harry F. Blaney, Irrigation Engineer, Bureau of Agricultural 

 Engineering, United States Department of Agriculture. 



** "Evaporation from the Surfaces of AVater and River-bed Materials," by 

 R. B. Sleight (Journal of Agricultural Research. Vol. X, No. 5. July .30, 1917) and 

 "Evaporation from Free Water Surfaces," by Carl Rohwer, IJ. S. Department of 

 Agriculture Technical Bulletin No. 271. 



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