Jan., 1908.] Recent Evaporation Investigations. 4^9 



Evaporation in inches per hour: 



W 



(e.-ed)X(l+^) 



E= 



60 or 



W 

 E = 0.0166 (e„,— ed)X(l+-5-). 



e^ and Cj, have the same value as in the Dalton formula 

 above, and W represents the wind velocity in miles per hour. 



His formula showing the evaporation in inches per day is 

 E = 0..3984 (e,,— ej) X (1 + 0.0208 w,) . 



In this W represents the wind movement in miles for the day. 



After the break in the Colorado River had been closed and it 

 was known that the great Salton Sea in southern California must 

 be reduced by evaporation in ten or twelve years, it was deter- 

 mined to take the opportunity to study evaporation on a large 

 scale in the arid regions. 



The importance of determining what the real evaporation is 

 from irrigation and water supply reservoirs, especially in the 

 arid region, can hardly be overestimated. In some instances- 

 reservoirs built at a large expense are nearly or quite dry during 

 most of the year. 



It has been estimated that the evaporation in southern 

 Arizona is about 6 feet per year. If this is true the loss of 

 water from evaporation from a reservoir like the Roosevelt 

 Reservoir covering 16,320 acres w^ould be sufficient to irrigate 

 48,960 acres of land. 



The true evaporation is not known however, therefore after 

 a Board of Conference had visited the Salton Sea region, the 

 work of investigation was placed in the care of the U.S. Weather 

 Bureau, and Professor Frank H. Bigelow was put in charge of it. 



Professor Bigelow found that when the results were brought 

 together from the different formulas that have been in use the 

 constants do not agree. He thought it wise then to determine 

 the cause for the discrepancy and to ascertain a correct formula 

 if possible. 



Consequently, he established five towers 40 feet in height in 

 and about the Reno, Nevada, reservoir. On these towers evap- 

 orating pans were located at different altitudes, and pans were 

 located at different points in the reservoir. Twenty-nine pans 

 were distributed in this way andf observations were made every 

 three hours during August 1 to September 15, 1907. 



From these investigations Professor Bigelow determined that 

 a vapor blanket always overlays any body of evaporating water, 

 and that pans were found to evaporate at very different rates 

 according to their location. In fact the rate of evaporation 

 seems to be controlled largely by the action of this invisible 

 vapor covering water surfaces, irrigated fields, etc. 



