Jan., 1908.] Recent Evaporation Investigations . 
419 
Evaporation in inches per hour: 
(e w —e d ) X (1 +~2 ) 
E = 
60 or 
E = 0.0166 (e w —e d )X(l + ^). 
e w and e d 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 w — e d ) 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 would 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 andfobservations 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. 
