IOWA ACADEMY OP SCIENCE 
185 
EVAPORATION FROM FREE WATER SURFACES. 
BY ARTHUR G. SMITH. 
The very extensive work being done by the United States Reclamation Service 
in the storing and preserving of the waters of the arid and sub-arid regions 
of this country, has brought prominently before irrigation engineers the ques- 
tion of the amount of evaporation taking place at different places and under 
various external conditions. 
Taking for example the annual evaporation from a reservoir in Arizona, as 
amounting to six feet, and supposing the amount of water needed for satisfac- 
tory irrigation as two feet then 'we see that in a reservoir like the Roosevelt 
reservoir with a surface of 16,000 acres area, it means that the efficiency of the 
reservoir is reduced by nearly 50,000 acres. A loss that is very considerable in 
comparison with the total capacity of the reservoir. 
If we suppose the value of the irrigation to an acre of ground to be say five 
dollars a year, then this evaporation means a loss of a quarter of a million 
dollars a year to the arid region surrounding this particular reservoir. 
While the amount of evaporation from the water surface is a matter of both 
scientific and economic importance in the arid west, it is also of considerable 
interest in the humid regions of the middle west. 
During the summer of 1906 through the kindness of Mr. A. H. Horton, 
United States Hydrographic Engineer, an evaporation gaging station was estab- 
lished at Iowa City, the reader having charge of the work. 
It is a well known fact that the evaporation consists of two steps; first the 
liquid must be changed to the vapor state forming a film as it w^ere upon the 
water surface and then this film must be removed by the atmospheric move- 
ments in order that another dryer layer of air may take the place of this 
saturated film and take up the succeeding evaporation. The amount of evapora- 
tion will of course depend upon vapor pressure and this will depend upon 
temperature. The removal of the film of saturated air and its mixture with 
the upper layers of the air, depends very largely upon the wind. If we think 
of the wind as advancing across a considerable body of water and thus becom- 
ing more or less saturated with water vapor, the removal of the film will be- 
come less and less effective. It may then be expected that the average evapora- 
tion will not be so great from a large body of water as from a smaller area 
for the unit area. During the midsummer months the water surface is cooler 
than the air above and the tendency is for the cooled air in contact with the 
water surface to hang upon the surface sinking rather than rising unless the 
wind is sufficient to remove it. Later in the year, say from the middle of 
August on the water surface except in the middle of the day is w^armer than 
the air above and the air warmed by contact with the water not only takes 
up the water vapor but then rises from its expansion and gives place to dryer 
