FFB 1 - 1909 
The Ohio £A£aturalist, 
PUBLISHED BY 
The Biological Club of the Ohio State University. 
Volume IX. JANUARY, 1909. No. 3. 
TABLE OF CONTENTS. 
Smith— Recent Evaporation Investigation 1 !. 417 
Dktmers —Annual Report on the Plants New to the Ohio State I.ist tor 1907-08 . 421 
Scott— The Ohio Species of the Genus Disonycha. 423 
Johnson —Variation in Syndesmon and Hepatiea... 431 
RECENT EVAPORATION INVESTIGATIONS* 
J. Warren Smith. 
Evaporation depends in general upon the dryness of the air, 
the velocity of the wind, and the temperature of the evaporating 
water. It makes no appreciable difference whether the evap¬ 
orating surface is in the sunshine or in the shade. 
The evaporation from a saturated soil covered with growing 
plants is greater than from a water surface, but becomes less 
when the level of complete saturation falls below the surface of 
the ground. It has been calculated that when the water table is 
six inches below the surface of the land the evaporation is 95% 
of what it is from an open tank. 
Evaporation is greater from a forest of evergreen trees than 
one of leafy trees; greater from leafy trees than from grass, and 
greater from grass than a bare soil. 
Newell states that the runoff over any watershed is from 36% 
to 47% of the rainfall, and the balance is evaporation, including 
in that of course transpiration of plants. Over the Muskingum 
watershed the average annual rainfall is 39.7 inches while the 
runoff is 13.1 inches. 
Formulas for the annual evaporation over a watershed have 
been worked out as follows: 
E = 15.50 + 0.10X annual rainfall. (1) 
or if thought best to consider the temperature, 
E = 15.50+ (0.16XR) X (0.05XT—1.48) (2) 
R.=mean annual rainfall; T=mean annual temperature. 
*Read at the meeting of the Ohio Academy of Science. 
417 
UIBRAR'V 
NEW YORK 
BOTANICAL 
garden. 
