THE ARTESIAN WATER SUPPLY OF EASTERN FLORIDA. 
101 
deposit, is almost universally present. This sand receives and 
stores the rainfall with great readiness. 
(3) RAINFALL ENTERING THE EARTH. 
Of the water which enters the earth a part is ultimately re¬ 
turned to the atmosphere by evaporation. The water retained in 
soils is slowly given up through evaporation during dry weather. 
As the evaporation takes place near the surface, the capillary attrac¬ 
tion draws a new supply from beneath, thus maintaining to some 
extent the moisture content of the soil. The amount of water thus 
brought to the surface and evaporated, while varying with climate 
and with soils, is, in the course of a year, considerable. 
To the evaporation from the surface of the soil must be added 
that from the leaves of plants. This in turn varies greatly with 
the different plants and with different climatic conditions. King, 
in 1892, in one experiment, found that a crop of peas evaporated 
477 pounds of water for each pound of dry matter formed, while 
corn under the same conditions evaporated in one instance 238 
pounds of water per pound of dry matter.* Assuming that a citrus 
tree evaporates approximately as much as the European oak 
(Quercus cerris), the water evaporated from the leaves of a fifteen- 
year-old orange tree is estimated by Hilgard at 20,000 pounds a 
year, or about 1,000 tons of water per acre of 100 trees.f This is 
equivalent to about nine inches annual rainfall over the same area. 
Water is the chief vehicle for conveying plant food absorbed from 
the soil by the roots. This enormous evaporation from the leaves 
is in part for the purpose of disposing of the water thus taken up 
by the plant. It serves, chiefly, however, the purpose of preventing, 
through the conversion of water into vapor, an injurious rise of 
temperature during the hot sunshine and dry weather. 
It is impossible to estimate within even approximate limits the 
loss of water by evaporation from the surface of the ground, and 
from the leaves of plants in the area under consideration. The at¬ 
mosphere in Florida is relatively humid. On the other hand, the 
temperature throughout most of the year is high. Much of the 
country is uncultivated, and practically all of the soil is of medium 
coarse texture. 
It is probable that almost one-half of the rainfall entering the 
earth is re-evaporated from the surface of the ground and from the 
*20th Ann. Report Wis. Agriculture Experiment Station, p. 320, 1904. 
fBased on weighings made by R f H. Loughridge of the leaves of a citrus 
tree at Riverside, Calif. Soils, by E. W. Hilgard, p. 263, 1906. 
