THE ARTESIAN WATER SUPPLY OF EASTERN FLORIDA 1

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 returned 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 attraction 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 I892, 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 fifteenyear-old orange tree is estimated by Hilgard at 20,000 pounds a year, or about x,ooo tons of water per acre of ioo trees.t 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 waterthus 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 atmosphere 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-evaForated from the surface of the ground and from the
*20th Ann. Report Wis. Agriculture Experiment Station, p. 320, 1904.
t'Based on weighings made by R. H. Loughridge of the leaves of a citrus tree at Riverside, Calif. Soils, by E. W. Hilgard, p. 263, 1906.

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