July 18, 1913] 



SCIENCE 



71 



winds, just at the time when the evapora- 

 tion is the greatest and the forest vegeta- 

 tion is especially active. It seems, there- 

 fore, that the amount of moisture evapo- 

 rated within the more moist region of the 

 United States can influence the conditions 

 of humidity, not only in the States close to 

 the ocean, but also in the region into which 

 the prevailing moist winds flow. The more 

 moisture there is evaporated from the 

 ground in the southern and southeastern 

 portions of the United States, the moister 

 must be the air in the central states and 

 the more precipitation must fall there. 



POEEST THE GREATEST EVAPORATOK OF 

 WATER 



What are the sources from which the 

 evaporation on land is the greatest? The 

 evaporation from a moist, bare soil is, on 

 the whole, greater than from a water sur- 

 face, especially during the warm season of 

 the year when the surface of the soil is 

 heated. A soil with a living vegetative 

 cover loses moisture, both through direct 

 evaporation and absorption by its vegeta- 

 tion, much faster than bare, moist soil and 

 still more than free water surface. 



The more developed the vegetative cover 

 the faster is moisture extracted from the 

 soil and given oi£ into the air. The forest 

 in this respect is the greatest desiccator of 

 water in the ground. 



The latest experiments of Russian agron- 

 omists and foresters, corroborated by sim- 

 ilar observations in France and Germany, 

 have proved that in level or slightly hilly 

 regions the forest has a desiccating effect 

 upon the ground, causing the water table 

 to be lower under forest than in the ad- 

 joining open fields. Professor Henry, in 

 his recent investigations on the effect of 

 forests upon ground waters in level coun- 

 try, has found that the minimum depres- 

 sion of the water table produced by the 



transpiration of forest trees in the Mondon 

 forest near Luneville, France, amounts to 

 11.8 inches. With a porosity of the soil 

 strata ranging between 45 and 55 per cent., 

 such depression would correspond to a 

 rainfall of 5.9 inches, which amount to 21,- 

 443 cubic feet per acre. This amount of 

 water given off by the forest into the air 

 obviously contributes greatly to the mois- 

 ture content of the atmosphere above the 

 forest. Dr. Franz R. von Hohnel, of the 

 Austrian forest experiment station at 

 Mariabrunn, carried on observations for a 

 period of three years (1878-1880) upon 

 the amount of water transpired by forests. 

 He found that one acre of oak forest, 115 

 years old, absorbed in one day from 2,227 

 to 2,672 gallons of water per acre, which 

 corresponds to a rainfall of from 0.09 to 

 0.115 inch per day, or 2.9 to 3.9 inches per 

 month. Taking the period of vegetation 

 as five months, the absorption of water 

 would be 158,895 cubic feet, which repre- 

 sents a rainfall for this period of 17.7 

 inches. This amount of water is given off 

 merely through transpiration from the 

 leaves and does not include the physical 

 evaporation from the surface of twigs, 

 branches, and leaves. These figures, while 

 only approximate, give an idea of the enor- 

 mous quantities of water given off by for- 

 ests into the air, which has justly given 

 them the name of the "oceans of the con- 

 tinent. ' ' 



The most valuable and complete work 

 on the subject is by Otozky, a Russian geol- 

 ogist and soil physicist, which appeared as 

 a publication of the forest experiment sta- 

 tions. Otozky worked up an enormous 

 amount of observations, both his personal 

 and those furnished him by other people, 

 and did not find a single contradictory 

 fact. His conclusion is that the forest, on 

 account of its excessive transpiration, con- 

 sumes more moisture, all other conditions 



