FIFTH NATIONAL CONSERVATION CONGRESS 187 



If the clearing of the forest in the Atlantic plain and southern Appalachians 

 is a necessity, it should be done only under condition that the cleared land is to 

 be devoted to intense cultivation, as, after forests, crops contribute most to the 

 moisture of the air. 



The highest organic production is in harmony with the safeguarding of the 

 himiidity in the regions which lie in the path of the prevailing winds. Cleared 

 land which becomes waste or poor pasture or grows up to weak vegetation means 

 so much evaporation lost to the passing air currents. 



THE RELATION OF FORESTS AND WATER 



INTRODUCTORY. 



THE influence of the forest upon climate and the supply of water in 

 streams and the regularity of their flow is one of the most important in 

 human economy. Yet the great number of factors which make up this 

 influence, the difficulty of observing them with precision, and the wide range of 

 economic interests which it affects has caused considerable divergence of opinion 

 on the subject. This report aims to bring together impartially, without attempt- 

 ing to pass, however, the final word on the subject, all the well-established scien- 

 tific facts in regard to the relation of forest to climate and water supply. 



I. 

 FORESTS AND CLIMATE 



THREE main factors determine the climate of any locality: (1) Tempera- 

 ture of the air, (2) wind, (3) precipitation. These factors further de- 

 termine the temperature of the soil, the relative humidity of the air, 

 evaporation, and other secondary physical factors which form the climate of the 

 region. 



TEMPERATURE. 



The forest lowers the temperature of the air inside and above it. The verti- 

 cal influence of forests upon temperature extends in some cases to a height of 

 5,000 feet. 



The physical and physiological processes which accompany any plant growth 

 must necessarily reduce the temperature of the air, at least during the vegetative 

 period. First, because, the leaves evaporate water. Second, because the heat 

 of the sun is consumed in this evaporation, and the plant can not become heated 

 to the same extent as, for instance, a rock or soil without any vegetative cover. 

 Similarly the ground under plants can not become greatly heated on account of 

 shading. Third, the surface from which heat radiates at night is much greater 

 when vegetation is on the ground than when the ground is bare. The cooling 

 effect on the air by crops has been experimentally proven. For every pound of 

 dry substance produced it has been found that corn evaporates 233 pounds of 

 water, and turnips 910 pounds. 



Under good cultivation an acre may produce about 7 tons of dry substance. 

 If the evaporation of water be only 500 times more than the amount of dry sub- 



