THE RELATION OF FORESTS AND WATER 



By the Sub-Committee on Forest Investigations. 



Chairman, Raphael Zon, Washington, D. C. 



F. B. LvANEY, Denver, Colo. C. G. Bates, Denver, Colo. 



Waetee Mulfoed, Ithaca, N. Y. AlEc. G. McAdie, Cambridge, Mass. 



Presented by Mr. Raphael Zon, Monday evening, November 17, 1913. 



SYNOPSIS 



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. 



FORESTS AND CLIMATE 



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

 vertical influence of forests upon temperature extends in some cases to 

 a height of 5,000 feet. 



The forest lowers the temperature of the soil in summer and increases the 

 temperature of the soil in winter. This influence extends to a depth of at least 

 4 feet. 



The relative humidity of the air during the summer is higher in the forest 

 than in the open. 



Forests increase both the abundance and frequency of local precipitation over 

 the areas they occupy, the excess of precipitation as compared with that of ad- 

 joining unforested areas amounting in some cases to more than 35 per cent.* 



The influence of mountains upon precipitation is increased by the presence 

 of forests. The influence of forests upon local precipitation is more marked in 

 the mountains than in the plains. 



The reason for an increase in the total amount of precipitation over wooded 

 areas as compared with that of barren and deforested ones is due to : 



1. The tendency of moisture-bearing currents to precipitate their moisture 

 more readily above or near the forests than over bare or cultivated fields at the 

 same elevation because of the dampening and chilling effect of the forests upon 

 the atmosphere, which induces a greater condensation of the water vapor. 



2. The air from forests contains a much larger amount of moisture than that 

 over bare or cultivated fields. 



3. The mechanical action of the trees themselves. When a cloud in the 

 mountains passes through a forest, the branches and the leaves of the trees retard 

 its movement. It comes, therefore, into a state when it can no longer retain its 

 moisture in suspension, just as a river carrying sediment deposits part of it as 

 soon as the rapidity of its flow is diminished. The moisture from such clouds is 



* Professor Walter Mulford, as a member of the Committee, does not agree with the 

 conclusions of the majority of the committee with regard to the effect of forests upon 

 precipitation. 



