INFLUENCE OP EOBESTS 



267 



regions of the West, states it as his belief that the local springs 

 and streams are *'raore diminished by the water used by the trees 

 than by evaporation in their absence." 



It has been shown^ that the total amonnt of moisture returned 

 into the atmosphere from a forest by transpiration and evapora- 

 tion from the trees and underlying soil, is about 75% of the 

 total precipitation. For other forms of vegetation it varies 

 between 70% and 90%, the forest as a rule being surpassed by 

 the cereals, while the evaporation from a bare soil is but 30% 

 of the precipitation. To this should be added the fact that 

 the activity of evaporation from forested areas is continued 

 throughout a longer period of each year, as a rule, than in 

 non-forested, for the simple reason that the grasses and cereals 

 early ripen, and practically cease to exhale altogether. This 

 is particularly the case in cultivated areas and prairie regions. 

 Hence, while the daily evaporation from given areas might for 

 a time be nearly equal, the annual amount is likely to be greatest 

 for that which is forested. 



Further, it has been shown that only 70% as much rainfall 

 reaches the soil in the woods as in the open fields, the rest 

 being caught in the leaves, branches, and trunks, whence it is 

 returned directly to the atmosphere by evaporation. These 

 percentages naturally vary with the character of the forest 

 growth. In this connection the following tables, showing the 

 measured amounts of water at varying depths in a loamy soil 

 under forests of spruce, twenty-five, sixty, and one hundred 



Water Contents ow a Loamy Sand; Eesults by Seasons expressed in 

 Percentages of the Weight of the Soil 





Spkxtce 



SXASON 



25 Teaks Old 



60 Tbaes Old 





16incli 



82 inch 



Average 



16 inch 



82 inch 



Averag-e 



Winter (January and February) . 

 Spring (March to May) . , , . 

 Summer (June to August) . . . 

 Fall (September to NoTember) . . 



/o 

 20.23 



18.62 



15.10 



16.67 



% 

 17.00 



18.02 



16.22 



17.57 



% 

 18.61 



18.32 



16.96 



17.07 



% 

 18.06 



15.29 



14.42 



13.49 



0/ 



k 

 17.76 



16.28 



17.03 



16.52 



/o 

 17.91 



15.78 



15.72 



15.00 



See Bull. No. 7, Forestry Division, IT. S. Dept. of Agriculture, 1893. 



