132 FOREST INFLUENCES. 



many of our western forest areas would be considerably smaller, es- 

 pecially as the rains usually fall with great force, and much of the pre- 

 cipitation is in the form of snow. Although branches and foliage catch 

 a goodly amount of this the winds usually shake it down, and conse- 

 quently but very little snow is lost to the ground by interception of the 

 foliage. 



There is also a certaiji amount of water intercepted by the soil cover 

 and held back by the soil itself, which must be saturated before any of 

 it can run off or drain away. This amount, a part of which is eventually 

 dissipated by evaporation and transpiration, depends, of course, upon 

 the nature of the soil and its cover, especially upon their capacity to 

 absorb and retain water. 



Altogether an appreciable amount of the precipitation does not run 

 off or drain through the forest cover but is retained by it; yet while this 

 is apparently a loss, we shall see further on that this moisture retained 

 in the upper strata fulfills an important office in checking a much greater 

 loss due to evaporation, and thus becomes an element of conservation. 



Evaporation. 



The loss by evaporation, after the water has reached the ground, de- 

 pends in the first place upon the amount of direct insolation of the soil, 

 and hence its temperature, which again influences the temperature of 

 the air. The nature of the soil cover, the absolute amount of moisture 

 in the atmosphere, and the circulation of the air are also factors deter- 

 mining the rate of evaporation. 



A considerable amount of experimental data is available showing 

 the rate and total quantity of evaporation in different climates and at 

 different seasons. Eecent observations made by the U. S. Signal 

 Service and the Geological Survey show that the evaporation from a 

 water surface on the western plains and plateaus of the United States 

 may amount to from 50 to 80 inches, and in special localities to even 

 100 inches, in a year, while the rainfall (diminishing in reverse ratio) 

 over this area is from 30 to 12 inches and less. Thus in Denver, where 

 the maximum annual precipitation may reach 20 inches, the estimated 

 evaporation from a water surface during one year was 69 inches. 



These experiments, made by the Signal Service and the Geological 

 Survey, have thus far referred only to the rate of evaporation from 

 water surfaces; the much more difficult question of the rate of evapora- 

 tion from soil surfaces has been the subject of tentative experiment, 

 but as yet without such results as can here be generalized. 



If the loss by evaporation from an open field be compared Avith that 

 of a forest- covered ground, it will, as a matter of course, be found to 

 be less in the latter case, for the shade not only reduces the influence 

 of the sun upon the soil, but also keeps the air under its cover relatively 

 moister, therefore less capable of absorbing moisture from the soil by 

 evaporation. In addition, the circulation of the air is impeded be- 



