28 PRINCIPLES OF AMERICAN FORESTRY. 



to the kind of trees which form the forest, as well as the 

 density and age of the growth. 



"A light drizzling rain of short duration may be almost 

 entirely intercepted by the foliage and at once returned to 

 the atmosphere b}^ evaporation ; if, however, the rain con- 

 tinues, although fine, the water will run off at last from 

 the foliage and along the trunks. 



^'Altogether for the rainfall conditions of Austria, Prussia, 

 and Switzerland, where measurements have been made, a 

 dense forest growth will on the average intercept 23 per 

 cent, of the precipitation ; but if allowance be made for the 

 water running down the trunks, this loss is reduced to not 

 more than 12 per cent. 



''The amount of interception in the open growths which 

 characterize many of our Western forest areas would be 

 considerably smaller, especially as the rains usually fall 

 with great force, and much of the precipitation is in the 

 form of snow. Although branches and foliage catch a 

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

 consequently but very little snow is lost to the ground by 

 interception of the foliage. 



"There is also a certain 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, 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. 



"The water capacity of litter depends upon its nature 

 and of course its thickness to a certain degree, but is much 

 greater than that of soils. 



"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 



