34 ELEMENTARY FORESTRY. 



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 consid- 

 erably 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 sat- 

 urated 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 coven 

 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 fulfills an impor- 

 tant office in checking a much greater loss due to evaporation, 

 and thus become an element of conservation." 



Evaporation. The loss by evaporation after the water has 

 reached the ground depends 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 relative amount of moisture in the atmosphere 

 and the circulation of the air are also factors determining the 

 rate of evaporation. The importance of evaporation as an ele- 

 ment of dissipation may be learned from the experiments of 

 Prof. T. Russell, Jr., of the U. S. Signal Service, made in 1888. 

 We learn from these that the evaporation on the western plains 



