ELEMENTS OF DISSIPATION. 27 



atmosphere by evaporation: if, however, the rain continues, 

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

 and along the trunks. 



"Altogether for the rainfall conditions of Austria, Prus- 

 sia 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 con- 

 siderably 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 

 fulfills an important office in checking a much greater Joss due 

 to evaporation, and thus becomes an element of conservation. 



"Evaporation. The loss by evoporation 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 atmos- 

 phere, and the circulation of the air are also factors determin- 

 ing the rate of evaporation. The importance of evaporation 



