306 A NATIONAL PLAN FOR AMERICAN FORESTRY 



chases of forest land made since 1924 have been made primarily for 

 watershed protection. 



In direct opposition to the popular idea regarding beneficial forest 

 influences have been the doubts from time to time implied or expressed 

 by various small groups of engineers, geologists, and meteorologists. 

 The Mississippi River Commission, for example in its 1927 plans for 

 controlling floods in that stream, set up grounds "to justify rejection 

 of reforesting as an element of flood control in the lower Mississippi 

 River", and has ignored the possibility that proper management of 

 the 20 percent of the watershed still in forest may reduce flood crests 

 by the critical feet or inches that often spell the difference between 

 mere high water and disaster. Other men of scientific standing from 

 time to time attempt to prove that because forests and similar vege- 

 tation are well known to appropriate to their use considerable quan- 

 tities of ground water, particularly at seasons when streams are low, 

 their influence is detrimental rather than beneficial. In the face of 

 criticism of this character it is desirable to summarize here the more 

 important available experimental evidence on the relation of forests to 

 stream flow under American conditions of climate, soil, and vege- 

 tation. 



HOW FOREST COVER INFLUENCES RUN-OFF 



The average yearly rainfall (including snow) in the United States 

 varies from less than 10 inches in the more arid portions of the South- 

 west to more than 100 inches in the Pacific northwest and in portions 

 of the southern Appalachian Mountains. In some parts of the country 

 the rainfall is concentrated within a few months, or in a few heavy 

 storms, while in others it is so well distributed that the precipitation 

 for the wettest month is rarely more than twice that for the driest 

 month. These figures are averages for a period of years; irregular- 

 ities both excesses and deficiencies in rainfall, often of extraordinary 

 magnitude are common to practically all sections of the country, 

 although particularly marked in a few. 



Whether the rain and snow falling on any watershed is as fully 

 useful to mankind as it might be depends almost wholly on the 

 character of its run-off. Of that which sinks into the ground that is, 

 is absorbed by the surface soil or percolates through it to greater 

 depths the greater part becomes available for the growth of plants 

 useful to man or his domestic animals, or in time appears in streams 

 capable of furnishing fairly constant supplies of water for domestic, 

 industrial, and irrigating use, of generating water power, and of 

 transporting freight. Or it may be stored in natural underground 

 reservoirs available to human use. The precipitation which quickly 

 reaches the streams by flowing over the surface of the ground, on the 

 other hand, causes much erosion and many floods. This general 

 classification of subsurface run-off as useful, and flashy surface run-off 

 as detrimental, is of course subject to many exceptions. Not all vege- 

 tation using rain that has penetrated the ground is directly useful to 

 us, and some of this water is lost through chemical combination in the 

 soil and through seepage to great depths. Even subsurface waters 

 when they reach the streams may contribute to floods, and the flashy 

 run-off under some conditions may be stored above or below ground, 

 and thus be prevented from causing destructive floods or being lost 

 to human use during dry seasons. These instances are, however, so 



