158 FOREST INFLUENCES. 



EETAEDATION OF THE WATERFLOW. 



It is in the first place this mechanical obstruction which a forest 

 floor more than most other kinds of vegetation offers, which changes 

 the distribution in time of surface waters, that constitutes the forest an 

 influential factor in water flow. 



Direct measurements as to the dilference in time which it takes for 

 water to run ofl" from watersheds of different conditions are difficult or 

 almost impossible, because it would be necessary that not only the same 

 amount of water should fall upon the two areas under comparison, but 

 also that the topography, angles of slopes, and length be the same, and 

 a ready means of measurement be found. In fact, the absorption and 

 obstruction to surface flow acting in the same sense, it would be impos- 

 sible, and for practical purposes also irrelevant, to credit each with its 

 separate quota of influence. We shall, therefore, have to be content 

 with general reasoning and more or less inaccurate observations to prove 

 this retardation of surface drainage. But without any evidence fur- 

 nished by experiments, we can at once understand that the surface runoff 

 IS impeded by any kind of mechanical obstruction, such as is offered by 

 the vegetation of a meadow or of a forest. 



The great number of inequalities which the forest floor otters, in ad- 

 dition to the trunks and stumps and fallen trees, forces the run-oft' to 

 many detours, thus retarding its flow and its collection in the open runs 



and brooks. 



The retardation iii the waterflow begins even before the rain has 

 reached the soil, for the leaf canopy catches and reevaporates, as we 

 have seen from 12 to 25 per cent of the total fall, and certainly retards 

 the fall of the water to the ground, as can be readily observed; long 

 after the rain has ceased the water keeps on dripping from the foliage. 

 Thus, although most of the water reaches the ground at last, except in 

 case of very light showers, yet the devious ways in which it reaches 

 the soil makes the flow of water from a forest-covered hill longer in time 

 than if the rain had fallen on a bare slope. As the result of a long- 

 continued precipitation, it would be under the same conditions by an 

 unforested slope, but this stage occurs in the forest later than on un- 

 forested soil and later still than on naked soil. 



The great importance of the factor of time in surface drainage, both 

 as regards dangers from freshets and erosion of soil, will be more readily 

 appreciated when we remember that the dangerous waters in the moun- 

 tains are generally of short duration. 



A difterence of 1,000 to 2,000 cubic feet of water per second from a 

 square mile of watershed may often determine whether a dangerous 

 flood is experienced or not. And since a square mile of moss-covered 

 forest floor is capable of absorbing from 40,000,000 to 50,000,000 cubic feet 

 in, say, ten minutes (a humus cover is capable of taking up 50 per cent 

 of its own weight), nearly all of which the naked soil would give up 

 Bome tNvelve to fifteen hours earlier, the surface conditions of the water- 



