1899- 



AMERICAN FORESTRY ASSOCIATION. 



279 



Effect of Forests on Water Supply. 



II. INVESTIGATIONS REGARDING CAPILLARY ACTION AND THE EFFECT OF FOREST 



COVER AS RELATED TO WATER SUPPLY. 



The mechanics of granular soils present 

 some particularly interesting features. It 

 can readily be demonstrated that, if the 

 granules are true spheres and of one 

 iunform diameter, the voids form one 

 constant percentage of the total cubical 

 contents, irrespective of the actual diam- 

 eter of the spheres, and also that the area 

 of passages between the spheres bears a 

 constant ratio to the area of the circum- 

 .scribing cross section, irrespective of 

 diameter. This is a property of uni- 

 formity of size. By mixing different 

 sizes together in such proportions that 

 each succeeding smaller size enters the 

 interstices of the preceding larger grains of 

 soil may be made impervious to water, 

 save by capillary action. This feature of 

 mixtures will frequently explain the im- 

 perviousness of stream beds in sandy gravel. 

 Although the void* and water passages bear 

 a constant ratio to the total volumes and 

 areas with grains of uniform size, the 

 rate of the passage of water is higher the 

 larger the grains. With very minute 

 grains the passages become capillaries en- 

 tirely and gravitation is overcome. 



Capillary action is one of surface ten- 

 sion. Imagine a membrane enclosing 

 each grain and stretched thereon. The 

 tension of this imaginary membrane is 

 analogous to surface tension. The surface 

 tension increases with decrease of radius. 

 The sharper the curvature the greater the 

 tension. When neighboring interstitial 

 spaces are filled with water to a greater 

 or less degree surfaces or films of sharper 

 or flatter curvature are produced. The 

 surfaces are not in equilibrium and a 

 movement from the flat to the sharper 

 curves takes place, and continues until, by 

 re-adjustment of the curves, equilibrium is 

 established. 



This is the nature of capillary action ; 

 it takes place in all directions according to 



the surrounding conditions. In soils the 

 conditions are usually such that the action 

 is upward and opposed to gravity. Evap- 

 oration at the ground surface depletes the 

 interstitial spaces, the films around the 

 grains grow sharper of curvature and a 

 movement takes place toward them from 

 the lower interstices refilling the upper. 

 Forests reduce surface evaporation and re- 

 tard the capillary depletion of ground 

 water. In chalk the limit of capillary 

 action exceeds 16 feet. In sandy soils 

 one and a half feet has been found to be 

 an extreme. In very open coarse material 

 the limit may be but a few inches. 



The rate of percolation is affected by 

 the temperature. The viscosity, or inter- 

 nal friction, of water increases with de- 

 crease of temperature. Assuming the 

 viscosity at 32 Fahrenheit to be 100, the 

 viscosity at 77 is found to be 50; at S6, 

 45; and at 11 2, 31. The viscosity of 

 gases, contrary to that of fluids, increases 

 with increase of temperature and as air is 

 frequently used in making permeability 

 tests of soils grave errors are liable to be 

 introduced, unless these opposite character- 

 istics are duly accounted for. 



Tests at ten German forest stations show 

 that the general effect of forests is to raise 

 the soil temperatures during the cold 

 months and lower them during the warm 

 months. This has the effect of facilitat- 

 ing percolation during the rainy periods 

 and retarding capillary upward action dur- 

 ing the warm months when little rain 

 falls. The surface tension of water is 

 also lowered by increase of temperature, 

 causing less capillary resistance to gravi- 

 tation and increasing percolation. 



The rate or velocity of percolation is 

 very variable. It varies with every soil, 

 from no movement whatever, to over 100 

 feet per hour. Each soil is more or less a 

 law unto itself and must be studied by it- 



