148 FOREST INFLUENCES. 



That is to say, the increase in water capacity ceases with about 8-inch 

 depth. 



The quantity of water which the soil cover can contain appears from 

 the followiug measurements of Dr. Ebermayer : On the 17th of August, 

 1885, after rainy weather, the moss cover in a 60-year-old spruce 

 growth contained 72.33 per cent at the top, 76.64 per cent on the lower 

 side, and 71.67 per cent in the humus soil beneath. 



After a rainstorm lasting one and a half days, on September 9, 1885, 

 the moss cover contained 80.45 per cent at the top, 74.61 per cent on the 

 lower side, and 74.42 per cent in the top soil. 



In this connection the following note regarding the enormous water 

 capacity of moss covers in Alaska may be of interest: 



In the interior plateau of the Cordillerau and St. Elias regions of Alaska, accord- 

 ing to Mr. C. W. Hayes, surface degradation is greatly retarded by the luxuriant 

 growth of moss, which covers practically tbe entire surface of the country. The 

 annual precipitation is largely confined to the winter mouths, and the water from 

 the melting snow is held by the sponge-like moss, which remains saturated through- 

 out the short but hot and dry summer. Thus, with a rainfall which, in lower lati- 

 tudes, would condition an arid region, a large part of the surface is swampy, quite 

 irrespective of slope— that is, wherever the material composiug it is sufficiently com- 

 pact to become impervious to Avator on freezing. On account of this slow and 

 imperfect surface drainage, the slopes are not cut into the ravines and arroyas so 

 characteristic of arid regious. 



WATER CONDUCTIVITY OF SOILS. 



Of still more importance for the runoff than the water capacity is the 

 water conductivity of the soil, or the rate of water absorption— filtra- 

 tion. 



The rapidity with which the water is conducted from above down- 

 ward must necessarily influence the nature of the run-off. 



Gravity tends to drain the water downward, capillarity to carry it 

 upward; the difference of these two forces in the main must, besides 

 the mechanical obstructions of the soil particles, determine the rapidity 

 of drainage. Experiments to establish the rate under various conditions 

 are very few and unsatisfactory. 



The capillary conduction from below has frequently been made the 

 subject of investigation, but the downward movement has not yet been 

 studied with sufiBcient detail, and it has hardly yet been recognized by 

 the experimenters that this depends upon the difference of gravity and 

 capillarity as two opposed forces. According to E. Wollny's experi- 

 ments in 1883 and 1884— 



(1) Water is conducted downward the more rapidly the larger the soil particles 

 (i. €., the less capillary attraction exists). 



(2) The noncapillary interstices of the soil accelerate the downward movement of 

 the water (t. e., the less mechanical obstruction of soil particles). 



(3) In granular soil the water penetrates faster than in powdery soil (i. p., pene- 

 tration is the slower the denser the stratification). It is most rapid in quartz and 

 slowest in clay; in humus at a rate between these two, but in a mixture of clay soil 

 and humus faster than the average of the two. 



