at least, somewhat regular long-run variations in precipita- 

 tion. The curves of rainfall in figure 5 are from two stations 

 at which records have been kept for considerable periods. 

 They suggest that there may be regular alternations of 

 periods of greater and of lesser precipitation. It should be 

 observed, however, that the spans in these curves of annual 

 precipitation vary greatly from about 25 to SO years; and 

 what is perhaps more striking, that for areas so close together, 

 relative to the entire United States, as Ohio and Minnesota, 

 at any particular time the precipitation characteristic of 

 one is the opposite of that of the other. 



Attention should be directed especially to the importance 

 to Man of the store of ground water. It is less visible and 

 measurable than precipitation and the surface waters, and 

 therefore less commonly understood, but there is nothing 

 essentially mysterious about it. Its importance can hardly 

 be overestimated. 



As absorption and infiltration proceed during precipitation, 

 the penetration may be at first quite rapid. It becomes slower 

 as the surface soil becomes puddled and partially sealed, and 

 as pores, sun checks, and earthworm and root perforations 

 are closed by fine particles and the swelling of colloidal mate- 

 rial in the soil. It will also be slowed if it reaches a more 

 impervious layer. Continuing to move down and laterally, 

 eventually it strikes impervious rock or other materials, 

 and if there is a sufficient supply, fills the underground porous 

 masses completely. Here it is held as in a great reservoir, 

 from which generally there is leakage along porous strata 

 which permit much of the water again to find the surface 

 through seeping and springs. As the late Allen Hazen has 

 said: "the most instantaneous and effective of all fresh-water 

 reservoirs is the soil. 3 



"Even in the better-watered sections", says W. J. McGee 

 the rainfall during the growing season seldom suffices for the production of 

 a full crop, so that generally the productivity of this country is essentially 

 dependent on the water stored in soil and subsoil and underlying rocks 

 within reach of draft by the growing plants. Moreover this store is the 

 chief source of springs and streams whence animals drink; it is the supply 



3 Flood Flow, New York 1930, p. 154. 



