THE WORK OF GROUND WATER 57 



feet deep ; while in the Colorado Plateau, where the surface is cut 

 by deep canyons, it is sometimes 3500 feet beneath the surface; or 

 it may be entirely absent, except where water-bearing strata conduct 

 water from other areas. In the Navaho Reservation in Arizona, for 

 example, no water except artesian (p. 59) is encountered below a 

 depth of 100 feet. (H. E. Gregory.) 



The water table varies with the slope of the land, being farther 

 from the surface on the hills than in the valleys (Fig. 32). The greater 

 depth beneath the surface of the hills is due to gravity, which between 

 rains and during dry seasons tends to pull the water downward to 

 the level of the valleys, but is unable entirely to do so because of 

 capillarity and friction of the water with the grains of rock. As a 

 result, the water table is never flat in a hilly region, although it is 

 more nearly so after a prolonged drought. It necessarily follows 

 that the depth of the water table in any place will depend largely 

 upon (1) the slope of the land, (2) the porosity of the rock, and 

 (3) the frequency and character of the precipitation, slow, soaking 

 rains accomplishing more than sudden and brief downpours. 



In forested areas it is found that the water table is lower than 

 under similar conditions of moisture, rock, and topography in other 

 regions, because of the great quantity of water abstracted by the 

 roots of the trees and lost by evaporation through the leaves. 

 The headwaters of streams, however, should be kept forested, 

 since much of the water of excessive rains is retained in the thick 

 layer of forest mold, from which it slowly drains away and thus 

 tends to prevent great floods. 



Wells. — When wells are sunk, it is necessary that they penetrate 

 to a permeable rock or to a much fractured one (Fig. 33) below 



^ifliil 



. • SATURATED ZON€ . 



/ 









■""T^^^—fir-*- 





Fig. 32. — Diagram showing the water table or level of underground water A A A A 

 and the effect upon natural and artificial depressions. 



the water table (Fig. 32), for otherwise they do not afford a 

 perennial supply of water. The value of wells, both for drink- 

 ing purposes and for irrigation, is inestimable. It is stated that 

 in India more land is irrigated from wells than from streams, and 



