CHAPTER XIV 

 THE TRAVELS OF THE UNDERGROUND WATER 



The descent within the unsaturated zone. Of the moisture 

 precipitated from the atmosphere, that portion which neither 

 evaporates into the air nor runs off upon the surface, sinks into 

 the ground and is described as the ground water. Here it descends 

 by gravity through the pores and open spaces, and at a quite 

 moderate depth arrives at a zone which is completely saturated 

 with water. The depth of the upper surface of this saturated zone 

 varies with the humidity of the climate, with the altitude of the 

 earth's surface, and with many other similarly varying factors. 

 Within humid regions its depth may vary from a few feet to a few 

 hundred feet, while in desert areas the surface may lie as low as a 

 thousand feet or more. 



The surface of the zone of the lithosphere that is saturated 

 with water is called the water table, and though less accentuated it 

 conforms in general to the relief of the country (Fig. 188). Its 



FIG. 188. Diagram to show the seasonal range in the position of the water table 

 and the cause of intermittent streams. 



depth at any point is found from the levels of all perennial streams 

 and from the levels at which water stands in wells. 



During the season of small precipitation the water table is 

 lowered, and if at such times it falls below the bed of a valley, 

 the surface stream within the valley dries up, to be revived when, 

 after heavier precipitation, the water table has in turn been raised. 

 Such streams are said to be intermittent, and are especially char- 

 acteristic of semiarid regions (Fig. 188). 



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