GROUND-WATER SURFACE 



beneath the Great Plains of the West there is much water which 

 fell on the eastern slopes of the Rocky Mountains. It has flowed 

 beneath the surface to the plains, where some of it is drawn out for 

 purposes of irrigation in regions where rainfall is deficient. 



Ground-water surface. Water-table. If a well 60 feet deep fills 

 with water up to a point 20 feet below the surface, it is because the 

 material in which it is sunk is full of water up to that level. When 

 the well is made, the water leaks into it, filling it up to the level to 

 which the rock (or subsoil) is itself full. This level, below which 

 the rock and subsoil (down to unknown depths) are full of water, is 

 known as the ground-water surface, or water-table. 



In a flat region of uniform structure and composition, the 

 ground-water surface is essentially level, though it rises during wet 

 weather, and sinks in times of drought. Its rise is due simply to 

 the descent of rain water; but its sinking is due to several things: 

 (i) Where there is growing vegetation, its roots draw up water from 

 beneath; (2) evaporation goes on independently of vegetation; (3) 

 the water is drawn out through wells, mines, etc., and runs out as 

 springs; and (4) it flows underground from places where the water 

 surface is higher to those where it is lower. In these and other 

 minor ways the ground-water surface is depressed. 



A well sunk to such a level as to be supplied with abundant 

 water in a wet season may dry up during a period of drought, be- 

 cause the ground-water level is 





depressed below its bottom. 

 Thus either well shown in Fig. 

 19 will have water during a 

 wet season when the water- 

 level is at a; but well i will 

 go dry when the water surface 

 sinks to b. 



Where the toppgraphy is not 

 flat, the ground-water surface is 

 not level. As a rule it is higher (though farther below the surface) 

 under an elevation than under surrounding lowlands, as illustrated 

 by Fig. 21. The reason is as follows: If a hill of sand is rained 

 upon, most of the water falling on it sinks in. If the rain continues 

 long enough the hill of sand will be filled with water, the water 

 filling the spaces between the grains. The water in the hill tends 

 to spread, but since the movement involves friction, the spreading 



Fig. 19. Diagram illustrating the 

 fluctuation of the ground-water surface; 

 a = wet- weather ground-water level; b = 

 ground-water level during drought. 



