THE WORK OF RUNNING WATER. 



71 



If wells be sunk in a flat region of uniform structure and composition 

 the water in them is generally found to stand at a nearly common level. 

 The meaning of this fact is not far to seek. If a hole 60 feet deep fills 

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

 material in which the well is sunk is full of water up to that level. 

 "WTien the well is dug the water leaks into it, filling the hole up to the 

 level to which the rock for subsoil) is itself full. This level, below 

 which the rock and subsoil fdo\M:i to unknown depths) are full of water, 

 is known as the ground-icater level, ground-water surface, or water -table. 



The ground-water level fluctuates. In a wet season it rises, because 

 more water has fallen and sunk beneath the soil; but several processes 

 at once conspire to bring it do^\TL again. Where there is growing vegeta- 

 tion its roots draw up water from beneath, and evaporation also goes on 

 independently of vegetation. The water is drawn out through wells and 

 runs out through openings. It may also flow underground from one 

 region to another where the ground-water surface is lower. All these 

 processes depress the ground-water surface. 



A w^ell sunk to such a level as to be supplied with abundant water in 

 a wet season may go dry during a period of drought because the ground- 

 water level is depressed below its bottom. Thus either well sho^^i in 

 Fig. 52 will have water during a wet 

 season when the water-level is at a; 

 but well Xo. 1 will go dry when the 

 water surface is depressed to b. 



The principles applicable to wells 

 are apph cable to valleys. When a 

 valley has been deepened until its 

 bottom reaches below the ground- 

 water level, water seeps or flows into 

 it from the sides. The valley is then 

 no longer dependent on the run-off 

 of showers for a stream. It T\ill be 

 readily seen that at some stage in its development, the bottom of a 

 valley may be below the ground-water level of a wet season without 

 being below that of a dry one. Thus the valley represented in cross- 

 section by the line 2-2, in Fig. 53, will have a stream when the ground- 

 water level is at aa, but none when this level is depressed to hh. If 

 the rainfall of the year were concentrated in a single wet season, the 

 intermittent stream would flow not onlv during that season, but for so 



t z 



a 



FiG. 52. — Diagram illustrating the 

 fluctuation of a ground-water sur- 

 face, a = wet-weather ground-water 

 level; 6= ground- water level during 

 drought. Well Xo. 1 will contain 

 water during the wet season, but will 

 go dry in times of drought. Well 

 Xo. 2 will be permanent. 



