GROUND-WATER 31 



dug to about the same depth in order to secure an equal supply of 

 water. Other wells at c and d will also have to be about equally 

 deep. Under these circumstances, the water in the several wells 

 will stand at about the same level. This means that the rocks and 

 subsoil of the region, below the level of the water in the several wells, 

 are full of water. The surface below which the subsoil and rocks 

 are full of water in any given region is the water surface (also called 

 water table) for that region. In one region, the water table may 

 be 10 feet below the surface of the land, and in another, 100 feet. 

 In dry regions it may be even deeper; but where the precipitation 



Fig. 20. Diagram showing a series of wells sunk in a flat tract of land. 



is enough for successful farming, the water surface is seldom more 

 than a few score feet below the surface of the land. 



Amount of ground-water. The amount of ground-water is not 

 known, for there is no way of measuring it accurately. The best 

 estimates which have been made indicate that the water in the soil, 

 rocks, etc., of the land would probably make a layer something like 

 1,000 feet deep over the land, if spread out over its surface. 



The movement of ground-water. Ground-water is constantly 

 moving, as shown in many ways. (1) If all the water is pumped 

 out of a well, it soon fills again about to its former level, because 

 water flows in. (2) The constant flow of springs shows that ground- 

 water is in movement. (3) The flow of water into mines, quarries, 

 etc., tells the same story. 



The reasons why ground-water moves may be readily under- 

 stood. The rain does not fall equally everywhere. If there is a 

 heavy shower in one part of a flat region where the ground-water 

 surface is level, the soil and rock where the rain falls become filled 

 with water, and as a result, the ground-water surface is there raised 

 temporarily. Under these conditions, the ground-water will flow 

 from the place where the water surface is higher, to places where it 



