Origin of Ground Water 429 



THE ORIGIN OF GROUND WATER AND ITS 

 RELATION TO THE SURFACE 



To understand the laws governing the flow of 

 water into tile drains and ditches, it is necessary to 

 know how the flow into streams and lakes takes 

 place, and how the surface of the water in the 

 ground is related to that in the streams and lakes 

 into which it is continually draining. 



The rains which fall upon the surface tend, first 

 of all, to sink vertically downward until they reach 

 the level at which the pores in the soil or rock are 

 completely filled with water. There are no soils and 

 very few rocks through which there can be abso- 

 lutely no flow, but the downward percolation is very 

 much slower in some than it is in others. This 

 being true, everywhere beneath the land surface a 

 place may be reached where the pores are filled with 

 water, and the level at which this occurs is called 

 the ground -water surface. 



This ground-water surface is seldom horizontal, 

 but usually rises and falls much as does that of the 

 ground above it, but with gradients less steep. In 

 Fig. 132 is represented a section of land adjoining a 

 lake, where the differences in level of the surface are 

 shown by means of contour lines passing through all 

 places, having the height above the lake indicated by 

 the number set in the line ; while in Fig. 133 the 

 surface of the ground water for the same area is 

 also indicated in like manner. The data for the levels 

 of the ground water were procured by sinking wells 



