CHAPTER XVIII 

 SURFACE WATER, DRAINAGE, AND IRRIGATION 



What Becomes of the Rainfall. When water falls upon 

 the earth as rain, it may do any one of three things. It 

 may evaporate and return to the air ; it may run off 

 in little streams to join other streams and finally reach the 

 ocean ; it may sink into the ground and reach under- 

 ground streams or remain in the soil as capillary water. 



How the rainfall is disposed of depends upon the nature 

 of the earth upon which it falls and also upon the intensity 

 of the rainfall. If the rain falls gently upon loose soil or 

 upon 'soil that is covered with vegetation, most of it will 

 sink into the earth ; but if it falls rapidly upon a hard, 

 sloping surface, most of the water will run off as surface 

 water. Many of us have noticed the little streams that 

 so quickly assume torrential proportions after a very hard 

 rain. 



Ground Water. Under the influence of gravity the 

 free water that sinks into the soil percolates downward 

 until it reaches an impervious layer of rock. It now flows 

 over the top of that layer in the direction of its inclination 

 until it finds an outlet as a spring or line of seepage on the 

 side of a hill or valley. If rock layers occur in alter- 

 nately porous and impervious strata and dip toward 

 a fault fissure which appears at the surface at a lower 

 level than the outcrop of the rock layers, water will 

 be forced out of the fissure. This is the same principle 



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