92 



SouKCE AND Disposition of Watkr. 



Source. — It is deemed ndvisable to consider first the jreological condi- 

 tions wliich govern jjround water. All terrestrial water is drawn primar- 

 ily from the ocean, from whence it is taken by evaporation and carried by 

 winds to be deposited upon the surface of the ground, principally as rain 

 Ijut partially as snow, mist, fog, or dew. There can be no other source of 

 gi"(mnd water available to man in any portion of the globe, with the prob- 

 able exception of the special cases iu which sea water penetrates througli 

 the pores of the rocks for a considerable distance inland in coral and other 

 islands of a porous material. 



Evaporation. — The rainfall is disposed of in a variety of ways. A 

 great portion of it is returneil to the atmosphere in the form of vapor by 

 evaporation. This may be made to include the great amounts given off 

 by vegetation in transpiration. A small portion of water is used in supply- 

 ing the organic needs of the plants. The proportion that evaixirates from 

 the surface of the soil varies greatly under different conditions. Wind^. 

 a warm temperature, sunshine, etc., are very conducive to evaporation. 

 The character of the soil and soil covering also has a great influence upon 

 the amount of water returned to the air, a mulch of any character reduc- 

 ing the same. 



/'iin-off. — Another portion of the water which falls upon the earth is 

 known as run-off. This may be divided into two classes: surface run-off 

 and sce]>age run-off. That portion of the preciiiitation which flows over the 

 surface of the ground into streams and rivers without gaining access to 

 the soil is known as surface or superficial run-off. By seepage run-off is 

 meant that portion of the rainfall which sinks into the earth but wliicli 

 later reapi)ears on lower levels as si>rings, seeps, etc., and joins the surface 

 run-oft'. Another i)ortion of the water is known as deeiJ-seejiage, and this 

 sinks into the .soil to such deiiths that it does not later reajipear on the 

 drainage basin. 



I'NDERGROUND WATER. 



The amount of water wliich enters the soil, rocks, and other materials. 

 varies greatly with the nature of the materials, the iiorous mediums ab- 

 sorbing the most water. The porosity of a soil or rock is determined by 

 the fracfi(»nal part of it wiiicii is occupied by the oiien sjiaces. 



//( Drift. l>ritl is a ]ict('i'(.genoiis mixture of clav. s.nul. L^ravel. and 

 ixMilders left by gl.Mciers. It v.aries from very ])orous to imiiervious. ae 



