92 



Source and Disposition of Water. 



Source.- — It is deemed advisable to consider first the geological condi- 

 tions which govern ground water. All terrestrial water is drawn primar- 

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

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

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

 ground water available to man in any portion of the globe, with the prob- 

 able exception of the special cases in which sea water penetrates through 

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

 islands of a porous material. 



EvajHUution. — The rainfall is dispdsed of in a variety of ways. A 

 great portion of it is returned to the atmo.sidiere in the form of vapor by 

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

 by vegetation in transpiration. A small i)ortion of water is used in supply- 

 ing the organic needs of the plants. The proi>ortion that evaiwrates from 

 the surface of the soil varies greatly under different conditions. Winds, 

 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 mulcli of any character reduc- 

 ing the same. 



I{in\-t)ff. — Another jiortidu of the water which falls upon the earth is 

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

 and seei)age nni-off. That iiortion of the precipitation which flows over the 

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

 the soil is known as surface or suixM-ficial run-off. P,y seepage run-off is 

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

 later rcaitjiears on lower levels as s])rings, seeps, etc.. and joins the surface 

 run-off. Another portion of the water is known as dtK'p-seepage, and this 

 sinks into the soil to such depths that it does not later reappear on the 

 drainage basin. 



Underground Water. 



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

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

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

 the fractional part of it which is occui>ied by the open spaces. 



In Drift. — Drift is a heterogenous mixture of clay, sand, gravel, and 

 boulders left by glaciers. It varies from very porous to impervious, ac 



