16 CONSERVATION OF GROUND WATER 



gravity. Where the openings or pores in rocks are sufficiently 

 minute, the force of molecular attraction is powerful enough 

 to prevent any movement of water through them. 



The effectiveness of the solar energy in pulling water back 

 to the atmosphere varies from time to time and from place 

 to place, but this force is operating at all times upon all water 

 within reach. Water is evaporated from oceans, lakes, reser- 

 voirs, streams, canals, swamps, vegetation, and moist earth; 

 it may evaporate as soon as it hits the ground — or even before 

 — in hot deserts, and it may return to the atmosphere from 

 snowbanks or ice fields at temperatures far below freezing; 

 it is transpired by all vegetation, and thus the "reach" of 

 this solar force extends below the land surface as far as plants 

 can send their roots for water, or in the absence of vegetation, 

 as far down as air can circulate. 



The science of hydrology embraces all phases of the hydro- 

 logic cycle. So complex is each of these phases that hydrologists 

 must also be specialists in one or more of the closely related 

 sciences of meteorology, soil science, geology, chemistry, phys- 

 ics, biology, forestry, and agronomy. Partly because of the com- 

 plexities of the hydrologic cycle and partly because of the high 

 degree of specialization of technical research, there are only 

 a few small areas where we yet have a reasonably complete 

 and quantitative description of the operations of the hydro- 

 logic cycle. We know how variable the paths of a particle of 

 water in the hydrologic cycle can be in various parts of the 

 country. Of the water that reaches the land surface by pre- 

 cipitation, some may evaporate where it falls; some may infil- 

 trate into the ground; some may run off overland to evaporate 

 or infiltrate elsewhere or to enter streams. Of the water that 

 infiltrates into the ground, some may be evaporated; some 

 may be absorbed by plant roots and then transpired; some 

 may percolate downward to ground-water reservoirs or into 

 voids and crevices in relatively impermeable material. Of the 

 water that enters ground-water reservoirs, some may move 

 laterally until it is close enough to the surface to be subject to 

 evaporation or transpiration; some may reach the land sur- 



