WATER-SUPPLY. 49 



precipitation is an important fadlor in the calculation 

 of the relative percentages of rainfall and run-off. A 

 mild rain continued through many hours will give but 

 little run-off, while the same amount falling in one- 

 fourth or an eighth the time will give a greatly 

 increased run-off. Unfortunately the climatic condi- 

 tions are such in nearly all the arid portions of the 

 world that what little rain does fall comes in the form 

 of hard, driving storms. On the great plains of 

 America it is by no means unusual for a two-inch rain 

 to fall in as many hours, while instances are of yearly 

 occurrence in which four inches or more fall within 

 one hour. Under such conditions almost all the water 

 runs off, except in the most sandy places. 



The most important of all the conditions affedting 

 the run-off is the charadler of the ground upon which 

 the water falls. A loose, porous soil will absorb a 

 large portion of a rainfall, as will also a sandy surface, 

 while a close, compa(5l soil sheds the greater part of it. 

 An area composed principally of a close-grained shale, 

 and soil resulting therefrom, which generally has a corn- 

 pad: clay subsoil, has inferior absorptive properties, 

 while one composed principally of sandstone and sand 

 will allow but little water to run away. The flood 

 plains of rivers frequently have little power of absorp- 

 tion. In times of overflow a thin layer of a fine- 

 grained sediment is deposited, which is partially 

 cemented by an organic mucilage produced by the 

 decomposition of mineral matter of one kind or another. 

 This material is almost entirely impervious to water, 

 a thin layer of it being sufficient to prevent downward 

 percolation, no matter how sandy the soil below may 



