PROBLEMS FROM LAND OCCUPANCY 191 



charged by precipitation is unknown, but, as a rough guess, 

 it is hazarded that all the ground-water reservoirs capable of 

 yielding large quantities of water to wells may be dependent 

 for their replenishment upon infiltration in less than a quarter 

 of the area of the nation. 



Effects of Urban Use of Land 



Urban areas cover a relatively small part of the total area 

 of the country, but they include many of the areas where man 

 has changed the land surface most profoundly. The topog- 

 raphy has been modified by extensive cutting and filling, and 

 natural drainage patterns have been revised materially. So 

 far as the hyclrologic cycle is concerned, some of the most sig- 

 nificant changes have resulted from construction of buildings, 

 pavements, and other impermeable surfaces upon the land. 

 These have prevented infiltration into the ground in exten- 

 sive areas. Precipitation upon them is carried elsewhere by 

 drains to the sewer system or by snow-removal crews or other 

 means. The water is generally not wanted immediately under- 

 ground because of the damage it might do to utilities, or base- 

 ments, or other subsurface structures. 



The effects of urban development upon water supplies are 

 quite varied. New York's Manhattan Island is an example of 

 an area that contains no ground-water reservoir worthy of 

 the name, and the storm sewers merely act as substitutes for 

 the natural drains that carried the runoff when the island 

 was worth $24. Storm sewers of several cities empty into 

 streams or other water bodies where the water is available 

 for further use. In many cities the storm sewers are cut deeply 

 enough to carry ground water as well as storm runoff from the 

 city. In many residential areas, the water collected from roofs 

 is discharged into the ground, so that infiltration may be 

 equivalent to that which existed before the residential de- 

 velopment. 



Several cities are located on the recharge areas of important 

 ground-water reservoirs, and of these there are some where 



