192 CONSERVATION OF GROUND WATER 



the practice has been to pack the buildings so closely as to 

 leave very little of the land surface uncovered. Brooklyn, 

 south Philadelphia, and Baltimore are perhaps the best ex- 

 amples. About half of Brooklyn's 70 square miles is covered by 

 buildings, paved streets and sidewalks, and other impermeable 

 surfaces. Water from precipitation upon these areas is carried 

 out to the shores of Long Island in storm sewers. It has been 

 estimated that the replenishment to the ground-water reser- 

 voir in Brooklyn has been reduced by 30 to 40 million gallons 

 a day by land occupancy (see page 125). This is equivalent to 

 the current rate of pumping from wells in the borough. 



Some residential communities that take their water supplies 

 from wells deplete the ground-water reservoir considerably 

 less than the total pumpage might indicate. In Pasadena, 

 Calif., an intensive study of the Raymond Basin 29 showed that 

 1 acre-foot of water pumped and used in sewered areas within 

 the basin depletes the reservoir by 0.65 acre-foot, but 1 acre- 

 foot used on unsewered areas depletes the reservoir by only 

 0.19 acre-foot. Pumping of about 28,000 acre-feet a year has 

 not drawn on the reservoir as much as expected, partly be- 

 cause of the return flow after use. Nearly half the ground- 

 water replenishment comes from infiltration of precipitation 

 over the basin, a far greater proportion than in undeveloped 

 desert areas. This high contribution is attributed to the fact 

 that there is never any great deficiency in soil moisture be- 

 cause of the use of water on lawns and gardens, etc., through- 

 out the growing season. 



Irrigation and Drainage 



Significant changes in ground-water storage have resulted 

 both from activities involving the irrigation of crops in arid 

 regions and drainage of swamps or lake beds. In many cases 

 the changes of ground-water levels have created difficulties 



29 Report of Referee, Pasadena v. Alhambra, Pasadena C-1323, Superior 

 Court of Calif., p. 188. For the complete reference to this case, see footnote 

 17 on page 60 of this book. 



