190 CONSERVATION OF GROUND WATER 



the Chino Basin in southern California. 280 There the average 

 deep penetration of rainfall was found to be about 4% inches 

 a year in the period from 1927 to 1947. In the wettest year 

 (1941) the penetration was about 16 inches, or about 19 per 

 cent of the total deep penetration from rainfall in the period. 

 About 60 per cent of the total penetration in the 20-year pe- 

 riod occurred in the four wettest years. 



In an area where it is intended to increase the proportion of 

 precipitation that reaches a ground-water reservoir, there 

 should be detailed data as to the precipitation cycle of the lo- 

 cality and its relation to the growing season, and as to the 

 ground-water hydrology. Indiscriminate activity on lands, 

 without this fundamental data, may get water into the ground 

 in places where it can be of no value to the developed ground- 

 water reservoirs and might even be harmful to the water re- 

 sources. For example, many saline springs in the West serve 

 only to contaminate the usable waters in streams, reservoirs, 

 and wells. Operations in ignorance of the areal hydrology 

 might increase the recharge to the aquifers that produce these 

 springs, thus increasing the outflow of brine and aggravating 

 the contamination of usable supplies. 



The need for discrimination in projects to increase ground- 

 water replenishment is clearly seen by reference to Plate I, 

 which shows that only half the area of the country is occupied 

 by ground-water reservoirs capable of yielding moderate to 

 large supplies of water. In the blank areas of that map, so far 

 as known, fresh water occurs only in rock materials that are 

 generally too impermeable to supply more than a few gallons 

 a minute to wells. But many aquifers cannot be replenished at 

 all places by downward percolation from the land surface. 

 Wherever water occurs under artesian pressure, it is likely 

 that the water enters the ground elsewhere, at some point 

 where artesian conditions do not exist. The actual area of the 

 places where the nation's ground-water reservoirs are re- 



28o Muckel, D. C, and V. S. Aronovici, "Progress Report on Rainfall and 

 Irrigation Water Penetration and Consumptive Use in the Chino Basin, Santa 

 Ana River Valley," U.S. Soil Cons. Service, Mirneo. rept., 1948, 96 pp. 



