164 CONSERVATION OF GROUND WATER 



Because of the draft upon water resources by vegetation, 

 practices that are effective in preventing despoliation of the 

 soil resources do not necessarily increase the net water sup- 

 plies available for man's use. The effects of various agricul- 

 tural practices upon the soil and soil water are treated below, 

 and the effects of those practices upon ground water are con- 

 sidered subsequently. 



EFFECTS UPON INFILTRATION AND OVERLAND FLOW 



The disposal of water that falls on the land surface as precipi- 

 tation — whether by infiltration into the ground or by flow 

 over the surface — is of major significance to the agriculturist. 

 Overland runoff can carry away his precious soil, cut his fields 

 by gullying, and then create problems for downstream users 

 of water in streams and reservoirs. Water infiltrated into the 

 ground, on the other hand, may remain in the soil and be used 

 by his crops. The alternative methods of disposal are thus the 

 alternatives of benefit or detriment, of conservation or waste. 



The soils and rock materials that form the land surface vary 

 tremendously from place to place in their capabilities for ab- 

 sorbing water. The surficial materials in some areas are so ex- 

 ceedingly permeable that overland runoff rarely occurs, and 

 in other areas they are so impermeable that there is negligible 

 infiltration of water into the ground. By far the greater part 

 of the land surface is formed by materials moderately to 

 slightly permeable. 



The high natural infiltration capacities of gravels, sands, 

 boulders, and some other rock materials have generally not 

 been modified by man's use of the land. Although these ex- 

 tremely permeable materials cover only a small proportion of 

 the area of the country, there are some drainage basins in 

 which a high percentage of the precipitation is absorbed into 

 the ground and the flow in streams is derived chiefly from 

 ground water discharged at relatively constant rates. The 

 Metolius River in Oregon, draining permeable basalts, and 

 the Loup River in Nebraska, draining the Sand Hills, are out- 



