28 CONSERVATION OF GROUND WATER 



tween rains is long enough. It has been amply demonstrated 

 that man can increase the infiltration of water into the soil by 

 conservation practices such as strip cropping, contour plow- 

 ing, and terracing, and progressive farmers have found this 

 augmented storage of especial value for maintaining crops in 

 rainless periods. 



Soil holds water by molecular attraction against the force 

 of gravity. Only the excess above this field capacity moves 

 downward to become ground water. 



Ground-water reservoirs provide all the water yielded by 

 wells and springs and constitute the source of the base flow 

 of streams — the flow sustained throughout rainless periods. 

 These reservoirs probably hold several times as much usable 

 water as the combined capacities of all lakes and surface res- 

 ervoirs, but we do not have enough information to provide 

 a reliable estimate as to how much water is in them. Some 

 examples provide an indication of the volume of water in 

 storage in small areas of the country: it has been estimated 4 

 that the total volume of water that could be yielded by the 

 ground-water reservoir under a 6,700-square-mile area in the 

 Southern High Plains of Texas is about five times the capacity 

 of Lake Mead, our largest surface reservoir. The alluvial 

 valley of the Mississippi River below the mouth of the Ohio 

 is estimated 5 to contain about 1,000 cubic miles of alluvial 

 material. The water in this saturated material need be only 

 13 per cent of the total volume, a reasonable or even a con- 

 servative figure, to equal the quantity discharged annually 

 by the river into the Gulf of Mexico. (The Mississippi River 

 discharges about one-third of the total runoff from the Na- 

 tion's borders.) 



The term "ground-water reservoir" is commonly used to 

 designate the water-bearing material, or aquifer, from which 



* Barnes, J. R. et al., "Geology and Ground Water in the Irrigated Region of 

 the Southern High Plains of Texas," Texas Board of Water Engineers, Progress 

 rept. 7, p. 41, 1947. 



s Fisk, H. N., "Geological Investigation of the Alluvial Valley of the Lower 

 Mississippi River," Mississippi River Commission, pp. 17-18, 1944. 



