OUR WATER SUPPLY— MEINZER 301 



ground water through stream flow and through evaporation and 

 transpiration. Greater aggregate increase in recharge is Ukely to 

 result from general improvements in agricultural practice and from 

 structures designed to retard soil erosion. 



Great as are the variations in precipitation from place to place and 

 from year to year in the same place, the variations in ground-water 

 recharge are still greater. It is now known, for example, that per- 

 ennial supplies amounting to many millions of gallons a day are 

 available to wells through natural recharge of the sand and gravel in 

 the fill of the coastal valleys and the Great Valley of Cahfornia, the 

 glacial outwash sands and gravels of Long Island, the Rhine Valley 

 and the plain of northern Germany, the dune sands of Holland, the 

 creviced limestone in the RosweU artesian basin in New Mexico, the 

 broken lava rocks of the Snake River Plain in Idaho and the islands 

 of Oahu and Maui, and the water-bearing rocks of various other areas. 

 On the other hand, there are areas in which ground-water recharge 

 is extremely small, either because the surface terrane is impermeable 

 or because the water absorbed from scant precipitation is nearly all 

 evaporated or utiUzed by plants before it reaches the bottom of the 

 root zone. Large areas in the arid and semiarid parts of this country 

 have only very meager recharge because the precipitation is light and 

 occurs largely in the growing season. Yet many of these areas are 

 underlain by water-bearing formations that contain large stores of 

 accumulated water which they will yield freely to wells so long as the 

 supply lasts. In the coastal region of Cahfornia and in the Great 

 Valley the soil normally becomes desiccated during the long dry sum- 

 mers. In winters of subnormal precipitation the precipitated water 

 is here largely required to restore the soil moisture, and there may be 

 httle ground-water recharge; in exceptionally wet winters, however, 

 the water-retaining capacity of the soil is satisfied long before the end 

 of the rainy season and very large quantities of water percolate to the 

 water table, either locally through the soil or through the channels of 

 the influent streams. In the relatively humid eastern part of the 

 United States there is normally considerable recharge not only in 

 winter and spring but also in wet periods in summer, but in the drought 

 of 1930-31 some localities were devoid of recharge for nearly a year. 

 In cold regions with only moderate precipitation nearly all the re- 

 charge may occur in a very short time in the spring when the snow 

 melts and the frost leaves the soil. In such regions there are also 

 great differences in the annual crop of ground water. 



Relation of the water table to the 'plant kingdom. — It has already been 

 pointed out that in most places the roots of the plants do not obtain 

 their water supply from the zone of saturation. Throughout the 

 greater part of the extensive and productive interior agricultural 

 region of our country, the staple crops depend on the soil moisture 



