302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



derived directly from the rain and snow and fail if that supply be- 

 comes exhausted, regardless of the quantities of water that are stored 

 below the water table. On the other hand, however, water from the 

 zone of saturation is utiUzed, either habitually or in times of drought, 

 by native and cultivated plants in many low places, including large 

 parts of the Atlantic and Gulf Coastal Plain, the glaciated region, and 

 the stream valleys and structural valleys in other parts of the country. 

 Thus the relation of the water table to forest and fruit trees, staple 

 crops such as wheat, corn, and alfalfa, garden truck, and native 

 grasses is a subject of great consequence. 



It is estimated that in the eastern part of the United States at least 

 one-third of the water dischai^ed from the zone of saturation is dis- 

 charged by transpiration or evaporation, the rest being discharged 

 chiefly as stream flow. In some of the summer months the discharge 

 by transpiration and evaporation may greatly exceed the discharge by 

 stream flow. In going toward the less humid parts of the country 

 the total annual supply of ground water decreases but the proportion 

 discharged ])y plants increases, until in some of the arid sections 

 vh'tually all the water discharged from the zone of saturation is 

 through plants, and the phreatophyte vegetation, which taps the 

 zone of saturation, stands in strildng contrast to the other desert 

 plants. 



Probably the greatest deficiency in hydrologic knowledge, especially 

 in this coujitry, is in the important practical subject of the relation 

 of the native and cultivated plants to the water table. Not enough 

 attention has been given to the water table by botanists, silvicultur- 

 ists, or agronomists, and adequate information is not available on 

 such important subjects as the phreatophytic habits of trees and their 

 relation to forestation in the arid and semiarid regions, the depth to 

 which different cultivated plants mil extend their roots to reach the 

 water table, the optimum depth to the water table, and the increase 

 in crop production resulting from use by the plants of water from the 

 zone of saturation. The intensive drainage developments that have 

 been made m this country have been based too largely on the concept 

 that the water table is a detrimental feature, and not enough consid- 

 eration has been given to the value of the water table to plants under 

 proper conditions. On the other hand, the madequacy of specific 

 information on this subject is largely responsible for exaggerated 

 statements that are made from time to time as to the disastrous results 

 to agriculture from general lowering of the water table. 



Relation of underground storage to stream jiow and to water supplies 

 from wells. — The principal function of a reservoir is to store water 

 for future use. The underground reservoirs function naturally like 

 lakes and ponds in equalizing the stream flow, but they are more 

 effective because of the retardation of the ground water by the 



