216 CONSERVATION OF GROUND WATER 



moisture. The consumptive-use draft on the surface- and 

 ground-water resources varies from place to place and varies 

 seasonally, but is increasing progressively as the human popula- 

 tion and its standard of living rise. Perhaps a separate category 

 should be made for the water taken from its place in the hydro- 

 logic cycle and put in storage for an indefinite period in plant 

 and animal tissues or in canned foods or other products. The 

 release from this storage enables the sugar-cane processing in- 

 dustry, for instance, to show a "negative" consumptive use of 

 water: the water for disposal is greater than the amount used 

 in processing, because of the water squeezed out of the 

 canes. 



Consumptive use accounts for only a fraction of the water 

 in the major categories of use by man. It is negligible in pro- 

 duction of power, navigation, disposal of wastes, and recrea- 

 tional use, except for the evaporation from water surfaces 

 exposed for those uses. Of the water applied for irrigation, as 

 little as half may be used consumptively, and probably con- 

 siderably less than half the municipal and domestic supplies 

 is actually consumed. Generally only a small proportion of 

 industrial water is used consumptively, but there is a wide 

 range among the different industries. The consumptive use 

 of water in irrigation, industry, and public supply may be of 

 the order of 50 to 80 billion gallons a day, which is about 4 

 to 6 per cent of the average stream flow. On the basis of a 

 natural return to the atmosphere of 3 trillion gallons a day, 

 our consumptive use of surface and ground-water resources 

 has increased this return by 2 or 3 per cent. This is by no means 

 the total consumptive use of the water that falls as precipita- 

 tion. Nearly three-fourths of the precipitation water is re- 

 turned to the atmosphere by evapotranspiration, of which an 

 unknown proportion represents the consumptive use of agri- 

 cultural crops. Unless the crops are irrigated, however, their 

 consumptive use is generally not derived from the net surface- 

 water and ground-water resources. 



It has become rather common practice to label all transpira- 

 tion as "consumptive use" by vegetation. Many plants have 

 very little utility for mankind, and some could be classified as 



