INCREASING REQUIREMENTS 215 



volumes of water wherever regulations specify that the waste 

 products be amply diluted. The use of water for navigation is 

 a ticklish question in the development of the Missouri River 

 Basin, where navigation will require minimum flows of water 

 that might be used alternatively for irrigation or other pur- 

 poses. But to compute the amount of "use" for navigation is 

 difficult. If navigation "use" is defined as the quantities re- 

 leased from reservoirs to maintain a minimum depth of chan- 

 nel for river navigation, the quantity may be significant on 

 such streams as the Missouri. It is no easier to compute the 

 "use" of water for recreation. The recreational value of a lake 

 or reservoir becomes nil when it is emptied in order to use 

 the water for other purposes, and if the lake level is held con- 

 stant for best recreational use, that "use" might be computed 

 as equivalent to the quantities which have not been taken for 

 other purposes. Computation of the total water used for power, 

 waste disposal, navigation, and recreation might easily give fig- 

 ures far greater than the total runoff from the country. 



It is obvious that some water is being used more than once, 

 and some a good many times. In order to relate usage of water 

 to water resources, it is necessary to distinguish the quantities 

 that are consumed by use, from those which remain in the 

 liquid state although the chemical or physical properties of 

 the water may be changed by use. The water that is consumed 

 is taken from surface or subsurface resources and returned as 

 vapor to the atmosphere and thus completes its hydrologic 

 cycle sooner than it might under natural conditions. 



Consumptive Use and Waste 



Water returns to the atmosphere by evaporation or in the 

 special cases where it has first served living processes, by 

 transpiration or perspiration. Of the water used by men, these 

 are the consumptive uses. They constitute elements of varying 

 importance in the domestic, industrial, and agricultural use 

 of water. Transpiration by the plants of economic value is a 

 use of great benefit to man, but that water is derived from soil 



