proper division of the supply between contending uses need first 

 of all a knowledge of the amounts consumed by crops and native 

 vegetation; without such data there is little likelihood that re- 

 sults of investigations can be final. Moreover, in planning new 

 irrigation projects, consideration must often be given to differ- 

 ences in amounts of water used by irrigated crops and those used 

 by the native vegetation replaced by the crops. These differences 

 largely determine the extent of the available water supply and 

 show how much must be obtained from other sources. 



By "Use of Water," the title of this report, "consumptive 

 use" is principally intended. Consumptive use, sometimes called 

 "evapo-transpiration, " is the sum of the volumes of water used by 

 the vegetative growth of a given area in transpiration or building 

 of plant tissue and that evaporated from adjacent soil, snow, or 

 intercepted precipitation on the area in any specified time. If 

 the unit of time is small, such as a day or week, the consumptive 

 use may be expressed in acre-inches per acre or depth in inches; 

 whereas, if the unit of time is large, such as a crop-growing 

 season or a 12-month year, the consumptive use may be expressed 

 in acre-feet or depth in feet. Such terms as "irrigation require- 

 ment" and "water requirement," sometimes used to designate, 

 respectively, the quantity of irrigation water applied to crops, 

 or the total quantity including rainfall required for their normal 

 and profitable production under field conditions, include some 

 unavoidable losses by deep percolation. Such losses are not in- 

 cluded in the definition of consumptive use, which designates only 

 the \inrecoverable portion of the water supply. 



Investigations by which consumptive use is ascertained do 

 not involve the determination of amounts of water "evaporated from 

 adjacent soil, snow, or intercepted precipitation on the -area," 

 since such evaporation is difficult -- often impossible -- to 

 ascertain and of no particular interest as a separate element in 

 the total unrecoverable portion of the water supply. On the other 

 hand, the relation of consumptive use to evaporation from water in 



