There are four general methods by which consumptive use of 

 water by native vegetation may be determined: tank studies, soil- 

 moisture investigations, stream-flow studies and water-table fluc- 

 tuations. Tank data on use of water by native vegetation are 

 available for several kinds of native vegetation in western States. 



The most satisfactory method of water-table control in 

 tanks appears to be that using the Mariotte supply tank as develop- 

 ed by the Division of Irrigation. This equipment is automatic and 

 permits regular observations of water consumed by tank growth. 



Annual consumptive use of water by saltgrass varies from 

 more than 40 inches when the water table is 12 inches from the 

 surface to as little as 10 inches when depth to water exceeds 

 3 to 4 feet. Other factors than depth to water table also influ- 

 ence these values, as is evident from the considerable differences 

 in water used for the same depth to water table at different loca- 

 tions in western States where investigations have been made. 



Wire rush appears to be a heavy user of water when a plen- 

 tiful supply is available, but the number of investigations with 

 this plant are insufficient for conclusive data. 



Willows usually grow where the roots extend into the ground- 

 water region, and they appear to use the approximate equivalent of 

 evaporation from a water surface. Investigations with willows are 

 limited, and this relation may vary for different localities. 



Tules and cattails grow with their roots in water and con- 

 sume greater quantities than other varieties. Sedges compare in 

 consumptive use with other aquatic growth. 



The natural growth of brush and weeds found on outwash 

 slopes in arid and semiarid regions depends entirely upon precip- 

 itation for moisture. As this varies widely from season to season 

 it is evident that there is no definite water requirement for such 

 vegetation. In years of light precipitation all moisture entering 

 the ground is consumed within the root zone, but as precipitation 

 increases to proportions iinnecessary for plant life, moisture 

 passes beyond the roots as a contribution to the underground-water 



