145 

 supply. 



Weeds growing along ditch banks or in irrigated fields are 

 consumers of large quantities of water. Many weeds are adapted to 

 the use of a limited water supply, but if water is abundant they 

 use increased amounts. Canal bank growth, such as willows, alders, 

 and tules, are also consumers of large quantities of water, but is 

 sometimes useful as a means of canal bank protection. 



Vegetation grown in tanks must be surrounded by similar 

 growth if consumptive-use measurements correctly represent losses 

 in open fields. Unnatural exposure of tank growth to sun and 

 wind results in increased losses that may lead to erroneous re- 

 sults. Too often this very important factor has been overlooked 

 in extending tank data to field losses. In considering reduction 

 factors to be applied to data obtained from fully exposed tanks, 

 it seems probable that there is little difference in consumptive 

 use between tank growth and field growth of the various native 

 grasses. Investigations have shown, however, that actual swamp 

 consumptive use of water by tules and cattails lies between l+O 

 and 50 per cent of the consumptive use as indicated by growth in 

 tanks outside their natural environment. Factors for other 

 species probably are found between 50 and 100 per cent. 



Riparian growth along streams, as willows, alders, syca- 

 mores, and cottonwoods, obtain moisture from underground water 

 traveling tovrard stream channels or from waters percolating from 

 the stream bed. They have first use of the water supply in the 

 stream. Regardless of the showing of a considerable use of water 

 by vegetation and an inadequate water supply for much of the west- 

 ern area, it is not the purpose of this report to advocate destruc- 

 tion of vegetation. 



Daily water-table fluctuations in areas of high ground 

 water are usually the result of consumptive use by the overlying 

 vegetation. This is evident from the fact that fluctuations in- 

 crease as the plants approach their maximum growth and decrease 

 as they mature. Fluctuations respond to those factors of weather 



