66 DIVISION OF WATER RESOURCES 



but no basis exists for computing the proper factor for use with willows. 

 Because there is a great natural difference in the habits of growth of 

 tules and willows, the same factor will not apply to both. Tules 

 naturally grow in swamp areas, whereas willows are found in scattered 

 areas of dry land, sometimes in isolated clumps, small groups, or in 

 large bodies of ))rush. AVhere isolated growth occurs, conditions of 

 temperature, sunlight, and wind movement are the same as for tank 

 growth, and consumptive use is nearly the same in all cases. In dense 

 grovrths of brush, however, conditions are changed to reduce the 

 factors mentioned and transpiration also is less. In this case, an adjust- 

 ment factor should be applied to the observed consumptive use by 

 willoM'S grown in tanks to arrive at the correct figure for field con- 

 ditions. In many instances willows grow as a fringe along water 

 courses, and endjankments of ditches. They are also found in large 

 areas interspersed with open places that are sometimes of considerable 

 size. Considering differences in the spread of willow growth it is evi- 

 dent that an adjustment factor is not a constant that can be used under 

 all conditions, but is a variable depending upon density and size of 

 brush. Due to the present lack of data, any factor arrived at must be 

 only an estimate, subject to revision later when further evidence is 

 available. For willow growth in the Santa Ana Kiver basin, which is 

 ]iartly in solid blocks of brush and partly scattering, it is estimated 

 that consumptive use is 75 to 100 per cent of the amount of water 

 necessary for isolated tank growth, with an average of 85 per cent. 



Use of Water by Wire Rush 



Wire rush (Juncus hallicus) grows in limited areas of high ground 

 water in the Prado basin where it is found in association with salt grass. 

 In appearance it is a heavy, tough, Avire-like grass growing from a thick 

 creeping rootstalk. In places where it was observed, it did not exceed 10 

 or 12 inches in height. Some of this growth was transplanted into a 

 small tank at the Santa Ana station in the summer of 1930 for the pur- 

 pose of making a consumptive use of water study, as indications were 

 that it might be a large user of water. During the first year of the 

 study a considerable amount of salt grass Avas included with the wire 

 rush, but in the following year the heavier growth had croAvded out 

 the salt grass. 



A fixed water level was maintained at a depth of 2 feet in the 

 tank, although where a natural growth occurred in tlie fields there were 

 seasonal fluctuations of ground water at greater depths. This depth 

 was the same as that chosen for investigations of consumptive use by 

 salt grass, Bermuda grass, and willows. During the first winter, the 

 wire rush tank was protected from rainfall, as were all soil moisture 

 tanks, but during the second year it was exposed to all rains. 



A summary showing monthly use of water b,y wire rush is given 

 in Tables 14 and 15. The maximum use for any month was 13.75 acre- 

 inches per acre during July, 1931. The total use for a ten-month 

 period was 83.45 acre-inches per acre or 141.8 ])er cent of the evapora- 

 tion from a Weather Bureau pan for the same period. In comparison 

 with consumptive use by grasses and willows, the wire rush has 

 appeared to be a heavy user of water and it is fortunate that the area 

 restricted to its growth is limited. 



