64 



of the sun by surrounding grass. Crop tanks in bare fields may 

 have a somewhat higher rate of water use, but data bearing out 

 such a conclusion are lacking. 



No definite figure is available for use as an adjustment 

 factor for willow or other brush grown in tanks. In dense growths 

 of brush the effect of sunlight and wind is modified by surround- 

 ing vegetation and consumptive use under such conditions will be 

 less than by isolated growth in field or tank. Considering dif- 

 ferences in willow distribution it is evident that an adjustment 

 factor is not a constant, applicable to all conditions, but a 

 variable depending upon density and size of brush area. Owing to 

 present lack of evidence, any factor selected must be only an es- 

 timate. For willow growth in the Santa Ana Valley, where such 

 growth is partly in solid blocks of brush and partly scattered, 

 it is estimated that consumptive use varies from 75 to 100 per 

 cent of consumptive use by willows grown in exposed tanks. A 

 tentative factor of 85 per cent is adopted. 



Use of water by wire rush grown in a tank at the Santa Ana 

 station exceeds that of any other growth except tules and cattails. 

 While the tank was not set in a field of similar growth, it was 

 surrounded by grass and weeds. It is possible, since the wire 

 rush did not grow in its natural moist-land habitat, that change 

 of environment was responsible for the high use of water, but it 

 seemB more probable that an ample water supply close to the sur- 

 face and an unusually heavy growth were the direct causes. 



A summary of tank investigations showing estimated adjust- 

 ment factors and consumptive use of water in moist areas is pre- 

 sented in Table 13. 



