62 



coefficient, determined by experiment to be 0.70, must be appliec 

 to pan records to obtain equivalents for large bodies of water. 

 Application of this value to Weather Bureau pan data at Victor- 

 ville indicates an equivalent lake evaporation of 57.72 inches. 

 Using this value it is found that the consumptive use by tules 

 in the swamp was 135.9 per cent of the computed lake evaporation, 

 which classes aquatic growth as a heavy user of water. 



ADJUSTMENT FACTORS FOR LARGE AREAS 



Tank records of consumptive use of water by aquatic growth 

 are not suitable for extension to large areas unless modified by 

 proper coefficients which too often have not yet been determined. 

 Consequently, in many cases, tank records may be of little actual 

 value for the determination of consumptive use. Save for the re- 

 lation of swamp consumptive use to evaporation, determined as 95 

 per cent at the Victorville station in Mohave Valley, reduction 

 coefficients in southern California are lacking. The Santa Ana 

 Valley tank records should be reduced through application of the 

 Victorville factor. 



Table 12 has been prepared to show the consumptive use of 

 water by tules and cattails in swamp areas as percentages of ob- 

 served consumptive use in exposed tanks, the coefficient of 0.95 

 being used to reduce evaporation values to estimated swamp values. 

 Swamp consumptive use as a percentage of observed tank use varies 

 from 29 to 55 per cent with an average of 40 per cent. The fig- 

 ures indicate, on the whole and as far as a determination is 

 possible, that consumptive use of water by natural growt^h in 

 swamp areas in southern California averages but 40 per cent of 

 the observed consumptive use as indicated by tank records. 



No adjustment factor is necessary for grasses grown in 

 tanks surrounded by fields of similar growth, as conditions are 

 so nearly those of the field that factors for these crops have 

 been taken as 100 per cent. It is emphasized that tanks should 

 be set low in the ground with their rims protected from the rays 



