25 

 coefficient which must be applied to tank results. Grasses, 

 particularly the original crop of an undisturbed soil, are most 

 likely to have the same density of growth as under field condi- 

 tions, and the use of water by the tank crop will be approximate- 

 ly the same as by grass in the open field. For native shrubs it 

 is difficult to obtain the same density of growth as under natu- 

 ral conditions, since this tj'pe of vegetation does not grow in 

 an orderly manner. The wide spacing of some shrubs and the close 

 growth of others make the correct unit area per plant a matter of 

 conjecture . 



Aquatic plants, as tules and cattails, may grow with the 

 same approximate density in tanks as in swamps, although the 

 plants around the edges are more stunted than in the center, 

 owing apparently to greater exposure to sun and wind. The n\imber 

 of stems per unit area is likely to vary in different tanks. A 

 comparison of the density of tules in the Santa Ana investigation 

 (4), showed a tank 6 feet in diameter to have a density of 57 

 stems per square foot of area and to use 12.43 acre-inches of 

 water in September, while a 2-foot tank having 87 stems per foot 

 used 19.37 acre-inches. Both tanks had the same exposure. Carry- 

 ing the comparison further, the consumptive use of v/ater per indi- 

 vidual stalk was the same regardless of density of growth or the 

 size of tank in which it grew. However, the tank growth was 

 stunted in comparison with normal swamp growth. 



The limitations of tank groiirth, as affected by environment, 

 are extremely important in determining the quantity of water used. 

 It is emphasized that tanks containing vegetation must be sur- 

 rounded by the same tjrpe of growth; otherwise there can be no true 

 comparison of the amount of water used by the tank vegetation and 

 similar growth in the field. 



The injury to roots caused by transplanting vegetation into 

 soil tanks, or through cutting the roots, temporarily limits the 

 plant growth and temporarily affects the amount of water consumed. 

 Plants with running roots (saltgrass or brush for example) are 



