18 



a previously saturated soil by gravity, measured as a percentage 

 of the total volume. Fluctuations of the water table, caused by 

 transpiration losses, show a decline by day when transpiration is 

 greatest, and a recovery by night when it is least. Measurement 

 of the fluctuations by continuous recorders provides a basis for 

 calculating the consumptive use. Investigations have shown that 

 the daily fluctuations usually do not occur \inless there is vege- 

 tative discharge. They respond directly to weather conditions, 

 increasing in amplitude with sunshine, temperature, and a clear 

 sky, and decreasing with lower temperature, greater humidity, and 

 increasing cloudiness. 



This method was first proposed by Dr. G. E. P. Smith of 

 the University of Arizona in an unpublished paper read before 

 the Geological Society of Washington, November 22, 1922. It has 

 been successfully used by White (38) in the Escalante Valley, 

 Utah, and by Troxell (33) in the Santa Ana Valley, Calif. It is 

 an ingenious method of translating a natural phenomenon into 

 vegetative discharge, having the distinct advantages of using 

 soil that has never been disturbed and of measuring consumptive 

 use by vegetation in its native habitat. On the other hand it 

 involves the obvious difficulty of obtaining average specific 

 yield of a large noniiniform soil mass underlying a given basin. 



TANK MEASUREMENTS 



Metal tanks have been used extensively in plant investiga- 

 tions for many years. Those best adapted for consumptive use of 

 water studies are of the double type having an annular space for 

 water between the inner and outer walls, with perforations 

 through the inner wall to insure a thorough distribution of 

 water throughout the soil mass. Tanks of this design usually 

 are from 24 to 36 inches in diameter by 4 to 6 feet deep. They 

 should be of heavy galvanized iron to withstand corrosion. In 

 acid or alkali soils the metal should be treated with a protective 

 coating. 



