68 THE FUTURE OF ARID LANDS 
water used by plants. Experiments which attempt to measure 
the water loss from a leaf or a branch detached from the plant, 
or from isolated plants in special pots, are highly artificial, and 
generalizations from such studies have sometimes been greatly 
in error. The only method that measures the evapotranspiration 
from a field or any other natural surface without disturbing the 
vegetation cover in any way is the so-called vapor transfer method 
(14). Water vapor when it enters the atmosphere from the ground 
or from plants is carried upward by the moving air in small 
eddies or bodies of air that are replaced by drier eddies from 
above. If we determine the rate at which the air near the ground 
is mixing with that above it and at the same time measure the 
difference in water vapor content at the two levels, we can deter- 
mine both the rate and the amount of evapotranspiration. 
This method is not easy to understand or to use. It requires 
physical measurements more precise than are usually made. 
However, the method can and should be perfected for it will 
answer many important questions for climatology and biology. 
There are other ways of determining both water use and water 
need. In some irrigated areas rainfall, irrigation water, and water 
outflow are all measured. The fraction of the applied water that 
does not run off is the evapotranspiration. In a few isolated 
places, mostly in western United States, irrigation engineers 
have determined the evapotranspiration from plants growing in 
sunken tanks filled to ground level with soil in which water 
tables are maintained at different predetermined depths beneath 
the soil surface (19). 
Since 1946 increasing thought has been devoted to the problem 
of measuring the water use of plants under optimum soil moisture 
conditions, and an instrument has been developed and standard- 
ized (4, 18). It consists of a large soil tank so constructed that 
plants can be grown in it under essentially field conditions and 
can be provided with water as they need it. The tanks are 4 square 
meters in area and contain soil to a depth of approximately 70 
centimeters. They have means for subirrigation from a supply 
tank designed so that actual amounts of water used can be accu- 
rately measured, or they can be irrigated by sprinkling from above. 
