RESEARCH AND DEVELOPMENT Ss) 
the amount of water which will return to the atmosphere from a 
surface completely covered with vegetation when there is in the 
soil sufficient moisture for the full use of the vegetation at all 
times, and this he calls the “potential evapotranspiration.” 
Geen 73:) 
One way of determining evapotranspiration is by the “vapor 
transfer’ method based on the rate at which air near the ground 
is mixing with air above it at a given height and by measuring 
the difference in water vapor content at the two levels. 
It is possible also to measure rainfall, the inflow of irrigation 
water, and the outflow water, regarding the amount which does 
not run off as evapotranspired. 
Latterly specially designed soil tanks 4 square meters in area 
and 70 centimeters deep, in which plants can be grown under 
field conditions, have been set up in a number of places, each 
tank being surrounded by a large buffer area to ensure greater 
accuracy in results, but not enough are yet in use to give the 
range of variation from one area to another. Meantime Thorn- 
thwaite and his colleagues have come to the conclusion that the 
computation of potential evapotranspiration for any place can 
be done from data on air temperature and latitude alone. With 
these it is possible to determine the water needs of an area and, 
as it were, keep accounts whereby the most economical use may 
be made of irrigation water. 
Underground Water 
Having briefly referred to rainfall, using that as a general term, 
we may naturally turn to what happens to the rainfall apart 
from evapotranspiration. Some infiltrates the soil and other layers 
and goes underground, where it may be stored or move slowly in 
suitable layers gradually toward the sea. The most generally used 
method of determining infiltration is to examine the data of the 
use and fall of water in wells, although today radioactive isotopes 
are available to make evident the movement of water through 
permeable strata. There are in the United States about seven 
thousand observation wells, and approximately 5% of these have 
automatic recording equipment. 
The examination of an area for underground water in the first 
