Climatology in Arid Zone Research 
C. W. THORNTHWAITE 
Laboratory of Climatology, Centerton, New 
Jersey 
The problems of the arid regions are climatic in origin and 
stem from the imbalance between the water supply and the 
water need. Two obvious proposals have been suggested in order 
to relieve this disparity between water need and supply: (1) in- 
crease the water supplies through artificial induction of rainfall; 
(2) make better use of the existing water supplies by study of the 
water requirements of crops and avoidance of overirrigation. 
This paper will be directed toward the second—ways in which 
existing water supplies may be utilized more effectively. 
One cannot determine the amount by which precipitation fails 
to supply the water needs of crops unless these water needs are 
known. Thus, it is first necessary to determine the water need. 
This most important climatic element is defined as the amount of 
water which will return to the atmosphere from a surface com- 
pletely covered with vegetation when there is sufficient moisture 
in the soil for the use of the vegetation at all times. I have called 
this the potential evapotranspiration (10, 17). 
Measurement of Evapotranspiration 
Precipitation is easily measured by means of rain gages and 
has been recorded in most settled areas of the world. It is not 
easy to measure evapotranspiration, however. In fact, no weather 
service in the world yet determines this important element, and 
the little known about its distribution has been pieced together 
from various scattered determinations. 
Scientists have tried various ways to determine the amount of 
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