History and Problems 
of Arid Lands Development 
H. L. SHANTZ 
Santa Barbara, California 
Definition of the Arid Zone 
The arid zone has not been precisely defined. Probably in no 
zone on the earth are there greater swings in precipitation, tem- 
perature, and aridity than in this zone. It is customary to charac- 
terize this area in terms of the minimum of precipitation and the 
maximum of heat and aridity. Rainfall in this zone is rare, local- 
ized, irregular, and often violent: precipitation in the more arid 
portions of the zone averages less than an inch a year, but more 
than the yearly average may fall in a single storm. The precipi- 
tation in the semi-arid parts may be as high as 30 or 40 inches a 
year, enough, if maintained, to move the area into the humid 
zone. The relative humidity in the extremely arid parts may drop 
to less than 5%. The highest air temperature on earth, 136° F, 
was recorded 1n 1922 in this zone at Aziza in Libya. 
To define such a zone properly would probably not be possible, 
for if done the definition would be bound largely to a single factor 
of the environment. A definition based on climatic data would not 
be the same as one based on soils, on vegetation, on animal dis- 
tribution, or on land use. 
I would be inclined to include all the area from extremely arid 
to semi-arid in this belt. In this whole range the only safe assump- 
tion is that any year may be extremely arid. The more humid 
years will take care of themselves, but the more arid years set the 
pattern of use and must be anticipated and planned for if man is 
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