THE FUTURE OF ARID LANDS 
78 
amount decreases, so that in dry regions one might expect great 
yearly changes in the extent of the arid and semi-arid zones. As 
part of the inventory it is necessary to determine the year-to-year 
shifts in the extent of the climatic regions and to make available 
formation in graphical or cartographic form. Good examples 
are the maps showing the year-to 
over the United States which I prepared 
of Agriculture in 1 
the in 
year variations in climatic zones 
for the U. S. Department 
941 (g). From these data it was possible to de- 
f the different climatic zones 
f years each o 
termine the number o 
2 shows the frequency of arid 
arid climates over the United States based on the period 
© 
occurred over different areas. Figure 
and semi 
f this type, when coupled with the de- 
formation o 
tailed data on the moisture regions of the world 
much more adequate basis for the interpretation of moisture prob- 
lems than is presently available. 
—39. Inf 
Igol 
would provide a 
) 
year fluctuations in the 
f the broad-scale moisture regions it is 
to 
ar- 
In addition to determining the ye 
eographic distribution o 
desirable to know the probabilities of occurrence o 
oO 
5 
f values of the 
other important factors which constitute the water balance. In 
FREQUENCY OF CLIMATES, SEMIARID AND DRIER 
ANNUAL 
ORR) 
PERCENT OF YEARS 
ear] 100 SSE 
[EES] 7-99 ZA \-25 
= s5:-75 BSQ o 
. AAA ANI yoy. 
Frequency of semiarid and drier climates in the United 
Figure 2. 
States. 
