WATER RESOURCES IN ARID REGIONS 87 
within the network of the nation itself, of an association of states 
permitting compensation, or on an international scale. 
This raises inevitable political problems, and it can be stated 
from the record that the prosperity of arid regions has been ex- 
tremely sensitive to political conditions. 
Economic Planning 
In arid regions planning should be for long periods. The five- 
year period, often used in temperate countries, is too short in the 
arid zones. We give two examples. 
Tunisia had established a plan for agricultural development 
for the years 1949-53, based on statistics for preceding years. 
The years 1942-48 had been very dry, but the years 1949-53 
were extremely humid. Agricultural production increased con- 
siderably, but it is very difficult to determine what part of the 
increase was the result of the measures taken under the plan. 
With the same thought (8) Mexico established a six-year plan 
providing for a 50% increase in wheat production for the period 
1953-59, 1n comparison with the period 1948-51. Argentina 
aimed at a 27% increase in wheat production before 1958, taking 
for base the production of 1947-51. Variability in climate can 
cause as great variations, despite the fact that these countries 
are larger than Tunisia. When these plans terminate, it will 
perhaps be difficult to determine whether the objectives have 
been attained; yet it is necessary to check their efficacy. 
A rational procedure should include (a) planning for more 
than 5 years, depending on the variability of the climate (20 or 
30 years, for example), and (4) submitting the economic statistics 
to climatic corrections resulting from a parallel study by statis- 
ticlans, agronomists, and hydrologists. 
Variability in Rainfall 
Rainfall is the most variable element of climate and the one 
that most conditions the economy of arid regions. Variability in 
rainfall has repercussions on harvests, where dry culture is used, 
on the volume of water reaching the water table, and on the 
volume of runoff. 
