86 THE FUTURE OF ARID LANDS 
Effects of Variability of Climate on Economic Life of Arid Regions 
Arid zones are characterized by lack of water and variability 
of climate. This variability entails numerous economic, social, 
and political consequences. The enumeration of some of these 
will be useful for they may orient studies of hydrology and 
climatology. 
Nomadism 
The rainfall must exceed certain amounts to be usable either 
for agriculture or for cattle-raising. These amounts are not 
exceeded everywhere at one time. The population must plant 
where the rain has fallen and likewise transport their flocks there. 
Nomadism is therefore obligatory for complete utilization of 
very arid areas. 
Storing 
By storing is meant all the methods of storing resources or 
products from one year or from one group of years to another. 
Storing first has to do with the water supply, and in arid 
regions it necessitates regulation continuing over several years 
in the interest of the oaglereround aquifers which alone are 
capable of resisting loss by evaporation. 
Storing should also cover all agricultural products, the 
principal ones in Tunisia being wheat, vegetables, oil, and fodder. 
Stocking of these products requires the intervention of the state 
or of powerful cooperatives, and seed supplies should be furnished 
by the state in many areas. 
In any case, material means for storing are not sufficient and 
should be supplemented by a system of credit. 
Exchanges 
Despite all the methods of storing that an arid country can 
utilize, its possibilities of demographic expansion remain very 
limited. Beyond these possibilities, and without the importation 
of water from surrounding humid lands, it is necessary to proceed 
to exchanges of agricultural products with regions where the 
climate is temperate or complementary. Such exchanges can be 
